Motion Time Histories Research Articles

DAMAGE EVOLUTION SPECTRA FOR SEISMIC ANALYSIS OF HIGH-RISE BUILDINGS

Seismic response spectra are amongst one of the most important tools for characterizing earthquake ground motions. In design practice, the response spectra are presented without including any load history, hence the nonlinear analysis of structures based solely on conventional earthquake response spectra is theoretically unsound, particularly for long-period or vertically irregular high-rise buildings. In this paper, a concept of seismic damage evolution is introduced and the method of analysis for characterizing the process of seismic damage to structures under earthquakes is presented. Seismic damage evolution spectra for analysis and design of high-rise buildings are then developed as an effective means of describing and simplifying earthquake ground motions. These spectra are shown to be very useful in selecting the ground motion-time history and, particularly, validating the equivalent static-load analysis and design of high-rise buildings under near-fault pulse-like ground motions. Case studies of the seismic inelastic performance of two vertically irregular, tall buildings are presented considering the seismic damage evolution spectra.

Applying the PD controller on the roll reduction and track keeping for the ship advancing in waves

This study aims to develop a PD control system based on the neural network algorithm to reduce the ship rolling motion in the desired track through rudder and fin actions. The mathematical model including sea-keeping and maneuvering characteristics is developed in the present paper and the nonlinear time history of ship motions is solved by the fourth order Runge–Kutta method. In order to achieve the purpose of roll reduction and track keeping, the rudder and fin stabilizer are used as the control tools for the ship advancing in the seaway. In addition, the PD controller based on the self-tuning neural network algorithm is applied to achieve the goals of multi-input and multi-output in the control system. Four different types of control modes on a container ship model are studied and the performances are investigated in different sea states. The results indicate that the PD controller based on the self-tuning neural network algorithm applying to the stabilizer fin control for roll reduction and the rudder control for track keeping in the seaway would be suggested.

Calibration of the Specific Barrier Model to the NGA Dataset

The seismic design of structures sometimes necessitates the use of synthetic strong ground motion time histories (European Committee for Standardization 2003; International Code Council 2000). Toward this end, the stochastic method with a point-source representation of the seismic source is a fast and efficient way to generate synthetic time histories ( e.g. , Boore 2003). While other modeling procedures may be more refined and physically realistic ( e.g. , full finite-fault simulations), they generally require a larger number of input parameters for which calibration relations do not exist (see Douglas and Aochi 2008, and references therein). Even for well-recorded earthquakes, these parameters are difficult to develop calibration relations for because of (1) the observed variability in the results from earthquake source process inversion and its dependence on available data ( e.g. , Custodio et al. 2005) and (2) the overall lack of confidence in even the best estimates of the parameters controlling fault rupture (Monelli and Mai 2008; e.g. , Monelli et al. 2009). The simple seismological models also have advantages over empirical ground motion predictive equations (GMPE) (Abrahamson and Shedlock 1997; Power et al. 2008), although the forms of the source, path, and site functions in the simple seismological models may be somewhat less flexible than empirical GMPEs when trying to fit existing data. Seismological models can provide information on the physical nature of the parameters controlling the strong motion while the GMPEs cannot ( e.g. , Boore 2003; Olafsson et al. 2001; R. Sigbjornsson and Ambraseys 2003). Moreover, despite the increase in strong motion recordings over the last couple of decades, and presumably the corresponding increase in knowledge, there are still considerable differences among the median ground motions estimated using the various GMPEs. These differences are a measure of epistemic uncertainty and have not been …

CHAOTIC CHARACTERISTIC ANALYSIS OF STRONG EARTHQUAKE GROUND MOTIONS

This paper aims to analyze and understand the irregularity and complexity of earthquake ground motions from the perspective of nonlinear dynamics. Chaotic dynamics theory and chaotic time series analysis are suggested to examine the nonlinear dynamical characteristic of strong earthquake ground motions. Based on the power spectral analysis, principal component analysis and modified false nearest neighbors method, it is illustrated qualitatively that the acceleration time series of earthquake ground motions exhibit chaotic property. Next, the chaotic time series analysis is proposed to calculate quantitatively the nonlinear characteristic parameters of acceleration time histories of near-fault ground motions. Numerical results show that the correlation dimension of these ground motions is fractal dimension. Their Kolmogorov entropy is a limited positive value, and their maximal Lyapunov exponent is larger than 0. It is demonstrated that the strong earthquake ground motions present the chaotic property rather than the pure random signals, and the severe irregularity and complexity of ground motions are the reflection of high nonlinearity of earthquake physical process.

추계학적 그린함수법으로 합성된 지반운동에 대한 단층 파라미터의 영향

이 연구에서는 추계학적 그린함수법에 의한 단층 모델을 이용하여 지진파를 합성하고 단층 파라미터의 변화에 의한 지반 운동의 차이를 평가하였다. 모멘트 규모 6.5의 단층을 예제로 선정하였고 아스페리티 면적의 통계값을 이용하여 슬립의 분포를 모델링하였다. 평가를 위해 고려된 단층 파라미터들은 진원의 위치, 전단파 속도 대비 파열 전파속도 비, 상승시간, 절점주파수 그리고 고주파감쇠 필터 등 이었다. 요소지진원에 적용된 파라미터들은 구조권역별 특성이 다른 지역의 값을 사용하였고 다른 파라미터들은 발생 가능한 임의의 값을 사용하였다. 생성된 지반운동 시간이력으로부터 응답스펙트럼을 작성하였으며, 파라미터의 값을 달리하여 비교하였다. 이로부터 각각의 단층파라미터에 의해 영향을 받는 주파수 구간 및 스펙트럼 가속도의 차이를 평가하였다. In this study, the ground motion was synthesized using the finite fault model by the stochastic green function method, and the difference in the ground motions was evaluated by using various values of the source parameters. An earthquake with a moment magnitude of 6.5 was assumed for the example fault model. The distribution of the slip in the fault plane was calculated using the statistical data of the asperity area. The source parameters considered in this study were the location of the hypocenter in the fault plane and the ratio of the rupture to the shear wave velocity, the rise time, the corner frequency of the source spectrum, and a high frequency filter. The values of the parameters related to the stochastic element source model were adjusted for different tectonic regions, and the others were selected for several possible cases. The response spectra were constructed from the synthesized ground motion time history and compared with the different parameter values. The frequency range affected by each parameter and the differences of the spectral accelerations were evaluated.

Enrichment of seismic ground motion databases using Karhunen–Loève expansion

SUMMARYBest estimate seismic analysis are generally based on time‐domain simulations of structural responses. The seismic load is then modeled by a stochastic process representing ground motion. For this purpose, the analyst can use recorded accelerograms or work with synthetically generated ones. The number of ground motion time‐histories available for a given scenario and site condition is limited and generally not sufficient for carrying out more advanced probabilistic structural response analysis. It is then necessary to have at our disposal methods that allow for generating synthetic accelerograms that realistically characterize earthquake ground motions. However, most of the methods proposed in literature for generating synthetic accelerograms do not accurately reproduce the natural variability of ground motion parameters (such as PGA, cumulative absolute velocity, and Arias intensity) observed for recorded time histories. In this paper, we introduce a new method for generating synthetic ground motion, based on Karhunen‐Loève decomposition and a non‐Gaussian stochastic model. The proposed method enables the structural analyst to simulate ground motion time histories featuring the properties mentioned above. To demonstrate its capability, we study the influence of the simulation method on different ground motion parameters and on soil response spectra. We finally compute fragility curves to illustrate the practical application of the proposed method. Copyright © 2012 John Wiley & Sons, Ltd.

A procedure for rapid visual screening for seismic safety of wood-frame dwellings with plan irregularity

This paper highlights the development of a rapid visual screening (RVS) tool to quickly identify, inventory, and rank residential buildings that are potentially seismically hazardous, focusing on single-family, wood-frame dwellings with plan irregularity. The SAPWood software was used to perform a series of nonlinear time-history analyses for 480 representative models, covering different combinations of plan shapes, numbers of floors, base-rectangular areas, shape aspect ratio, area percentage cutoffs, window and door openings, and garage doors. The evolutionary parameter hysteresis model was used to represent the load–displacement relationship of structural panel-sheathed shear walls and a 10 parameter CUREE hysteresis model for gypsum wallboard sheathed walls. Ten pairs of ground motion time histories were used and scaled to four levels of spectral acceleration at 0.167, 0.5, 1.0, and 1.5g. An average seismic performance grade for each model was generated based on the predicted maximum shear wall drifts. Five seismic performance grades: 4, 3, 2, 1, and 0, are associated with the 1% immediate occupancy drift limit, 2% life safety limit, 3% collapse prevention limit, 10% drift, and exceeding 10% drift, respectively. The obtained average seismic performance grades were used to develop a new RVS tool that is applicable for checking the seismic performance of either existing or newly designed single-family, wood-frame dwellings. It examines the adequacy of the structure’s exterior shear walls to resist lateral forces resulting from ground motions, including torsional forces induced from plan irregularity.

The effect of near‐fault directivity on building seismic collapse risk

SUMMARYForward directivity may cause large velocity pulses in ground motion time histories that are damaging to buildings at sites close to faults, potentially increasing seismic collapse risk. This study quantifies the effects of forward directivity on collapse risk through incremental dynamic analysis of building simulation models that are capable of capturing the key aspects of strength and stiffness degradation associated with structural collapse. The paper also describes a method for incorporating the effects of near‐fault directivity in probabilistic assessment of seismic collapse risk. The analysis is based on a suite of RC frame models that represent both past and present building code provisions, subjected to a database of near‐fault, pulse‐like ground motions with varying pulse periods. Results show that the predicted collapse capacity is strongly influenced by variations in pulse period and building ductility; pulse periods that are longer than the first‐mode elastic building period tend to be the most damaging. A detailed assessment of seismic collapse risk shows that the predicted probability of collapse in 50 years for modern concrete buildings at a representative near‐fault site is approximately 6%, which is significantly higher than the 1% probability in the far‐field region targeted by current seismic design maps in the US. Copyright © 2012 John Wiley & Sons, Ltd.

強震動を対象とした海溝型巨大地震の震源モデルをより単純化する試み

A new simplified source model is proposed to explain strong ground motions from a huge subduction earthquake. The proposed model is simpler, and involves less model parameters, than the conventional characterized source model, which itself is a simplified expression of actual earthquake source. In the proposed model, the spacio-temporal distribution of slip within a subevent is not modeled. Instead, the source spectrum associated with the rupture of a subevent is modeled and it is assumed to follow the omega-square model. By multiplying the source spectrum with the path effect and the site amplification factor, the Fourier amplitude at a target site can be obtained. Then, combining it with Fourier phase characteristics of a smaller event, the time history of strong ground motions from the subevent can be calculated. Finally, by summing up contributions from the subevents, strong ground motions from the entire rupture can be obtained. The source model consists of six parameters for each subevent, namely, longitude, latitude, depth, rupture time, seismic moment and corner frequency of the subevent. Finite size of the subevent can be taken into account in the model, because the corner frequency of the subevent is included in the model, which is inversely proportional to the length of the subevent. Thus, the proposed model is referred to as the "pseudo point-source model". To examine the applicability of the model, a pseudo point-source model was developed for the 2011 off the Pacific coast of Tohoku earthquake. The model comprises nine subevents, located off Miyagi Prefecture through Ibaraki Prefecture. The velocity waveforms (0.2-1 Hz) and the Fourier spectra (0.2-10 Hz) at 15 sites calculated with the pseudo point-source model agree well with the observed ones, indicating the applicability of the model. Then the results were compared with the results of a super-asperity model of the same earthquake, which can be considered as an example of characterized source models. Although the pseudo point-source model involves much less model parameters than the super-asperity model, the errors associated with the former model were comparable to those for the latter model for velocity waveforms. Furthermore, the errors associated with the former model were much smaller than those for the latter model for Fourier spectra. These evidences indicate the usefulness of the pseudo point-source model.

The structural safety assessment of a tie-down system on a tension leg platform during hurricane events

The performance of a rig tie-down system on a TLP (Tension Leg Platform) is investigated for 10-year, 100-year, and 1000-year hurricane environments. The inertia loading on the derrick is obtained from the three-hour time histories of the platform motions and accelerations, and the dynamic wind forces as well as the time-dependent heel-induced gravitational forces are also applied. Then, the connection loads between the derrick and its substructure as well as the substructure and deck are obtained to assess the safety of the tie-down system. Both linear and nonlinear inertia loads on the derrick are included. The resultant external forces are subsequently used to calculate the loads on the tie-down clamps at every time step with the assumption of rigid derrick. The exact dynamic equations including nonlinear terms are used with all the linear and second-order wave forces considering that some dynamic contributions, such as rotational inertia, centripetal forces, and the nonlinear excitations, have not been accounted for in the conventional engineering practices. From the numerical simulations, it is seen that the contributions of the second-order sum-frequency (or springing) accelerations can be appreciable in certain hurricane conditions. Finally, the maximum reaction loads on the clamps are obtained and used to check the possibility of slip, shear, and tensile failure of the tie-down system for any given environment.

Soil–structure interaction analysis of cable-stayed bridges for spatially varying ground motion components

In this study, it is intended to determine the effects of soil–structure interaction (SSI) and spatially varying ground motion on the dynamic characteristics of cable-stayed bridges. For this purpose, ground motion time histories are simulated for spatially varying ground motions, depending on its components of incoherence, wave-passage and site-response effects. The substructure method, which partitions the total soil–structure system into the structural system and the soil system, is used to treat the soil–structure interaction problem. To emphasize the relative importance of the spatial variability effects of earthquake ground motion, bridge responses are determined for the fixed base bridge model, which neglects the soil–structure interaction (no SSI) and for the bridge model including the soil–structure interaction (SSI). This parametric study concerning the relative importance of the soil–structure interaction and spatially varying ground motion shows that these effects should be considered in the dynamic analyses of cable-stayed bridges.

Causal Analysis of a Tugboat Capsizing Accident in Rough Weather Condition Based on a Dynamical Simulation

Tugboats are widely used near harbors to assist with various operations such as the berthing and deberthing of very large vessels and the towing of barges. Capsizing accidents involving tugboats occasionally take place when the tugboat makes rapid turns in harsh weather conditions. When there is little evidence suggesting how the accident occurred and when the crew members are missing, it is necessary to predict the time history of the towing vessel’s attitude and trajectory from its departure point to when and where it capsized, depending on various input parameters using a numerical simulation. In this paper, the dynamics of a tugboat and a towed barge in conjunction with the external force and moment were established, and the possible input parameters and operational scenarios which might influence the large roll motion of the tugboat were identified. As a result of analyzing the simulated time history of the excessive roll motion of the tugboat, it was found that roll motion can take place when the tugboat is situated on the crest of a wave and when it is pulled by a towed barge through a towing line. The main cause of the accident would be the parameters that primarily influence such situations. These are the wave parameters, course changing scenario, and the amount of tension.

Modelling and simulation of spatially varying earthquake ground motions at sites with varying conditions

In a flat and uniform site, it is reasonable to assume that the spatially varying earthquake ground motions at various locations have the same power spectral density or response spectrum. If an engineering site with varying soil conditions at different structural supports is considered, this assumption is no longer valid because of different local site amplification effect. This paper presents an approximate method to model and simulate spatially varying ground motions on the surface of an uneven site with non-uniform conditions at different locations in two steps. In the first step, the base rock motions at different locations are assumed to have the same intensity, and are modelled by a filtered Tajimi–Kanai power spectral density function or other stochastic ground motion attenuation models. The base rock ground motion spatial variation is modelled by an empirical coherency loss function. The power spectral density functions of the surface motions on the site with multiple soil layers are derived based on the deterministic 1D wave propagation theory, neglecting the wave scattering on the uneven canyon surface, and assuming that the base rock motions consist of out-of-plane SH wave or in-plane combined P and SVwaves propagating into the site with an assumed incident angle. In the second step, a stochastic method to generate spatially varying time histories compatible with non-uniform spectral densities and a coherency loss function is developed to generate ground motion time histories on an uneven site. Two numerical examples are presented to demonstrate the proposed method. Each generated ground motion time history is compatible with the derived power spectral density at a particular point on the site or response spectrum corresponding to the respective site conditions, and any two of them are compatible with a model coherency loss function.

Building damage scenarios based on exploitation of Housner intensity derived from finite faults ground motion simulations

In this paper earthquake damage scenarios for residential buildings (about 4200 units) in Potenza (Southern Italy) have been estimated adopting a novel probabilistic approach that involves complex source models, site effects, building vulnerability assessment and damage estimation through Damage Probability Matrices. Several causative faults of single seismic events, with magnitude up to 7, are known to be close to the town. A seismic hazard approach based on finite faults ground motion simulation techniques has been used to identify the sources producing the maximum expected ground motion at Potenza and to generate a set of ground motion time histories to be adopted for building damage scenarios. Additionally, site effects, evaluated in a previous work through amplification factors of Housner intensity, have been combined with the bedrock values provided by hazard assessment. Furthermore, a new relationship between Housner and EMS-98 macroseismic intensity has been developed. This relationship has been used to convert the probability mass functions of Housner intensity obtained from synthetic seismograms amplified by the site effects coefficients into probability mass function of EMS-98 intensity. Finally, the Damage Probability Matrices have been applied to estimate the damage levels of the residential buildings located in the urban area of Potenza. The proposed methodology returns the full probabilistic distribution of expected damage, thus avoiding average damage index or uncertainties expressed in term of dispersion indexes.

Generation of response-spectrum-compatible artificial earthquake accelerograms with random joint time–frequency distributions

The sustained dissemination of databases of recorded accelerograms along with the increasing number of strong-motion networks installed worldwide revealed that the current methodologies for simulating artificial earthquakes possess the drawback that the simulated time–histories do not manifest the large variability of the seismological parameters as well as of the joint-time frequency distribution observed for natural accelerograms. As a consequence, the dispersion of the output of structural response analysis can be underestimated. In order to take into account the natural variability of earthquakes a methodology for simulating artificial earthquake accelerograms matching mean and mean ± standard deviation response spectra is proposed in this paper. This dispersion can be determined from attenuation relationships or evaluated from selected accelerograms of a strong-motion database. The procedure requires the definition of an evolutionary response-spectrum-compatible power spectral density function with random parameters. It is shown in the paper that the simulated ground motion time–histories will manifest variability similar to that one observed in natural records.

Analyzing the Sufficiency of Alternative Scalar and Vector Intensity Measures of Ground Shaking Based on Information Theory

The seismic risk assessment of a structure in performance-based design (PBD) may be significantly affected by the representation of ground motion uncertainty. In PBD, the uncertainty in the ground motion is often represented by a probabilistic description of a scalar parameter, or low-dimensional vector of parameters, known as the intensity measure (IM), rather than a full probabilistic description of the ground motion time history in terms of a stochastic model. In this work, a new procedure employing relative sufficiency measure is introduced on the basis of information theory concepts to quantify the suitability of one IM relative to another in the representation of ground motion uncertainty. On the basis of this relative sufficiency measure, several alternative scalar- and vector-valued IMs are compared in terms of the expected difference in information they provide about a predicted structural response parameter, namely, the seismically induced drift in an existing reinforced-concrete frame structure. It is concluded that the most informative of the eight considered IMs for predicting the nonlinear drift response are two scalar IMs and a vector IM that depend only on the spectral ordinates at the periods of the first two (small-amplitude) modes of vibration.

Optimal probabilistic design of seismic dampers for the protection of isolated bridges against near-fault seismic excitations

A probabilistic, simulation-based framework is presented in this paper for risk assessment and optimal design of supplemental dampers for multi-span bridge systems supported on abutments and intermediate piers through isolation bearings. The adopted bridge model explicitly addresses nonlinear characteristics of the isolators and the dampers, the dynamic behavior of the abutments, and the effect of pounding between the neighboring spans against each other as well as against the abutments. Nonlinear dynamic analysis is used to evaluate the bridge performance, and a realistic stochastic ground motion model is presented for describing the time history of future near-fault ground motions and relating their characteristics to the seismic hazard for the structural site. A probabilistic foundation is used to address the various sources of structural and excitation uncertainties and ultimately characterize the seismic risk for the bridge. This risk is given by the expected value of the system response over the adopted probability models. Stochastic simulation is used for evaluating the multi-dimensional integral representing this expected value and for performing the associated optimization when searching for the most favorable damper characteristics. An efficient probabilistic sensitivity analysis is also established for identifying the importance of each of the uncertain model parameters in affecting the overall risk. An illustrative example is presented that considers the design of nonlinear viscous dampers for the protection of a two-span bridge.

Correlation of Significant Duration with Amplitude and Cumulative Intensity Measures and Its Use in Ground Motion Selection

This manuscript examines, and develops parametric equations for, the correlation of significant duration with several amplitude- and cumulative-based ground motion intensity measures. The correlations are determined using ground motions from active shallow crustal earthquakes in the Next Generation Attenuation database, and recently developed ground motion prediction equations. It is found that significant durations tend to be negatively correlated with high-frequency amplitude-based intensity measures, weakly negatively correlated with moderate-frequency amplitude-based intensity measures, and weakly positively correlated with low-frequency amplitude-based intensity measures and cumulative absolute velocity. Particular attenuation is given to the physical interpretation of the observed correlations, which can largely be explained as a result of: (a) the 5–75% significant duration () representing approximately the duration of body wave arrivals; and (b) the limited amount of energy contained in a ground motion time history. Finally, the practical application of the developed correlation equations in determining conditional distributions of ground motion duration for use in ground motion selection is demonstrated.

A note on predetermined earthquake damage scenarios for structural health monitoring

It is shown that the damage scenarios for site-specific building models, responding in highly nonlinear manner to strong earthquake ground-motion pulses, can be used in real-time health-monitoring systems. When it can be shown that such predictions produce robust results and are not sensitive to the details of the complete time history of strong ground motion, the predetermined earthquake damage scenarios (PEDS) method can produce reliable predictions of the location(s) and the degree(s) of structural damage in essentially real time. It is shown that for a full-scale building, damaged during the 1994 Northridge earthquake in California, PEDS based on one-dimensional wave propagation in the layered building model would have produced accurate and realistic predictions of damage. In contrast to the vibrational health-monitoring methods that track changes in the frequency and stiffness of the characteristic functions, PEDS methods provide invaluable information for spatial and temporal identification of damage. Copyright © 2011 John Wiley & Sons, Ltd.

Effectiveness of the 30%-rule at predicting the elastic seismic demand on bridge columns subjected to bi-directional earthquake motions

The adequacy of the 30%-rule to predict the seismic demand on bridge columns subjected to bi-directional earthquake components is examined for two North American sites in areas of moderate seismic hazard: Montreal, in the east, and Vancouver, along the west coast. For both sites, historical and simulated ground motion earthquake ensembles are considered. Time history analyses were performed on generic bridge models to determine the critical response of the columns under pairs of orthogonal seismic ground motion time histories. Response spectrum analyses were also carried out to determine the maximum response in each direction and the critical response was estimated using the 30%-rule. The results show that the combination rule is tributary of both ground motion and bridge characteristics. Results also show that the 30%-rule as currently prescribed in codes leads to an adequate estimation of the bi-directional elastic seismic demand on regular bridge columns.