| Cold-formed steel(CFS)structure is according with the developing trend of building industrialization and green building advocated by Chinese government,because it exhibits many advantages,such as low-weight,high-strength,environmental friendly,easy installation,and etc.Thus it has extensive application prospects in China.Due to the basic situation of more-people less-land in China,mid-rise CFS structures are more desired for the civil construction.Therefore,Prof.Ye's research group developed a novel mid-rise CFS residential system,which comprehensively considers the vertical bearing capacity,lateral capacity,fire resistance and other requirements.A systematic study is carried out in this paper for some key aspects of the developed system,including prefabricate construction technology,large-scale shaking table test,simplified numerical method,seismic collapse mechanism,seismic risk,seismic robustness,and etc.The main contents and conclusions are:1.In order to simplify the on-site construction procedures of mid-rise CFS structures,and to enhance the level of industrialization of such system,prefabricated mid-rise CFS composite shear wall system(PM-CFS-CSWS)is proposed.Four 5-story hierarchical shaking table tests were conducted for the proposed system,and the contributions on seismic performance of CFS frame,novel strengthened beam-to-column joint,reinforced and traditional CFS composite shear wall are studied and compared.Following conclusions are made:(1)During the examinations on four shaking table test models,the key connections of CFS members between each modulus were not failed,which demonstrates the reliability of the proposed system.(2)Buckling of concrete-reinforced cold-formed steel tube(CFRST)columns at the first story as well as failure of the beam-to-column joints were typical failure modes of the CFS frames,failure of screw connections(tilted,pulled out and torn)as well as failure of wallboards(cracked,partial fell off and detached)were typical failure modes of the CFS composite shear walls.(3)The novel strengthened beam-to-column joints,reinforced and traditional CFS composite shear walls could improve the seismic performance of the system,the contribution of traditional CFS wall was more obviously subjected to relative large earthquake,and the reinforced CFS wall provided the last defense line to prevent collapsing of the system.(4)The test models basically meet the seismic requirements of mid-rise and high-rise steel structures in Chinese code,and exhibit comparable seismic capacity with mid-rise and high-rise steel structures.(5)The deformation check under rare earthquake is still suggested for mid-rise CFS structures,and the inter-story drift ratio of them is proposed as 1/30~1/25 according to the tests.2.A simplified analytical method for seismic performance simulation of mid-rise CFS structures is proposed taking basis of OpenSess numerical platform.The contents include:(1)Both of the steel tubes and concrete in CFRST columns are simplified as built-up sections with new material properties,and nonlinear numerical analysis of CFRST columns is realized by these simplified sections.(2)A CFS composite shear wall is simplified as two crossed nonlinear springs,and nonlinear numerical analysis of the walls is realizedby inserting hysteretic parameters of them by Pinching04 materials.(3)The mechanical behavior of hold-down connections is modeled by linear spring,and the stiffness of the spring is determined by quasi-static tests of CFS walls.(4)The composite floor is simplified as rigid plane according to the rigid floor estimation,and case study is conducted to valid such estimation.The shaking table test results are used to validated the simplified analytical method,errors in most cases on maximum drift of first story,cumulative energy dissipation of CFS walls in first story,maximum drift of the structure and structural cumulative energy dissipation between tests and numerical results are about 10%,and the maximum value is 20.8%.The time-history and energy dissipation curves obtained from the analytical model accurately tract the deformation and energy dissipation processes of the test models.Such findings demonstrate the rationality and high computational accuracy of the proposed analytical model.3.Due to the limitation of traditional structural vulnerability theory(SVT)in application to framed structures,the improved SVT(ISVT)is developed in this paper.The following conclusions can be obtained:(1)The connection failure process from rigid joints to pinned ones is considered in the ISVT,thus it could identifies the structural collapse modes which cannot be found by traditional SVT.(2)The ISVT accurately predicts the weaken-story collapse mode of the 4-story steel moment frame,and predicts the forming sequence of the plastic hinges at the first story.Thus the rationality of the seismic design concepts of“strong-joint weak-member” and “strong-column weak-girder” are verified theoretically.(3)The ISVT accurately predicts the damage locations and collapse modes of 2-story traditional and reinforced CFS composite shear walls,and finds that the capacity for collapse prevention is closely related to the stiffness of the end studs,which needs attentions in structural design.(4)The possible collapse modes of 5-story CFS composite shear wall building are analyzed by ISVT,and the collapse mode with maximum vulnerability index is same with the failure mode of the test model.Besides,the priority unzipped components are same with the most serious damaged components of the test model.Such findings demonstrate the corrective of ISVT in collapse analyzing of CFS structures.Finally,the “strong-frame weak-wallboard” seismic design conception is proposed.4.The redundancy characteristics of the mid-rise CFS composite shear wall structures are analyzed taking basis of response-based sensitivity analysis method.The contents include:(1)The sensitivity of strain energy to elastic modulus for CFS members and the sensitivity of dissipation energy to axial stiffness for CFS walls are used for redundancy indexes of CFS members and CFS walls,respectively.(2)To overcome the boundedness of modern numerical tools for sensitivity analysis of CFS structures,the “stacked-type”double-element numerical modelling method is proposed,and the double-element is used to input hysteretic and sensitivity parameters.The feasibility of the proposed method is demonstrated by classical structural dynamic theory.(3)The limit of redundancy scale factor(RSF)is proposed to select the key members of the structures.Finally,such method is used to calculate the redundancy of the 5-story CFS composite shear wall shaking table test model,and the analytical results are compared with the test results,it is found that the selected key members are exactly same with the serious damaged components of the test model.Such findings indicate that the weakness of the CFS structures could be identified by redundancy,and the collapse mechanism of the structures could then be revealed.5.A typical 6-story CFS composite shear wall building is modeled by the proposed analytical method,and the potential multiple uncertainties involving in the processes of structural design and servicing life are quantified for seismic risk assessment of the building.The conclusions can be drawn:(1)The uncertainty in defining limit states is proposed as an uncertainty factor,it is quantified by statistic analysis of 108 groups of test data firstly,and then is corrected by shaking table tests.The inter-story drift ratio for IO,SD,CP limits are suggested as 0.5%,1.3% and 3.5,respectively.The logarithmic standard deviation(LSD)of suchlimits are suggested as 0.300,0.215 and 0.336,respectively.(2)The statistic analysis method is suggested to calculate the modeling uncertainty,totally 36 times comparisons between test and numerical results are used as database,and the LSD of the modeling uncertainty subjected to OpenSees-based modeling method is 0.493.(3)The LSD of record-to-record uncertainty of the case building is 0.286~0.344,which is lower than the LSD of wooden structures(0.39)but larger than the LSD of steel frame structures(0.19~0.26),indicating that the CFS composite shear wall system exhibits characteristics both of the wooden and steel frame structures.(4)The LSD of structural uncertainty of the case building is 0.125~0.161,which is lower than the LSD of wooden structures(0.18)but quite similar with the LSD of steel frame structures(0.19~0.26).The researchers and designers could select the suitable LSD of structural uncertainty between0.15 to 0.2 according to the construction standard and environment.(5)The failure probability in servicing life(50 years)of the CFS structures is obviously increased if the multiple uncertainties are considered.Thus,it is suggested that all of the uncertainties should be included in seismic risk assessment of CFS structures,and none of them can not be ignored.6.The vulnerability-fragility combined method for structural robustness assessment is proposed,in which the separateness in structural vulnerability theory and the exceedance probability for each limit state in fragility analysis are used as consequence index and probability index,respectively.The conclusions are made as following:(1)The proposed method could considers the effects on disproportion both of consequence and probability,and it is more flexible and comprehensive in assessing structural robustness,thus it has good application prospect.(2)The conditional robustness against progressive collapse of mid-rise steel frame structures is assessed based on the proposed method,the robustness results are related to the conception results from structural mechanics theory,which demonstrates the corrective of the proposed method.(3)The failure of key member is used as local damage event,the seismic conditional robustness of the shaking table model based on the proposed method is assessed.Positive correlation relationship can be found between the robustness values and the redundancy index of the damage events,which demonstrates the corrective of the proposed method in assessing seismic conditional robustness.(4)A seismic overall robustness assessment method is proposed by considering whole probability in serving life of structures,the estimation for conditional robustness assessment used in most of references is eliminated,and the local damage events are selected as IO,SD and CP limit states instead of the pre-defined events.It is found that the more ductile structure has the higher seismic overall robustness,which finding is same with the theoretical conclusion by Prof.Baker,demonstrating the corrective of the proposed method.Main innovative ideas:(1)The prefabricated mid-rise CFS composite shear wall system that can be modular constructed is preferentially proposed,and large-scale hierarchical shaking table tests of mid-rise CFS composite shear wall structures were firstly completed in China.(2)A more delicate improved structural vulnerability theoretical method is established by aiming at the shortage of the traditional SVT in applying for framed structures,and the collapse mechanism of mid-rise steel frame and CFS structures are revealed on the viewpoint of vulnerability.(3)The conception on uncertainty in defining performance limits and test-based statistic method for quantifying uncertainties are proposed,and seismic risk of mid-rise CFS structures considering multiple uncertainties are preferentially assessed.(4)The vulnerability-fragility combined method for structural robustness assessment of building structures is preferentially developed,and a preliminary answer is given for the challenge issue presented by Prof.Baker on 2008,in which how to apply the risk-based robustness framework to practical engineering structures?... |
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