An Assessment to Benchmark the Seismic Performance of a Code-Conforming Reinforced-Concrete Moment-Frame Building

摘要

This report describes a state-of-the-art performance-based earthquake engineering methodology udthat is used to assess the seismic performance of a four-story reinforced concrete (RC) office udbuilding that is generally representative of low-rise office buildings constructed in highly seismic udregions of California. This “benchmark” building is considered to be located at a site in the Los udAngeles basin, and it was designed with a ductile RC special moment-resisting frame as its udseismic lateral system that was designed according to modern building codes and standards. The udbuilding’s performance is quantified in terms of structural behavior up to collapse, structural and udnonstructural damage and associated repair costs, and the risk of fatalities and their associated udeconomic costs. To account for different building configurations that may be designed in udpractice to meet requirements of building size and use, eight structural design alternatives are udused in the performance assessments. udOur performance assessments account for important sources of uncertainty in the ground udmotion hazard, the structural response, structural and nonstructural damage, repair costs, and udlife-safety risk. The ground motion hazard characterization employs a site-specific probabilistic udseismic hazard analysis and the evaluation of controlling seismic sources (through uddisaggregation) at seven ground motion levels (encompassing return periods ranging from 7 to ud2475 years). Innovative procedures for ground motion selection and scaling are used to develop udacceleration time history suites corresponding to each of the seven ground motion levels. udStructural modeling utilizes both “fiber” models and “plastic hinge” models. Structural udmodeling uncertainties are investigated through comparison of these two modeling approaches, udand through variations in structural component modeling parameters (stiffness, deformation udcapacity, degradation, etc.). Structural and nonstructural damage (fragility) models are based on uda combination of test data, observations from post-earthquake reconnaissance, and expert udopinion. Structural damage and repair costs are modeled for the RC beams, columns, and slabcolumn connections. Damage and associated repair costs are considered for some nonstructural udbuilding components, including wallboard partitions, interior paint, exterior glazing, ceilings, udsprinkler systems, and elevators. The risk of casualties and the associated economic costs are udevaluated based on the risk of structural collapse, combined with recent models on earthquake udfatalities in collapsed buildings and accepted economic modeling guidelines for the value of udhuman life in loss and cost-benefit studies. udThe principal results of this work pertain to the building collapse risk, damage and repair udcost, and life-safety risk. These are discussed successively as follows. udWhen accounting for uncertainties in structural modeling and record-to-record variability ud(i.e., conditional on a specified ground shaking intensity), the structural collapse probabilities of udthe various designs range from 2% to 7% for earthquake ground motions that have a 2% udprobability of exceedance in 50 years (2475 years return period). When integrated with the udground motion hazard for the southern California site, the collapse probabilities result in mean udannual frequencies of collapse in the range of [0.4 to 1.4]x10ud-4ud for the various benchmark udbuilding designs. In the development of these results, we made the following observations that udare expected to be broadly applicable: ud(1) The ground motions selected for performance simulations must consider spectral udshape (e.g., through use of the epsilon parameter) and should appropriately account for udcorrelations between motions in both horizontal directions; ud(2) Lower-bound component models, which are commonly used in performance-based udassessment procedures such as FEMA 356, can significantly bias collapse analysis results; it is udmore appropriate to use median component behavior, including all aspects of the component udmodel (strength, stiffness, deformation capacity, cyclic deterioration, etc.); ud(3) Structural modeling uncertainties related to component deformation capacity and udpost-peak degrading stiffness can impact the variability of calculated collapse probabilities and udmean annual rates to a similar degree as record-to-record variability of ground motions. udTherefore, including the effects of such structural modeling uncertainties significantly increases udthe mean annual collapse rates. We found this increase to be roughly four to eight times relative udto rates evaluated for the median structural model; ud(4) Nonlinear response analyses revealed at least six distinct collapse mechanisms, the udmost common of which was a story mechanism in the third story (differing from the multi-story udmechanism predicted by nonlinear static pushover analysis); ud(5) Soil-foundation-structure interaction effects did not significantly affect the structural udresponse, which was expected given the relatively flexible superstructure and stiff soils. udThe potential for financial loss is considerable. Overall, the calculated expected annual udlosses (EAL) are in the range of $52,000 to $97,000 for the various code-conforming benchmark udbuilding designs, or roughly 1% of the replacement cost of the building ($8.8M). These losses udare dominated by the expected repair costs of the wallboard partitions (including interior paint) and by the structural members. Loss estimates are sensitive to details of the structural models, udespecially the initial stiffness of the structural elements. Losses are also found to be sensitive to udstructural modeling choices, such as ignoring the tensile strength of the concrete (40% change in udEAL) or the contribution of the gravity frames to overall building stiffness and strength (15% udchange in EAL). udAlthough there are a number of factors identified in the literature as likely to affect the udrisk of human injury during seismic events, the casualty modeling in this study focuses on those udfactors (building collapse, building occupancy, and spatial location of building occupants) that uddirectly inform the building design process. The expected annual number of fatalities is udcalculated for the benchmark building, assuming that an earthquake can occur at any time of any udday with equal probability and using fatality probabilities conditioned on structural collapse and udbased on empirical data. The expected annual number of fatalities for the code-conforming udbuildings ranges between 0.05*10ud-2ud and 0.21*10ud-2ud, and is equal to 2.30*10ud-2ud for a non-code udconforming design. The expected loss of life during a seismic event is perhaps the decision udvariable that owners and policy makers will be most interested in mitigating. The fatality udestimation carried out for the benchmark building provides a methodology for comparing this udimportant value for various building designs, and enables informed decision making during the uddesign process. udThe expected annual loss associated with fatalities caused by building earthquake damage udis estimated by converting the expected annual number of fatalities into economic terms. udAssuming the value of a human life is $3.5M, the fatality rate translates to an EAL due to udfatalities of $3,500 to $5,600 for the code-conforming designs, and $79,800 for the non-code udconforming design. Compared to the EAL due to repair costs of the code-conforming designs, udwhich are on the order of $66,000, the monetary value associated with life loss is small, udsuggesting that the governing factor in this respect will be the maximum permissible life-safety udrisk deemed by the public (or its representative government) to be appropriate for buildings. udAlthough the focus of this report is on one specific building, it can be used as a reference udfor other types of structures. This report is organized in such a way that the individual core udchapters (4, 5, and 6) can be read independently. Chapter 1 provides background on the udperformance-based earthquake engineering (PBEE) approach. Chapter 2 presents the udimplementation of the PBEE methodology of the PEER framework, as applied to the benchmark udbuilding. Chapter 3 sets the stage for the choices of location and basic structural design. The subsequent core chapters focus on the hazard analysis (Chapter 4), the structural analysis ud(Chapter 5), and the damage and loss analyses (Chapter 6). Although the report is self-contained, udreaders interested in additional details can find them in the appendices.