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Contents : Durability and detail design - the result of 15 years of systematic improvements Fran ois W. Kropf EMPA Swiss Federal Laboratories for Materials Testing and Research Abstract Most weather exposed wooden structures do not fail due to structural errors but due to decay of essential parts of the carrying system. To avoid further substitution by concrete and steel wooden structures must be built for a service life of 25-50 years. Chemical wood preservation being increasingly criticized it was necessary to rethink environmentally less harmful possibilities. By following old and proven design principles a number of essential constructive details were developed and tested. Service life of exposed wooden structures can be comparable to other building materials. With a minimum of maintenance there is no quick failure mechanism to destroy a covered bridge. As wood reacts quickly to adverse influences shorter maintenance intervals are recommended for exposed wood structures to keep repairs if needed easy and cheap - a feature substitute materials cannot offer. The quintessence of fifteen years of supervision of pilot structures is being presented to enhance the awareness of designers as well as craftsmen for the importance of proper detail design for the durability. Keywords: bridge durability design detail exposure preservation wood weathering Introduction Exposed wooden structures have a long tradition in all countries where timber has been readily available as construction material. In Europe the oldest exposed wood constructions still in service today date back to the 11th century (1). In Switzerland alone about 250 covered bridges are still in good working condition and many more would be around had they not been carried away by floods or burned down for strategic reasons during the innumerous wars of the last 500 years. Of all the wooden structures that have disappeared with time only very few failed due to structural errors but most of them degraded progressively as a consequence of decay by fungi. (Termites are beyond the considerations of this paper.) Nevertheless much engineering effort is still being invested to optimize the structure itself but only little research work goes into finding better ways to prevent the decay of structural parts. For the last 15 years a project team of EMPA has concentrated its efforts on extending the service life of weather exposed wood structures. The first pilot structure of this program was built in 1979. For each of the following projects the experiences gained on the previous ones were systematically included in the planning phase until approx. 40 structures could be included in the supervision program. Every structure is visited periodically by an experienced specialist and developments are being recorded during its entire service life. 321 Failure mechanism of weather exposed wood The following (known) sequence leads ultimately to the failure of weather exposed wood members. material into disrepute for a long time in many parts of the world. As steel and concrete became cheap and readily available these became the materials of choice for bridge construction even for minor crossings where wood would have performed perfectly well. For many decades new wooden bridges were therefore out of discussion except for temporary crossings or in remote areas with difficult access. Rediscovering wood for exposed engineering structures In the aftermath of the first oil crisis of 1973 public awareness for ecological thinking and green ideas made it attractive for decision makers (politicians) to resort to natural materials for all kinds of applications - and wood took much benefit of this trend. The speed of decay depends on exposure and a number of climatic factors. Often damage go unnoticed until major repairs are necessary. Measures to extend service life To extend service life the above chain of events must be interrupted at some point either by - preventing the development of cracks and delaminations: Protecting the essential members of the structure from heating up by providing sunshields and possibly a surface treatment of an appropriate color (not too dark) or - stopping water from entering the fissures: Covering the cracked surfaces where rain could enter or - poisoning the fungi: Applying pressure treatment all the way down to the bottom of the deepest potential cracks. Considering the difficult penetration in the usual construction wood (softwoods) one cannot count on it for larger cross sections. Consequences for design The major risk factors for the supporting elements are rain and direct sunshine but not the changes of ambient moisture. The most effective weather protection is a good roof which covers the entire structure. Such a roof is costly and reduces the design load and many bridges were built without. In our climate it is then a matter of 5-15 years until decay makes such structures unsafe for their desi

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