Moment Frame Connections Revisited: A Fundamental Fracture Mechanics Approach

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9703864 Chen The 6.8 magnitude earthquake that shook California's San Fernando Valley on January 17th in 1994, did not cause the collapse of any steel structures but connections, confidently designed and constructed in the past with traditional code simplifications and common site-welding techniques, were discovered no to meet our expectations. This project will study the performance of structures depends on the performance of connections. This will be accomplished by analyzing the fracture strength and toughness of the moment connection. The load effect as determined by a structure's global response, and its performance as controlled by local material properties and geometric configuration will be considered. The interaction and relative importance of horizontal and vertical seismic forces will be evaluated. The effect of composite action (concrete floor-slab on steel girder) and panel zone deformation will be included. Locally, a detailed nonlinear finite element model will provide the stress state information needed to perform a fracture analysis. The fracture strength of the moment connection will consider the effect of reduced toughness in the heat treated region; line and semi-elliptic cracks will be used to simulate weld flaws; stress intensity factors at these locations will quantify the effect these weaknesses have on the ultimate strength of the joint. It has been recognized that the best means to describe the strength performance of structural members, must now be followed by a similar "back to the basics" approach to connection design. Fracture mechanics is proposed to provide that foundation: empirical relationships based upon theoretical analyses will be presented to estimate the ultimate strength resistance of moment connections to seismic loads and brittle fracture ***