Steel bridges launching and safety against patch loading

There are a large variety of construction methods used during bridges erection; one of the most interesting, particularly when bridges have long spans or have to overpass deep valley or rivers, is the launching of the bridge from an abutment. The launching phase, generally, is an important phase but last relatively a very short time compared to the total service life of the structure; consequently it is necessary that an accurate preliminary analysis of the technical approach of the erection method statements is adopted. It is also relevant to carry out precise, complete and consistent calculations to understand more exactly the mechanisms that the might structure develop, during the erection phases, in order to assign minimal safety coefficients required by the codes in force or in the technical specifications prepared for this temporary condition. Herein some specific investigations are proposed in order to solve satisfactorily the difficult problems met during the bridge erection: patch loading is one of the latter. Briefly some experiences gained in bridge launching is discussed, focusing on interesting solutions of some practical aspects and on erection techniques, like pull launching of the bridge in a wide curved or long length spans, using special but suitably designed equipments with due account taken of the cost-benefit aspect, when choosing a solution amongst those that are technically practicable. The interaction between patch loading and bending moment in the elastic domain has been exhaustively analysed and described for design procedure. The proposed relationships are valid for (Figure Presented) global stability of plates with a/h > √2 (class 4 of Eurocode 3-1-1), for which instability shapes are different in the two main directions of symmetry and also because those rectangular panels are commonly used in steel girders. The conclusions of the analysis are likely to be a contribution to the design of patch loaded web panels. The computational procedure described allows to evaluate in a simple way the reduction of elastic critical loads for patch loading due to the interaction with bending moments. It should be necessary to develop a comparison between more extensive numerical analyses and corresponding experimental results, in order to definitively validate the proposed method. On the other hand, the solution of a complex problem like the evaluation of the elastic critical load of web girders during bridge launching, subjected to combined patch loading and bending moment, is simple and accurate by using the proposed method. © 2006 Taylor & Francis Group.