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Recommendations for the Design of Beams and Posts in Bridge Falsework
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Recent field observations in bridge falsework identified potential deficiencies in the design of falsework resulting in localized bending in sill and cap beam flanges and lateral buckling in other beams. Possible limits states associated with the bearing of timber and steel posts on cap and sill beams are investigated in this report. The critical limit states are related to flange bending, post crushing or yielding, web yielding, web crippling, lateral web buckling and corbel crushing. Two methods are investigated for predicting the capacity of the flange. The first accounts for an interaction between flange bending and post compression strength, found to be most effective for timber posts, while the second uses an effective bearing area of the post while is more effective with steel posts. For beams with relatively thick webs, such as those typically used in bridge falsework, the web was found to have a greater capacity than the flange and posts. The critical web limit state was web yielding with variations of existing equations found to be appropriate for predicting the capacity. Web crippling is found to generally not govern the design. Lateral web buckling may govern the design, particularly when two sill beams are stacked on top of each other. Blocking may be used to increase the flange bending and web yielding capacity, although lateral bracing or stiffeners are recommended to increase lateral buckling capacity. Design equations are presented in allowable stress design format for the consideration of the critical limit states in a falsework bent. These are compared to current Caltrans design practice and other relevant specifications. Two design examples are also provided to demonstrate the application of these equations, one using timber posts and a second using steel posts.
Report No. CCEER-05-11