EVALUATION ON PLASTIC DEFORMATION CAPACITY OF STEEL BEAM ENDS WITH LOCAL BUCKLING AND FRACTURE UNDER CYCLIC LOADING USING FE ANALYSIS

Abstract

 In Tohoku earthquake (March 11, 2011), highrise steel buildings of the Tokyo downtown area shook for a long moment under the influence of long-period ground motion. To prevent damage under such ground motion, evaluation on deformation capacity of steel member (welded beam end, etc.) has been required under multi-cycle loading.

 In this paper, deformation capacity of the steel beam with local buckling and ductile fracture is assessed by the FE analysis based on the fracture rule (cyclic damage rule and monotonic damage rule) and recontact of removed element. The cyclic damage rule is a fatigue damage law based on Continuum Damage Mechanics (CDM), and the monotonic damage rule is a damage rule for a large ductility factor equivalent to monotonic loading. The validity of this fracture rule and re-contact of removed element is verified by the simulation of past experiments under cyclic loading. Two past experiments are selected. One is an element experiment using specimens modeling beam flange and web, in which tensile and compressive repeated force is applied in the axial direction. The other is a partial frame experiment using field welding type specimen having a ¼ circular weld access hole of compound circle with R25 and R10 at the beam-end, in which constant cyclic displacement is applied. For an element experiment, simulation of monotonic loading and repeated loading with constant amplitude is performed using FEM with solid elements and shell elements. We can simulate the reduction of peak load using FE analysis, and it is found that the deformation capacity under multiple cyclic loading conditions can be evaluated. For a partial frame experiment, simulation using shell elements is carried out. As a result of the FE analysis, it is found that a crack occurs at the toe of the weld access hole on the beam flange, and propagates in the flange width direction. Using FEM it is possible to simulate the situation where the welded beam-end fractures, and it has been verified that the load and deformation relationship of FE analysis has good agreement with test results.

 In addition, as a result of conducting the analysis with the beam web thickness changed, it demonstrates that the initial crack initiation timing is delayed and the fracture timing is delayed accordingly because the stress concentration at the weld access hole bottom with thinner beam web is relaxed.