Seismic Performance And Design Procedure Of Self-centering-beam Steel Moment-frames

Steel moment frames can provide significant inelastic deformation capacity after plastic hinges occurred at frame members.However,inelastic deformations in structural members caused by earthquake effects often lead to significant residual drift of the structures.if the building with residual post-earthquake drift must be reused,it will no longer behave the designed manner and is difficult to repair damaged frame members.Self-centering-beam(SCB)is an innovative shop-fabricated structural member incorporating the gap-opening mechanism into the body.This new beam was capable of self-centering characteristic by using high strength post-tensioning(PT)strands and dissipating seismic energy by combining pretensioned bolt frictional devices.However,the force distribution of current SCB is not well understood due to its complicate configuration,which limits the widespread application of the structures in a seismic region.In term of these circumstances,this dissertation focused on the further improvement and seismic evalution of self-centering-beam moment-frames(SCB-MFs)through theoretical derivation,experimental analysis and numerical simulation.The detailed reseach results are listed as follows:(1)The force transfer distribution at the SCB-MF is investigated to derive the beam end moment of SCB.A non-uniform factor was firstly presented for measuring the uneven force distribution at two SCB ends.Sensitivity analysis for a one-story one-bay frame was conducted to identify the significant parameters for the mitigation of uneven force distribution in the SCB.Nonlinear dynamic analyses of multi-story SCB-MFs with different combinations of design parameters indicated that the decreasing axial stiffness ratio of the upper beam component to the lower beam component could significantly mitigate uneven force distribution at beam ends.(2)Based on the new configuration of friction-damped SCBs,static cyclic tests are conducted on friction-damped SCB-MF.The integrity of the SCB is maintained by the restrainers eccentrically compressed at the beam ends after the strands are prestressed.Eleven approximately half-scale tests are conducted to isolate and investigate the effects of design parameters such as initial stress in the PT strands,and slip resistance in the friction device on the system strength and stiffness when subjected to quasi-static cyclic loading.After cyclic displacements up to drift levels of 3%,all specimens experienced nearly zero residual drift and no yielding was identified in the any of the structural members.Experiments revealed that the initial stiffness was related to the PT force as increasing PT force further compressed the bearing surfaces and partially mitigated the effect of fabrication tolerances and uneven bearing surfaces.(3)Concepts related to the initial stiffness of self-centering systems are investigated and the SCB-MF configuration is further improved to increase the intial stiffness of the frame.Using this new understanding about the sources of flexibility in self-centering systems,methods for increasing the stiffness of the SCB-MF are proposed and evaluated.The equivalent moment of inertia of the SCB was derived to be twice the moment of inertia of the upper and lower beam.Based on changes made in the beam-to-column connection,the SCB-MF exhibited a similar initial tangent stiffness as the corresponding MF.(4)Parametric analyses are performed on seismic performance of the SCB-MF using the proposed self-centering hinge model.In order to increase the computional efficiency,non-linear rotational springs with hysteretic,flag-shaped moment rotation curves at each beam end was created to simulate and analyze the cyclic responses of SCBs.On basis of the nonlinear dynamic analyses on 3-story,6-story,9-story and 12-story SCB-MFs,the effects of initial stiffness ratio and gap-opening strength ratio on the the peak drifts and residual drifts were analyzed.The results can shed light on further design procedures of SCB-MFs.(5)Performance-based design procedure is proposed for the SCB-MF by the equivalent design between SCBs and traditional beams.A multi-element discrete-springs model was developed for SCBs in OpenSees software and calibrated to simulate the experimental behavior of the SCBs with different self-centering ratio.One important feature was that the modeling strategy could reflect the slight rotation of end plates due to the beam length difference in the upper and lower beams,and therefore simulate the initial stiffness related to the initial posttensioning force.Nonlinear time history analyses were conducted to these frames under a same ensemble of 20 historical earthquakes.The results suggested that the proposed SCB-MFs with fabrication tolerances can achieve comparable maximum seismic performance and eliminating residual drifts compared to MFs with the equivalent beam size and fully restrained connections.(6)The effects of infill walls on the SCB-MF are analyzed using cyclic tests and numerical analyses.The phenomenological hysteretic model is built for masonry infill walls in the SCBMF.Static cyclic tests on the SCB-MF with infill walls were firstly conducted.Uniform cracking model of infill walls was seen in the experiment.The reloading behavior of infill walls was considered based on the Menegotto-Pinto expression to represent the continuum nature of the infill walls subjected to cyclic loading.The cyclic deterioration rates are controlled by the rule developed by Rahnama.The model was calibrated on the basis of different available experimental results in the literatures.The seismic analyses of 3-story,6-story,9-story and 12-story SCB-MFs with and without infills were carried out to analyze the effects of infills on the peak drifts and residual drifts responses under the selected ground motions.