Seismic Response of a Tall Building to Recorded and Simulated Ground Motions

Seismological modeling technologies are advancing to the stage of enabling fundamental simulation of earthquake fault ruptures, which offer new opportunities to simulate extreme ground motions for collapse safety assessment and earthquake scenarios for community resilience studies. With the goal toward establishing the reliability of simulated ground motions for performance-based engineering, this paper examines the response of a 20-story concrete moment frame building analyzed by nonlinear dynamic analysis under corresponding sets of recorded and simulated ground motions. The simulated ground motions were obtained through a larger validation study via the Southern California Earthquake Center (SCEC) Broadband Platform (BBP) that simulates magnitude 5.9 to 7.3 earthquakes. Spectral shape and significant duration are considered when selecting ground motions in the development of comparable sets of simulated and recorded ground motions. Structural response is examined at different intensity levels up to collapse, to investigate whether a statistically significant difference exists between the responses to simulated and recorded ground motions. Results indicate that responses to simulated and recorded ground motions are generally similar at intensity levels prior to observation of collapses. Collapse capacities are also in good agreement for this structure. However, when the structure was made more sensitive to effects of ground motion duration, the differences between observed collapse responses increased. Research is ongoing to illuminate reasons for the difference and whether there is a systematic bias in the results that can be traced back to the ground motion simulation techniques

Classification algorithms for collapse prediction of tall buildings and regional risk estimation utilizing SCEC CyberShake simulations

Quantification of collapse risk of buildings in seismically active regions is one of the key elements for informed decision making for building design and establishment of public policies to promote seismic safety and resilience. This paper focuses on development, testing and application of efficient and reliable collapse classification algorithms using machine learning tools. To this end, a large database of structural responses is developed by performing around two million nonlinear time history analyses of an archetype 20-story tall building. Unscaled seismograms simulated for the Los Angeles region as part of the Southern California Earthquake Center (SCEC) CyberShake project are used as inputs for the analysis. Feature selection is performed using regularized logistic regression to identify intensity measures with strong predictive power for classification of collapse. Results of regularization generally confirm the understanding of important predictors as gained from scaling of recorded motions as well as highlight additional important features. Logistic regression and support vector machine (SVM) binary classifiers are then trained on the data to develop collapse prediction models. The resulting collapse assessment models achieve high values of precision and recall and show good performance when tested using benchmark collapse responses. Finally, trained collapse classifiers are utilized to perform regional estimation of collapse risk. Collapse predictions are made using CyberShake data from 336 sites across Southern California where there are around 500,000 simulated seismograms at each site. Regional estimation of mean annual frequency of collapse is performed to generate maps of collapse risk. Higher values of risk correlate well with geologic features such as presence of sedimentary basins and the surface trace of the San Andreas fault

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

This report describes a state-of-the-art performance-based earthquake engineering methodology that is used to assess the seismic performance of a four-story reinforced concrete (RC) office building that is generally representative of low-rise office buildings constructed in highly seismic regions of California. This “benchmark” building is considered to be located at a site in the Los Angeles basin, and it was designed with a ductile RC special moment-resisting frame as its seismic lateral system that was designed according to modern building codes and standards. The building’s performance is quantified in terms of structural behavior up to collapse, structural and nonstructural damage and associated repair costs, and the risk of fatalities and their associated economic costs. To account for different building configurations that may be designed in practice to meet requirements of building size and use, eight structural design alternatives are used in the performance assessments. Our performance assessments account for important sources of uncertainty in the ground motion hazard, the structural response, structural and nonstructural damage, repair costs, and life-safety risk. The ground motion hazard characterization employs a site-specific probabilistic seismic hazard analysis and the evaluation of controlling seismic sources (through disaggregation) at seven ground motion levels (encompassing return periods ranging from 7 to 2475 years). Innovative procedures for ground motion selection and scaling are used to develop acceleration time history suites corresponding to each of the seven ground motion levels. Structural modeling utilizes both “fiber” models and “plastic hinge” models. Structural modeling uncertainties are investigated through comparison of these two modeling approaches, and through variations in structural component modeling parameters (stiffness, deformation capacity, degradation, etc.). Structural and nonstructural damage (fragility) models are based on a combination of test data, observations from post-earthquake reconnaissance, and expert opinion. 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 building components, including wallboard partitions, interior paint, exterior glazing, ceilings, sprinkler systems, and elevators. The risk of casualties and the associated economic costs are evaluated based on the risk of structural collapse, combined with recent models on earthquake fatalities in collapsed buildings and accepted economic modeling guidelines for the value of human life in loss and cost-benefit studies. The principal results of this work pertain to the building collapse risk, damage and repair cost, and life-safety risk. These are discussed successively as follows. When accounting for uncertainties in structural modeling and record-to-record variability (i.e., conditional on a specified ground shaking intensity), the structural collapse probabilities of the various designs range from 2% to 7% for earthquake ground motions that have a 2% probability of exceedance in 50 years (2475 years return period). When integrated with the ground motion hazard for the southern California site, the collapse probabilities result in mean annual frequencies of collapse in the range of [0.4 to 1.4]x10 -4 for the various benchmark building designs. In the development of these results, we made the following observations that are expected to be broadly applicable: (1) The ground motions selected for performance simulations must consider spectral shape (e.g., through use of the epsilon parameter) and should appropriately account for correlations between motions in both horizontal directions; (2) Lower-bound component models, which are commonly used in performance-based assessment procedures such as FEMA 356, can significantly bias collapse analysis results; it is more appropriate to use median component behavior, including all aspects of the component model (strength, stiffness, deformation capacity, cyclic deterioration, etc.); (3) Structural modeling uncertainties related to component deformation capacity and post-peak degrading stiffness can impact the variability of calculated collapse probabilities and mean annual rates to a similar degree as record-to-record variability of ground motions. Therefore, including the effects of such structural modeling uncertainties significantly increases the mean annual collapse rates. We found this increase to be roughly four to eight times relative to rates evaluated for the median structural model; (4) Nonlinear response analyses revealed at least six distinct collapse mechanisms, the most common of which was a story mechanism in the third story (differing from the multi-story mechanism predicted by nonlinear static pushover analysis); (5) Soil-foundation-structure interaction effects did not significantly affect the structural response, which was expected given the relatively flexible superstructure and stiff soils. The potential for financial loss is considerable. Overall, the calculated expected annual losses (EAL) are in the range of $52,000 to$97,000 for the various code-conforming benchmark building designs, or roughly 1% of the replacement cost of the building ($8.8M). These losses are 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, especially the initial stiffness of the structural elements. Losses are also found to be sensitive to structural modeling choices, such as ignoring the tensile strength of the concrete (40 EAL) or the contribution of the gravity frames to overall building stiffness and strength (15 change in EAL). Although there are a number of factors identified in the literature as likely to affect the risk of human injury during seismic events, the casualty modeling in this study focuses on those factors (building collapse, building occupancy, and spatial location of building occupants) that directly inform the building design process. The expected annual number of fatalities is calculated for the benchmark building, assuming that an earthquake can occur at any time of any day with equal probability and using fatality probabilities conditioned on structural collapse and based on empirical data. The expected annual number of fatalities for the code-conforming buildings ranges between 0.05*10 -2 and 0.21*10 -2 , and is equal to 2.30*10 -2 for a non-code conforming design. The expected loss of life during a seismic event is perhaps the decision variable that owners and policy makers will be most interested in mitigating. The fatality estimation carried out for the benchmark building provides a methodology for comparing this important value for various building designs, and enables informed decision making during the design process. The expected annual loss associated with fatalities caused by building earthquake damage is estimated by converting the expected annual number of fatalities into economic terms. Assuming the value of a human life is$3.5M, the fatality rate translates to an EAL due to fatalities of $3,500 to$5,600 for the code-conforming designs, and $79,800 for the non-code conforming design. Compared to the EAL due to repair costs of the code-conforming designs, which are on the order of$66,000, the monetary value associated with life loss is small, suggesting that the governing factor in this respect will be the maximum permissible life-safety risk deemed by the public (or its representative government) to be appropriate for buildings. Although the focus of this report is on one specific building, it can be used as a reference for other types of structures. This report is organized in such a way that the individual core chapters (4, 5, and 6) can be read independently. Chapter 1 provides background on the performance-based earthquake engineering (PBEE) approach. Chapter 2 presents the implementation of the PBEE methodology of the PEER framework, as applied to the benchmark building. 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 (Chapter 5), and the damage and loss analyses (Chapter 6). Although the report is self-contained, readers interested in additional details can find them in the appendices

Guidelines on nonlinear dynamic analysis for seismic design of steel moment frames

Nonlinear dynamic (response history) analysis is being used increasingly in design practice for the performance-based seismic design of buildings. In contrast to nonlinear static analysis, dynamic analyses require more explicit modeling of cyclic response including strength and stiffness degradation as well as special consideration to selection and scaling of ground motions, definition of viscous damping, and other dynamic effects. To help bridge the gap between state-of-the-art in research and practice, the National Institute of Standards and Technology has funded ATC-114 project to develop improved modeling criteria and guidelines for nonlinear dynamic analysis. The guidelines address both overall considerations, such as recommendations for modeling floor diaphragms and equivalent viscous damping, along with component modeling criteria that are specific to steel moment frame systems. Nonlinear analysis provisions for steel moment frames include new parameters for concentrated hinge models to facilitate modeling of strength and stiffness degradation under random cyclic loading. The new parameters are calibrated to testing and detailed finite element analyses of beam-to-column connections and columns subjected to bending and axial loads. The guidelines also include recommendations for modeling fracturecritical welded connections using fiber-type hinge models. An example analysis of a four-story steel moment frame building is included to illustrate application of the guidelines, following the new ASCE 7-16 requirements for nonlinear response history analysis

Fracture Investigation of Welded Cruciform Connections

As one of the main failure modes of steel structures, fracture in welded connections has widely been discussed based on experimental investigations and numerical simulations. However, the mechanical properties of the weld and Heat Affected Zone (HAZ), such as stress-strain relationships and fracture strains under various stress states, have rarely been considered in these analyses. Therefore, in this paper, the fracture process of welded connections is discussed to investigate the effects of the inhomogeneity of mechanical properties in the weld zone. Tensile tests are conducted on welded cruciform specimens fabricated using 8 mm or 12 mm fillet welds and finite element models are developed by considering or ignoring the material inhomogeneity in the weld zone. The simulation results are compared with the experimental and it is concluded that the assumption of homogenous properties within the weld zone using the properties of the base metal will underestimate the strength of the welded cruciform specimens and using the mechanical properties of the three material areas in the weld zone will increase the accuracy of the simulation results. Using the free parameters calibrated by the fracture strains of the three material areas, the fracture process of the welded cruciform specimens is simulated using the fracture model LMVGM, and the comparison shows that the mechanical properties of the weld and HAZ should be included in the investigation of fracture in welded connections to obtain reliable simulation results

Strengths and Fracture Strains of Weld and HAZ in Welded Connections

This paper investigates the strengths and fracture strains of weld and heat affected zone (HAZ) in welded connections for both the longitudinal and transverse directions and compares them to those of the base metal. A series of miniature coupons, including miniature flat plates, notched round bars and grooved plates, were extracted from the three zones of a butt weld and tested using a custom-built jig. The true stress-strain relationships and fracture strains of the base metal, weld and HAZ materials were obtained for both directions from the miniature coupon tests and corresponding numerical simulations. The fracture strain data were used to calibrate the Lode angle modified void growth model (LMVGM) for predicting the fracture strain of the three material zones at any given stress state. The following major conclusions were drawn: (1) The weld was generally isotropic in terms of both strength and fracture strain. The weld also had the highest values of yield and tensile strengths among the three materials in both directions, but the lowest fracture strain in both directions except for the longitudinal direction with stress triaxiality above 0.21 to 0.30, for which the base metal had the lowest fracture strain. (2) The HAZ had higher yield and tensile strengths but smaller fracture strain in the longitudinal direction than in the transverse direction. The same anisotropic characteristic applied to the base metal. Meanwhile, the HAZ had higher yield and tensile strengths than the base metal as well as similar but slightly larger fracture strains in both directions. (3) The yield and tensile strengths of the weld and HAZ can be approximated using the empirical hardness-strength correlation functions, except that the functions tend to overestimate the strengths of the weld by about 10%. (4) For the weld, HAZ and base metal, the fracture surfaces tilted towards stress states with high stress triaxiality and low Lode angle parameter, indicating that fracture can initiate more easily at these stress states. Note that the above conclusions are limited to the tested AS350 grade steel and the selected welding parameters

Revised ASCE-41 modeling recommendations for moment-resisting frame systems

Nonlinear static and dynamic analysis is utilized by engineers to evaluate the seismic behavior of new and existing structures in the context of performance-based earthquake engineering. Numerous experiments on steel moment-resisting frames and their components have been conducted over the past two decades. The findings from these tests suggest that the current ASCE-41-13 nonlinear component models do not adequately simulate the steel MRF component behavior. As part of the ATC-114 project, new modeling recommendations are proposed for several structural steel components of new and existing MRFs including, steel beams, columns, the beam-to-column web panel zone, column bases and column splices. These recommendations are based on a consistent methodology that takes advantage of unique experimental data as well as insights from detailed finite element analyses. For each structural component of interest a set of equations is developed to predict their first-cycle envelope and monotonic backbone curves that can be directly used in nonlinear frame analysis. The proposed equations also include information related to the associated modeling uncertainty of each of the input model parameters. Through an array of illustrative examples, it is shown that the new recommendations reflect much more accurately the behavior of structural steel components from the onset of damage through the loss of their load carrying capacity

ASCE-41 and FEMA-351 Evaluation of E-Defense Collapse Test

A welded steel moment-frame building is used to assess performancebased engineering guidelines. The full-scale four-story building was shaken to collapse on the E-Defense shake table in Japan. The collapse mode was a side-sway mechanism in the first story, which occurred in spite of a strongcolumn and weak-beam design. Computer analyses were conducted to simulate the building response during the experiment. The building was then evaluated using the Seismic Rehabilitation of Existing Buildings and Seismic Evaluation and Upgrade Criteria for Existing Welded Steel ) for the collapse prevention performance level via linear and nonlinear procedures. The guidelines had mixed results regarding the characterization of collapse, and no single approach was superior. They mostly erred on the safe side by predicting collapse at shaking intensities less than that in the experiment. Recommendations are made for guideline improvements