Bracing of steel bridges during construction; theory, full-scale tests and simulations

Sammanfattning: Popular Abstract in English Bracings are structural components that assure the stability of the load bearing structures is maintained so as a desired level of load-carrying capacity of them is achieved. The present study as a whole involves investigations of either the temporary or permanent bracings that are often required during the construction phase of steel bridges. Current knowledge concerning bracing requirements for steel bridges during construction is also discussed. Investigations of possible brace alternatives to the conventional ones were also of interest. Bridges are an important part of a country's road network. New bridges are often built over busy roads or railways. Traditionally, bridge construction involves the in-place casting of concrete, this requiring both time and a large space, this often causing serious traffic problems. To reduce disruptions of this sort, it is extremely important that a bridge's assembly be performed as quickly, smoothly, and as safely as possible. A type of bridge that makes smooth and relatively quick installations possible, and that during the last few decades has taken over a large part of the bridge market, is that of the so-called composite steel-concrete bridge. The installation of steel-concrete bridges, however, is a critical matter in the designing of such bridges, it is often controlling the size of both the steel girders and the bracings. In recent years, a number of accidents during the construction of steel bridges have occurred due to various instability phenomena during concreting phase in bridge construction. An example of such accidents is the collapse of Bridge Y1504 over the Gide River in Sweden that occurred in 2002. The investigations that took place following the accident required considerable costs and efforts in themselves, and replacing the bridge cost approximately twice the original budget for building of the bridge. Although two workers dropped down to the ground when the bridge collapsed, there were fortunately no fatalities, since the bridge was not particularly high. There have also been failures in the construction of composite bridges due to problems caused by the instability of their falseworks (the timber made elements that hold the fresh concrete during concreting phase till the concrete deck has hardened). The falsework failure in connection with the Älandsfjärden Bridge in Sweden in 2008 is an example of such an accident, five construction workers there falling 20 meters down to the ground, two of them being killed, and two severely injured. There have been also recent bridge collapses in Norway (Trondheim Bridge on May 8, 2013, two persons killed) and in Denmark (in Aalborg in June of 2006, one person killed; and in Helsingør in September of 2014) all of which occurred during concreting of their deck. Despite the fact that bracings are generally highly effective in controlling the out-of-plane deformation of steel bridges, relatively few guidelines and design recommendations are available regarding this matter. It was found that providing slight bracings can effectively enhance the resistance of steel girders. By using improper bracings, too large a lateral deflection of the bridges of this type could easily occur. The financial support this Ph.D. study received were from the “J. Gust Richert Stiftelse” Dnr 2012/05, “The Lars Erik Lundbergs Stipendiestiftelse” Dnr 2013/07 and Dnr 2014/05, this including also a research grant from Byggrådet to support the laboratory testing. Moreover, Britek AB and also Structural Metal Decks (SMD) Ltd provided the project with the scaffoldings and the corrugated metal sheets that were needed during the laboratory tests.