Patch loading resistance of plated girders : ultimate and serviceability limit state

Sammanfattning: Patch loading or partial edge loading of steel girder webs is a load case where a concentrated force is introduced perpendicular to the flange of a girder. This usually induces a local failure of the girder web in the vicinity of the loaded flange. In structural applications concentrated forces are a common load case for girders introduced for example; at supports, by purlins, from crane wheels and during launching of bridges. For fixed loads, the problem of concentrated forces are usually solved by transverse stiffeners but for moving loads this is nor practically possible neither an economical solution. Further, it would be possible to use longitudinal stiffeners when the load is moving but stiffeners are expensive to fit and for girders with web depth below 3 m longitudinally stiffeners are not economically justified. Instead, the girder web itself has to resist the applied load in such cases. From the fifties and later a large amount of studies on this subject have been performed, starting with investigations on the elastic buckling of plates where only a part of the edge was loaded and followed by many test series and resistance functions. The earlier proposed resistance models were usually divided into two separate checks, one for yielding and one for instability. However, the test results do not show of any clear distinction between those two cases. This thesis deals with patch loading of plated girders without longitudinal stiffeners in both the ultimate and the serviceability limit state. A resistance model in the ultimate limit state is proposed, that have a continuous transition from yielding to buckling and hence, that is harmonized with the procedure for other buckling problems. The model contains three significant parts; the yield resistance, the elastic buckling load used to establish the slenderness and a reduction factor that relates the slenderness to the actual resistance. The advantage with the design model presented herein, which is a modification of the work presented by Lagerqvist and later introduced in EN 1993-1-5, is that the same equations are used irrespective of failure mode. The in this thesis proposed design procedure for patch loading gives a better accuracy of the predicted resistance compared to the design rule in EN 1993-1-5. Most of the experimental studies performed by others contain tests with very short loaded lengths, i.e. very concentrated loads. In order to gain more knowledge of the influence of the loaded length, three patch load tests were conducted where only the loaded length was varied. By means of the test results and a parametric study with the finite element method it was concluded that the variation in loaded length could be well described by the design procedure proposed in this thesis. For a bridge girder, the problem concerning resistance to patch loading usually occurs during launching. Bridge launching is a common method to erect steel and composite bridges and means that the bridge girders are assembled on ground behind the abutment and then pushed out over launching shoes into the final position. The launching shoe on which the girder travels will introduce a concentrated force to the girder, which can be of a magnitude that governs the web thickness and even a small increase of the web thickness can add a substantial amount of steel. Therefore, it is important to find a suitable criterion for the serviceability limit state for patch loading, i.e. for bridge launching. Compared to the ultimate resistance the amount of available research considering the serviceability limit state is very small. In fact, only one serviceability limit criterion proposed by Granath, which was developed for stationary loadings, was found. A number of FE-analyses of the launching process were carried out to investigate at what loads different girder cross sections will have a repeatable behaviour. A girder section of a bridge girder was subjected to several repeated travelling loads corresponding to a long bridge girder launched over several supports together with a co-existing bending moment. By means of these FE-analyses a serviceability criterion is established with a limit criterion not allowing any effective plastic membrane strains. The bridge designer can beneficially use the proposed serviceability criterion for bridge launching.