Wetterfester Stahl in tragenden Teilen und im Geländer von

Transcrição

2. Internationale Holzbrückentage 12
Wetterfester Stahl in tragenden Teilen und im Geländer von Holzbrücken | H. Kepp, T. Dyken, T. Stensbye und J. Veie
Wetterfester Stahl in tragenden Teilen
und im Geländer von Holzbrücken
The use of Weathering Steel in Structural Parts and
Vehicle Restraint Systems on Timber Bridges
Hauke Kepp
Staatlich Norwegische Straßenverwaltung
NO-Lilleström
Tormod Dyken
NO-Oslo
Trond Arne Stensbye
NO-Hamar
Johannes Veie
NO-Hamar
1
2
The use of Weathering Steel in
Structural Parts and Vehicle Restraint
Systems on Timber Bridges
Summary
Weathering steel is maintenance free in the right conditions. This is because the rust coat
protects the members from further rusting by reducing the access to oxygen. To achieve
this, the steel needs to be in an environment that is not too wet and not too dry. The
design and the environment play a vital role in achieving this. Bad design may result in
shorter service life expectancy.
The paper is based on preliminary inspections and gives a preview of expected good and
bad designs, with respect to weathering steel, used on Norwegian timber bridges.
1.
Introduction
Weathering steel is an interesting construction material offering new possibilities within
aesthetics as well as maintenance. The natural colour of the material makes the structures appear less “technical” and thus blend into the landscape. Additionally, the material
has the ability to heal when exposed to scratches. These are important aspects for a road
owner. The Norwegian Public Roads Administration (NPRA) has tested weathering steel
on a large scale in order to gain wide experience with the material. It has been used
on the E6 motorway north of Oslo in several road equipment objects, on the vehicle
restraint system and in steel parts of timber bridges.
Weathering steel has great advantages when used correctly, but is probably more vulnerable to improper use and poor design than most construction materials. The use of
this type of steel requires both insight and experience, with careful attention to the
details of the design. The scope of this paper is limited to the use of weathering steel in
timber bridges.
1.1. Types of weathering steel
The most well-known brand-name is COR-TEN-steel, developed by the United States
Steel Cooperation, originally for the railroad industry. The product has been used in
bridges, pipes, storage tanks, railings, cladding panels and containers. Two types of
COR-TEN steel are used, COR-TEN A and B. The corrosion resistance of type A is better; however, type B is more commonly used in bridges due to higher impact resistance
and better resistance against brittle fracture. All of the steel used by the NPRA is an
alloy corresponding to COR-TEN type B. Other types will be tested in the future. NAWTEN-steel has shown good resistance in marine environments /1/ and is therefore of
special interest.
Several different weathering steel products are available on the market today as shown
in Figure 1. Weathering steels are all low-alloyed steels.
3
4
Figure 1: Some weathering steel available /1/
1.2. Inspection status to date
As the first structures with weathering steel were constructed in 2011, no thorough
inspections have been conducted thus far, as it seems too early to do so. Some early
inspections show both good and bad results, but the rust products have not yet been
analysed. However, a comprehensive follow up programme is planned.
2. Why weathering steel?
Ideally, weathering steel should be maintenance free. However, even after construction
and some years into service, one has the option of surface treating those parts which are
exposed to unfavourable climate. These parts can be treated locally, whereas normal
carbon steel needs to be treated on all surfaces. Thus, without taking the production
process into account, it is likely that weathering steel is more environmentally friendly
than carbon steel.
In practice one often sees damages to the protecting surface after transportation and
assembly, as shown in Figure 2. Damages repaired on site are expected to become weak
points during operation. Such scrapes on weathering steel, after the first layer of rust has
built up, will be visible for some time. But they are expected to be self-repairing and pose
more of an aesthetic problem than a maintenance problem.
Figure 2: Not uncommon damages to the surface on site
Weathering steel also has a matt surface finish, which looks more natural and can be
aesthetically pleasing. In combination with wood one achieves a very harmonic look, different from the industrial look of painted steel and the cold look of concrete surfaces.
Figure 3: Weathering steel railing together with copper and creosote treated wood
3.
How (not) to use weathering steel
Weathering steel needs to be thicker than carbon steel, due to the allowance for loss of
material. On exposed surfaces the added thickness can be expected to be up to 1-2 mm
/2/. The use of untreated weathering steel also requires a lot more attention to the detailing of connections, drainage, horizontal surfaces and pockets gathering dirt and
thereby holding on moisture. Too much moisture or a very harsh environment, especially
with respect to salts, may diminish the durability of weathering steel.
3.1. Horizontal surfaces
Some guidelines for design are given in /2/ and /3/. The avoidance of water pockets
requires special attention as shown in Figure 4. It should, however, also be good design
practice to avoid this on surface treated steel. Figure 5 shows a detail of the railing of
Klemetstrud Pedestrian Bridge in Oslo, opened in 2004. The horizontal stiffeners used on
the railing posts of the Klemestrud Pedestrian Bridge create quite unnecessary pockets
and resulted not only in discolouring of the members, but in the long run also a maintenance problem. A far better solution would have been to use massive or hollow profiles.
Weathering steel is unlikely to withstand a moisture trap as shown in Figure 5.
5
6
Figure 4: Avoidance of water pockets /2/. A=bad B=good
Figure 5: Detail of a post at Klemetsrud Bridge after only 7 years
Figure 6: Beam made of weathering steel on a timber bridge. The places where the snow collects may become
problem areas.
The beam in Figure 6 should probably have had the sides covered or used hollow profiles.
As the beam is situated above a highway, one can expect the water spray to deposit
de-icing salts in these places. The Finnish road authorities state in /4/ that corrosion
rates are higher in panels sheltered from rain when subjected to de-icing salts.
3.2. Connections
Connections need special attention, not unlike the details of timber bridges. Special considerations are:
1) Connections often include different materials and thus electrochemical considerations
have to be taken
2) Connections often create crevices and water pockets.
Different electrochemical potential will result in one of the materials acting sacrificial.
Stainless steel, zinc or cadmium plated bolts should all be isolated /2/. In the case of
Figure 7 the weathering steel will act as a sacrificial anode, however, since the surface of
weathering steel is a lot bigger than the surface of the stainless steel, this might not be a
problem. Ideally the stainless steel should have been painted or passivated. In Figure 8
the connecting steel is surface treated. However, if the coated material is carbon steel, a
scratch in its surface treatment might make it a very small anode connected to a very
large cathode (the weathering steel), if electrically connected and the weathering steel is
electrochemically more noble. This also applies for galvanized bolts, as the example
shown in Figure 9. If the zinc layer is sufficiently thick /2/ states that the service life may
still be adequate, implying that the rate of corrosion might be small enough. A light coloured stain must be expected though.
The temperature expansion joint in the railings create crevices as shown in Figure 9 and
will likely become an issue during the service life of the bridge. Water and dirt can gather
in between the rails. In Figure 10 the area around one bolt was wet. One possible explanation is that dirt gathered around the bolt was holding onto moisture. Figure 8 and show
good examples in sense of crevices. In water is not channelled into the railing, and the
connection is open and will probably not gather dirt.
7
8
Figure 7: Stainless steel in direct contact with weathering
steel
Figure 8: Good connection in the sense of crevices,
but also electrochemically?
Figure 9: Temperature expansion joint in the railing
Figure 10: Bolts different wet possibly due to dirt
Figure 11: An open connection
3.3. In contact with other materials
Weathering steel members buried or embedded in concrete, soil or gravel should according to /3/ be protected with a coating similar to those used for carbon steel. Runoff water
from weathering steel will discolour surfaces below. Especially porous materials, like concrete, will be difficult to wash. Water should therefore be channelled away from the front
surface. In the case of Figure 12, runoff water is channelled from the top side of the
beams onto the hinge. This is not uncommon, but may not be a good solution when using
weathering steel.
Figure 12: Neither runoff water nor protection of the concrete surface has probably been considered adequately. The concrete has also been poured poorly, without covering the steel.
4.
Vehicle restraint system
The vehicle restraint system used on the bridges using weathering steel was designed in
cooperation between the NRPA and AB Värmforzinkning. The posts shown in Figure 13
and Figure 14 have little surface and no water pockets. In design they probably work well
in weathering steel, time will show if they are capable of withstanding the environment
they are subjected to. On bridges with underlying roads the vehicle restraint system also
uses grillage panels between the posts, as shown in Figure 15, thus preventing snow
chunks from falling off the deck during ploughing.
Figure 13: Posts attached to vertical surfaces
Figure 14: Posts attached to horizontal surfaces
9
10
Figure 15: Vehicle restrain system with grillage panels for snow ploughing
5.
Concluding remarks
Designing a bridge with weathering steel is in some sense quite similar to designing timber bridges: The details are of great importance. Weathering steel has great advantages,
but also significant weaknesses. The use of weathering steel is perhaps not the best
choice for all the steel parts of these bridges, but we hope to gain valuable experience
during the years to come. Time will show what works and what does not.
6.
References
[1]
Johannesen, Arve: Værbestandig karbonstål – bruk, erfaringer og korrosjonsmekanismer i ulike miljøer (Weather resisting carbon steel – use, experience
and corrosion mechanism in different environments); Report nr 3030-11-0435;
Teknologisk Institutt; 2011
[2]
Brown, C W: The Use of Weathering Steel in Bridges; European Convention for
Constructional Steelwork (ECCS); ISBN 92-9147-000-64
[3]
Weathering Steel, A technical overview of weathering steels for bridges and general
construction; Mittal Steel USA – Plate; 2004
[4]
The use of weathering steels in bridges – Field test 1982-2003; Tutkimusraportti
NRO BTUO75-041245; VTT

Wetterfester Stahl in tragenden Teilen und im Geländer von

Transcrição

Documentos relacionados

eco worktop cupboards with upstand

Filtro em Y revestido| Catálogo

ECO PASS THROUGH HOT CUPBOARDS

ECO PASS THROUGH WORKTOP CUPBOARDS

worktop cupboards with upstand

worktop cupboards

MI. E5070.EN- 28012016

Welcome to Grade 6! We are very much looking forward to meeting

dimout - Hornbach

The Sky Disc of Nebra - Aspects of the aesthetics and astronomy

Characterization of natural and consolidated stones