WEB BREATHING OF STEEL PLATED GIRDERS IN BRIDGES UNDER TRAFFIC LOAD

Since road and railway bridges are under repeated service loading, fatigue analysis and design of such structures' components are imperative. Limit state design method allows web panels of steel plate girders to be loaded beyond the elastic shear buckling load to make use of tension field reserve of strength. Under repeated load in excess of the elastic shear buckling load, due to initial imperfections, repeated out-of-plane deformations result in the web panel which, in turn, leads to relatively high secondary bending stress ranges at panel borders where the panel intersects flange and vertical stiffener. This phenomenon is known as web breathing effect. High secondary stress range in combination with membrane stress causes fatigue cracking and premature failure at the web panel borders. Stress states at critical regions of the panels play an important role in fatigue crack initiation. Therefore, in the present study, an extensive numerical analysis on steel plated girders was performed to explore the state of stresses in breathing webs. Postbuckling of web panels was simulated using a FE procedure incorporating both material and geometrical nonlinearities, and it was shown that maximum principal surface stresses occur at corner borders of web panels where the tension field is anchored. In these critical regions, fatigue cracks initiate, and it is in agreement with experimental evidences. In this numerical study, the influences of various geometrical parameters, including slenderness ratio, imperfection factor, and boundary members' stiffness, on the stress response of panels were investigated. It was observed that by increasing slenderness ratio, the rate of increase in maximum principal stresses for very slender panels is much higher than stockier panels. Moreover, in small imperfections, the response curves of maximum principal stresses versus applied load become nonlinear, whereas, for large imperfection factors, the curves are almost linear. The effect of the boundary members' stiffness on the stress response of breathing web panels becomes more pronounced in higher imperfection scale factors. In most codes of practice, no specific detail based on nominal stress is available to predict fatigue life of breathing webs. In this study, the method of geometric stress at critical regions adjacent to the weld toe is employed to predict fatigue life of breathing webs.