Synthetic Ground Motions Research Articles

Seismic vulnerability assessment of tilt-up concrete structures

This paper focuses on seismic vulnerability assessment for one-story tilt-up concrete structures. To capture the potential failure mechanisms, an analytical modelling approach using nonlinear properties is developed and verified with measured data from a shake table test documented in the literature. Nonlinear dynamic analyses using synthetic ground motions for Memphis, Tennessee, are performed to assess dynamic behaviour of the buildings. Then, probabilistic demand models for multiple limit states that represent potential failure mechanisms are developed with a Bayesian updating approach. These demand models are used in conjunction with appropriate capacity limits to develop fragility curves that provide a probabilistic measure of the seismic vulnerability of typical tilt-up concrete buildings. This study shows that the vulnerability of typical tilt-up structures in Mid-America is significant when seismic hazards are high. In addition, it is found that the aspect ratio of building geometry has a significant impact on the seismic performance and fragility estimates of tilt-up buildings.

Special Issue on Recent Advances in Seismic Design, Analysis, and Protection of Highway Bridges

Special Issue on Recent Advances in Seismic Design, Analysis, and Protection of Highway Bridges

Ground motion record simulation for structural analysis by consideration of spectral acceleration autocorrelation pattern

A novel approach is introduced to generate simulated ground motion records by considering spectral acceleration correlations at multiple periods. Most of the current reliable Ground Motion Record (GMR) simulation procedures use a seismological model including source, path and site characteristics. However, the response spectrum of simulated GMR is somewhat different when compared with the response spectrum based on recorded GMRs. More specifically, the correlation between the spectral values at multiple periods is a characteristic of a record which is usually different between simulated and recorded GMRs. As this correlation has a significant influence on the structural response, it is needed to investigate the consistency of the simulated ground motions with actual records. This issue has been investigated in this study by incorporating an optimization algorithm within the Boore simulation technique. Eight seismological key parameters were optimized in order to achieve approximately the same correlation coefficients and spectral acceleration between two sets of real and simulated records. The results show that the acceleration response spectra of the synthetic ground motions also have good agreement with the real recorded response spectra by implementation of the proposed optimized values.

Investigation on the performance of the six DOF C.G.S., Algeria, shaking table

Shaking tables are devices for testing structures or structural components models with a wide range of synthetic ground motions or real recorded earthquakes. They are essential tools in earthquake engineering research since they simulate the effects of the true inertial forces on the test specimens. The destructive earthquakes that occurred at the north part of Algeria during the period of 1954-2003 resulted in an initiative from the Algerian authorities for the construction of a shaking simulator at the National Earthquake Engineering Research Center, CGS. The acceleration tracking performance and specifically the inability of the earthquake simulator to accurately replicate the input signal can be considered as the main challenge during shaking table test. The objective of this study is to validate the uni-axial sinusoidal performances curves and to assess the accuracy and fidelity in signal reproduction using the advanced adaptive control techniques incorporated into the MTS Digital controller and software of the CGS shaking table. A set of shake table tests using harmonic and earthquake acceleration records as reference/commanded signals were performed for four test configurations: bare table, 60 t rigid mass and two 20 t elastic specimens with natural frequencies of 5 Hz and 10 Hz.

Performance evaluation of natural rubber seismic isolators as a retrofit measure for typical multi-span concrete bridges in eastern Canada

More than 46% of the 2672 multiple-span bridges in the Province of Quebec (Canada) are concrete girder bridges. Due to the advanced age and the lack of seismic detailing, these bridges may be vulnerable to earthquake events. In an effort to prevent an interruption of the transportation network, which could be catastrophic for Quebec, a comprehensive research program was conducted whose aims were to assess the seismic vulnerability of typical bridge classes retrofitted with natural rubber seismic isolator devices through the development of fragility curves. The understanding of the impact of various modeling parameters on structural-component responses is the first step in a forward-vulnerability assessment of bridges in Quebec. This paper presents part of a comparative study using detailed three-dimensional nonlinear models that was conducted to assess the longitudinal and transverse responses of multi-span continuous (MSC) and multi-span simply supported (MSSS) concrete bridges in their as-built and retrofitted configurations using different synthetic ground motions (SGM) for the eastern part of Canada. Deterministic responses in terms of column curvature demand, abutment footing deformations, and abutment wall deformations are provided for the longitudinal and transverse directions of bridge models. The influences of various modeling parameters were assessed using analysis of variance (ANOVA) and design-of-experiments (DOE) principles to perform screening of 12 structural parameters. This study revealed that the most important modeling parameters that affect critical components responses of MSC and MSSS bridges are the isolator effective stiffness, the abutment stiffness, and the gap between abutment and deck, in addition to the variation in gross bridge geometry and ground-motion contents. This screening study also gives an indication that the variation in other modeling parameters such as structural damping and skew angle affect the seismic response of MSC and MSSS bridges and should be considered carefully in a seismic-vulnerability assessment for these portfolios of concrete bridges. In addition, it was found that the use of seismic isolation devices effectively reduces the curvature demand placed on columns but has a negative impact on the deformation demand placed on abutment walls.

Evaluating the Compatibility of Dynamic Rupture-Based Synthetic Ground Motion with Empirical Ground-Motion Prediction Equation

We performed an assessment of a database of synthetic ground motion generated by a suite of dynamic rupture simulations to verify compatibility of the peak ground amplitudes with respect to empirical ground‐motion prediction equations (GMPEs). The dynamic rupture model database, developed by Dalguer and Mai (2011), is composed of 360 earthquake scenarios with moment magnitudes in the range of 5.5–7, for three mechanisms of faulting (reverse, normal, and strike slip) and for both buried faults and surface rupturing faults, having depth‐ and non‐depth‐dependent normal stresses. Initial shear‐stress distribution follows a von Karman stochastic distribution. Overall, we show quantitatively that the upper frequency limit of the suite of simulations is 1.0 Hz. Up to this frequency, the synthetic data are compatible with the empirical model, which means that the residuals (defined as the differences between observed and predicted ground motions) fall in the range of ± σ of the empirical GMPE. Characteristics of the mean values along distance (10–45 km) and period T ≥1.0 s, including standard deviation of the synthetic response spectra and peak ground velocity, are comparable to their counterpart empirical GMPE. At very near source (<1–2 km), synthetics show supersaturation of the mean peak values, which is different from the saturation features predicted by the GMPEs. The effect of source parameters (such as stress drop, peak slip velocity, and rupture speed) and of surface and buried rupture, as well as hanging wall and footwall, are highly sensitive to ground motion, suggesting that these effects contribute to the variability of ground motion near the source and that inclusion of them in the source terms may contribute to reducing uncertainties in GMPEs.

Behavior of a seismically isolated bridge crossing a fault rupture zone

The effect of the fault rupture zone traversing a seismically isolated bridge is investigated utilizing a finite element model of a section of the Bolu Viaduct and a set of synthetic broadband strong ground motions simulated for the Bolu Viaduct site due to the 1999 Duzce earthquake. Both the original and a potential retrofit seismic isolation system designs are considered in the analyses. The results show double isolation system demands when fault crossing is considered, as compared to the case where fault crossing is ignored. The pier drift demands, however, remain comparable in both cases. Furthermore, the location of fault crossing along the bridge length, as well as the fault orientation with respect to the bridge longitudinal direction are shown to influence substantially the response of the seismically isolated bridge. Isolation system permanent displacements are greatly influenced by the restoring force capability of the isolation system when fault crossing effects in the excitations are ignored. In the case of fault crossing, the permanent displacements of the isolation system are dominated by the substantial permanent tectonic displacement along the fault trace which is imposed upon the structure. The results of this study contribute to developing a better understanding of how seismically isolated bridges respond when traversed by fault rupture zones. The lack of analyses and design guidelines for bridges crossing faults in international standards renders this study a useful reference for the profession.

Variability in Wood-Frame Building Damage using Broad-Band Synthetic Ground Motions: A Comparative Numerical Study with Recorded Motions

Earthquake damage to light-frame wood buildings is a major concern for North America because of the volume of this construction type. In order to estimate wood building damage using synthetic ground motions, we need to verify the ability of synthetically generated ground motions to simulate realistic damage for this structure type. Through a calibrated damage potential indicator, four different synthetic ground motion models are compared with the historically recorded ground motions at corresponding sites. We conclude that damage for sites farther from the fault (>20 km) is under-predicted on average and damage at closer sites is sometimes over-predicted.

Stochastic strong ground motion simulations in sparsely-monitored regions: A validation and sensitivity study on the 13 March 1992 Erzincan (Turkey) earthquake

Stochastic simulations have recently become quite popular for estimating synthetic ground motion time histories. For seismically active regions that are not well-monitored or studied extensively, input parameters of the simulations should be carefully selected as the reliability of the simulation results directly depends on the accuracy of the input parameters. In the first part of this study, 13 March 1992 Erzincan (eastern Turkey) earthquake (Mw=6.6), which is recorded at only three strong ground motion stations, is simulated using the stochastic finite-fault method. The source and regional path parameters for this event are adopted from previously validated studies whereas the local site parameters are derived herein. In the second part of the paper, sensitivity of the simulation results with respect to small changes in selected input seismic parameters is investigated. The parameters for which sensitivities are computed include stress drop, crustal shear-wave quality factor and kappa operator. A change of 20% in stress drop value results in 14% change in PGA, whereas a 20% difference in the Q0 value causes 17% change in PGA, and a 20% variation in kappa leads to 15% difference in PGA. Numerical experiments presented in this study prove that the ground motion simulations are prone to trade-off between the source, path and site filters. Hence, input models must be implemented carefully for reliable synthetic ground motions.

Finite-Fault Simulation of Broadband Strong Ground Motion from the 2010 Mw 7.0 Haiti Earthquake

Abstract Because of the socioeconomic environment in the small island nation of Haiti, no seismic recording stations were operating in the source region at the time of the 2010 M w 7.0 earthquake. The lack of strong motion instruments has hindered the estimation of the amplitude, duration, and frequency content of the ground shaking caused by the seismic event. In this study, synthetic broadband strong ground motions are generated in the vicinity of the 2010 Haiti earthquake. The low‐frequency components of the synthetic motions are simulated using a slip model compatible with seismological observations, geologic field data, and space geodetic measurements. The computations are carried out using the discrete wavenumber representation method and the generalized transmission and reflection coefficient technique. The high‐frequency ground‐motion components are generated using the stochastic modeling approach and the specific barrier model. The two independently derived ground‐motion components are subsequently combined using matched filtering at a crossover frequency of 1 Hz to generate broadband ground‐motion time histories and response spectra for the city of Port‐au‐Prince and other sites in the vicinity of the earthquake. Finally, the synthetic strong ground motions are compared against peak ground‐motion estimates inferred from the U.S. Geologic Survey ShakeMap of peak ground accelerations, ground‐motion prediction equations, and observed structural damage. The synthetic time histories and response spectra generated in this study should be seen as plausible, but not necessarily unique, ground motions that capture the primary features of the 2010 Haiti earthquake.

Strong ground motion simulation during the November 1759 Earthquake along Serghaya Fault in the metropolitan of Damascus, Syria

The seismic hazard potential for metropolitan of Damascus, Syria is mainly controlled by earthquakes along Serghaya Fault which is a branch of Dead Sea Fault System. In this study, strong ground motion due to the November 1759 Earthquake along the fault of Serghaya was estimated with a numerical simulation technique. In the simulation, the Kostrov-like slip-velocity function was used as an input to the discrete wave number method to simulate the strong ground motions in a broadband frequency range. In order to model the incoherent rupture propagation which can excite large high-frequency waves, random numbers are added to arrival time of circular rupture front. MMI intensities calculated from the synthetic ground motions are compared with the observed values by Ambraseys and Barazangi (J Geophys Res 94:4007-4013, 1989). The calculated intensities are in good agreement with the observed ones at the most sites that validate appropriateness of the proposed source model. The PGA and PGV in the eastern region of Damascus city are higher than those in the western region due to the effects of local site amplification. The simulated high-frequency (1.0–6.0 Hz) ground motions for the sites in the Damascus city are higher than the design requirements defined by the Syrian building code. Furthermore, the simulated high-frequency ground motions for sites in the focal region are bigger than the design requirements in the case of the near-fault factors and are not considered. That demonstrates the appropriateness of considering the near-fault factors for a site near the focal region as introduced by the new building code.

Source Estimation by Full Wave Form Inversion

We consider the inverse problem of estimating the parameters describing the source in a seismic event, using time-dependent ground motion recordings at a number of receiver stations. The inverse problem is defined in terms of a full waveform misfit functional, where the objective function is the integral over time of the weighted $$L_2$$ L 2 distance between observed and synthetic ground motions, summed over all receiver stations. The misfit functional is minimized under the constraint that the synthetic ground motion is governed by the elastic wave equation in a heterogeneous isotropic material. The seismic source is modeled as a point moment tensor forcing in the elastic wave equation. The source is described by 11 parameters: the six unique components of the symmetric moment tensor, the three components of the source location, the origin time, and a frequency parameter modeling the duration of the seismic event. The synthetic ground motions are obtained as the solution of a fourth order accurate finite difference approximation of the elastic wave equation in a heterogeneous isotropic material. The discretization satisfies a summation-by-parts (SBP) property that ensures stability of the explicit time-stepping scheme. We use the SBP property to derive the discrete adjoint of the finite difference method, which is used to efficiently compute the gradient of the misfit. A new moment tensor source discretization is derived that is twice continuously differentiable with respect to the source location. The differentiability makes the Hessian of the misfit a continuous function of all source parameters. We compare four different gradient-based approaches for solving the constrained minimization problem; two non-linear conjugate gradient methods (Fletcher---Reeves and Polak---Ribiere), and two quasi-Newton methods (BFGS and L-BFGS). Because the Hessian of the misfit has a very large condition number, the parameters must be scaled before the minimization problem can be solved. Comparing several scaling approaches, we find that the diagonal of the Hessian provides the most reliable scaling alternative. Numerical experiments are presented for estimating the source parameters from synthetic ground motions in two different three-dimensional models; one in a simple layer over half-space, and one using a fully heterogeneous material. Good convergence properties are demonstrated in both cases.

Site Response Evaluation of Agartala City Using Geophysical and Geotechnical Data

In this paper, the study addressed how the local geology and soil condition influences on incoming ground motion. Subsurface Geotechnical (SPT) and geophysical (MASW) data in 27 locations at Agartala city have been obtained and used to estimate the surface level response. The vulnerable seismic source (Sylhet fault) has been identified based on deterministic seismic hazard analysis (DSHA). The stochastic point source seismological model has been used to generate synthetic ground motion at 27 locations where MASW tests were conducted. The site response analyses have been performed using SHAKE2000.The results are presented in the form of contour maps in terms of PGA, amplification factor (AF) and spectral accelerations of periods (0.20sec, 1.0sec). The highest amplifications (2 to 3) were observed in the intermediate periods between 0.20 to 1.0sec ranges. However, the mean response spectrum has also been developed for both the rock and ground surface that has mean PGA 0.285g at rock and 0.35g at the surface. Further, the IS code and NEHRP provisions were compared with the mean surface level response spectrum. The IS code provision overestimates the mean response spectrum for all the periods except the periods from 0.26-0.42 sec. Based on the present study, the average shear wave velocity (Vs30=236±52 m/s) of Agartala actually falls a site class-D category (180 m/s &lt;Vs30&lt;360 m/s) as per NEHRP provisions and the design response spectrum has been developed. The developed site specific design response spectrum and other results would be very useful for city planners/designers and the Govt. for earthquake resistant design, city planning, also to identify vulnerable locations, structures suitability identification and also better disaster mitigation planning of the city area.

Seismic reliability assessment of classical columns subjected to near‐fault ground motions

SUMMARYA methodology for the performance‐based seismic risk assessment of classical columns is presented. Despite their apparent instability, classical columns are, in general, earthquake resistant, as proven from the fact that many classical monuments have survived many strong earthquakes over the centuries. Nevertheless, the quantitative assessment of their reliability and the understanding of their dynamic behavior are not easy, because of the fundamental nonlinear character and the sensitivity of their response. In this paper, a seismic risk assessment is performed for a multidrum column using Monte Carlo simulation with synthetic ground motions. The ground motions adopted contain a high‐ and low‐frequency component, combining the stochastic method, and a simple analytical pulse model to simulate the directivity pulse contained in near source ground motions. The deterministic model for the numerical analysis of the system is three‐dimensional and is based on the Discrete Element Method. Fragility curves are produced conditional on magnitude and distance from the fault and also on scalar intensity measures for two engineering demand parameters, one concerning the intensity of the response during the ground shaking and the other the residual deformation of the column. Three performance levels are assigned to each engineering demand parameter. Fragility analysis demonstrated some of the salient features of these spinal systems under near‐fault seismic excitations, as for example, their decreased vulnerability for very strong earthquakes of magnitude 7 or larger. The analysis provides useful results regarding the seismic reliability of classical monuments and decision making during restoration process. Copyright © 2013 John Wiley &amp; Sons, Ltd.

An isolation device for near-fault ground motions

Seismic isolation is an appreciable control strategy that reduces the vibrations of structural and nonstructural systems induced by strong ground motions. However, under near-fault (NF) ground motion, the seismic isolation devices might perform poorly because of large isolator displacements caused by long-period large velocity and displacement pulses associated with such strong motion. The objective of this paper is to assess the effectiveness of a new seismic isolation device, referred to as roll-in-cage (RNC) isolator, in protecting against NF ground motions. The device is intended to achieve a balance in controlling isolator displacement demands and structural accelerations. The RNC isolator provides in a single unit all the necessary functions of rigid support, horizontal flexibility with enhanced stability, and energy dissipation characteristics. Moreover, it is distinguished from other isolation devices by two unique features: (i) it has a built-in energy-absorbing buffer to limit the design displacement under strong excitation, and (ii) it has a built-in linear recentering mechanism that prevents residual displacement after earthquakes. The seismic response of multistory buildings isolated by the RNC isolator is investigated under three recorded NF earthquakes and three synthetic ground motions. The results show that the RNC isolator is a convenient isolation system in protecting against NF earthquakes.

Synthetic earthquake ground motions on an array

Linear and especially non-linear analyses of spatially extended structures, such as pipelines and bridges, often requires specification of time histories of ground motion at an array of closely spaced points. As the number of dense accelerograph arrays worldwide is small, and the number of earthquake observations is limited, synthetic motions with desired characteristics become necessary. This paper presents a method for synthesizing such motions, which is an extension of the SYNACC method, developed first in the early 1970s for synthetic accelerations, velocities and displacements at a point, and later extended to synthetic near surface strains, rotations and curvatures of ground motion at a point. It consists of unfolding in time a site specific Fourier amplitude spectrum of ground acceleration, obtained by an empirical scaling model, by representing the ground motion as a superposition of traveling wavelets of Love and Rayleigh waves and body waves, which propagate with phase and group velocities consistent with the dispersion characteristic of the site geology, approximated by parallel layers. Uniform hazard Fourier spectra or any specified target Fourier spectrum can also be used. Derivations of the point strains, rotations and curvatures are also presented. The method is illustrated for scenario M6.5 and M7.5 earthquakes and three dispersion models.

Seismic Fragility of a Highway Bridge in Quebec

The seismic vulnerability of the Chemin des Dalles Bridge over Highway 55 located in Trois-Rivieres (Quebec, Canada) was evaluated through fragility analysis using field- and laboratory-validated models. This approach offers an effective means to capture the uncertainties in ground-motion realizations, the demands placed on key structural components, and the capacity of the components to resist various levels of seismic excitation. A series of 180 synthetic ground-motion time histories (GMTHs) compatible with eastern Canada was used to capture the uncertainties related to the hazard. Nonlinear time history analyses were performed with these GMTHs and statistically analyzed to define the probabilistic seismic demand model for the abutments, bearings, and columns, which are the critical components. Data from the literature, coupled with sectional and damage mechanics analyses, were used to define the limit states (LSs) for these components. Bridge component and system fragility curves were used to evaluate the likely failure modes of the bridge and potential targets for retrofit, while accounting for key sources of uncertainty in the performance assessment. The results reveal the seismic vulnerability of this specific bridge and even offer insight into the seismic vulnerability of a typical multispan girder-type concrete bridge in Quebec.

Seismic Fragility Relationships of a Cable-Stayed Bridge Equipped with Response Modification Systems

This paper presents the development of fragility curves of a benchmark cable-stayed bridge retrofitted with response modification systems. Fragility relationships are an accepted technique used for seismic risk assessment of structures. The example used herein is a benchmark cable-stayed bridge. Synthetic ground motions are used to excite the structure. Semiactive control is applied to the structure to evaluate and quantify its impact on response mitigation. The semiactive devices used herein are magnetorheological (MR) dampers. The performance of the structure retrofitted with MR dampers is compared with the structure retrofitted with passive, active, and no devices. Fragility relationships based on deck displacement, deck shear, and overturning moment are investigated for comparison. Analyses of peak and time history responses are used to support the accuracy of the fragility relationships. An investigation of the cables exceeding an appropriate tension range and control effort required for the control cases is also provided. The resulting fragility relationships show that MR dampers are effective at mitigating the effects of seismic excitation on the cable-stayed bridge.

Sensitivity of ground-motion scenarios to earthquake source parameters in the Tehran metropolitan area, Iran

The northern Tehran fault (NTF) is a principal active fault of the Alborz mountain belt in the northern Iran. The fault is located north of the highly populated Metropolitan Area of Tehran. Historical records and paleoseismological studies have shown that the NTF poses a high seismic risk for the Tehran region and the surrounding cities (e.g. Karaj). A series of ground-motion simulations are carried out using a hybrid kinematic-stochastic model to calculate broadband (0.1–20Hz) ground-motion time histories for deterministic earthquake scenarios (M7.2) on the NTF. We will describe the source characteristics of the target event to develop a list of scenario earthquakes that are probably similar to a large earthquake on the NTF. The effect of varying different rupture parameters such as rupture velocity and rise time on the resulting broadband strong motions has been investigated to evaluate the range of uncertainty in seismic scenarios. The most significant parameters in terms of ground-shaking level are the rise time and the value of the rupture velocity. For the worst-case scenario, the maximum expected horizontal acceleration, and velocity at rock sites in Tehran range between 128 and 1315cm/s/s and 11–191cm/s, respectively. For the lowest scenario, the corresponding values range between 102 and 776cm/s/s and 12 to 81cm/s. Nonlinear soil effects may change these results but are not accounted for in this study. The largest variability of ground motion is observed in neighborhood of asperity and also in the direction of rupture propagation. The calculated standard deviation of all ground-motion scenarios is less than 30% of the mean. The capability of the simulation method to synthesize expected ground motions and the appropriateness of the key parameters used in the simulations are confirmed by comparing the synthetic peak ground motions (PGA, PGV and response spectra) with empirical ground-motion prediction equations.

Archeoseismic study of damage in Roman and Medieval structures in the center of Cologne, Germany

The causes of damage observed in archeological records or preserved monuments are often difficult to be determined unequivocally, particularly when the possibility of secondary earthquake damage exists. Such secondary damage has been previously proposed for the Roman Praetorium, the governor’s palace in the center of Cologne. Ongoing excavations since 2007 revealed additional damage. The existing ground that has been uncovered and documented extends the affected area to 175 × 180 m. We present a comprehensive virtual model of the excavation area based on 200 3D laserscans together with a systematic analysis of the damage patterns and an improved model of the terrain during Roman/Medieval times including geotechnical parameters of the subsurface. Five locations with different damage patterns, including a Roman sewer, the octagonal central part of the Praetorium, a section with strongly inclined massive walls, a 13 m deep deformed well, a collapsed hypocaust, and damages in the Medieval mikveh are analyzed in detail. We use site-specific synthetic strong ground motion seismograms to test the possibility of earthquake-induced ground failures as a cause for the observed damage. This subsurface model is also used to test the possibility of hydraulically-induced damages by seepage and erosion of fine-grained material from stray sand. Heavy rainstorms can induce a direct stream of surface water through the fine sand layers to the ground water table. Simulated ground motion for assumed worst-case earthquake scenarios do not provoke slope instability at the level necessary to explain the structural damages.