Correspondence on Thoughts on the Structural Efficiency of Cable-Stayed and Catenary Suspension Brid

A proof test is described on a short-span reinforced concrete T-beam bridge without construction drawings, in the absence of which the bridge was posted by using only engineering judgment, to a maximum vehicle weight of 9t. A systematic proof test conducted with the help of controlled incremental loads, and some instrumentation, confirmed that the posting on the bridge can be safely increased to 14t. Observed tensile strains on the bottom faces of the concrete girders were found to be extremely small. It is suggested that the behaviour of the girders of the bridge is significantly enhanced by an internal arching action. As evidence of the arching action, two examples of thin deckslabs are presented, which are supported by girders at spacing of up to 13.5 times their thickness, and which contain no tensile reinforcement. B. Bakht and L.G. Jaeger

The range of standard pretensioned bridge beams was developed between 1955 and 1975. The bridge stock built using these beams has been remarkably durable, and the major durability problem with these, as with bridges of in situ concrete and steel beam construction, has been chloride attack from road salt channelled through deck joints onto often poorly detailed and maintained substructures. H.P.J. Taylor

This paper describes the development of a general 3-dimensional collapse mechanism approach for the assessment of shear in concrete beam-and-slab bridges. The present empirical rules for shear assessment of existing structures do not allow for the enhancement in shear capacity which an individual beam within a concrete structure might exhibit over and above its isolated (2-dimensional beam) strength. A rational approach, using the theory of plasticity in its standard form, is applied to this problem in an attempt to model the overall collapse mechanism which would have to occur in a concrete beam-and-slab bridge for the structure to actually fail. Some improvement (over the existing British assessment Codes of Practice for concrete bridges) in the predictions for shear strength of such structures is found. While the theory of plasticity relies on several assumptions of doubtful validity for concrete structures, the present method is shown to have potential as a possible assessment tool for bridges. T.J. Ibell, C.T. Morley and C.R. Middleton