Investigation of Web sidesway buckling phenomenon in Steel Girders

In previous experiments، practice and in some condition of loading، geometry and support configurations، a type of instability has been observed in which tension flange moves laterally which it has been addressed in design codes as “web sidesway buckling”. Web sidesway buckling is a case of instability that it was observed in beam with restrained top flange and no constraint for bottom flange. Primary studies and experiments show that web sidesway buckling is due to both local instability in the web just under loaded zone and global instability of tension flange along length of the girder. In the present paper، a concise study has been carried out on behavior and the mechanism of this instability occurrence in web sidesway and how to evaluate loading capacity of the girders in the light of experiments، then a simple model which is a modification on the existing model has been proposed. Experimental work has been conducted to investigate the effect of tension flange on the load capacity of beams. The specimen''s dimensions were adjusted in order to show web sidesway buckling. In addition، the supports configuration was made compatible with this instability. The objectives of the experiment were to obtain; mechanism of instability initiation، deformation pattern، effect of tension flange width and nonlinear deformations underneath the loading point. A closer view in load-displacement behavior of test specimens shows at first، loading accompanies lateral displacement due to imperfection and then rate of displacement reduces. After reaching to maximum load، girder has still capacity for load carrying but with excessive lateral displacement in tension flange. The results of experiments also show that web sidesway buckling generally accompanied by local buckling or crippling of the web under loaded zone. Deformation initiates with lateral movement of tension flange. Then local buckling and yielding occur، and finally، web sidesway buckling develops along the beam length. From this time onwards، load capacity is approximately constant. Furthermore، it can be seen that the rate of lateral displacement is directly dependent on width of tension flange. On the other hand، the results from design equations of design codes for estimating load capacity of girders against web sidesway buckling are too conservative in comparison to the proposed model. The critical load is affected by tension flange clearly، and occurrence of this type of instability is credible in other sections such as T- shape beam but in a lower critical load with respect to the I-shape sections. Also the results from the new model are in good agreement with that of experimental data. Finally، • It seems that only the existence of an area in the web affected by a tension stress field do not cause web sidesway buckling and transformation of tension stress field into compression field determents on initiation of instability.