FAILURE ANALYSIS OF STRUCTURAL STEEL CONNECTIONS IN THE NORTHRIDGE AND LOMA PRIETA, CALIFORNIA EARTHQUAKES

摘要

There was extensive damage to steel beam-to-column moment connections in several hundred buildings caused by the January 17, 1994 Northridge, CA earthquake. Similar damage was found during 1995 inspections of buildings that experienced the 1989 Loma Prieta near San Francisco. Presented herein are stress analyses, results of full scale connection tests, and metallurgical fracture surface analyses that demonstrate that the state of stress and force distributions used in the design rationale of these steel moment connections was not correct. These analyses show that the stress, strain and force distributions were so unfavorable as to cause the "pre-Northridge" connection to fail due to low cycle fatigue. Results of cyclic cantilever beam-to-column connection tests are used herein to evaluate both the fracture modes and the accumulated seismic connection damage of the pre-Northridge, Dogbone and Slotted Web connections. These tests were made using the ATC-24 test protocol with the exception that the tip displacements of the beam-column assemblies were cycled at constant displacements from the initiation of the test until the connections fractured. Results of these tests are then used to demonstrate how accumulated seismic connection damage may be evaluated and to compare the performance of the three connections based upon a composite seismic interstory drift spectrum for a steel moment frame building. These concepts and methods may then be used to create Probable Maximum Loss (PML) surveys for buildings in seismic regions.