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During an earthquake, buildings are simultaneously excited by two horizontal and one vertical ground motion components. Modern seismic codes and guidelines such as ASCE/SEI 41-06 (Seismic rehabilitation of existing buildings, American Society of Civil Engineers), EUROCODE 8 (1998-1) (Design provisions for earthquake resistance of structures, European Committee for Standardization, 2003), FEMA 356 (Prestandard and Commentary for Seismic Rehabilitation of the Buildings) and FEMA P-2082 (NEHRP Recommended Seismic Provisions for new buildings and other structures) require the consideration of the effects of two horizontal orthogonal ground motions in seismic design of buildings. Therefore, the main objective of this study is to evaluate the simultaneous effect of two horizontal orthogonal ground motion components to seismic behavior of buildings. A four-story steel frame is modeled, and it is subjected to a set of twenty ground motion pairs recorded distances between x and y kilometer from epicenter. Three methods for combining peak response to individual component of ground motions is used to estimate the displacement responses. The combination rules used in this present study are 30%, SRSS, and 20%. The response of the four-story steel frame is investigated within the context of linear response history analysis and the results are compared to the peak responses obtained from time history analyses under bidirectional and unidirectional ground motion. The structural response includes the following parameters: nodal displacements and the critical angle of excitation. The output results showed that the maximum response under two components was, on average, 23 % more than the maximum response under a single component, and the two horizontal orthogonal seismic excitations increased the structure displacement response compared to unidirectional excitation.
The dynamic behavior of buildings in the horizontal components of motion has been extensively studied, allowing for horizontal demands to be routinely considered in the design process. In contrast, although there has been growing awareness of the effects of vertical ground motions on buildings, little understanding has been gained regarding how structures respond in the vertical direction. The two main pieces of information needed in dynamic analysis to estimate the vertical structural response of buildings subjected to earthquakes are the vertical ground motion component and the building's dynamic parameters in the vertical direction. Previous studies of the vertical component of motion have mainly focused their efforts on understanding the characteristics of the vertical component of ground motions with comparatively less research devoted to the vertical dynamic characteristics of buildings. The studies of ground motion characteristics have led to the addition of a vertical design response spectrum in ASCE 7, which accounts for the site-dependent maximum considered earthquake as well as soil type conditions. Even though the inclusion of the design response spectrum furthers the advancement in structural codes, giving structural engineers a tool to incorporate the effects of the vertical ground motion in structural design, building design standards are lacking in guidelines and requirements for how to accurately account for the effects of the vertical ground motions from the dynamic point of view. For example, except for the long-stablished static approach of using 0.2SDSD as the primary method to include the vertical ground motion loads in the design process, ASCE 7 provides little additional guidance or criteria for considering vertical ground motion effects in design. This dissertation characterizes the dynamic behavior of buildings in the vertical component using parametric relationships and examples of how buildings respond to vertical ground motions. We studied the influence of modal dynamic parameters of buildings in the vertical component obtained from eigenvalue analysis--modal periods, mode shapes and modal participation factors--on the structural response, and we contrasted the vertical modal dynamic parameters characteristics to those of the lateral components. We also validated the vertical dynamic parameters' characteristics obtained from theoretical models, using a structural system identification procedure to calibrate the set of dynamic parameters that best reproduce the recorded response of instrumented buildings subjected to earthquakes. Additionally, we present the patterns of contribution of modes in the vertical component to the structural responses most affected by vertical ground motions, and we propose modeling practices to accurately estimate structural responses in the vertical component. Furthermore, we propose two modal combination rules in response spectrum analysis that accurately predict the maximum response of buildings in the vertical component and that exceed the performance of existing modal combination rules widely used for the response to horizontal components of ground motions. Finally, probabilistic risk studies of reinforced concrete and steel columns designed for gravity loads demonstrate that risks of exceeding strength limit states to vertical ground accelerations are generally relatively low, due to the inherent overstrength introduced through load factors that are applied to dead and live loads in design, along with other behavioral effects.
Databases of recorded motion are limited despite the increasing amount of data collected through strong motion instrumentation programs. Particular lack of data exists for large magnitude events and at close distances as well as on earthquakes in deep sedimentary basins. Additionally, databases of recorded motions are also limited in representation of energy at long periods due to the useable frequencies of recording instruments. This lack of data is currently partially addressed through assumption of ergodicity in development of empirical ground motion prediction equations (GMPEs). Nevertheless, challenges remain for calibration of empirical GMPEs as used in conventional approaches for probabilistic estimation of seismic hazard. At the same time, limited data on strong earthquakes and their effect on structures poses challenges for making reliable risk assessments particularly for tall buildings. For instance, while the collapse safety of tall buildings is likely controlled by large magnitude earthquakes with long du- rations and high long-period content, there are few available recorded ground motions to evaluate these issues. The influence of geologic basins on amplifying ground motion effects raises additional questions. Absent recorded motions from past large magnitude earthquakes, physics-based ground motion simulations provide a viable alternative due to the ability to consider extreme ground motions while being inherently site-specific and explicitly considering instances not well constrained by limited empirical data. This thesis focuses on utilization of physics-based simulated earthquake ground motions for performance assessment of tall buildings with three main goals: (1) developing confidence in the use of simulated ground motions through comparative assessments of recorded and simulated motions; (2) identifying important characteristics of extreme ground motions for col- lapse safety of tall buildings; (3) exploring areas where simulated ground motions provide significant advantages over recorded motions for performance-based engineering. To gain confidence in the use of simulated motions for full performance assessment of tall buildings, a 'similar intensity measure' validation study was performed. Structural responses to ground motions simulated with different methods on the Southern California Earthquake Center (SCEC) Broadband Platform (BBP) are contrasted to recorded motions from PEER NGA database with similar spectral shape and significant durations. Two tall buildings, a 20-story concrete frame and a 42-story concrete core wall building, are analyzed at increasing levels of ground motion intensity, up to structural collapse, to check for statistically significant differences between the responses to simulated and recorded motions. Considered demands include story drift ratios, floor accelerations and collapse response. These comparisons yield similar results in most cases but also reveal instances where certain simulated ground motions can result in biased responses. The source of bias is traced to differences in correlations of spectral values in some of the stochastic ground motion simulations. When the differences in correlations are removed, simulated and recorded motions yield comparable results. Moving beyond validation, the thesis also explored areas where the use of simulated motions provides advantages over approaches based on limited databases of recorded motions for performance-based engineering. One such area is seismic risk in deep sedimentary basins which is studied by examining collapse risk and drift demands of a 20-story archetype tall building utilizing ground motions at four sites in the Los Angeles basin. Seismic demands of the building are calculated form nonlinear structural analyses using large datasets (~500,000 ground motions per site) of unscaled, site-specific simulated seismograms. Seismic hazard and building performance from direct analysis of SCEC CyberShake motions are contrasted with values obtained based on 'conventional' approaches that rely on recorded motions coupled with probabilistic seismic hazard assessments. The analysis shows that, depending on the location of the site within the basin, the two approaches can yield drastically different results. For instance, at a deep basin site the CyberShake-based analysis yields around seven times larger mean annual frequency of collapse ( c) and significantly higher drift demands (e.g. drift demand of 1% is exceeded around three times more frequently) compared to the conventional approach. Both the hazard as well as the spectral shapes of the motions are shown to drive the differences in responses. Deaggregation of collapse risk is performed to identify the relative contributions of earthquake fault ruptures, linking building responses with specific seismograms and contrasting collapse risk with hazard. The effect of earthquake ground motions in deep sedimentary basins on structural collapse risk is further studied through the use of CyberShake earthquake simulations in the Los Angeles basin. Distinctive waveform characteristics of deep basin seismograms are used to classify the ground motions into several archetype groups, and the damaging influence of the basin effects are evaluated by comparing nonlinear structural responses under comparable basin and non-basin ground motions. The deep basin ground motions are observed to have larger durations and spectral intensities than non-basin motions for vibration periods longer than about 1.5 seconds, which can increase the relative structural collapse risk by up to 20 percent between ground motions with otherwise comparable spectral accelerations and significant durations. Two new metrics, termed sustained amplitude response spectra (RSx spectra) and significant duration spectra (Da spectra), are proposed to quantify period-dependent duration effects that are not otherwise captured by conventional ground motion intensity measures. The proposed sustained amplitude response spectra and significant duration spectra show promise for characterizing the damaging effects of long duration features of basin ground motions on buildings and other structures. The large database of CyberShake simulations is utilized to re-examine the relationships between engineering demand parameters and input ground motions on structural response. Focusing on collapse response, machine learning techniques are applied to results of about two million nonlinear time history analyses of an archetype 20-story tall building performed using CyberShake ground motions. The resulting feature selection (based on regularized logistic regression) generally confirms existing understanding of collapse predictors as gained from scaled recorded motions but also reveals the benefit of some novel intensity measures (IMs), in particular the RSx spectral features. In addition, the statistical interrogations of the large collection of hazard-consistent simulations demonstrate the utility of different IMs for collapse predictions in a way that is not possible with recorded motions. A small subset of robust IMs is identified and used in development of an efficient collapse classification algorithm, which is tested on benchmark results from other CyberShake sites. The classification algorithm yields promising results for application to regional risk assessment of building performance.
This study presents the influence of vertical component of ground motion on the response of a tall building; earthquakes with reverse fault and strike-slip with short or far distance to the fault are considered. The main focus has been on the change of structural response due to inclusion of vertical exitation in the analysis.The first part of this study discusses the description of the structural model and modeling techniques in detail which is based on Model 2B of PEER's Case Studies of the Seismic Performance of Tall Buildings (Moehle, et al. 2011). All elements are modeled with specific effective stiffness and nonlinear materials to capture the accurate behavior of the building under the nonlinear response history analysis. Backbone curves of the components is presented. The model is validated by comparing the behavior of the structural model for this study and the response of the Model 2B of PEER's Case Studies of the Seismic Performance of Tall Buildings (Moehle, et al. 2011).The second part of this thesis focuses on the nonlinear response history analysis, the ground motion selection and information on the selected records. The effect of vertical component of the ground motion on the structural behavior is discussed in detail by comparing the structural response due to the lateral components of the ground motion and the structural response due to all three components of the ground motion. The structural response includes the following parameters: maximum inter-story drift ratio, the axial force of columns and shear walls, story shear, peak floor vertical acceleration and peak floor vertical displacement. Equations for estimating the effect of vertical components of ground motions on tall buildings are presented based on the results.
This book presents the main outcomes of the first European research project on the seismic behavior of adjustable steel storage pallet racking systems. In particular, it describes a comprehensive and unique set of full-scale tests designed to assess such behavior. The tests performed include cyclic tests of full-scale rack components, namely beam-to-upright connections and column base connections; static and dynamic tests to assess the friction factor between pallets and rack beams; full-scale pushover and pseudodynamic tests of storage racks in down-aisle and cross-aisle directions; and full-scale dynamic tests on two-bay, three-level rack models. The implications of the findings of this extensive testing regime on the seismic behavior of racking systems are discussed in detail, highlighting e.g. the confirmation that under severe dynamic conditions “sliding” is the main factor influencing rack response. This work was conceived during the development of the SEISRACKS project. Its outcomes will contribute significantly to increasing our knowledge of the structural behavior of racks under earthquake conditions and should inform future rack design.
Rotational components of ground motions, torsion about the vertical axis and rocking about the horizontal axes, have caused significant damage to engineering structures and failures to bridges. Several analytical and experimental studies have been conducted to investigate the effect of these components on structures. Rotational components of ground motions cannot be measured directly and have been measured by rotational sensors only for explosions and by strong motion arrays only for far-field seismic events. Therefore, in the absence of near-fault records of differential ground motions, the characterization, parameterization, modeling and simulation of strain, rocking, and torsional ground motions in the vicinity of the fault, as well as the systematic investigation of their effects on the dynamic response of engineering structures becomes an important issue. In this study, the dynamic ground deformations generated by Mavroeidis and Papageorgiou (2010a) for two well-documented seismic events (i.e. 1979 Imperial Valley and 1999 Izmit earthquakes) based on the discrete wavenumber representation method (Bouchon and Aki, 1977; Bouchon, 1979a) are utilized to obtain torsional and rocking ground motions and their associated distributions on the gridded region in the vicinity of each earthquake. These synthetic ground deformations are used in this study to investigate the effect of the biaxial action of recorded translational ground motions and synthetic torsional ground motions on the response of symmetric and asymmetric structures. In the current seismic deign codes and standards, this torsional ground motion accounts by the shifting of the center of mass to produce the desired results. In order to investigate the effects of torsional motions on the structural responses, a software has been developed to study the linear and nonlinear response of buildings under biaxial and torsional seismic ground excitation. The program is able to perform nonlinear time history analysis based on the force- and displacement-based formulation methods developed by Spacone (1992). The biaxial and uniaxial Smooth Hysteresis Models developed Simeonov et al. (2000) are employed to model the hysteresis behavior of elements in the context of the moment-curvature relationship. The uniaxial and biaxial smooth hysteresis behaviors of the material, similar to the widely used Bouc-Wen model are employed in this research. Various numerical approaches such as the implicit Runge-Kutta, Newton-Raphson and Newmark methods are used to solve the differential equations that govern the dynamic response of the system. Finally, parametric linear and nonlinear analyses are performed for a series of symmetric and asymmetric single-story buildings to investigate the influence of the natural and accidental torsional eccentricity on the response of structures. The structural models are subjected to bidirectional recorded translational motions and synthetic low-frequency angular accelerations from the 1979 Imperial Valley and 1999 Izmit earthquakes. In order to examine the response of structures subjected to synthetic torsional motions containing high-frequency components, the bidirectional translational records from the 1986 Taiwan earthquake at FAT-1 station and the associated synthetic torsional motion, generated by the Surface Distribution Method, are also used to conduct parametric nonlinear analysis. The equivalent accidental eccentricity is developed through the mathematical formula for structures subjected to the combination of the bidirectional translational motions and torsional ground motions. The torsional amplifications developed in structures either by accidental torsion or by synthetic ground differential deformations are not significant for the lower periods. The nonlinear behavior of the structure imposed by strength eccentricity is also explored, while the results are displayed in the biaxial Base Shear and Torque (BST) Surface, inferred for the possible collapse mechanisms regardless of the analysis results.
This study quantifies the impact of different ground motion selection methods on the seismic performance evaluation of steel special moment frames. Two methods are investigated: a "traditional" approach, herein referred to as the Pacific Earthquake Engineering Research (PEER) method, and a newer approach known as the Conditional Mean Spectrum (CMS) method. The PEER method selects ground motions using the Riskbased Maximum Considered Earthquake (MCER) as the target spectrum, while the CMS method uses the conditional mean spectrum that anchor to the MCER at multiple conditioning periods. Three special moment frames of 4-, 8-, and 16-stories are designed in accordance with ASCE/SEI 7-10 to represent archetype steel frame buildings as found in regions of high seismicity. The seismic performance of these frames is assessed with the nonlinear dynamic procedure prescribed in ASCE/SEI 41-13, using ground motions selected and scaled in accordance with both methods. The performance of the buildings is evaluated at the Collapse Prevention (CP) performance level for a far-field site located in Los Angeles, CA. The CMS method results in lower mean and median response in terms of demand-to-capacity ratios in the reduced beam sections and column hinges. Ground motions selected and scaled using CMS result in a smaller dispersion of the output parameters in most of the beam and column elements, if the conditioning period that results in the highest mean demand-to-capacity ratio is the fundamental period, ??1. The results of this study show that the ground motion selection process can cause significant differences in structural response that may lead to different retrofitting decisions. These results provide motivation for engineers to consider the use of the CMS method as an alternative ground motions selection approach when assessing building performance.
Performance-Based Seismic Design (PBSD) is a structural design methodology that has become more common in urban centers around the world, particularly for the design of high-rise buildings. The primary benefit of PBSD is that it substantiates exceptions to prescribed code requirements, such as height limits applied to specific structural systems, and allows project teams to demonstrate higher performance levels for structures during a seismic event.However, the methodology also involves significantly more effort in the analysis and design stages, with verification of building performance required at multiple seismic demand levels using Nonlinear Response History Analysis (NRHA). The design process also requires substantial knowledge of overall building performance and analytical modeling, in order to proportion and detail structural systems to meet specific performance objectives.This CTBUH Technical Guide provides structural engineers, developers, and contractors with a general understanding of the PBSD process by presenting case studies that demonstrate the issues commonly encountered when using the methodology, along with their corresponding solutions. The guide also provides references to the latest industry guidelines, as applied in the western United States, with the goal of disseminating these methods to an international audience for the advancement and expansion of PBSD principles worldwide.