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Investigation of flange failures in falsework cap and sill beams : recommendations for the design of beams and posts in bridge falsework.
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2008-12-01
[PDF-3.14 MB]
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Alternative Title:Recommendations for the design of beams and posts in bridge falsework.
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Abstract: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. Different methods are investigated for predicting the capacity of the flange. The first assumes a uniform
stress distribution resulting in bending of the flange. This is adequate for timber posts, but not as accurate at the second
more elaborate method which accounts for an interaction between flange bending and post compression strength, found
to be effective for timber posts. The third method uses an effective bearing area of the post, which 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 (referred to as web
crippling in the Caltrans Falsework manual) with variations of existing equations found to be appropriate for predicting
the capacity. Web crippling (as defined by ASCE) is found to generally not 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.
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