Masashi KITAGAWA

Evaluation of ultimate bending strength of composite I-girder under sagging bending moment

Masatugu NAGAI, Eiji IWASAKI

This paper presents ultimate bending strength of the composite I-girder under sagging bending moment. It has been reported that ultimate bending strength of the composite I-girder subjecting to the sagging bending moment is plastic bending moment. Japan Highway Bridge Specifications stipulated that ultimate bending strength is yield moment, which is around 25% lower than plastic moment. Based on Eurocode (EU) and AASHTO/LRFD (USA), if plastic neutral axis is within the concrete slab, plastic bending moment is expected even the I-girder has web with a considerably high depth-to-thickness ratio. From a viewpoint of reducing the construction cost, it is very exciting to identify the above suggestion.
Using composite girder models with different web depth and thickness, elasto-plastic finite displacement analyses were carried out. The width and thickness of the concrete slab were set to be 5,000 millimeters and 320 millimeters, respectively. Two web height‚“ of 2,500 and 3,000 millimeters were chosen and a ratio of the web depth to thickness chosen was from 124 to 294. The Three different material grades used were three types, they are SM400(ƒÐy = 235MPa), SM490Y(ƒÐy = 355MPa) and SM570(ƒÐy= 455MPa), in which ƒÐy is yield point of steel materials. In all models dealt with, neutral axes were in the concrete slab.
In the nonlinear analyses, the residual stress was neglected, because it has been reported that its effect on ultimate strength is minor. Hence, only the initial deflection of the web was taken into account.
From this study, it is concluded that ultimate strength of all the models in this study was plastic bending moment.