Wheel loadings on web panels of overhead crane box-girders.

In torsion-box design of twin girder overhead cranes, the bridge rail on which the lifting unit runs is positioned eccentrically on the girder, directly above one of the web plates. This web is subjected to in-plane patch loading produced by the spread of a wheel load through the overlying rail and flange. This study concerns the load carrying capacity of plate box-girder web panels subjected to a wheel load at the midspan of the panel. Distribution of a wheel load through a rail and flange is investigated from recordings made of in-plane vertical stress distribution profiles along the upper edge of a web panel of a short-span model box-girder. The girder was loaded through various interfaces above the web by a wheel load. A simple method is proposed for relating a distributed wheel load to an equivalent uniform patch load. Methods for estimating distributed wheel loading lengths are investigated. It is shown that crane web panels are generally subjected to patch loads of short length, occupying less than one-quarter of the panel length. A computer analysis is presented to determine elastic buckling coeffic-ients for flat rectangular plates subjected to a uniform in-plane patch load centrally disposed on one edge and supported by shear stresses on the adjacent edges. Patch loads of various lengths are considered over a range of plate aspect ratios for plates with various combinations of simply supported and clamped edges. Also considered are some non-uniformly distributed patch loads modelling approximately a distributed wheel load. For the large majority of geometries considered, it is the support condition along the loaded edge which has greatest influence on the buckling load. Correlation with buckling loads estimated from experimental measurements on a model crane girder web panel indicated that an assumption of simply supported panel edges is over-conservative and that it is probably more representative to consider the edges attached to the flanges as clamped. Ultimate load carrying capacity is considered. A plastic mechanism analysis originally presented by Roberts and Rockey is studied and a modified form derived which reveals the transition region from collapse initiated by direct web yielding for girders with stocky webs to failure by a mechanism of out-of-plane web deformation for girders with slender webs. Certain approximations in the original analysis are shown to involve the omission of terms which can contribute significantly to the plastic work expression. Inclusion of these terms, however, whilst offering potential refinement, increases considerably the complexity of the analysis. Results are presented of a series of collapse tests conducted on short-span model box-girders subjected to a wheel load above one of the webs. The effect on the failure load of rail size, web thickness, panel aspect ratio, and longitudinal web stiffening is investigated. Snap buckling was exhibited by several of the test web panels. From the results, a simple expression is developed for predicting collapse loads of plate girders subjected to narrow patch loads. The main findings of the work are used as a basis for a series of recommendations to aid the structural designer in taking account of patch loading on slender web panels.