Wetterfester Stahl in tragenden Teilen und im Geländer von
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Wetterfester Stahl in tragenden Teilen und im Geländer von
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 Staatlich Norwegische Straßenverwaltung NO-Oslo Trond Arne Stensbye Staatlich Norwegische Straßenverwaltung NO-Hamar Johannes Veie Staatlich Norwegische Straßenverwaltung NO-Hamar 1 2. Internationale Holzbrückentage 12 2 Wetterfester Stahl in tragenden Teilen und im Geländer von Holzbrücken | H. Kepp, T. Dyken, T. Stensbye und J. Veie 2. Internationale Holzbrückentage 12 Wetterfester Stahl in tragenden Teilen und im Geländer von Holzbrücken | H. Kepp, T. Dyken, T. Stensbye und J. Veie 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 2. Internationale Holzbrückentage 12 4 Wetterfester Stahl in tragenden Teilen und im Geländer von Holzbrücken | H. Kepp, T. Dyken, T. Stensbye und J. Veie 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 2. Internationale Holzbrückentage 12 Wetterfester Stahl in tragenden Teilen und im Geländer von Holzbrücken | H. Kepp, T. Dyken, T. Stensbye und J. Veie 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 2. Internationale Holzbrückentage 12 6 Wetterfester Stahl in tragenden Teilen und im Geländer von Holzbrücken | H. Kepp, T. Dyken, T. Stensbye und J. Veie Figure 4: Avoidance of water pockets /2/. A=bad B=good Figure 5: Detail of a post at Klemetsrud Bridge after only 7 years 2. Internationale Holzbrückentage 12 Wetterfester Stahl in tragenden Teilen und im Geländer von Holzbrücken | H. Kepp, T. Dyken, T. Stensbye und J. Veie 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 2. Internationale Holzbrückentage 12 8 Wetterfester Stahl in tragenden Teilen und im Geländer von Holzbrücken | H. Kepp, T. Dyken, T. Stensbye und J. Veie 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 2. Internationale Holzbrückentage 12 Wetterfester Stahl in tragenden Teilen und im Geländer von Holzbrücken | H. Kepp, T. Dyken, T. Stensbye und J. Veie 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 2. Internationale Holzbrückentage 12 10 Wetterfester Stahl in tragenden Teilen und im Geländer von Holzbrücken | H. Kepp, T. Dyken, T. Stensbye und J. Veie 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