| A novel shape memory alloy(SMA)bracing system was put forward to solve the problem of small strength capacity,low redundancy,and expensive construction for existing SMA wire-based device.The seismic collapse performance of steel moment-resisting frame under extremely rare earthquake or mainshock-aftershock sequence events were improved by utilizing the self-centering property and energy dissipation capability of the novel SMA bracing system.Furthermore,the SMA bracing system failure was taken into account in the numerical modeling for seismic performance assessment.A combination of theoretical analysis,experimental research,and numerical simulation were utilized to systematically study the seismic and collapse performance of SMA-braced steel frame structure in this dissertation.The main research contents and corresponding conclusions are as follows:(1)A novel SMA brace with simple configuration and a clear working mechanism was put forward based on SMA cables.The details of configuration,advantages as well as working mechanism were presented.Two novel anchoring systems for SMA cables were proposed to solve the problem of end connection of cables in the SMA bracing system.The detailed constructions and corresponding assembly procedures of the two anchoring systems were illustrated.(2)The SMA cables with two different anchorages were designed and manufactured for the cyclic tensile tests under different strain amplitudes and different loading frequencies.The experimental results reveal that both two types of SMA cable anchoring systems work effectively.Nevertheless,the second anchoring system was recommended for the end connection of SMA cables as the Specimen-II owns less initial damage on cables,higher strength capacity,larger ultimate deformation capability,and lower residual strain than Specimen-I.The hysteretic curves of SMA cables show a typical flag-shaped characteristic with good energy dissipation capacity and superior self-centering behavior.The SMA cables have a specific frequency dependence and excellent fatigue performance.The ultimate strains of the SMA cables with two different anchoring systems reached 8.92% and 12.83%,respectively.(3)Two novel SMA brace specimens were designed and manufactured for experimental characterization on mechanical behavior.The reciprocating dynamic tests were carried out under various strain amplitudes and loading frequencies.The results illustrated the feasibility and stable property of the SMA bracing system.The SMA brace shows a typical flag-shaped hysteretic curve with good energy dissipation and superior self-centering capabilities.The SMA brace has a low-frequency correlation and good anti-fatigue performance.The ultimate deformation capacity of the SMA brace reached 50 mm.(4)A new three-element mechanical model with failure consideration was put forward for the SMA bracing system.The calculation formulas for the new three-element mechanical model were derived.The hysteretic curves of experimental and fitted results were compared,and the dynamic history analysis of SMA braced one-story steel building considering SMA brace failure was conducted to illustrate the feasibility of the new mechanical model.The results reveal that the three-element mechanical model fits the experimental hysteretic curves very well with a maximum only 0.84% difference among all mechanical property parameters.The new three-element mechanical model considering failure can be used to achieve the failure of SMA brace in dynamic time history analysis.(5)A four-story steel frame-building model has been developed,calibrated,and verified in OpenSees.The seismic performance of SMA braced steel frame buildings with different SMA brace initial stiffness and ultimate deformation capacity were examined by utilizing static nonlinear Pushover analysis,dynamic time history analysis and incremental dynamic analysis.The results indicate that it is crucial for the consideration of SMA brace failure in collapse performance assessment.The higher the coefficients of SMA brace initial stiffness and ultimate capacity,the better the collapse performance of the structures.The structural strength increase with increasing SMA brace initial stiffness;nonetheless,it is easy to form a weak story once the SMA brace failed when the brace with high stiffness while insufficient ultimate deformation capacity.Increasing the ultimate deformation capacity of SMA brace shows more effective on the enhancement of seismic and collapse performance of the whole structure,as compared to improve the initial stiffness on the SMA brace.(6)The probabilistic seismic demand model(PSDM)was improved by conducting efficiency and sufficiency analyses relate to 5 different intensity measures based on a traditional logarithmic linear model.The probabilistic seismic hazard analysis was carried out with incorporation of improved PSDM for the steel frame buildings with different design parameters.The results show that improved PSDM owns smaller standard error and better goodness of fits.The SMA brace can effectively mitigate the seismic response of the structure associated with seismic performance improvement and residual deformation reduction.The enhancement of overall seismic and collapse performance of SMA braced steel frames requires the improvement of both the strength of the main structural components and the ultimate deformation capacity of SMA brace.(7)A total of 968 mainshock-aftershocks were constructed on the basis of 22 far-field earthquake ground motions provided by FEMA P695.The incremental dynamic analyses were performed for the conventional steel moment-resisting frame and SMA braced frame with different damage indexes and different damage states under mainshock-aftershock sequences.A new mainshock damage index based on SMA brace failure was put forward for the self-centering structure system.The results verified the feasibility and effectiveness of the SMA brace failure based mainshock damage index for seismic collapse performance assessment under mainshock-aftershock sequences.The SMA brace can effectively reduce the mainshock damage so that seismic performance under aftershocks get improved,nevertheless,the aftershock collapse capacity will remarkably reduce by up to 76% once SMA brace failed under mainshock earthquake.The aftershock collapse capacity decreased with the increasing of mainshock damage,while under the circumstance of same mainshock damage state,the aftershock seismic performance is affected by the different structures,the characteristics of aftershock ground motions(i.e.,frequency spectrum,duration,peak acceleration)and the polarity of aftershock ground motion. |
