development of post-northridge steel moment connections

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DEVELOPMENT OF POST-NORTHRIDGE STEEL MOMENT CONNECTIONS Bozidar STOJADINOVIC1, Subhash C GOEL2 And Kyoung-Hyeog LEE3

SUMMARY Fractures of steel moment connections in recent earthquakes sparked an unprecedented research effort to find out why these connections failed and how to design better, earthquake-resistant connections. A summary of the work on fully-restrained steel moment connections done at the University of Michigan is presented in this paper. A comprehensive test series on pre-Northridge steel moment connections, organized by the SAC Joint Venture, demonstrated that these connections are vulnerable. Even in laboratory conditions, they suffered brittle failures, usually stemming from the beam flange complete joint penetration welds. Development of a new connection design in the SAC Steel Program focused on deploying weld fracture mitigation measures, which comprise the use of notch-tough weld metal and improvements in weld detailing practice. The original connection configuration was not changed. A series of parametric tests of SAC post-Northridge connections was done at the University of Michigan. On average, the specimens developed a total plastic rotation of 0.015 radians. All specimens fractured in the same way: beam flanges cracked in the zone between the weld and the end of the beam access hole. The response of these connections did not satisfy the AISC prequalification criteria for special or intermediate moment frames. A comprehensive suite of finite element analyses of steel moment connection models, conducted at the University of Michigan, showed that boundary restraints significantly affect the stress flow in the connection. The restraints produce a self-equilibrated stress field that overloads the flanges and causes excessive local flange bending. This overload condition is a potential source of connection failures. Thus, fracture mitigation measures alone may not be sufficient for agood connection design. Connection configuration must also be changedto mitigate flange overload. Two new connection configurations weredeveloped and tested at the University of Michigan. One is anindirect beam connection which utilizes cover plates and verticalribs. The other is the Free Flange connection. Tests of these connections show that they should perform very well in real earthquakes.

INTRODUCTION It has been almost five years since we learned that traditional fully restrained steel beam-to-column moment

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Asst Professor, Dept of Civil and Env Eng, The University of Michigan, Ann Arbor, MI, USA, e-mail: [email protected] Professor, Dept of Civil and Environmental Eng, The University of Michigan, e-mail: [email protected] Transportation Engineer, California Dept of Transportation, Oakland, CA 94623, e-mail: [email protected]

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