Steel self-centering moment frames with collector beam floor diaphragms

Following a major earthquake, self-centering moment resisting frames (SC-MRFs) are capable of returning to their original plumb position and sustaining little structural damage. Under earthquake loading, these frames are characterized by a gap that develops between one of the beam flanges and the column face at the connection. This gap causes the frame to expand and therefore imposes special design requirements on the floor diaphragm and on the floor system (e.g. filler beams and slab) that are connected to it. Through nonlinear analyses of several floor diaphragm designs, this paper examines the influence that the floor diaphragm stiffness, strength, and configuration have on the seismic response of SC-MRFs. The floor diaphragm is represented by collector beams that transfer the inertia forces from the building floor to the SC-MRF. This work also presents several equations that are used to predict the effect that the floor diaphragm has on the axial force and moments that develop in the SC-MRF beams. This effect is shown to not be negligible. Finally, practical considerations related to the construction of SC-MRFs that use collector beams as the floor diaphragm are discussed.