Serviceability Limit State for Steel Girders during Bridge Launching - A Patch Loading Problem

Abstract: Incremental launching is a construction method in which bridge girders are first assembled in the vicinity of the future permanent position of the bridge. When the elements have been welded together the girders are launched to their permanent position. During the launching they often slide on launching shoes placed in the positions of the future permanent support bearings.

At the launching stage relatively large support reactions are introduced into the steel girders as they ride on the launching shoes. These forces are quite concentrated (patch loading) and travel along the length of the girders as these are launched to their permanent positions. It is not uncommon that girders receive residual plastic deformation during the launching stage.

Until now the design for patch loading has been made using methods that focus on the ultimate limit state, i.e. on the load-carrying capacity of the girder. This thesis presents a design method that focuses on avoiding plastic deformations in girder webs subjected to a travelling patch load.

The method is based on a description of the elastic deformation that occurs in a girder web subjected to patch loading. The description was found by finite element simulations of laboratory tests on girders. It is concluded that no yielding should be allowed in the web plate since it may accumulate into residual deformations that could be potentially harmful for a travelling patch load. The design criterion was used in a parametric study where acceptable load levels were calculated by means of the finite element method. Regression analysis was then used to find a closed form solution that is easy to use for any structural engineer. The method deals with doubly symmetric I-shaped steel girders subjected to patch loading and bending moment.

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