Research On Global Seismic Performance Factors And Comprehensive Response Modification Factors Of RC Frame Structures

Global seismic performance factors (SPFs) include response modificationcoefficient R, system overstrength factor RSand displacement amplification factorCd. The values of SPFs are fundamentally critical in the specification of designseismic action, in which the R factors are used in the current building codes toestimate strength demands for seismic-force-resisting systems designed using linearmethods. Moreover, the SPFs are also the key parameters to determine the eual-ductility inelastic response spectra in performance-based seismic design.In order to take into account the influences of different ductility levels ofstructures, the design seismic force is generally determined by the responsemodification factors to reduce the elastic responses under the design intensity. Thisconventional design approach has been widely utilized in seismic codes in theUnited States, Europe, Japan and other countries. However, the values of the SPFsin the current seismic codes all over the world are based largely on judgments ofengineers, so there are significant differences in the values of the SPFs in the codesor standards in different countries. To reasonbaly quantify the SPFs of differentstructures, recently, the FEMA P695report has recommended a methodology forassessing if the structures designed according to the values of the SPFs specified bythe current seismic codes could satisfy the performance objective of collpaseprevention under major earthquakes.In Chinese seismic codes which had been issued before1978, the seismicdesign forces were established by applying a seismic influence coefficient (thereciprocal of response modification coefficient) to the elastic earthquake forcesunder the design seismic intensity. While since1989, Chinese seismic design codeshave discarded the concept of structural influence coefficient, instead the seismicdesign forces are directly determined by the elastic design response spectrum underminor earthquake. Therefore, many researchers in the mainland of China began tore-think the shortcomings in the current elastic design method and the problem ofthe current seismic design method. The studies on global seismic perfomancefactors of steel structures have been carried out by many researchers, while therelevant research for reinforced concrete structures has been relatively little. Therefore, a thorough research on reasonably quantifying global seismicperformance factors of reinforced concrete frames has been made in thisdissertation. From the viewpoint of the author, the quantification and assessment ofthe global SPFs is not only a key scientific problem in the paradigm transition fromthe elastic seismic design force theory to the ductility-based seismic design forcetheory, but also a basic scientific problem of performance-based seismic design.The solution to this problem will have great both theoretical and realistic meaningsfor improving the science, rationality and economy of seismic design, and furtherfor promoting the applications of performance-based seismic design (PBSD) inChina.In this dissertation,17RC frame buildings with different fortificationintensities and storeys are designed according to the current Chinese Codes. Thesestructures are modeled and analyzed in the platform OpenSees. By comparison ofthe quasi-static test data in Tsinghua university and the shaking table test data of theauthor’s research group for RC frame structures with the analytical datacorrespondingly, the OpenSees models are verified and validated. For these17RCframes, the values of the “capacity” and the “demand” of the global seismicperformance factors are analyzed by nonlinear static procedures (NSP) andnonlinear dynamic procedures (NDP) respectively. A capacity-demand ratio λ isproposed for deterministically assessing the acceptability of the values of the globalseismic performance factors implicitly adopted by the current design codes.Furthermore, the adjusted collapse margin ratio (ACMR) and the demand-capacity-factor method (DCFM) are jointly utilized to comprehensively evaluate the globalseismic performance factors from the viewpoint of randomness, and the reasonablevalues of structural response modification factors are suggested. Considering thefact that the seismic robustness of damaged structures are rarely accounted for inthe current global seismic performance factors, in this thesis, a comprehensiveresponse modification factor is put forward by introducing seismic robustnessindices for both sidesway and vertical progressive collapse failure modes into theconventional structural response modification factor. In this way, the effects ofprogressive collapse of damaged structures under major earthquakes can beconsidered in seismic design of structures.The main contents of this dissertation are as follows:1)17RC frame buildings with different fortification intensities and storeys are designed according to the current Chinese seismic code. The nonlinear finiteelement models of these structures are set up in the platform OpenSees. Throughcomparing with the shaking table test data of RC frame structures performed by theresearch group of the author, and with the quasi-static test data of RC framestructures performed by Tsinghua University, these OpenSees models are verifiedand validated to be adequate to describe the dynamic nonlinear behavior of thestructures in consideration. The analyses of seismic performance are then carriedout for the17RC frames by nonlinear static procedures (NSP) and nonlineardynamic procedures (NDP). On the basis of this, the “capacity” values of the globalseismic performance factors and their variation rules with structural storeys anddesign fortification intensities are analyzed and derived.2) The seismic performance of the17RC frames is evaluated by the staticcapacity spectrum method, the dynamic capacity spectrum method and the timehistory analysis method, respectively. And then, the “demand” values of the globalseismic performance factors of these17RC frames are evaluated. The concepts andthe formula of capacity-demand ratios for the four seismic performance factors areput forward correspondingly to assess the acceptability of the default values ofthese global seismic performance factors implicit in the current design codes.Furthermore, the seismic influence coefficient curves under minor earthquakes inthe current Chinese seismic code is improved by the “demand” values of responsemodification factors obtained in this thesis.3) The adjusted collapse margin ratio (ACMR) proposed by FEMA P695andthe demand-capacity-factor method (DCFM) adopted by FEMA350are jointlyutilized to comprehensively evaluate if the consistent probability levels of seismiccollapse and the performance objectives of collapse prevention of the structuresdesign according to the current design codes could be achieved, and toprobabilistically assess the global seismic performance of these structures from theviewpoint of uncertainty. Based on the above analytical study, the recommendedvalues of the response modification factors for RC frame structures are suggested.4) For the sidesway progressive collapse failure mode of structures, theresistance for sidesway progressive collapse prevention of these RC framestructures is analyzed and evaluated by four kinds of alternative load path (ALP)based approaches, namely, ALP-based pushover method, ALP-based static capacityspectrum method, ALP-based IDA method, and ALP-based DCFM. The reserve load carrying capacities of the damaged structures are evaluated from the viewpointof strength, deformation, and energy dissipation, respectively. The traditionalrobustness indices based on load carrying capacity are extended to the newrobustness indexes based on spectral acceleration and deformation. By introducingthe seismic robustness index RR1for sidesway progressive collapse into theresponse modification factor, the comprehensive response modification factors forRC frame structures considering sidesway progressive collapse modes are derived.5) For the vertical progressive collapse failure mode of structures, theresistance for vertical progressive collapse prevention of these RC frame structuresis analyzed and evaluated by two kinds of alternative load path (ALP) basedapproaches, namely, ALP-based pushdown method, which considers the effect ofloading scheme for simulating column loss; and ALP-based vertical IDA method,which considers the effect of duration of element removing. The reserve loadcarrying capacity of the damaged structures and the corresponding robustnessindices are analyzed and evaluated for three different limit states: the local yieldlimit state, the global yield limit state, and the global failure limit state. Thesidesway robustness structural index RR1and the vertical robustness index RR2aresimultaneously introduced into the response modification factor, which results inthe comprehensive response modification factor. The improved comprehensiveresponse modification factor can be used to obtain the seismic design forces, and torealize the performance based seismic design which considers the effects ofprogressive collapse failure modes.Through the study of this dissertation, it has been founded that the RC framesdesigned using the global seismic perfomance factors implied in the currentChinese seismic design code can meet the expected safety goal against collapseunder major earthquakes, however, the global seismic perfomance factors arerelatively conservative, and the reasonable values of the response modificationfactors are suggested. This study will provide theoretical reference for transformingthe elastic seismic design theory under minor earthquakes into the ductility-basedseismic design theory under moderate earthquakes. Meanwhile, based on thecomprehensive response modification factor put forward in this thesis, the sideswayrobustness index and the vertical robustness index can be simultaneouslyconsidered, and the influence of progressive collapse failure modes can be takeninto account in seismic design of structures.