11.52: Local buckling behavior and plastic deformation capacity of H‐shaped beams under reversed axial forces

Abstract

ABSTRACTIn general, beams with concrete slabs in the moment resisting steel frames are assumed to be rigid axially when pushover or dynamic analyses are conducted. Consequently, axial forces acting on the beams are not considered in a design procedure. However, steel beams are subject to axial forces when a frame has buckling restrained braces during the earthquake. Ultimate strength, plastic deformation capacity, and cumulative plastic deformation capacity of beams with axial forces decrease and severe damage may occur to compare with beams with no axial force.The purpose of this research is to elucidate the characteristics of H‐shaped beams under alternating axial force, which models the reversed axial forces from the dampers. Firstly, this paper conducts monotonic and cyclic loading tests on beams to understand local buckling behaviors under different loading protocols of axial force (no axial force, constant compressive axial force, and alternating axial force). Secondly, influences of the loading protocol of axial force on the ultimate strength, plastic deformation capacity, and cumulative plastic deformation capacity are investigated. Thirdly, the differences of capacity due to the magnitude of axial force and the number of cycles are examined. Finally, the effects of the width‐thickness ratio of beams on the buckling behaviors and performances under the alternating axial force are summarized.As a result, beams under the alternating axial force show local buckling under the compressive axial force and the pinching effect under the tensile axial force. Although the performances of beam under alternating axial force degrade to compare with those under no axial force, larger ultimate strength, plastic deformation capacity, and cumulative plastic deformation capacity than those under the constant compressive axial force are obtained. This enhancement of performances under alternating axial force is originated by the stretch of residual deformation due to local buckling under the compressive axial force when tensile axial force is applied to the beam.