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PREFACE
This work forms the first part of a research programme undertaken by BRANZ
to prepare design information for the fire performance of connections for
fire-rated timber members. The information from current overseas
standards, codes of practice and design guides is either unconfirmed or
inadequate for local practices.
ACKNOWLEDGEMENTS
The authors wish to acknowledge the assistance of Alyson Gabriel, Eppie
McDougall and Mary Coley of the BRANZ Library. Thanks are due to Howard
Harman, Don McConnochie, Bob Holbrook and Joop de Ruiter for their highly
skilled assistance with the experimental work, and Dave Kay and Chris
Becker for their backup from the BRANZ Workshop. The authors would also
like to express their gratitude to their colleagues Jason King, Denis
Bastings, Alan Woodside and Colleen Wade, also Andrew Buchanan of the
University of Canterbury, Bryan Walford of Forest Research Institute and
Jack Barnes for valuable discussions.
NOTE
The mention of trade names and construction systems in this report does
not imply exclusion of other products or practices for these applications,
nor specific endorsement by the Association.
This report is intended for fire and structural engineers and other
workers in the field of fire engineering research.
THE FIRE PERFORMANCE OF UNLOADED NAILED GUSSET CONNECTIONS FOR FIRE-RATED
TIMBER MEMBERS
P.K.A. Yiu
A.B. King
Study Report SR21
REFERENCE
Yiu, P.K.A. and King, A.B. 1989. The fire performance of unloaded nailed
gusset connections for fire-rated timber members. Building Research
Association of New Zealand. Study Report SR21. Judgeford.
b
-I
BUlLDlNG RESEARCH
ASSN. OF N.Z.
I
KEYWORDS
From Construction Industry Thesaurus - BRANZ edition: Bibliographies;
Coating materials; Failure; Fire; Fire properties; Fire protection; Fire
Resistance; Gusset plates; Gypsum plasterboard; Intumescent; Joints;
Loads ; Nails ; Performance concepts; Plywoods ; Properties; Steel;
Structural design; Structural members; Testing; Thermal insulation;
Thickness; Timber.
ABSTRACT
The fire performance of heavy glulam timber members in portal frame
structures is well understood. However, less is known about the fire
resistance of their nail-on plywood or steel gusset connections. Some
work has been done overseas to establish standards, codes of practice and
design guides in this field. Unfortunately, few of these are readily
applicable in New Zealand.
This work forms the first part of a research programme aimed at
establishing the appropriate guidelines and providing comprehensive design
data for the fire design of these connections in New Zealand. Relevant
work has been reviewed and summarised. The experimental investigation
consisted of fire tests on both unprotected and protected unloaded gusset
samples as well as other timber samples. The results provide a useful
understanding of various design aspects and the relative performance of
different protecting materials and arrangements.
CONTENTS
INTRODUCTION
General
Scope of the Present Work
GENERAL REVIEW
Characteristics of Joint Component Materials in Fire
Glulam Timber
Plywood
Steel
New Zealand Situation
Joint Types
Code Recommendations
Fire Resistance Rating
Relevant Information
Fire Characteristics of Nails in Timber
Fire Performance of Unprotected Nailed Connections
Types and Forms of Protections
Fire Performance of Protected Details
Fire Characteristics
Load Levels
Design and Failure Criteria
TESTS
The Experimental Programme
Test Specimens
Block Test Specimens
Unprotected and Protected Gusset Test Specimens
Test Setup, Instrumentation and Test Procedure
Test Setup and Test Duration
Furnace Temperature Measurements
Specimen Temperature Measurements
Temperature Recording
Test Severity
Observations
RESULTS
Block Tests
Unprotected Gusset Tests
Protected Gusset Tests
Graphical and Diagrammatic Results
Behaviour of Samples
DISCUSSION
Block Tests
Performance of Solid Timber Sample
Performance of Glued Timber Samples
Charring Rate of Timber
Performance of Nailed Sample
The Influence of Nails
Performance of Paper-Faced Gypsum Plasterboard Protected Sample
Temperature Development Characteristics
Steel and Plywood Gussets
Nail Temperatures
Performance of Different Protection Materials
Performance of Different Protection Arrangements
Plywood and Steel Gussets
Nail Temperatures
Other Aspects
Structural Adequacy
Integrity
Insulation
Test Arrangements and Instrumentation
A Note on Protection Systems
Future Work
General
Loaded Joint Tests
Computer Analysis
CONCLUSIONS
MAJOR SOURCES
REFERENCES
APPENDIX:
DETAILS OF TEST SPECIMENS
FIGURES
FIGURE 1:
Common gusset joints in New Zealand
FIGURE 2:
Schematic diagram of the penetration of charred
zone in the vicinity of large and small nails (from
Aarnio and Kallioniemi, 1979, 1983)
FIGURE 3:
Block test specimens
FIGURE 4:
Unprotected gusset specimens
FIGURE 5:
Protected gusset specimens, arrangement A
FIGURE 6 :
Protected gusset specimens, arrangement B
FIGURE 7 :
Protected gusset specimen, arrangement C
FIGURE 8 :
A typical test setup
FIGURE 9:
Furnace temperature rise vs time
FIGURE 10: Block test temperatures, 50 mm from exposed face
FIGURE 11:
Extent of charred zone at the mid-section of block test
specimens
FIGURE 12: Glued and nailed block test specimens after test
FIGURE 13:
Temperatures of unprotected gusset specimens
FIGURE 14: Unprotected gusset specimens after test
FIGURE 15:
Extent of charred zone at the mid-section of unprotected
gusset specimens
FIGURE 16 :
Temperatures behind first layer of protection,
arrangement A
FIGURE 17: Temperatures behind the surface protection,
arrangement A
FIGURE 18: Glulam timber surface temperatures, arrangement A
FIGURE 19: Glulam timber side temperatures, arrangement A
FIGURE 20: Nail tip temperatures, edge distance
arrangement A
=
20 mm,
FIGURE 21:
Nail tip temperatures, edge distance
arrangement A
=
40 mm,
FIGURE 22:
Solid timber protected specimens after test
FIGURE 23:
Plywood protected specimens after test
FIGURE 2 4 :
Paper-faced gypsum plasterboard protected specimens
after test
FIGURE 2 5 :
Intumescent coatings protected specimen after test
FIGURE 2 6 :
Extent of charred zone at the mid-section of protected
specimens, arrangements A and C
FIGURE 27 :
Nail tip temperatures for protected specimens,
I
arrangement B
FIGURE 2 8 :
Extent of charred zone at the mid-section of protected
specimens, arrangement B
FIGURE 2 9 :
Temperatures observed in the section of 1 6 0 x 360 nun2
timber beam after 6 0 minutes of fire exposure (from
Sauvage, 1 9 8 5 )
FIGURE 3 0 :
Temperature distribution of nails
FIGURE 3 1 :
Sections through mineral wool insulated steel plates after 63
6 0 minutes of fire test (from Aarnio and Kallioniemi, 1 9 8 3 )
62
FIGURE 32: Temperatures for intumesecent coating protected specimens
64
FIGURE 3 3 :
65
Conditions of timber around nails after fire test
FIGURE 34: Arrangement for testing loaded connections
66
TABLES
TABLE 1:
Unprotected nailed steel gusset connections
TABLE 2:
Unprotected nailed plywood gusset connections
TABLE 3:
Fire performance of protected details
TABLE 4: Overseas standards
TABLE 5:
Details of block samples
TABLE 6:
Test series for protected gusset samples
INTRODUCTION
General
In New Zealand, increasing popularity of timber construction and the
versatile nature of glued laminated members have resulted in 'heavy'
timber structures becoming more varied in their uses (Hay, 1987). The
performance of 'heavy' timber structures in fire is commonly accepted and
often quoted as an advantage of such construction (Buchanan, 1987). In
this context, 'heavy' refers to members whose least dimension are not less
than 75 mm. The design of these members is based on the sacrificial
'method of char' which accepts that charring would occur but the residual
sections would still continue to perform their load carrying function for
the required period (Standards Association of New Zealand, 1987).
However, as in other structures, there exists the problem of connections
where metal or timber components are involved. Steel components absorb
heat from a fire and cause charring and softening of adjacent timber,
while timber or wood-based components are basically combustible materials
normally with incompatible fire resistance with the main members. Also,
bolts, screws or nails provide a heat path into the interior, which may
initiate local charring with subsequent loss of anchorage or rigidity.
Overseas work (see: Major Sources 1.) indicates that the load capacity of
exposed connections is reduced considerably during a fire.
Currently the local construction industry is paying little attention to
this problem or using protection details not validated by established fire
engineering approaches; although the importance of this problem has been
Smith, 1984) . This is due to lack of
recognised (Loughnan, 1984;
comprehensive design information in New Zealand, the ambiguous basis of
some overseas guidelines, and the inapplicability of overseas test results
to local construction.
This work forms the first part of a research programme undertaken by the
Building Research Association of New Zealand to remedy this situation. The
objective is to investigate the fire performance of various exposed and
protected connections . i n order to provide engineers and code-drafting
authorities with comprehensive information to ensure that the fire ratings
of connections are compatible with those of the connected members. Work
has also been directed towards formulating test procedures to enable the
connections to be rated during fire attack.
Scope of the Present Work
In the last ten years, there has been a dramatic increase in the use of
glulam timber in local industrial buildings (Smith, 1984). The predominant
building systems incorporate portal frames with moment-resisting
connections consisting of nail-on steel or plywood gussets (Walford,
1988). This phase of the work concentrates on the fire performance of
these connections.
A review of the literature as well as a pilot experimental investigation
of the fire performance of block samples and,. unloaded gussets have been
carried out. Both exposed and protected gussets have been included. The
information generated helps to identify the critical parameters in design
and to provide useful information and guidelines in cases to be considered
for loaded connections.
GENERAL REVIEW
Characteristics of Joint Component Materials in Fire
Very little information is available on the fire performance of nailed
gusset connections commonly adopted in New Zealand, though the behaviour
of some of their component materials is well-documented.
Glulam Timber
It is well-known that timber members of structural cross-sections have
good fire resisting properties. The characteristics of timber at elevated
temperatures are also well-understood (see: Major Sources 2.).
During normal combustion of the exposed section, a charred layer is formed
which continues to grow in thickness at a relatively slow but
approximately constant rate. This charring rate varies according to the
size, type and density of the timber, its degree of exposure,
permeability, moisture content, inherent defects and strain level, as well
as the intensity of combustion and ventilation (Buchanan, 1987; Lie,
1972;
chaffer, 1984; Sauvage, 1985). In the presence of a pilot flame,
timber will ignite at around 250'~. The charring temperature is generally
accepted as being around 288 to 300'~ (Hall et al, 1980; Malhotra, 1986).
The mechanical properties of timber, e.g., tensile, compressive and shear
strength and moduli generally degrade during a fire (Springer and Do,
1983).
Laminated timber members, glued with phenolic or resorcinol adhesives,
have charring rates equivalent to solid wood. Casein adhesive is
satisfactory if the outer laminations are relatively thick, but urea
adhesive allows increased charring and separation (Schaffer, 1977). Little
is published on the strength and deformation characteristics of glued
joints at elevated temperatures though it is known that the bond strength
of epoxy resin joints decreases with an increase in temperature (Avent and
Issa, 1984).
Plywood
Relatively little information is available on the mechanical and thermal
properties and charring rate of plywood at elevated temperatures. Ashton
(1965) considered that the charring rate of plywood may be greater than
solid timber, depending upon the type of veneer and adhesive. He suggested
that premature delamination may result if unsuitable glues are used,
giving a higher rate of destruction.
Steel
The strength and modulus of elasticity of steel decreases with an increase
in temperature (Carling, 1986). Mild steel members fail when the yield
strength reduces to less than the working stress, i.e., with normal safety
factors, the critical temperature is about 550°C. Also, the creep rate
increases drastically at temperatures above 400°c. There is also little
information on the behaviour of steel nails at elevated temperatures
subject to the loading and arrangement similar to that in a gusset
connection.
New Zealand Situation
Joint Types
Both nailed steel or plywood gusset joints are widely used for the knee
and apex joints in portal frames made with glulam timber. Common steel
gusset joints are of the 'Gibson' or 'Timbertek' types while the usual
form of plywood gusset joints is the 'Batchelar' type (see: Major Sources
3 . ) , (Figure 1).
An important feature of the design for nailed gussets is the determination
of the permissible nail load. Permissible nail loads are magnified by
1.25 due to the enhanced nail performance as a result of the presence of
steel gussets. Recent proposed amendments to the timber design code (NZS
3603, 1981) suggest that this magnifier be increased to 1.5 when the plate
thickness is sufficient to induce double curvature into the loaded nail,
and that the basic nail load for all nailed gussets be increased by 1.7
because of the nail group effect. The proposal limits the combined effect
of these and the load duration multipliers to three times the basic nail
load.
Code Recommendations
MP9 (1987) contains the following recomrnendations for joint detail
It
(1)
The charring rate of 0.6 mm per minute shall apply to exposed
timber surfaces and timber surfaces in contact with or
adjacent to unprotected metal details.
(2)
Timber members glued with thermosetting glue may be considered
as 'solid' timber for fire resistance design. However, members
nailed or bolted together shall not be treated as one section.
(3)
Cracks, gaps and concealed spaces on the timber surface are
likely to have a flue action during fire and should be
avoided.
(4)
Metal fasteners exposed to fire shall be embedded into the
residual section with the countersink holes plugged and/or
covered with suitable protecting materials, e.g., timber,
plasterboard, asbestos insulation board or equivalent.I!
Attention is also drawn to proper fixings of the protecting materials.
However, no information is provided regarding the thickness and
arrangement of the protections.
Fire Resistance Rating
In NZS 1900 Chapter 5 (1984), most of the industrial portal frame
buildings are in 'construction type 4', such that internal structures
require no fire resistance (Tables 1 and 3 in the stated document).
Nevertheless, for buildings near a boundary, the structure supporting the
exterior wall is required to have a fire resistance of one hour in most
cases. Thus the connections in this type of construction should also
achieve the same fire resistance.
Relevant Information
A preliminary literature search was carried out by Olsen (1986) and the
references of the present report provide up-to-date information on this
subject. During the present literature search, a survey on related
subjects by Carling (1986) was obtained and translated. It is
inappropriate to duplicate the details in these documents and consequently
this section provides a summary of the relevant findings as we11 as
important information not covered elsewhere.
Fire Characteristics of Nails in Timber
The charring characteristics of timber around nails have been investigated
by Aarnio and Kallioniemi (1979, 1983). As illustrated in Figure 2, they
found that the depth of the charred zone was 20 to 30 mrn more in the
immediate vicinity of the nails with that between the large nails having
somewhat more penetration, i.e., 30 mm. Generally, the charred zone
penetrated 5 to 10 mm more than the sections without nails; with the small
nails giving a greater increase of the penetration depth than the large
The tests were carried out under standard fire
nails, i.e., 10 mm.
conditions with a test period of 35 minutes.
Anderberg (1980) has also examined the withdrawal resistance of different
types of nails after 15, 30 and 60 minutes of the standard fire. The
results show that the withdrawal resistance after the fire tests, besides
being related to the type of nails, also depends on the anchorage length
and the maximum temperature at the tip of the nail.
Leicester et a1 (1979) suggested that nailed joints performed better in
fire than other types of joints studied, possibly because nails yield
under load; this causes the interface between the splice plates and the
main member to close tightly under load, thereby protecting the highly
stressed timber at the interface.
Fire Performance of Unprotected Nailed Connections
At present, no established analytical method is available to determine the
behaviour and loadbearing capacities of unprotected or protected nailed
connections in the event of a fire; although solutions for simple metal
details have been proposed (Barthelemy and Kruppa, 1978; Hertz, 1983).
Gibson (1984) performed a calculation on unprotected moment-resisting
nailed plates based on the time-temperature characteristics for a typical
industrial building fire combined with an assessment of the buckling
characteristics of steel plates at elevated temperatures (ECCS, 1974) and
indicated a fire resistance of about 12 minutes. Nevertheless, different
types of unprotected connections have been investigated experimentally.
Although none of them resemble the form nor geometry o f connections of
interest in this report, the, results do provide useful insight into
certain aspects. Thus, a summary of the findings is included in Tables 1
and 2.
Reference
Failure
time
(minutes)
Failure
criteria
Test
condition
Comment
Lihavainen
( I976)
7
insulation
unloaded
assumed failure is when timber surfam
temperature = 3000C
Silcock ell al
7 Po I 5
collapse
loaded
timber rooi trusses with nailed plate
connectors
Baldwin and
Ransom ( I978)
I0 to I 5
----
loaded
metal sudam iastenings in
trussed rafter roof
Rimstad (I 979)
and Bakke ( I 978)
14
insulation
unloaded
timber surface tempratlare = 300C
Ahlen and
Mansson (
96 Po 24
deformation
loaded
(shear)
fitted sleeve joint
Notes:
I Temperature of t h timber
~
underneath the gusset quickly reached the furnaw temperature, causing localised charring thus
impairing the integrity of the connedion. Increasing the gusset thickness would reduce the steel stresses but would not solve the
charring problem because of the good conductivity of steel. Also, the rate of charring of the timber at the steel-timber interface
increases as the load transfer across the intedam increases.
2
Wviid (I 980) considered that the loss of loadbearing q a c i l y 01 this type of joint is caused by the penetration of the connection's
steel components into the timbsr and is related to wood softening and charring. He measured the penetration rate of electrically
heated dowels into timber specimens while vatying the temperature, bearing stress, angle of wood grain and dowel dimensions.
Temperature and bearing stress were found to be the most dependent variables with the penetration rate increasing for both.
In association with these studies, Kordina and Meyer-Ottens (1983) pointed
out the risk of instability of timber gussets in fire and proposed minimum
thicknesses for gussets related to the dimensions of the connected members
as well as the fire resistance required. Also, Ahlen and Mansson (1979)
found that the rate of charring increases in proportion to the load when
transferred to the timber by steel.
AarwHo and Kallioniemi (1979, 1983) also suggested that although the use
of timber gussets may result in a larger undamaged timber section than
connections involving steel gussets, the fire resistance of timber gusset
connections is similar to that with steel gussets because of the problem
of lateral stability and the poor performance of nailed joints.
These studies indicate that unprotected loaded gusset connections cannot
achieve a one hour fire resistance irrespective of the failure criteria.
Fire affects both the strength and deformation characteristics of the
connection and any permanent degradation or damage to the component
materials is also undesirable. This leads to the need for protection of
the connections.
Types and Forms of Protections
It is an accepted practice for connections to be protected against fire by
using suitable covering materials (see: Major Sources 4.). In relation to
this, experiences from steel and timber member protections are
particularly valuable (see: Major Sources 5.). This usually takes the form
of passive fire protection systems such as proprietary products and
construction methods which provide insulation to the connection, or
alternatively by shielding and positioning the comection or its critical
components to avoid fire. The principal aim is to insulate as well as to
reduce radiation and oxygen supply (Barnett, 1984).
Common protections include the following or their combinations:
(1)
Sacrificial or non-combustible boards or specially moulded
encasements, e.g., timber, plywood, particle board, gypsum plaster,
"Vicuclad", mineral fibre and fibreglass-reinforced plaster.
(2 1
Fire resistant plasters, e.g., gypsum-vermiculite or gypsum-perlite.
TABLE 2: Unprotected nailed plywood gussd connections
Reference
Failure
time
(minutes)
Failure
criteria
Test
condition
Comment
Hviid and Olesen (1977)
12
collapse
loaded
gusset lire retardant impregnated
Leicester et al(1979)'
33
deformation
loaded
failure when the joint extended 10 mm
Aarnio (1979)z
17.5
collapse
loaded
nail heads pulled through the gusset
Jackman (1981)2
----
collapse
loaded
the plywood failed when its thickness
was reduced by charring to a point
where the stress in the gusset was
close to the ultimate strength of the
cold material
22
----
----
gusset of sawn timber or glulam
timber
30
deformation
loaded
deflection expected at the joint
but the design load would still b~
carried at end of test
Kordina and Meyetr-ottens
(1983)
Notes:
1. Leicester et al used solid timber gussets in their tests and observed that the nails yielded under load, causing shear resistance
to build up between member faces as they failed, thus slowing down the rate of failure.
2. Both Aarnio and Jackman observed that the high temperatureat the nail head caused the surrounding timbertochar, thus thenails
offered very little resistance against being pulled through. However,failure did not occur due to the nails losing their fixity as a result
of heat conduction intothetimber substrate. Whilst thenail pull-out strength would almost certainly Rave been reducedby theaction
of fire, the nail tip temperatures may be relatively low thus the nails are still fiied adequately to provide the necessaty strength up
to the point when the plywood fails.
(3)
Fire resistant coatings, e.g., mineral wool fibres or intumescent
coating.
The board protections are lightly fastened to the connection to keep them
in position during the fire. Golding (1984) stressed that the manner of
providing these fixings could control the degree of protection. In New
Zealand, intumescent coatings are also used in gaps or exposed interfaces
to control the fire penetration as well as to ensure the structural
integrity of the connection (Baber and Fowkes, 1984; Smith, 1985; Tan,
1988)
One of the problems which can be overlooked in the arrangement of
protections is the reduction in edge and end distances of the outermost
nails in the connection as a result of charring either in the main member
or the plywood gusset if the protection is provided on the gusset surface
only. The problem is aggravated by the fact that these nails are the most
highly stressed. In MZS 3603 (l98l), the minimum edge and end distances
are 5D and 20D (D = nail diameter) respectively. With common nails having
2.8 to 4.0 mm diameter, the corresponding minimum distances are 14 to 20
mm and 56 to 80 rnm respectively. This highlights the need to protect the
gusset as well as the connected members on all sides unless due
consideration is given to the reduced edge and end distances or recessing
the gusset arrangement.
Fire Performance of Protected Details
The fire performance of both steel and plywood plates or gussets have been
investigated and the findings are presented in Table 3.
These findings indicate that a one hour fire resistance can be achieved at
the connection if suitable protection is provided.
Fire Characteristics
Fire tests of connections are normally performed in accordance with timetemperature conditions in recognized standards, e.g., IS0 834 (1975), AS
1530: Part 4 (1985), BS 476: Part 23 (1987), ASTM El19 (1980) or the
standard of the particular country.
Leicester et a1 (1979) attempted to use a less severe fire condition,
i.e. , a typical fire in a residential building, as an alternative to the
standard fire condition. The nailed gusset joint performance improved
under such conditions when compared with that in the standard tests. It
has been pointed out that in assessing the fire risk, only 10 per cent of
all fire ignitions reach the point of flashover (Baldwin, 1974), and many
fires are brought under control in a shorter time than the code-specified
fire resistance period.
Load Levels
Some of the overseas tests were performed under loaded conditions, mostly
tension loads. But it is difficult to correlate them with the dead, live,
wind or snow loads specified in the particular standard.
Leicester et a1 (1979) have applied a mixture'of dead and live loads which
were roughly 0.36 times the average short term ultimate tensile strength
of the connection when loaded without a fire environment. This is a much
higher load than would normally be expected during a fire.
IA B L 3:
~
w e penormance or protectea a w l s
Unloaded
Steel Plates1
Protection
Lihavainen (1976)
30 mm thick mineral wool
Rimstad (1979)
and Bakke (1978)
3 layers of 'Unitherm'
fire resistant coating
8 and 16 mm thick steel plates: coatings
have no effect until beyond 150 to 200°C
and the temperature delay is about 10
minutes at 300 and 500°C.
20 mm thick fire resistant
plaster
the plaster is effective at an early stage
of the fire giving a temperature delay
of about 15 minutes.
Aarnio (1979) and
Kallioniemi (1983)2
Failure
Time (mins)
mineral wool:
30 mm thick
50 mm thick
Comment
assumed failure is when the timber
surface temperature reaches 300°C.
5.5 mm thick asbestoscement board placed
between timber and metal
plate
charred depth reduced by 30% compared
with uninsulated plate after 20 minutes of
testing.
glulam timber boxes:
40 mm thick
70 mm thick
temperature at gusset increased by
- 1~ o after
C 30 minutes
- no increase at all.
particle board boxes
filled with mineral wool
temperature in steel less than the 300oC
after 30 minutes, the particle board is
completely charred but mineral wool
is still in place.
Plywood Plates/Gussets
Protection
Failure
Time (mins)
Comment
Hviid and Olesen (1977)
Plywood
plywood is lire retardant impregnated;
loaded joint, failure by joint rupture.
Kordina and
Meyer-Ottens (1983)
Sawn timber or
glulam timber:
24 mm thick
timber protection on gusset only, loaded
joint, iailure by loss of loadbearing
capacity.
40 mm thick
Notes:
1. Both nailed, glued and nailed boxes as well as boxes provided with a 1 mm gap at the joints, have been studied. Where the boxes
were nailed on only, gaps in the joints appeared relatively soon. Thereafter, convection through these gaps dominated the heat
flow but there was no difference in the temperature development in the boxes with different wool thicknesses nor was there any
effect of the 1 mm gap provided initially.
2. Keith (1987) has also recommended the use of 16 mm thick fire-rated gypsum plasterboard for providing a one hour FR.
At present, MP9 (1987) specifies that structural members in fire should be
capable of withstanding the design loading of two-thirds of the wind load
based on a five year return period wind gust velocity together with 75% or
50% of the live load for storage occupancies and other areas respectively.
If the connections are expected to have compatible structural performance,
this implies that their fire resistance design should be based on the same
loading. Fire design codes overseas specify lower loads to be considered
in conjunction with fire.
Overseas standards specify various criteria for the design or failure of
unprotected connections or the protection requirements. These are
summarised in Table 4.
TABLE 4: Overseas standards
Reference
Protection
NS 3478 (1981) Norway
Comment
materials which are in direct contact with timber must be
properly insulated such that their temperature does not exceed
300oC during the fire
Finland Building Regulations
Part 5 (1977) Finland
timber, suitable boards
or mineral wool boards
Ditto, the loadbearing metal components should be insulated
BS 5268: Part 4 (1978)
Britain
timber board, asbestoscement board or
equivalent
metal parts should be buried deeply into the timber such that
they lie within the residual section or be covered with suitable
material.
DIN 4102: Part 4 (1981)
Germany
timber 24 mm and
40 mm thick respectively
corresponds to fire resistance of F30-B and F60-B respectively
DS 413 (1982) Denmark
wooden plug or other
types of insulation
assumes that the heat transmitting metal parts are protected
against heat damage
PFS 1984: 1 (1984) Sweden
assumes the loadbearing capacities of steel components
decrease at elevated temperatures, no load transfer
between components and charred timber and the
characteristic loadbearing capacity can be determined as the
permissible load value under normal conditions, increased
by 50 per cent; asbestos-cement boards are no longer
permitted.
Draft DR83201, part 4
(1983) Australia
follows BS 5268 to some extent but also gives the fire
resistance of unprotected nailed joints as 30 minutes
(see note)
Note:
In the revision to Draft DR83201, Part 4 (1983), fire resistance will not be given to nailed joints. It is understood
(Leicester, 1988) that in some tests where there were a lot of nails, the nails formed a rigid shear layer and the joint
opened up like that of a split ring connector, enabling heat penetration and resulting in early failure.
1
Basically, there exist strength, deformation and temperature criteria, all
of which are interrelated.
For strength, the connection should sustain the highest working load
expected during the fire so that the structural performance is retained.
However, failure of the connections usually occurs due to large local
deformations within the timber-gusset contact areas (Ahlen and Mansson,
1979; Jackman, 1981). For a structural frame, AS 1530: Part 4 (1985)
specifies that the vertical deflection of roof or roof-ceiling systems
should be less than one-thirtieth of the clear span; which' was based on
observation of deflection related to eventual collapse.
In Table 4, some standards use a charring temperature of 300'~ as the
design criterion. This normally refers to the surface temperature under
conditions of high heat transfer, which gives rise to steep temperature
gradients beneath the surface. But Hillis and Rozsa (1978) have examined
the softening characteristics of Pinus radiata at elevated uniform
temperatures under prolonged heating, using dried strip specimens under
torsion. It was found that at around 130°c, the specimens sheared apart.
Green timber began to change to a rubbery state in the 70 to 80°c region,
following which flexibility of the sample varied rapidly until rupture
occurred at about 120'~. They also concluded that specimens preheated to
100°c became more flexible at even lower temperatures compared with
undried wood. A reduction in rigidity has also been observed in the region
of 127 to 146'~ for dried Pinus radiata (Mackay, 1973). These highlight
that a lower temperature than 300°C can be critical on overall joint
rigidity. With plywood gussets, the softening at the gusset-nail bearing
locations would be more influential than the deformation of the timber
materials around the nail embedment. For steel gussets, the softening of
the timber in contact with the steel may be significant.
TESTS
The Experimental Programme
The aim of this series of pilot tests was to examine some fundamental
features and to gather information on the fire characteristics of
protected and unprotected gusset connections in order to identify critical
design parameters and to assess performance. The use of unloaded
conditions permits great simplification of the test arrangement without
limiting the information that is of interest. It should be pointed out
that load causes gaps to open up, or cracks to occur, and these allow heat
to penetrate and cause more adverse effect than unloaded joints.
It was not the intention in this phase of the work to produce
comprehensive design data. The protections considered do not encompass all
the types or combinations of types that are available but do include the
most common and easily available ones. With this in mind, only one sample
of each type of specimen was tested.
The block tests were used to study the performance of glued and nailed
joints as well as alternative types of insulation.
The gusset tests were used to assess the insblating properties of various
protecting materials and different insulation arrangements, as well as to
measure nail temperatures. Both plywood and steel gussets were included.
Test Specimens
Details of the test material and specimen preparation are given in the
appendix.
Block Test Specimens
Six glulam timber block samples were tested. The test arrangement, sample
dimensions and the thermocouple locations are shown in Figure 3. Details
of the samples are given in Table 5.
Unprotected and Protected Gusset Test Specimens
Two unprotected gusset samples, plywood and steel, were included. The test
arrangement and the sample dimensions are shown in Figure 4. Thermocouples
were attached to measure the timber surface temperatures as we11 as the
nail tip temperatures. Two thermocouples were placed at similar locations
to assess the data variation.
As it was we11 known that the unprotected arrangements would not provide
the one hour fire resistance and only the extent of damage was of
interest, it was unnecessary to test a complete connection. However, the
test specimens had to be large enough to simulate real conditions, yet
small enough to be tested in the furnace. Thus the present setup
(Figure 4) was similar to a quarter of the connection in terms of the
specimen dimensions and the mounting arrangements.
As shown in Figure 4, two nail patterns were used for both gussets. The
intention was to examine the effect of nail spacings on the nail tip
temperature. The closely spaced nail patterns for the plywood and steel
gussets were similar to those used in the 'Batchelar' and 'Timbertek'
types of connection respectively (Batchelar, 1984
Loughnan, 1984),
(Figure 1).
The details of the protected gusset test arrangements are shown in Figures
5 to 7 and the test series is presented in Table 6. Basically, there are
three types of protection arrangement and five types of protection.
The reasons for the specimen size and test arrangement of protected
samples were the same as those for unprotected ones. Test arrangement A
(Figure 5) simulated the condition where the connection was protected on
all sides. Arrangement B (Figure 6) simulated the case where the
protection was provided on the gusset surface only. The gussets as we11 as
the nails were set back from the edges by the notional charring depth and
the gusset was also protected at the sides but not the timber. In
arrangement C , the aim was to examine the performance of intumescent
coatings on steel, thus only the steel gusset of the corresponding sample
was exposed to the fire (Figure 7).
A simple nail pattern was used for all the protected samples, which
satisfied the code (NZS 3603, 1981) requirements of minimum nail spacings
and edge distances. Additional nails were provided in the intumescent
coating protected sample (Figure 7) to examine the insulating effect of
timber as related to the nail locations and also a different type of nail
tip temperature measurement technique.
TABLE 5: Details of block samples
Sample
number
Sample
tYPe
Detail (sample types, slice location and nailpattern are given in Figure 3)
glulam timber only
glulam timber, with nails
glulam timber, sliced and glued back together
glulam timber, sliced and nailed back together
glulam timber, sliced, glued and nailed back together
glulam timber with 3 layers of 14.5 mm thick paper-faced
gypsum plasterboard glued on
TABLE 6: Test series for protected gusset samples
Protection
Test arrangement *
A
40 mm thick solid timber
2 layers of 18 mm thick plywood
A and B
19 mm thick paper-faced gypsum plasterboard
A and B
2 layers of 14.5 mm thick paper-faced gypsum plasterboard
A
Intumescent coating
C
Notes:
Details of protection materials and specimen preparation refer to the Appendix.
Test arrangements refer to Figures 5 to 7.
Intumescent coatings were used on the steel gusset only. Intumescent coatings for timber normally delay the onset of
charring by about 17 minutes (Nullifire Limited). Thus it is not suitable as an independent protection for one hour fire
resistance and is not included.
All the protections are applied to both steel and plywood gussets unless othetwise stated.
Test Setup, Instrumentation and Test Procedure
Test Setup and Test Duration
The tests were conducted at the BRANZ Fire Laboratory at Judgeford. The
reinforced concrete frame containing the test specimens was sealed to the
2.2 m high by 1 m wide pilot furnace, and the temperature conditions
controlled as specified in IS0 8 3 4 (1975). A typical test setup is shown
in Figure 8.
The tests were terminated after the specimens had been exposed to fire for
60 minutes.
Furnace Temperature Measurements
The temperature of the atmosphere within the furnace was measured using
four chromel-alumel thermocouples distributed evenly on a vertical plane
100 mm from the exposed face of the specimens.
Specimen Temperature Measurements
The temperatures at the nail tips and those inside the timber were
measured using chromel-alumel thermocouples. The thermocouple hot junction
was inserted into a 2 mm diameter hole predrilled in the timber and glued
to ensure that it remained in position throughout the test. This is termed
'hot junction contact' technique in this report. Particularly for the
thermocouples measuring nail tip temperatures, electrical continuity was
checked where possible to ensure that good contact had indeed been made.
For the intumescent coating protected sample, an additional arrangement
for measuring the nail tip temperature was used (Figure 7). A 2 mm
diameter hole was first drilled through the timber at the specific nail
location. A 20 mm diameter and 45 mm deep hole was then drilled at the
same location from the unexposed side. After the nail was inserted, the
hot junction of the thermocouple was wound around, and glued to, the nail
tip. The enlarged hole was then packed with a mixture of sawdust and
resorcinol glue. Electrical conductivity was checked both at the beginning
and at the end of the test. This alternative is termed 'enlarge and
refill' technique.
The temperatures on the timber, gusset and protection surfaces were
measured using chromel-alumel disc thermocouples complying with Clause
4.1.4 of IS0 8 3 4 (1975). Pilot investigations indicated that this
technique is reliable in obtaining the material surface temperature, i.e.,
an alternative arrangement of recessing the thermocouple into the surface
and covering it with asbestos pad (Harmathy, 1985) would give the same
result.
The positions of the thermocouples are shown in Figures 3 to 7.
Temperature Recording
All the thermocouples were connected to a computer controlled data logging
system which sampled the temperatures at thirty second intervals.
Test Severity
A measurement of the severity of a fire resistance test can be established
by a comparison of the area beneath the time-temperature curve for the
test with the area beneath the standard time-temperature curve for
same period. Figure 9 shows the standard time-temperature curve for
834 (1975) in relation to the actual temperatures of a typical test.
fire severity of the tests as calculated by the above comparison for
duration of the test was generally 100 per cent.
the
IS0
The
the
Observations
Some of the specimens could be viewed from ports at the side of the
furnace and their behaviour was monitored during the test. After the
tests, the specimens were removed immediately from the furnace,
extinguished and thoroughly dowsed with water to prevent further damage.
After cooling, the specimens were examined to determine the extent of
damage. Some of the specimens were also sawn through to determine the
extent of the charred zone.
RESULTS
Block Tests
The temperature developments and the extent of the charred zones at the
mid-section of the block samples are shown in Figures 10 and 11
respectively.
The glued samples, whether with or without nails, performed similarly to
the solid timber sample without any sign of glue melting or delamination.
For the sliced sample with nails only (sample 2 in Table 5), the top layer
was completely charred and started shrinking and warping after 15 minutes.
At 20 minutes, a gap of about 1 to 2 nun appeared between the two top
layers. The second plate then deteriorated in a similar fashion with a gap
opening up between the plate and the base block after 45 minutes. Most of
the layers fell off after 55 minutes. At this point, the thermocouple
temperature increased dramatically (Figure 10).
Sections through the glued and nailed samples are shown in Figure 12.
Generally, the timber surface shrinkage pattern of the nailed samples was
influenced by the presence of the nails, i.e., more cracks were initiated
around the nails compared with the solid timber sample.
For the paper-faced gypsum plasterboard protected sample, the paper
quickly burnt away. The exposed face of the board had a crazed pattern of
fine cracks which became slightly wider near the end of the test. There
were no significant dimensional changes of the boards. At the end of the
test, the outermost board lost adhesion when water was sprayed, whereas
apart from some damage at the edges, the bond between the inner boards and
that with the timber was retained.
The timber surface temperatures and the nail tip temperatures are shown in
Figure 13. The state of the samples after the test is shown in Figure 14.
Although two temperature readings were taken at similar locations for all
the unprotected and protected gusset samples, all the corresponding
readings were reasonably close, illustrating that the results are
consistent; thus they are not shown twice.
After 20 minutes of testing, the plywood gusset had completely charred and
had closely spaced cracks over the whole surface, whereas the steel sample
still looked intact apart from the shrinkage cracks and charring at the
side of the timber. And it was not until 25 minutes when a gap appeared
between the timber and the steel. After 43 minutes, most of the plywood
gusset had disintegrated and fallen off. At the end of the test, there was
a gap of 10 to 15 mm between the steel gusset and the charred timber.
The extent of the charred zone at the mid-section for both samples is
illustrated in Figure 15.
Graphical and Diagrammatic Results
The temperatures at the timber, gusset and protection surfaces as we11 as
the nail tip temperatures for protection arrangement A, i.e., protected on
all sides, are shown in Figures 16 to 21. The corresponding temperatures
for the intumescent coating protected sample (arrangement C) are also
included. The state of these samples after the test is shown in Figures 2 2
to 2 5 . The extent of the charred zone at the mid-section of the samples is
illustrated in Figure 2 6 .
The temperatures at the timber, gusset and protection surfaces for
protection arrangement B, i.e., protection on surface and sides of the
gusset only, are similar to those with the same protecting materials in
arrangement A; thus they are not presented. The nail tip temperatures and
the extent of the charred zone at the mid-section, however, are presented
in Figures 27 and 28 respectively.
Behaviour of Samples
The behaviour of the same protecting material was similar in both
arrangements A and B. The difference arose at the sides of the glulam
timber which charred and shrunk because of their exposure in arrangement
B. Such shrinkage caused some of the joints between the gusset side
protections and the glulam timber to open up towards the end of the test.
The solid timber protections charred and shrunk with a characteristic
pattern of cracking in the charcoal, i.e., large chunks of charcoal
separated by deep fissures. The shrinkage pattern is related to natural
defects, the direction of the timber grain (radial, tangential and
transverse) as we11 as the location of the glued joints. Particularly on
the face perpendicular to the grain direction, the shrinkage of the
surface timber protection in the radial and tangential directions created
a few separations from the side protections in arrangement A. In some
samples, there was a tendency for the surface protections to warp, thus
causing premature disruption. Nevertheless, all the timber protections
remained in position at the end of the test.
The charring of plywood was different to that of solid timber, i.e., there
were more shallow cracks closely spaced in the veneers, a characteristic
related to the way the plywood was manufactured. Plywood deteriorated
particularly rapidly during the last 15 to 25 minutes of testing and most
of the plywood gussets eventually fell off. This consequently caused
damage to the gussets and the timber underneath. Occasional delamination
was also observed in the plywood.
In both the plywood and solid timber protected samples, there was no sign
of the glue melting. The majority of the glued joints maintained their
bond during the fire exposure both in the charred and uncharred wood
zones. However, separations occurred at some locations where the timber
shrinkage was significant or at joints ;where the shrinkage characteristics
between components in different directions were incompatible, e.g.,
between surface and side protections.
The paper-faced gypsum plasterboard behaviour was similar to that in the
block samples with the surface layer crumbling, i.e., the 19 mm thick
boards and the exposed layer of the 14.5 mm thick boards. The inner layer
of 14.5 nun thick board, although suffering from minor damage, was still
reasonably sound. However, the board-to-board joints showed signs of
distress; after the tests, the exposed boards all deteriorated and could
be removed by hand, thus it was impossible to assess the state of the
joints. On the other hand, the board-to-timber joints performed well with
a bond maintained till the end of the test for both board thicknesses.
For the intumescent coating protected sample, the onset of the intumescent
reaction was about 2.25 minutes. At first the reaction was relatively
violent, then gradually developed into a steady state of coating
expansion; changing the surface colour from white to black and eventually
yellow. The final thickness of the coating was about 65 to 75 mm. When
water was sprayed at the end of the test, the coating lost adhesion
(Figure 25). It should be noted that thoroughly dowsing an intumesced
'chart (meringue) is an extremely drastic treatment.
It is widely
acknowledged that the resultant expanded 'chart of an intumescent is
fragile. Nevertheless, numerous tests and experience confirm that the
'char' remains intact in a fully developed fire despite downdraughts and
other external agencies likely to be present, thereby meeting the
criterion of ',stickabilityt, i.e. , the ability of a protection system to
remain in place around the steel section being protected. Indeed, that
the 'chart is fragile is recogn-ised as one of the advantages of using an
intumescent in that the spent protection is easily removed to enable the
steel component to be recoated after a fire.
DISCUSSION
Block Tests
Performance of Solid Timber Sample
Timber is unique in that it insulates itself as it burns by the formation
of charcoal. The thermal conductivity of charcoal is about one half to one
third of unburnt timber (Hall et al, 1980).
In Figure 10, temperature development in the timber was characterised by
an initial plateau where the heat was inadequate to penetrate the timber
and the timber insulated itself well. This was followed by a gradual
increase in temperature. The temperature ride is related to the location
of the thermocouple in the timber and initially, the heat energy being
used in physical (driving off moisture) and chemical (decomposition,
pyrolysis) reactions. As the charred zone built up, the penetration of
heat into the interior was slowed down. The temperature results at the end
of the test correlate well with those presented by Sauvage (1985) as shown
in Figure 29.
Performance of Glued Timber Samples
The charring of resorcinol glued timber, i.e., samples 3 and 5 in Table 5,
was similar to that of solid timber (Figures 11 and 12). The temperatures
in these samples were practically the same indicating that the glue lines
had little effect (Figure 10). There was no glue melting or delamination,
the samples effectively burned as one section. This confirmed the code
(MP9, 1987) recommendations on resorcinol glued timbers.
Charring Rate of Timber
The charred zones at the mid-section of the solid and glued samples were
about 38 to 40 mm deep giving a charring rate of 0.63 to 0.67 mm per
minute which correlated we11 with the MP9 (1987) recommendations.
Performance of Nailed Sample
The nailed laminated sample developed gaps which opened between the timber
laminations as they charred and shrunk during the test (Figure 12).
An interesting feature was that even when a gap (2 to 3 mm wide) was
visible between the lamination and the base block, the timber temperature
(which was measured 5 mm behind the gap) was less than 100°c, the furnace
temperature being around 900°c at that time. As the gap became wider, the
timber lamination further disintegrated and eventually fell off. At this
point, the timber temperature began to increase drastically. Also, the
base block surface was largely exposed which led to more severe charring
(Figure 11).
This illustrates that nailed joints could open up between laminations
causing a flue action during a fire (through hot gas movements) which
would result in fire penetration. Thus nailed laminations should not be
treated as one section and their fire resistance rating should be assessed
by considering that fire may reach all faces of the lamination. Again, the
MP9 (1987) recommendations are justified. A possible improvement may be to
use intumescent materials in the gaps.
The behaviour of this sample correlates we11 with the 'gap situations' in
fire which were examined by Aarnio and Kallioniemi (1979, 1983; Carling,
1986) . They found that a gap width of 5 mm was a critical figure . When the
gap was wider, the timber changed into charcoal right across the gap and
the corners were rounded off; regardless of the width of the test beam and
the duration of the test. With narrower gaps, only the corners of the beam
were charred. They also found that the temperatures in the wider gaps were
higher. However, no explanation was given for this behaviour. The last
stage of temperature increase in the present test was not observed by
Aarnio et a1 because they maintained a constant gap width throughout their
test. Kordina and Meyer-Ottens (1983) recommended that gaps between beams
or beam and column should be less than 3 mm wide. Nevertheless, in lieu of
advanced information, the MP9 recommendations are appropriate.
The Influence of Nails
The provision of closely spaced nails in the solid and glued timber
samples caused only a small increase in the timber temperature (Figure
10). In practice, such differences can be ignored.
The additional penetration
of the charred zone around the nails
*
(Figure 11) was similar to that previously described (Aarnio and
Kallioniemi, 1979, 1983), i.e., the nail influence was local. This is
related to the fact that temperatures of unburnt timber beneath the
charcoal may be surprisingly low (Figure 2 9 ) . When heat is conducted
through the nails, there could be a thermal gradient along the nails with
the temperature decreasing towards the nail tip, associated with the
surrounding timber acting as a 'heat sink'.
Performance of the Paper-Faced Gypsum Plasterboard Protected Sample
'With reference to Figure 10, the initial performance of the boards was
inferior to timber. However, after about 35 minutes, the boards provided
better insulation and the timber block, apart from the edges, was
uncharred at the end of the test (Figure 11).
The degree of disintegration of the board material reduced the further the
boards were from the exposed face. The outermost board lost adhesion when
water was sprayed indicating the deterioration of the adhesive. However,
the second board-to-board joint and the board-to-timber joint were still
in a reasonable state because face insulation was provided. This indicated
that the joint performance of any protection system was also important.
Glass reinforced 'fibrous plaster' which does not have paper facing may
behave better in both these areas.
Temperature Development Characteristics
The evaporation of moisture at depth can account for the shape of the
temperature development curves in Figure 10. The temperature behind the
layers of plasterboard increases more rapidly than that behind the timber
layers possibly because of the higher thermal conductivity of
plasterboard, yet the rate of increase slows down considerably after 30
minutes. This is almost certainly due to moisture evaporation. The same
explanation may hold for the timber specimens, but higher temperatures are
observed, simply because of the high internal pressures generated at
depth. The fact that the sliced, nailed sample shows the same effect, but
at slightly lower temperatures, is not inconsistent with this explanation
as the pressures generated will be less in the thinner sections.
Both the plywood and steel gusset samples were damaged beyond repair and
could not carry load, confirming that unprotected gusset connections
cannot achieve a one hour fire resistance rating (Figure 14).
Steel and Plywood Gussets
Because of the good conductivity of the steel gusset, the charring
characteristics of the timber underneath was similar to that in the
exposed situation. The charred zone was about 4 2 to 4 6 mm deep giving a
charring rate of 0.70 to 0.76 mm per minute (Figure 15).
The plywood gusset, although completely disintegrated, did insulate the
timber underneath. The charred zone was on average about 26 mm deep
(Figure 15) which interestingly, together with the plywood gusset
thickness, gave a charred depth of 44 mm which was similar to that in the
steel gusset sample.
In Figure 13, the timber surface temperature for the steel gusset sample
followed the trend of the furnace temperature development (Figure 8).
However, the corresponding temperature for the plywood gusset sample
indicated that the plywood provided reasonable insulation for the first 20
minutes and after the moisture was driven off at around 100°c, the
efficiency of the insulation decreased. After 34 minutes, the temperature
increased rapidly to that of the furnace atmosphere; which was also
associated with the charred zone extended beyond the gusset.
Generally, the thermocouples near the corners of the gussets (Figure 4)
gave slightly higher readings than those near the middle of the sample.
This was due to the heat from the sides as we11 as from the surface.
However, the temperature developments were similar, thus they are not
presented.
Nail Temperatures
For both gussets, there were significant differences between the timber
surface temperature (which indicated the temperature near the nail head)
and the nail tip temperatures. This illustrated that there was a thermal
gradient along the nails (Figure 13).
In the plywood gusset sample, the temperature developments at the nail
tips for different nail locations were practically the same, except in the
last stage of testing when edge charring became influential.
The temperature developments of the centre nail and the widely spaced
nails in the steel gusset sample were similar to the corresponding
readings in the plywood gusset sample. However, the medium and closely
spaced nails showed higher temperatures throughout the test possibly
because these nails were shorter, i.e., 45 mm long compared with the 60 nun
long nails in the other cases. Thus less heat was lost. At the end of the
test, the temperature readings implied a linear relationship with the
length of the nails (Figure 3 0 ) , even at different locations and spacings.
This may be related to the quasi-linear relationship between temperature
and depth along charred timber in the range of 200 to 850°C in Figure 29.
Generally, temperatures are related to the spacings of the nails which, as
explained before, influenced the shrinkage crack patterns of the charcoal,
i.e., the closely spaced nails induce more shrinkage cracks than the
widely spaced nails, thus permitting the heat to influence more along
their lengths. It is also obvious that after the moisture is driven off
from the timber, nail temperatures increase more rapidly.
The results of the protected gusset tests illustrate that a one hour fire
resistance can be achieved for the connections.
Performance of Different Protection Materials
The temperatures developed underneath the same protection are similar,
irrespective of whether it is protecting plywood or steel gussets,
illustrating the consistency of the results (Figure 17).
Solid timber: The 40 mrn thick solid timber performed we11 as protection
for the gussets (Figure 17). This is related to its charring and
insulating characteristics already described. It also confirms the German
design data for this type of protection (DIN 4102, 1981).
With the timber dimensions commonly adopted in New Zealand, the use of a
nominal thickness of 45 mrn for a one hour fire protection would be
appropriate.
The charring rate of the timber is vital in determining the thickness of
the protection. The overall performance of the protection system, however,
also depends on the fire resistance of the joints between the boards and
the existence of cracks, gaps or concealed spaces in the timber; both of
which are related to flue actions and illustrate the importance of quality
control.
It is well known that: the joints between timber boards can be rapidly
exploited by fire, particularly when there is a differential pressure
between one side and the other. In the block tests, the samples were
relatively small and the resorcinol glue joints performed satisfactorily.
However, with the larger gusset samples, it was found that because of the
different shrinkage characteristics of timber in different directions
(between the surface and the side boards), the glued joint can be under
stress, opening as the glue softens, permitting flame to penetrate into
the gusset; particularly near the end of the one hour period. This caused
some damage to the plywood gusset and the timber member at the edges in
some locations (Figure 26). Also in real connection protections, the
surface boards require intermediate joints because of their sizes. These
joints would be under tension as the timber shrinks. The use of additional
nails would not improve this situation.
One of the possibilities of overcoming this problem is to use an
intumescent material which is capable of expanding during a fire to fill
any construction or shrinkage gaps through which fluing may occur. For
example, 'Nullifire WD' on timber can expand and delay the onset of char
by an average of 17 minutes, after which the substrate would char at the
notional rate of structural timber. Such usage has been attempted (Tan,
1988), however, performance in protecting gusset type joints requires
further investigation. Intumescent coatings also present the possibility
of reducing the thickness of sacrificial timber needed.
The performance of two layers of 18 mm thick construction
Plywood :
plywood as protection was inferior to other materials considered. The
insulation of the first layer of plywood deteriorated rapidly after 20
minutes of testing (Figure 16), with similar deterioration for the second
layer after 40 minutes (Figure 17). The complete charring of the plywood
gusset and the timber underneath the steel gusset (both about 18 mm thick)
rendered this thickness of plywood inappropriate for a one hour protection
(Figure 26).
The performance of plywood as a protection depends on the timber species
and particularly on the adhesive (Hall et all 1980). Plywood which
delaminates on heating, allowing the exposed veneers to peel back as they
burn, would have far less fire resistance to flame penetration than that
in which the glue lines remain effective and the burnt veneer contributes
to the build-up of charcoal. In the tests under discussion, local
delamination did occur but was not significant.
The effect of thickness is also important because it appears that with
thin boards of plywood or other wood-based materials, e.g., the timber
laminations in the nailed block sample, it is necessary to have a stable
layer of unburnt material behind the charred layer if this is to be
effective in controlling the rate of charring.
Surface cracks and gaps in the cross-band or the core also reduce the
performance of the boards. However, with the majority of the veneer being
rotary cut, surface cracks and inherent gaps are inevitable. Although the
performance may be improved by using sliced cut veneers or marine plywood
(NZS 3613, 1984), the associated cost increase may not justify such usage.
McNaughton and Harrison (1940) found that phenolic bonded plywood 43 mm
thick resisted the passage of flame for 57 minutes; and suggested this was
related to the build-up of a protective charcoal layer. They also found
that the number of plies had relatively little effect on the resistance to
burn-through compared with the thickness of the plywood.
With reference to the samples in Figure 26, it seems that 54 mrn thick
construction plywood may be adequate in controlling the charring of the
protected timber in a one hour period. However, the issue of cost qeeds to
be considered.
It is also important to note that because of the different charring
characteristics of construction plywood and solid timber, plywood should
not be used as a substitute for the same thickness of solid timber in fire
protections.
Paper-faced gypsum plasterboards: Paper-faced gypsum plasterboards were
included in the present investigation because of their widespread use as
linings for local timber stud framing.
The results show that their resistance to the penetration of heat is
largely determined by the thickness of the board although the board
materials are slightly different (Appendix). The two layers of 14.5 mrn
thick boards performed well as protections, similar to solid timber
(Figure 17), without any damage to the gusset or the timber (Figure 26).
The first layer of board also gave better insulation than plywood
(Figure 16). However, the 19 mm thick board showed reduced efficiency
after about 37 minutes of fire (Figure 17); eventually causing an average
charred depth of 7 nun in the plywood gusset (Figure 26). In relation to
this, "Plasterglass" board is also a potential protection.
In general, 19 mm thick board did not provide complete protection and the
two layers of 14.5 mm thick boards provided adequate protection for a one
hour fire. Depending on the failure criteria and the board materials, it
is possible to obtain the optimum thickness through further investigation.
An interesting feature to note is that, similar to the block samples, the
initial insulation performance of paper-faced gypsum plasterboards is
inferior to that of timber or plywood (Figures 10 and 17). On the other
hand, unlike timber, the dimensional changes of the boards in the tests
were insignificant. Judging by the amount of damage on the gusset samples,
the glued joints also seemed to be more efficient than those for timber
'
(Figure 26). However, there is still scope for improving the jointing
method for these board protections.
Intumescent coatings: With reference to Figure 17, the performance of
intumescent coatings was inferior to other protections except plywood.
The initial rapid increase in steel temperature was due to the fact that
the coatings only began to react at temperatures of around 150 to 250°C.
Thereafter, the insulation effect was secured until the end of the test;
with the steel temperature rising at a steady rate.
Generally, this type of protection for steel loses favour when the section
factor (ratio of perimeter exposed to fire to the cross-sectional area) of
the member becomes too high for the coatings to protect adequately, i.e.,
more heat will be absorbed the higher this ratio. In this case, the factor
is 206 m-' and the steel plate heated up quickly to 200-250°c, where the
steel began to lose its strength and rigidity. Also, the timber underneath
started to char after about 45 minutes (Figure 17). At this stage, the
timber may have ignited and could have been generating heat which might
have accelerated temperature increase in the steel. The depth of the
charred zone at the end of the test was about 3 to 5 mm (Figure 26).
Another important consideration is the coating loadings. The sample tested
had one of the heavier loadings recommended by the manufacturer. The
reliability of the applied protection depends on how we11 it sticks to the
substrate. With heavier loadings, the risk of the expanded coating losing
adhesion is higher. Moreover, with the heavier loadings, the application
requirements, i,e., a greater number of coats and the specific period
between each application, may not be attractive in terms of application
costs. Strict quality control is also required on site.
On the whole, this system is unsuitable as an independent protection for
the large steel gussets commonly used in New Zealand.
Performance of Different Protection Arrangements
Figures 26 and 28 show that for the same protecting material, protection
arrangement A (Figure 5) is more efficient than arrangement B (Figure 6).
Arrangement B had the gusset set back from the edges with the intention of
permitting the timber at the sides to be sacrificed. However, this
arrangement is unsatisfactory in the following ways:
(1)
Because of the variability of the charring rate, the charred zone
can be more than 36 nun deep.
(2
The charring of the timber normally extends to areas beneath the
gusset (Figure 28), affecting the integrity of the gusset and the
nails. Similar behaviour was observed by Aarnio and Kallioniemi
(1979, 1983; Figure 31).
(3)
The necessary recession of the gusset from the member edges in a
design situation may be significant, causing undesirable reduction
of the joint moment capacity for the same number of nails; bearing
in mind that nails are most efficient the nearer they are to the
edges of the member.
(4)
Beyond the charred zone, the moisture in the timber may be driven
out which affects the mechanical properties (Hollis and Rozsa,
1978).
Hence protection arrangement B is not recommended unless these issues are
resolved.
The nail tip temperatures in arrangement B (Figure 27) for the plywood
protected samples was similar to that in arrangement A (Figure 21). Thus
the influence of heat from the sides is insignificant.
Plywood and Steel Gussets
The temperature development at the timber surface underneath the plywood
gusset in the protected arrangements was similar to that for the
unprotected case. The plywood gusset effectively insulated the timber thus
timber surface temperatures were lower than those behind the protections
(Figures 17 and 18).
For the steel gussets, there was an air gap between the gusset and the
underside of the surface protection (Figure 5 and 6). Hence, unlike the
unprotected cases where the timber surface temperatures resembled those of
the furnace atmosphere, the timber temperature was also lower than that
underneath the protection.
Nail Temperatures
The nail tip temperatures for the protected samples were generally less
than 100'~ at the end of the tests (Figures 20, 21 and 27). The difference
in temperatures for different edge distances was also relatively small
indicating that the effect of the heat from the sides was not influential.
Comparing Figures 20 and 21 with Figure 18 which showed the timber surface
temperatures (an indication of the nail temperature near the nail head),
there was also a thermal gradient along the nails.
For the intumescent coating protected sample, the temperatures of the nail
tip generally decreased the further it was from the edges (Figure 32).
This illustrates the effectiveness of having a larger timber surround in
keeping the temperature low.
In the same sample, the nail tip temperatures measured with the 'enlarge
and refill' technique were higher than those obtained from the 'hot
junction contact' approach which was used to obtain all the other nail
temperatures (Figure 32). There are two possible reasons for this.
Firstly, the 'enlarge and refill' technique gave a reliable measurement
because the thermocouple was actually wound around and glued to the nail
and good electrical conductivity was confirmed both at the beginning and
at the end of the test. However, any loss of contact for the thermocouple
in the other technique would change the reading to the timber temperature;
which may be slightly different to the temperature at the nail tip.
Secondly, in the 'enlarge and refill' technique, the thermocouple was
actually located 10 mm from the nail tip. As there was a thermal gradient
along the nail, a higher temperature would be recorded.
Other Aspects
In Figures 16 to 19, it is noted that the temperatures stabilise at around
100'~ while moisture is being driven off. Similar characteristics are also
observed in other tests. Generally, the length of this period is related
to the heat intensity, the bulk of the moisture-bearing component and its
moisture content.
The temperatures behind the side protections should theoretically be the
same as that underneath the surface protections in protection arrangement
A (Figure 5). In Figures 17 and 19, although the trends of temperature
developments were similar for various protections, the temperature at the
sides were generally lower than that of the surface.
The fire resistance of protected gusset connections can be defined as the
time, expressed in minutes, to failure under one or more of the following
criteria:
Structural Adequacy
Failure in relation to structural adequacy should be deemed to have
occurred at collapse.
In order to prevent collapse, the fully loaded steel gusset has to operate
below the critical temperature, i.e. , 550°c, and the nails must maintain
their grip. However, the critical temperature can increase for steel
elements subjected to less than full load at the time of the fire. With
plywood gussets, besides the nail withdrawal resistance and the bearing
capacity, the resistance against being pulled through is also important.
The commonly adopted charring temperature is 300'~ and the charred zone is
assumed to have no strength contribution.
In relation to this, as the temperature increases in the steel gusset
connection, the difference in thermal expansion of steel and timber (Hall
et al, 1980) could induce additional stresses on the components which must
be accounted for in the design.
In previous investigations, the connections usually deform considerably
before collapse. Thus the deformation criterion may be more critical than
ultimate strength. For steel gussets, this is related to the loss of
stiffness and the creep effects taking place below the critical
temperature. Plywood gussets have similar behaviour because they,
presumably like timber, soften at temperatures lower than the charring
temperature (Hillis and Rozsa, 1978). Such. softening of the gussets may
also lead to lateral or local buckling as well as overall lateral
instability. There may also be unacceptable nail slip at these
temperatures.
These effects need to be included in the consideration of structural
adequacy for protected connections.
Integrity
The protection should resist the passage of flame into the gusset
connection. Failure in relation to integrity should be deemed to have
occurred upon collapse, or the development of visible cracks, fissures or
other openings in the protection system, through which flames or hot gases
can penetrate into the connection.
Insulation
If the conventional criteria for steel and timber members are applied to
the connections, the temperatures of protected steel and plywood gussets
should be less than 550°C and 300'~ respectively. For steel gussets, the
temperature at the timber surface is similar to that of the steel; thus
the timber charring temperature would be the controlling factor. This
implies that for both steel and plywood gussets, the insulation should be
deemed to have failed when the gusset or timber temperature reaches 300'~.
However, Hillis et a1 (1978) illustrated that at lower uniform
temperatures, e.g., below 120°C, timber would soften and thus effectively
fail', by losing rigidity. Their work was based on small clear specimens
under twist, its applicability to the bearing and grip situations requires
further investigation. Although Hviid (1980) has studied the penetration
rate of timber by steel dowels at elevated temperatures, the dowels used
were of 12 and 25 mm diameter and his result may not be directly
applicable to nails.
Another consideration is the performance of the connection after the fire.
Hillis et a1 (1978) also pointed out that timber preheated at 100°c
becomes more flexible at even lower temperatures compared with that for
undried wood; presumably relating to the loss of moisture in the members.
Thus it may be desirable to limit the critical temperature of the
components to 100°C. However, with reference to Figures 17 and 18, such a
requirement is particularly severe and impractical. It is more appropriate
to specify a limit on the 100'~ temperature period, e.g. , 15 minutes, for
the gusset surface (plywood gusset) or timber surface (steel gusset) at
the nail locations; so that a minimum moisture content still remains in
the components. Hence their long-term mechanical properties are not
significantly affected. The 100°c temperature criterion also limits the
nail temperatures thus their performance can be ensured. The differential
expansion problem for steel gussets is also minimised.
In association with this, Twilt and Witteveen (1974) also proposed a much
lower critical temperature for steel (550°c), i.e.', 200°c, realising the
occurrence of local defects in the timber protections.
Test Arrangements and Instrumentation
The block and gusset sample test arrangements provide versatile approaches
for examining the insulating efficiency of various materials and the
adequacy of the joint protections. A number of samples can be tested at
the same time even in a pilot furnace. The resulting information provides
useful guidelines for design as we11 as the possible behaviour of the
full-scale connections.
Generally, the block tests can be used in preliminary studies and the
gusset tests (arrangements A and B in Figures 5 and 6) for investigating
the behaviour of unloaded connections. If only the characteristics of an
exposed face are required, arrangement C (Figure 7) is suitable.
The thermocouple readings were consistent thus only two thermocouples at
similar locations in the same sample are necessary.
The disc thermocouples are adequate in obtaining the surface temperatures,
thus sophisticated techniques (Harmathy, 1985) are not necessary. The 'hot
junction contact' technique is also useful in measuring the timber
temperatures.
The means of obtaining the precise nail tip temperatures, however,
requires further consideration. One of the difficulties in measuring nail
tip temperatures or temperatures along a nail is that it is necessary to
disturb the wood around the measurement points for the installation of the
thermocouples. In this case, the 'enlarge and refill' technique provides
reliable readings but the disturbance is more pronounced. On the other
hand, the 'hot junction contact' technique is simple but may be subject to
error if contact is lost.
In both techniques, a 2 mm diameter hole is drilled at the nail location
to ensure the positioning of the nail and the thermocouple. The hole size
compared with the nail diameter, i.e., 3.55 mm, is still significant.
However, there are practical difficulties in drilling smaller holes
through the full-size samples. There is also a minimum size for the
thermocouple.
If the timber material is removed from the drilled hole, the materials in
contact with the nail would be looser than that around a nail driven
without the hole. This could affect the amount of heat conduction from the
nail thus the nail temperature and the charring of the surrounding timber.
In reality, it is difficult to quantify this effect, although visual
comparisons are possible. In Figure 33, the charring characteristics
around nails at similar locations with or without the 2 mm diameter holes
are presented , i.e., nails in Figures 33 (a) and (b) are at similar
locations, the same applies to Figures 33 (c) and (d). These indicate that
the charring characteristics are similar for corresponding nails. Hence
the effect of the hole is not significant. In Figures 33 (a) and (b), the
timber was charred only partly along the nails indicating a thermal
gradient which correlated well with the temperature readings.
From a scientific point of view, the technique for measuring nail
temperatures requires improvement. Practically, in protected gusset
situations, if the temperature at the nail head or the gusset surface is
sufficiently low, the nail behaviour could be similar to that in the cold
condition. Thus the nail temperatures are of no significance.
A Note on Protection Systems
The choice of a suitable protection system depends on efficiency, economy,
reliability,
durability,
compatibility,
space
and
installation
requirements, ease of replacement as we11 as appearance. Combined use of
protecting materials and the standardisation of protecti.on details may
provide the most efficient solution.
The ideal protecting material should have good insulating properties. It
is also desirable for board-type materials to be free from defects, have
low thermal movement and high resistance to cracking and disintegration
when heated.
Normally, the resistance to the penetration of heat is
largely determined by thickness and the property of the protection. The
jointing materials should also have compatible performance.
A connection may be surrounded on all sides by the fire. The only heat
loss is via conduction to the connected members. As these are also being
heated by the same fire, heat 1-oss is minimal. The size and shape of the
connection has a bearing on how fast the materials heat up to the critical
temperatures. It is essential to have a minimum amount of exposed steel
and that it is embedded at the critical locations.
The importance of good detailing in protection systems cannot be
overemphasised. Fire attacks thin sections and sharp corners much more
readily than flat smooth surfaces.
Also cracks, gaps and concealed
openings encourage an increased rate of destruction. For this reason, the
performance of nail-laminated members is inferior to glued laminated
members. However, delamination in glued joints caused by unsuitable glue
or bad gluing techniques, e.g. , entraining air bubbles or using too much
glue, would be detrimental to fire resistance; and care should be taken to
ensure that the glue lines are sound.
The principal objective in designing fire protection systems is to achieve
smooth, flat , unbroken surfaces with joints carefully detailed and
fabricated to fit closely. Workmanship is of great importance. It is
ironic that the fire resistance of connections in timber frames is
dependent on the performance of the joints of the protections.
This
illustrates that engineering excellence requires attention to the last
detail.
Future Work
General
Fundamental research to improve' the understanding of the disintegration
process; thermal and mechanical properties of timber and steel together
with their interaction at elevated temperatures; heat conductivity (or
insulation) of different materials; as well as the development and
experience collected on various protections and jointing systems - all
contribute to the development, improvement and application of protection
systems for nailed gusset connections. The development of comprehensive
computer programs to determine the behaviour of gusset connections in fire
and the extension to the study of other types of connection is also
necessary. However, this work would better fit into a long term research
programme.
As efforts are made to standardise design procedures and arrangements of
nailed gusset connections (Walford, 1988), this will lead to the
production of standard details and the protection systems can be
standardised accordingly. The block and gusset tests provide useful means
to assess the efficiency of alternative protections.
Loaded Joint Tests
An experimental study on loaded connections is essential to obtain useful
design information. A simple arrangement for testing moment-resisting
gusset connections is shown in Figure 34.
The connection must be tested full scale and the protection system applied
as in the real structure.
The temperature conditions should be in
accordance with recognised standards.
Instrumentation should include
temperature, load and deformation measurements; and the monitoring of
lateral stability.
The amount of working load to be considered during the fire situation
should include the dead load of the structure. Live lodings may not be
appropriate because it is unlikely that people will be present on the roof
of an industrial building during a fire. Earthquake loadings are also
unnecessary. However, wind loads may need to be considered. The wind
load specified in MB9 (1987) may be excessive because the fire duration
considered is only one hour and detrimental effects are normally more
pronounced in the last twenty minutes of fire. In this regard, only a
very low wind load could be justified.
After the fire test and the cooling of the connection, it is recommended
that the connection be tested with the full design load in the cold
condition.
In relation to the present work, it would be more appropriate to specify
the design and failure criteria of the protected connections after the
temperature and deformation characteristics of the loaded connection tests
are available.
In fire design, it is necessary to re-assess the suitability of the one
hour fire resistance requirements for gusset connections in relation to
economy. The protections required for half hour fire resistance ratings
would be significantly simpler. Lateral restraints to portal frames are
also important.
Computer Analysis
In parallel
with the loaded connection tests, if the load-slip
characteristics for wails at elevated temperatures are established by
tests, they can be incorporated into a computer model to assess the
deformations of the connections. Although experimental verification is
necessary, such work would enable the behaviour of a large number of
connection configurations to be examined analytically, using the
temperature results from this study.
Another important piece of work would be on the calculated strength of
loaded joints under fire exposure.
CONCLUSIONS
principal conclusions of this study are as follows:
There has been a great deal of testing carried out overseas on
unprotected and protected connections between timber members under
standard fire conditions. However, none of these resemble the large
nailed gusset connections commonly adopted for portal frame
structures in New Zealand and hence this overseas work has limited
application here.
Unprotected nailed plywood and steel gusset connections cannot
achieve a one hour fire resistance rating.
Protected connections can achieve a one hour fire resistance rating,
depending on the protection system used.
The block and gusset tests presented herein are versatile test
arrangements providing useful information in assessing the
performance of various protection systems.
Disc thermocouples
provide useful means to measure surface temperatures while the
technique for nail temperature measurements can be further improved.
Of the five protection materials tested, 40 mm thick solid timber
and two layers of 14.5 mm thick paper-faced gypsum plasterboards
provide superior insulation and can achieve a one hour fire
resistance rating.
However, 19 mrn thick paper-faced gypsum
plasterboard and two layers of 18 mm thick construction plywood
cannot.
Intumescent coatings are not suitable as an independent
protection for large steel gussets.
The effectiveness of a protection system depends on the thickness
and properties of the protection materials as we11 as the adequacy
of their attachments.
Construction plywood cannot be used as substitute for the same
thickness of solid timber in fire protection.
The arrangement of protecting the connections on all sides is more
efficient than protecting the gussets only.
The recommendations in MP9 (1987) regarding construction details
(clause 1.8.5) are all confirmed by the block tests.
Nail-laminated joints would not provide a one hour fire resistance.
Suitably glued joints can perform satisfactorily.
There exists a thermal
connections in fire.
gradient
along
the
nails
for
gusset
The design criteria for nailed gusset connections have been
discussed and can be specified for design after additional
information on loaded connection fire tests is available.
MAJOR SOURCES
1.
Ahlen and Mansson, 1979; Baldwin and Ransom, 1978; Carling, 1986;
Keith, 1987 ;
Kordina and Meyer-Ottens, 1977a,
Jachan, 1981 ;
1977b, 1977c, 1977d; Leicester, Seath and Pham, 1979; Odeen, 1985;
Olsen, 1986; Rimstad, 1979; Sauvage, 1985; Schaffer, 1961; Smith,
1985.
Barnett, 1984;
Bastings, 1985;
Buchanan, 1987 ;
Anon. , 1976 ;
Canadian Wood Council, 1977 ;
Do and Springer, 1983;
Forest
Products Laboratory, 1981;
German Standards Institution, 1981;
Hall et al, 1980; Hay, 1987; Hillis and Rozsa, 1978; J ackman ,
1981 ;
Jonsson and Pettersson, 1985; Keith, 1987; Kordina and
Meyer-Ottens, 1977a, 1977b, 1977c, 1977d, 1983 ; Malhotra, 1986 ;
Norum, 1985;
Odeen, 1985 ;
Sauvage, 1985;
Schaffer , 1984;
Spencer, 1984; Springer and Do, 1983; TRADA, 1979.
3.
Batchelar, 1984; Gibson, 1984; Loughnan, 1984; Walford, 1988.
4.
DIN 4102, 1981; Gotz et al, 1978; Keith, 1987; Kordina and MeyerOttens,
1977a, 1977b, 1977c, 1977d, 1983 ;
Loughnan , 1984;
Sauvage, 1985; Smith, 1984; Table 4 of this report.
5.
Allen, Stapurnzak and Galbreath, 1974; American Iron and Steel
Institute, 1981; Baber and Fowkes, 1984; Barnett, 1986; Bastings,
1984, 1985, 1986 ;
Clifton, 1985 ;
European Convention for
Constructional Steelwork, 1974, 1979, 1985; Gibson, 1984; Golding,
1984; HERA, 1985; Morrison Cooper and Partners, 1985; N o r m ,
OldnalX, 197-; Ridge, King and Walker, 1.972; Twilt and
1985 ;
Witteveen, 1974.
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Jonsson, R. and Pettersson, 0.
review of the existing state of knowledge and research requirements.
Swedish Council for building research D3:1985. Stockholm.
Keith, J .W. 1987. The consideration of fire resistant rated timber in
the design of structures. Seminar on Fire Protection of Buildings - New
Directions.
Building Science Forum of Australia, New South Wales
Division, Sydney.
1977a.
Brandverhalten von
Kordina, K. and Meyer-Ottens, C.
Holzkonstruktionen (in German, title translates to - Fire behaviour of
timber structures). Informationsdienst Holz. EGH. Munich.
1977b.
Fire behaviour of wood structures.
Tech. University
Braunschweig. FRG Inst. Baustoffkundee. Stahlbetonbau Braunschweig.
1977c. Tests of connections between laminated wood members during
fire exposure according to DIN 4102 Part 2. United States Department of
Agriculture, Forest Products Laboratory, Research Report N 77 16 9
MO/SCHR .
1977d.
Feuerwiderstandsklassen von Bauteilen aus Holz und
Holzwerkstoffen (in German, title translates to - Fire resistance ratings
for construction in timber and timber based products) .
Tech. Univ.
Braunschweig, Inst. fur Baustoffkunde and Stahlbetonbau. Braunschweig.
1983. Holz-Brandschutz-Handbuch. (in German, title translates to Timber - fire protection handbook).
German Association for Timber
Research. Munich.
Leicester, R.H. 1988. Personal communication.
Leicester, R.H., Seath, C.A. and Pham, L. 1979. The fire resistance of
metal connectors.
Proceedings Nineteenth Forest Products Research
Conference, Melbourne.
Lie, T.T. 1972.
Fire and Buildings. Applied Science Publishers.
Lihavainen,
P.
1976.
Liimapuurakenteiden
palonkestavyydesta
(Limtrakonstruktioners
brandmotstandsformaga)
( in
Finnish,
title
translates to - The fire resistance of glulam timber structures).
Helsinki Institute of Technology, Department of Building Engineering,
Diploma report. Otnas.
Loughnan, A.A.M. 1984. The Timbertek building system, its design and
application. Proceedings Pacific Timber Engineering Conference, Auckland:
134-141. Institution of Professional Engineers, New Zealand. Wellington.
Mackay, J.F.G.
1973.
The influence of drying conditions and other
factors on twist and torque in Pinus radiata studs. Wood and Fibre, 4:
264-271.
Malhotra, H.L. 1986. Report on the work of technical committee 44-PHT,
"Properties of materials at high temperatures". Materials and Structures,
15: 161-170.
McNaughton, G.C. and Harrison, C.A.
1940.
Fire resistance tests of
plywood-covered wall panels. USDA FPL. Report 1257. (Updated and reaffirmed in 1961).
Morrison Cooper and Partners. 1985. Fire protection manual, section 7,
passive fire protection of steel. New Zealand Heavy Engineering Research
Association, Report R4-34. Auckland .
New Zealand Heavy Engineering Research Association.
1985.
Collected
papers on steel protection, volume 2. HERA Report R7-13. Auckland.
Norum, W.A. 1985. Fire ratings of exposed wood and protected wood-frame
assemblies. Building Standards, 54(3): 4-8,52.
Norwegian Standards Institute. 1981. NS 3478 Brannteknisk dimensjonering
av bygningskonstruksjoner (in Norwegian, title translates to - Fire
resistance design of timber structures). Oslo.
Nullifire Limited.
Coventry.
Odeen, K. 1985.
21(1): 34-40.
Nullifire systems specifiers manual.
Fire resistance of wood structures.
1st edition.
Fire Technology,
Oldnall, R.F. 197-. Fire protection of steel columns: some examples.
Auckland City Council. Auckland.
Olsen, C.S.
1986. Personal communication.
Plywood Association of New Zealand.
Zealand. Auckland.
A complete guide to plywood in New
Ridge, M.J., King, G.A. and Walker, G.S. 1972. The resistance to fire of
building elements made from cast gypsum.
Commonwealth Scientific and
Industrial Research Organization, Division of Building Research, Technical
Paper No. 29. Highett, Victoria.
Rimstad,
N.O.
1979.
Staldeler
i
limtrekonstruksjoner
beskyttelsemetoder. (In Norwegian, title translates to - Steel components
Nordic Timber
in laminated structures - methods of fire protection).
Symposium, Aalborg. Nordisk tratidskrift No. 6: 129-133.
Sauvage, M.E. 1985. Determination of the behaviour of wooden building
components and wood-based panels exposed to fire.
Commission of the
European Communities, Report EUR 9485 EN.
Schaffer, E.L. 1961. The effects of fire on selected structural timber
joints. University of Wisconsin, M.S. thesis. Madison.
1977. State of structural timber fire endurance.
9(2):145-170.
1984. Structural fire design: wood.
450. Madison.
Wood and Fibre.
Forest Products Laboratory FPL
Silcock, A., Savage, N.P. and Robinson, D. 1977. Fires in dwellings - an
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Building Research Establishment Current Paper CP 51/77.
Fire Research Station, Borehamwood.
1984.
Design of low rise buildings in heavy timber
Smith , P.C.
construction. Proceedings Pacific Timber Engineering Conference, Auckland:
159-167. Institution of Professional Engineers, New Zealand. Wellington.
1985.
Timber joints in fire.
Construction, May: 22-26.
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Spencer, M.G.
1984.
Fire design of timber structures.
Proceedings
Pacific Timber Engineering Conference, Auckland: 683-690. Institution of
Professional Engineers, New Zealand. Wellington.
Springer, G.S. and Do, M.H. 1983. Degradation of mechanical properties
of wood during fire.
National Bureau of Standards, Department of
Commerce, Report No. NBS-GCR-83-433. Washington, D.C..
Standards Association of Australia. 1983. Draft Australian handbook for
timber engineering code, part 4: fire resistance of timber structures. DR
83201. Sydney.
1985. Fire-resistance tests of elements of construction.
Part 4. Sydney.
AS 1530,
Standards Association of New Zealand.
1971.
construction plywood. NZS 3614. Wellington.
The manufacture
of
1981. Code of practice for timber design. NZS 3603. Wellington
1984. Model building bylaw: fire resisting construction and means
of egress. NZS 1900: Chapter 5. Wellington.
1984. Code of practice for the general structural design and design
loadings for buildings. NZS 4203. Wellington.
1984. Plywood for marine craft. NZS 3613. Wellington.
1987.
Fire properties of building materials and elements of
structures. MP9: 1987. Wellington.
Swedish National Board of Physical Planning and Building (Statens
SBN approval list for the strength of structural
Planverk). 1984.
elements during fire. PFS 1984:l. Stockholm.
Tan, R.H.S.
1988.
Plimrnerton Family Fun Park - a case history.
Proceedings Annual Conference, The Institution of Professional Engineers,
New Zealand, Volume 1: 297-303.
Timber Research and Development Association. 1979. Timber and wood-based
sheet materials in fire. TRADA Wood Information, section 4, sheet 11.
1985. Mechanical fasteners for structural timberwork.
Information, section 2/3, sheet 9.
TRADA Wood
Twilt, L. and Witteveen, J. 1974. The fire resistance of wood-clad steel
columns. BOUW, 12. (Reprinted in Fire Prevention Science and Technology,
11: 14-20)
Walford, 6.B.
1988. Analysis of timber portal frame gusset joints.
Proceedings Annual Conference, The Institution of Professional Engineers,
New Zealand, Volume 1: 317-326.
Winstone Wallboards Limited. 1979. Trade literature for Gibraltar Board
fire-rated timber partitions. Auckland.
APPENDIX:
TABLE 1.1:
DETAILS OF TEST SPECIMENS
Test materials
Material
Details
Glulam timber
. 1
Framing Grade untreated Pinus radiata seasoned
12 per cent moisture content and sawn from
0x90 mm2 cross-sections
Plywood
18 mm thick tanalised construction plywood
(New Zealand Pinus radiata plywood) seasoned to
11 per cent moisture content
Steel
5 mm thick mild steel
Solid timber
40 mm thick No. 1 Framing Grade untreated Pinus
radiata, seasoned to 17 per cent moisture content
14.5 mm paper-faced
gypsum plasterboard
14.5 mm thick 'Fyrestopt Gibraltar board
19 mm paper-faced
gypsum plasterboard
19 rnrn thick 'Fyrelinet Gibraltar board
Intumescent coating
'Nullifire S60t,total loading was 2200 grams per
square metre
resorcinol glue
'Desford' resorcinol glue
TABLE 1.2:
Specimen preparation
Sample
Method of assembly
Glued timber block
sample
glued with resorcinol glue
gypsum plasterboard
block sample
plasterboards glued with 'Gib Cove' adhesive,
plasterboard glued to timber with 'Gib Fix' adhesive
Plywood or solid
timber protected
sample
the protections were glued together as well as glued
to the gusset sample using resorcinol glue, also
nailed with 7 5 ~ 3 . 1 5bright flathead nails at 150 mm
centres around the perimeter
Gypsum plasterboard
protected gusset
sample
the protections were glued together with 'Gib Cove'
adhesive where appropriate, glued to the gusset
sample with 'Gib Fix' and also screwed to the sample
at 150 mm centres around the perimeter with screws
in accordance with the manufacturer's recommendation
Intumescent coating
sample
the coatings were applied in accordance with the
manufacturer's recommendation
Note:
1.
Other details of the samples are shown in Figures 3 to 7.
portal rafter
-
-
5 8 mm thick steel
gusset (both sides)
5 8 mm thick steel
gusset (both sides)
portal leg
portal leg
(a) ' Gibson" type
(b) ' TimberteKQtype
I
portal rafter
thick plywood
sides)
IA
portal leg
(c) ' Batchelar" type
Figure 1: Common gusset joints in New Zealand
/ charred zone
I
uncharred
section
Figure 2: Schematic diagram of the penetration of charred zone in the
vicinity of large and small nails (from Aarnlo and Kallioniemi, 1979,1983)
Sample Types
Furnace side
Furnace side
1
Solid specimen
Furnace
Notation
----
2
Solid specimen
Solid specimen
Sample Types
Mounting Details
I;'
.
.
thermocouple
/ hot junction
Furnace side
specimen
-
2
,
mm diameter hole
Thermocouple Location
Mounting Details
200 x 180 x 90
glulam timber
specimen
3.15 x 75 bright flat
-
head nail
face grain
direction
Nail Pattern
Figure 3 : Block test specimems
Joint location
-r
60
face
-
grain
direction
0
In
<V
II
0
In
@
LD
-.
50
.A
Unprotected Plywood Gusset Specimen
(18 mm thick gusset)
K
= 3.55 x 60 galv nails
= 1 to 7 nail tip temp
0
= 8 to
Notation
.
360
' I
11 timber surface temp
(disc thermocouple)
= "Batchelar" pattern
3.55 x 60 galv nails
Mounting Detail
Face grain direction
-
Notation
= widely
K
Unprotected Steel Gusset Specimen
(5 mm thick gusset)
FIGURE 4: Unprotected gusset specimens
o
.
spaced pattern
3.55 x 60 F.H galv nails
= "Timbertek" pattern
3.55 x 45 F.H. galv nails
a 1 to 7 nail tip temp
= 8 to 11 timber surface
temp (disc thermocouple)
Face grain
direction
Front Vlew (protection not shown)
R C frame
Notation
b
3.55 x 60 FH galv nails
x = 1 to 4 nail tip temp
o = 5,6 temp behind protection ) disc
7,8 timber surface temp
) thermocouple
9,10 gusset corner temp
)
1 1,12 temp behind first
1
layer of protection
0 =
packing
I
'7 9
I)
O
I
I
360x360~90
glulam timber
I
I
I
I,
I
I
I
-360 x 360 gusset
(18 mm plywood or
5 mm mild steel)
I
l11,12
I
---13 mm air gap for steel
gusset only
Mounting Detail
Figure 5 : Protected gusset specimens, arrangement A
gusset
-
Face grain
direction
-
360 x 360 x 90 glulam timber
/
Front View (protection now shown)
Notation
3.55 x 60 F H galv nails
1 to 4 nail tip temp
5,6 temp behind protection ) disc
7,8 timber surface temp
) thermocouple
9,10 gusset corner temp )
1 1,12 temp behind first
1
layer of protection
Protection-
Packing
-
287 x 287 gusset
(18 mm plywood or
5 mm mild steel)
13 mm air gap for
steel gusset only -
-
360x360~90
glulam timber
Mounting Detail
Figure 6 : Protected gusset specimens, arrangement B
Face grain
direction
Front view (protection not shown)
'
;'.
0
7
I
Notation
e
x
x
=
=
=
0 =
3.55 x 60 FH g a l ~
nails
1 to 6 and 9 nail tip temp ("hotjunction contact")
7,8 nail tip temp ("enlarge and refill")
10,11 timber surface temp disc thermocouple
-4
packing
steel gusset
nail
protection
("Nullifire 560")
Mounting Detail
Figure 7 : Protected gusset specimen, arrangement C
A typical t e s t setup
FIGURE 8:
o
Furnace flverage
- IS0
0
10
20
Time
FIGURE 9:
30
834-1975
40
50
(minutes)
Furnace temperature rise vs time
60
Time
NOTATION
---------
minutes)
PROTECTION
:
:
:
:
:
:
solid
sliced, glued
sliced, glued and nailed
solid with nails
sliced, nailed
FIGURE 10:
Block test temperatures, 50 mm from exposed face
outline of
/SZ
test
furnace side
Uncharred Section
NOTATION
PROTECTION
solid
sliced, glued
sliced, glued and nailed
solid with nails
sliced, nailed
3 layers of 14.5 m m thick paper-faced gypsum plasterboard
FIGURE
Extent of charred
speciments
zone
at
the
mid-section of
block
test
(a>
Glued and n a i l e d specimen
(b)
Nailed specimen
FIGURE 1 2 :
Glued and n a i l e d block t e s t specimens a f t e r t e s t .
PLYWOOD GUSSET
Time
(minutes)
STEEL GUSSET
Time
(minutes)
NOTATION
--
-
:
:
:
:
..
FIGURE 13:
Timber surface temperature, under gusset
Nail tip temperature, centre nail
Nail tip temperature, widely spaced nails
Nail tip temperature, medium spaced nails
Nail tip temperature, closely spaced nails
Temperatures o f unprotected gusset specimens
FIGURE 1 4 :
Unprotected gusset specimens after test
I
I
Steel gusset remained
in place
furnace side
\
Uncharred Section
(a)
Unprotected steel gusset specimen
Outline of plywood
gusset at the beginning
of test
360 mm
1
-----
(b)
-
--
--
Unprotected plywood gusset specimen
FIGURE 15:
Extent of charred
gusset specimens.
zone
at
the mid-section of
unprotected
PLYWOOD GUSSET
Time
(minutes)
STEEL GUSSET
Time
NOTATION
-FIGURE 16:
(minutes)
PROTECTION
: 2 layers of 18 rnm thick plywood
- .,
: 2 layers of 14.5 rnrn thick paper-faced gypsum plasterboard
Temperatures behind first layer of protection, arrangement A
PLYWOOD GU
Time
(minuted
STEEL
20
G
30
f h e (minuted
NOTATION
-
--
PROTECTION
: 2 l a y e r s of 18 mni t h i c k plywood
1 9 mrn t h i c k p a p e r - f a c e d gypsum p l a s t e r b o a r d
_ _.
---FIGURE 1 7 :
: 2 l a y e r s of 1 4 . 5 mm t h i c k p a p e r - f a c e d gypsum p l a s t e r b o a r d
: 40 mm t h i c k s o l i d timber
: intumescent c o a t i n g s
Temperatures behind t h e s u r f a c e p r o t e c t i o n , arrangement A.
PLYWOOD GUSSET
Time
(minutes)
STEEL GUSSET
Time
NOTATION
(minutes)
PROTECTION
19 mm thick paper-faced gypsum plasterboard
40 mm thick solid timber
intumescent coatings
FIGURE 18:
Glulam timber surface temperatures, arrangement A.
PLYWOOD GUSSET
Time
(minutes 1
STEEL GUSSET
Time
PROTECTION
NOTATION
--
----
--
FIGURE 19:
(minutes)
:
:
:
:
19 nun thick paper-faced gypsum plasterboard
40 rnm thick solid timber
Glulam timber side temperatures, arrangement A.
PLYWOOD GUSSET
Time
(minutes)
STEEL GUSSET
Time
NOTATION
--__
-
-
----
(minutes)
PROTECTION
: 2 layers of 18 mm thick plywood
:a19 mm thick
: 2 layers of
: 40 mm thick
: intumescent
paper-faced gypsum plasterboard
14.5 mm thick paper-faced gypsum plasterboard
solid timber
coatings
FIGURE 2 0 : Nail tip temperatures, edge distance
=
20 nun, arrangement A.
PLYWOOD GUSSET
n
0
0
V
Q)
L
3
t'
fd
I
(I)
e
€
Q)
I-
Time
(minutes)
STEEL GUSSET
-
10
0
20
Time
NOTAT ION
40
50
60
(minutes)
PROTECTION
*
-
30
---
-
-- - -
FIGURE 21:
. .2
l a y e r s of
: 19 mm t h i c k
: 2 l a y e r s of
: 40 mm t h i c k
intumescent
t
18 mm t h i c k ' p l y w o o d
p a p e r - f a c e d gypsum p l a s t e r b o a r d
1 4 . 5 mm t h i c k p a p e r - f a c e d gypsum p l a s t e r b o a r d
s o l i d timber
coatings
N a i l t i p t e m p e r a t u r e s , edge d i s t a n c e
-
40 mm, arrangement A .
FIGURE 22:
FIGURE 23:
Solid timber protected specimens a f t e r t e s t .
Plywood protected specimens a f t e r t e s t .
FIGURE 24:
Paper-faced
gypsum
plasterboard
protected
specimens
after
test.
....
<$..
..
r%
; ;
9.
.:
I...
FIGURE 25:
Intumescent coatings protected specimen after t e s t .
Uncharred Section
(a)
Steel gusset specimens
N.B.
Specimens protected by 40 mm thick solid timber or 2 layers of
14.5 mm thick paper-faced gypsum plasterboard have uncharred glulam
timber cross-sections.
outline of plywood
gusset at the
beginning of test
Furance Side
Uncharred Section
(b)
Plywood gusset specimens
N.B. Specimen protected by 2 layers of 14.5 mm thick paper-faced gypsum
plasterboard has uncharred plywood gusset and uncharred glulam
timber cross-sections.
NOTATION
--
-----
ARRANGEMENT
PROTECTION
:
:
:
:
2 layers of
19 mm thick
40 mm thick
intumescent
18 mm thick plywood
paper-faced gypsum plasterboard
solid timbe.r
coatings
FIGURE 26: Extent of charred zone at the mid-section of protected
specimens, arrangements A and C.
PLYWOOD GUSSET
-
0
10
30
20
40
50
60
Time (minutes)
STEEL GUSSET
Time
(minutes)
NOTATION
EDGE DISTANCE (rnm)
FIGURE 27:
Nail tip temperatures for protected specimens, arrangement B.
7#
Steel gusset
remained in
place
I
I
I
furnace side
uncharred section
I
I(a>
Steel gusset specimens
outline of plywood
gusset at the
beginning
.--of test
r I,
furnace side
------------------.
---,--,-,---------
/#
1
1
uncharred section
NOTATION
-FIGURE 28:
PROTECTION
: 2 layers of 18 mm thick plywood
: 19 mrn thick paper-faced gypsum plasterboard
Extent of charred zone at
specimens, arrangement B.
the mid-section of protected
-
,-------------------I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
1
I
I
I
I
I
I
I
I
I
I
uncharred
section
I
I
I
I
I
I
I
1
I
I
I
I
I
I
I
Figure 29 :Temperature observed in the sectlon'of 160 x 360 mm2
timber beam after 60 minutes of fire exposure (after Sauvage, 1985)
steel gusset
,
temperature
nails
glulam
timber
-
Figure 30 :Temperature distribution of nails
1
steel plate /
width
150 mm
thickness = 8 mm
-
,
-
mineral wool
I
steel plate
= 150 mm
width
thickness = 8 mm
Figure 31 :Sections through mineral wool insulated steel plates after 60
minutes of fire test ( from Aarnlo and Kalilonleml, 1983)
Time
(minutes)
NOTAT I ON
---
----FIGURE 3 2 :
: timber surface temperature
: nail tip temperature, edge nails
("hot junction contact" technique)
: nail tip temperature, intermediate nails
"hot junction contact" technique)
: nail tip temperature, intermediate nail
("enlarged and refill" technique)
: nail tip temperature, centre nail
("hot junction contact" technique)
Temperatures for intumescent coatings protected specimen.
FIGURE 33:
Conditions of timber around nails after fire test
-
Reinforced
concrete
wall __CI
Furnace
atmosphere
Load
Load
connection
/
support
reinforced
concrete
Figure 34 : Arrangement for testing loaded connections
glulam
timber
beam
The Fire performance of unloaded nailed
gusset connections for fire-rated timber
YIU, P.K.A.
BUILDING RESEARCH ASSOCIATION OF NEW ZEALAND INC.
HEAD OFFICE AND LIBRARY, MOONSHINE ROAD, JUDGEFORD.
The Building Research Association of New Zealand is
an industry-backed, independent research and testing
organisation set up to acquire, apply and distribute
knowledge about building which will benefit the
industry and through it the community a t large.
Postal Address: BRANZ, Private Bag, Porirua

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