FINITE ELEMENT SIMULATION OF A STEEL BRACED FRAME STRUCTURE DAMAGED BY THE 2011 TOHOKU EARTHQUAKE

Abstract eng:
During post-earthquake reconnaissance after the March 11, 2011 Tohoku Earthquake, the writers examined a damaged parking ramp structure. The structure was a twostory steel structure with a 52.5 by 48.5-m floor plan which relied on chevron braces for lateral load resistance. The earthquake damaged nearly all first-story braces in their connection to the beam. The majority of bracing connections had fractured in the east-west frame while many bracing connections were distorted, but none of them fractured, in the north-south frame. The cause of the observed damage was examined by detailed finite element simulations. Planar models of the structure, one extracted from the east-west frame and another extracted from the north-south frame, were subjected to static cyclic loading and to a ground motion record obtained within 800 meters from the structure. The simulation captured the main features of the observed damage. The results suggest that the bracing connections in the eastwest frame failed under a compressive force much smaller than the buckling load of the brace. Although they failed similarly, the bracing connections in the north-south frame were almost strong enough to develop the compressive strength of the brace. This difference may explain the reason why the damage differed in severity between the two loading directions. While fracture was not explicitly modeled in the simulation, the computed plastic strain indicated that the observed fracture in the bracing connections was due to severe cyclic deformation produced while the brace was in compression.

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