STUDY ON ACCUMULATED DEFLECTION CAUSED BY INELASTIC EARTHQUAKE RESPONSE IN LARGE-SPAN MOMENT FRAMES MADE OF H-SHAPED STEEL

Abstract

 Large-span steel moment frames are used as the main structure in many gymnasiums. Several studies pointed out that significant deflection in such moment frames was caused by inelastic seismic response for horizontal earthquake ground motion. This paper explains how horizontal excitation causes large accumulated deflection, and theoretically formulates the relations between vertical deflection and inelastic deformation. The moment frames made of H-shaped steel is considered herein, where shear yield of panel zones occur in advance to forming flexural yield hinges.
 Since gymnasiums are used as shelters in earthquake disaster, we need a method to evaluate the structural damage quantitatively, by inspecting and measuring the deformed gymnasium. By measuring the deflection after earthquake, we might be able to estimate the plastic deformation. In this paper, the possibility is discussed using FE dynamic response analysis.
 The mechanism how horizontal deformation causes accumulated deflection is summarized as follows. In elastic behavior, the moment distribution in the beam for horizontal load is antisymmetric. After forming two yield hinges at two ends of the beam, the moment distribution does not change and is also antisymmetric. In the both cases, the deflection does not increase. However, when only one yield hinge is formed at the leeward side, the deflection increases with the horizontal load due to the change of moment distribution until two hinges are formed. In this paper, bi-linear constitutive relation between the shear strain and moment acting on the panel zone is assumed and theoretical solution on the behavior is derived. The theoretical solution was verified by pushover FE analysis and good agreement was observed.
 By applying the derived theoretical formulas, ductility factor on the both panels can be estimated from the deflection. Dynamic response analysis was also performed to verify the estimation. This could be a useful method to estimate structural damage in the moment frame after earthquake.