Quy Thanh PHAN

Evaluation of ultimate bending strength of composite I-shape girders under positive and negative bending moment by experiments.

Masatsugu NAGAI, Eiji IWASAKI


In Japan the ultimate bending strength of composite I-shape girders is defined by lower strength between yield point of lower flange and buckling strength. On the other hand, the European and American design standards have defined the ultimate bending strength at higher strength, which is known as plastic moment. Therefore introduction of plasticity design is neccessary for the purpose of cost reduction.

If Plastic Neutral Axis (PNA) is in the concrete slab, the ultimate bending strength of composite I-shape girders can achieve the plastic moment. If PNA is in the web, Dp/Dt[1] ? 0.15 in Eurocode, and Dp/Dt ? 0.1 in AASHTO-LRFD[2] then concrete slab will be destroyed ahead by compress (ductility condition), so the ultimate bending strength can not achieve the plastic moment.

In that case, there is a valuation formula for plastic moment. Our laboratory has already examined the ultimate bending strength by experiments and analyses. The results show that both classification formula of compact section & noncompact section and the ductility condition of Eurocode & AASHTO-LRFD are safe. However that results are limited to two experimental models, so this research is to study the evaluation of ultimate bending strength of composite I-shape girders under positive and negative bending moment by experiments, in oder to estimate new classification formula of compact section & noncompact section and ductility condition.

Concluding remarks obtained and future research needs are as follows:
1.In Japan ultimate bending strength of composite I-shape girders is defined by yield moment of lower flange. But in the experiment of positive bending moment, it is clarified that ultimate bending strength was achieved plastic moment. So it is possible to introduce plastic moment in cost reduction.
2.About ductivity condition, if Dp/Dt ? 0.15 in Eurocode and Dp/Dt ? 0.1 in AASHTO-LRFD, then ultimate bending strength cannot achieve plastic moment. However in the experiment of positive bending moment, with Dp/Dt = 0.24, ultimate bending strength could achieve the plastic moment. Therefore the review of ductivity condition is necessary.
3.In the experiment of negative bending moment, ultimate bending strength could not achieve plastic moment. However when paying attention to crack state, there were horizontal axis direction. By this phenomenon, the reason why ultimate bending strength could not achieve plastic moment was stud design. Therefore it is neccessary to study about stud design and plascement method in the future.

[1] Dp/Dt
Dp: distance from upper edge of concrete slab to PNA
Dt: total height of composite I-girder including concrete slab
[2] AASHTO- LRFD
Load and Resistance Factor Design (LRFD) specifications of the American Association of State Highway and Transportation Officials (AASHTO)