Seismic evaluation and upgrading of braced frame structures for potential local failures.

Abstract

This dissertation is concerned with a type of local failure which was observed during the 1989 Loma Prieta earthquake: a brace-to-beam connection in concentric braced frame structures. A building in the Palo Alto area which suffered structural damage due to the local failure was selected for detailed study. The performance of the damaged building was predicted quite accurately by analytical modeling. Inelastic dynamic analyses with the Loma Prieta earthquake showed that the building with adequate lateral support at the brace-to-beam connections could survive earthquakes of similar intensities without structural damage. Field observed local failure of the brace-to-beam connection was reproduced in the laboratory test specimen. The hysteretic loops of the locally failed specimen showed an excessive "pinching" when compared to those of bracing members with adequate lateral end support. The local failure was confirmed by a finite element analysis. A simplified model was developed which could capture the important characteristics of the test specimen behavior. A sensitivity analysis with the simplified model indicated that effective ways to prevent the connection failure should include increasing the flexural stiffness of the intermediate beam web. Several upgrading schemes were evaluated by finite element analyses. The analyses showed that the specimen with a diagonal web stiffener along the center line of the brace member axis performed best. The effectiveness of diagonal web stiffeners was verified through testing. Behavior of inverted V bracing was analytically studied to investigate the possibility of local failure under larger story drifts. When lateral support at the upper flange of beam was removed, twisting failure occurred due to lateral torsional buckling caused by vertical unbalanced force in the brace members. Forming a box section in the middle of the beam appears to be the best scheme to prevent local failure due to lateral torsional buckling. Inelastic dynamic analyses with "design level" earthquakes showed that performance of the building can be much improved by preventing the local failure and delaying the fracture of brace members. Providing the diagonal web stiffeners and concrete filling in the tubular brace members would greatly improve the seismic performance of the building under future severe earthquakes.