Mainshock-aftershock Seismic Sequences Research Articles

Collapse strength ratios for structures under mainshock-aftershock subduction seismic sequences

The effects of aftershocks on civil structures have called the attention of the earthquake engineering community mainly in the last two decades. However, neither seismic design codes nor seismic assessment guidelines for existing structures worldwide consider the possible detrimental effects of strong aftershocks nowadays. From this aim, this paper presents the results of a statistical study focused on computing the relative lateral strength to avoid collapse, named lateral strength ratio Rc, of degrading (i.e., having stiffness and in-cycle strength degradation) single-degree-of-freedom, SDOF, systems under mainshock-aftershock seismic sequences. For this purpose, the degrading SDOF systems have a trilinear force–displacement backbone mainly defined by their displacement ductility capacity, μc, and the negative slope after peak strength, αc. Therefore, median Rc ratios were computed for a site-specific mainshock-aftershock scenario at rock sites considering as-recorded and artificial seismic sequences. From the results of this investigation, it is shown that median Rc ratios that take into account strong aftershocks are smaller than those computed for only the mainshocks, which means that the yield strength coefficient, Cy, for structures subjected to mainshock-strong aftershock pairs should be greater than that for structures under only the mainshocks. The ordinates of median Rc ratios depend on the site class, period of vibration, T, as well as μc and αc. A predictive equation for obtaining estimates of period-dependent Rc ordinates under mainshock and mainshock-aftershock sequences at rock sites is introduced whose fitted coefficients depend on μc and αc. The predictive equation of Rc allows obtaining estimates of the yield strength coefficient required to avoid collapse, Cyc, under mainshock-aftershock sequences for the design of new structures.

Correlation between non-spectral and cumulative-based ground motion intensity measures and demands of structures under mainshock-aftershock seismic sequences considering the effects of incident angles

One of the pivotal steps in seismic assessment of structures is the definition of functional relationships between an Engineering Demand Parameter (EDP) and a ground motion Intensity Measure (IM). This paper investigates the correlation between widely used non-spectral and cumulative-based ground motion intensity measures and corresponding engineering demand parameters for regular and irregular structures as bidirectional single-degree-of-freedom (2D-SDOF) systems. The correlation is investigated under sequential earthquakes in terms of efficiency and sufficiency, considering various seismic incident angles. Structural performance is expressed as maximum inelastic displacement, MD, maximum inelastic absolute acceleration, MA, residual displacement, RD, and hysteretic energy, EH. The results of extensive parametric analyses show that if the MD, MA, EH and RD of regular systems are considered as demand parameters, the optimal IMs in terms of efficiency and sufficiency are vsq, arms, vrs and vsq, respectively.

A new endurance time analysis method for damage evaluation of high arch dams under the oblique incidence of mainshock-aftershock seismic sequences by wavelet decomposition

Pulse and oblique incidence features that may be generated by near-fault seismic sequences are neglected in current earthquake-resistant design codes for arch dams. To solve the shortcomings of endurance time analysis method which cannot consider near-fault pulse-like ground motions, this paper attempted to develop a new endurance time analysis method by wavelet decomposition to assess the differences in damage mechanisms of arch dams under the cross-stream and stream oblique incidences of near-fault seismic sequences. This study decomposed a typical near-fault earthquake into high-frequency and low-frequency ground motion through wavelet decomposition, which is used to perform several nonlinear dynamic analyses under different seismic intensities and seismic sequence incidence angles. On the basis of dynamic calculations, the local and global damage indices of the dam were presented, with a focus on the comparison of the damage distribution and damage volume ratios of arch dams under the action of single and sequence earthquakes and by analysing the damage mechanisms of arch dams under two diagonal incidence conditions at different peak accelerations. Then, the effects of different frequency components of ground motions on dam damage indices were compared, and finally, additional damage to the dam after the mainshock damage was quantified for various aftershocks. The proposed novel prediction model was used to estimate aftershock damage by considering mainshock damage. The numerical results indicate that the dam damage development is highly complex during original mainshock cross-stream and stream oblique incidences. This finding differs from dam damage that spread from the downstream face to the upstream face during the vertical incidence. The dam damage degree is smaller for the original mainshock stream oblique incidence than for the original mainshock cross-stream oblique incidence. The original aftershock induces stronger damage requirements on the damaged dam during the mainshock than during the high-frequency aftershock. Original and high-frequency seismic sequences with cross-stream and stream oblique incidences do not contribute to variations in dam damage patterns during aftershocks. Therefore, the influence of near-fault pulse impacts and the oblique incidence angle of seismic sequences need to be considered comprehensively in the seismic calculations and reinforcement studies of arch dams.

Seismic behavior of RC moment resisting structures with concrete shear wall under mainshock–aftershock seismic sequences

A structure may be subject to several aftershocks after a mainshock. In many seismic design provisions, the effect of the seismic sequences is either not directly considered or has been underestimated. This study examined the seismic behavior of reinforced concrete (RC) moment-resisting structures with concrete shear walls under seismic sequences. Two three-dimensional structures of short and medium height were designed and analyzed under seven real mainshock–aftershock seismic sequences. The models were loaded and designed according to the Iranian seismic code (4th ed.; Standard No. 2800) and ACI-318. The structures were analyzed using the nonlinear explicit finite element method. The maximum displacement, inter-story drift ratio, residual displacement, and ratio of aftershock PGA to mainshock PGA were investigated and assessed. Because of the high lateral stiffness of the shear walls in addition to their completely elastic behavior, the aftershocks did not increase the inter-story drift ratio or relative displacement in the short structure model. The medium height model under the seismic sequences showed a significant growth in the relative displacement (roughly 25% in some cases), inter-story drift ratio, plastic strain, and residual displacement (42.22% growth on average) compared to the structure that was only subjected to the mainshock. Remarkably, in some cases, the aftershock doubled the residual displacement. Unlike the moment-resisting frames under seismic sequences, the models showed no significant growth in the drift ratio with an increase in height. Assessments indicated that the ratio of aftershock PGA to mainshock PGA is a determinative parameter for structural behavior under seismic sequences, such that high values of this ratio (> 0.6) caused significant increases in the residual and relative displacements.

Seismic assessment of school buildings with short captive RC columns under subduction seismic sequences

This paper presents the results of a study focused on evaluating the seismic performance of school buildings located in a region of high seismicity in the Pacific Coast of Mexico. For this purpose, a family of three archetype school buildings having typical plan view with one-, two-, and three-story height were designed with out-of-date 1990s seismic requirements. Detailed analytical models representative of the archetype school buildings included modelling the nonlinear behaviour of captive reinforced concrete (RC) columns and the adjacent partial-height infill masonry walls commonly found in this type of structures. The analytical models were subjected to a set of 22 as-recorded mainshock-aftershock seismic sequences gathered at stations placed on the subduction zone of the Mexican Pacific Coast. Drift-based fragility curves for captive RC columns failing in shear, which represent the probability of reaching or exceeding four damage states, based on experimental tests are introduced in this study for evaluating their seismic performance. This investigation revealed that the two- and three-story school buildings exhibit larger inter drift demands at the ground story than those exhibited by the one-storey school building, which led to a weak first-storey mechanism under some mainshock earthquake ground motions. Furthermore, it was noted that aftershocks increased the state of damage at the ground storey due to the increase of damage in the captive RC columns. Findings of this investigation highlight the priority of implementing retrofit strategies, mainly for three-storey school buildings.

Seismic Damage of Submerged Intake Tower under the Sequence of Mainshocks and Aftershocks

ABSTRACT Reinforced concrete intake towers are considered as main hydraulic structures constructed in dams’ reservoirs, connected to the dam or as a separate structure. If an intake tower is damaged, the powerhouse or downstream of the dam may experience serious problems. In this research, the influence of mainshock-aftershock seismic sequences is studied on real intake tower. The intake tower of Encino dam is modeled by using the finite element method. The dynamic time history analyses are conducted with the real as-recorded earthquakes and the nonlinear behavior of concrete and steel bars is considered. The tower is modeled with reservoir and foundation, and their interactions are considered. The earthquake records are applied to the tower in three cases: (i) one horizontal component, (ii) two horizontal components, and (iii) two horizontal components with vertical component. The main dynamic responses of the tower including relative deflections and nonlinear material responses are presented. The obtained results are discussed, and the influence of the main shock-aftershock sequences are thoroughly examined.

Mainshock–aftershock low-cycle fatigue damage evaluation of performance-based optimally designed steel moment frames

Considering the widespread damages of connections in steel moment frame (SMF) structures due to destructive earthquakes, seismic assessment of these structures for cumulative low-cycle fatigue (LCF) is of utmost importance and must be carefully taken into account in the design process of SMFs. To calculate the LCF damage index for SMFs, the Palmgren-Miner’s rule is applied on the history of relative drift of the stories based on non-linear response history analysis and existing experimental fatigue curves. On the other hand, repeated earthquakes strongly affect the inelastic response of structures and can increase building vulnerability. Therefore, the primary objective of this study is to numerically assess the mainshock-aftershock LCF damage index for performance-based optimally designed SMFs. Two illustrative design examples of 6- and 12-story SMFs are presented and designed for optimal initial and total costs in the context of performance-based design. The LCF damage index is evaluated for these structures by conducting nonlinear response-history analysis for a suite of real strong multiple earthquakes. The results showed that the optimally designed SMFs are highly vulnerable against LCF damage due to mainshock-aftershock seismic sequences. Subsequently, a simple procedure is proposed to increase the safety of optimally designed SMFs against the LCF damage. To this end, performance-based optimization process is performed for 6- and 12-story SMFs by considering higher seismic confidence levels and the results demonstrate the effectiveness of this strategy in controlling the LCF damage of SMFs caused by seismic sequences.

Seismic Performance Assessment of SMRF Structures Subjected to Mainshock-Aftershock Seismic Sequences by Endurance Time Method

ABSTRACT Seismic performance of postmainshock buildings considering seismic mainshock-aftershock sequences are studied. The case studies correspond to three SAC steel MRF buildings of three, nine and twenty stories. The 2-D models are able to capture important properties of deterioration of strength and stiffness. Endurance time method is used to predict the response of structures in different hazard levels. Results are compared with those from of nonlinear incremental dynamic analysis. Maximum dynamic seismic responses of structures are captured during synthetic mainshock-aftershock seismic sequences which are produced by endurance time method. Interactional-conceptual curves for different seismic performance levels and residual collapse capacity of post-mainshock damaged buildings are presented. The collapse fragility curves of case studies are extracted. The results show that the residual seismic response of building caused by the mainshock and the direction of applied aftershocks are highly significant factors in evaluation of postmainshock buildings. It is found that in a seismic sequence which has two earthquakes with different intensities, occurrence of the stronger shake before the less intensive one, imposes higher seismic risk to the building.

Numerical investigation of AP1000 NIB under mainshock-aftershock earthquakes

This manuscript studies the seismic behavior of AP1000 nuclear power plant buildings (NPPB) under different mainshock and aftershock earthquakes. Three mainshock-aftershock seismic sequences, which satisfy RG 1.60 spectrum, are adopted in time history analysis. A 3D finite element model (FEM) is established to analyze the seismic response of AP1000 NPPB. Parametric analysis of maximum acceleration, peak displacement, acceleration response spectrum, and stress distribution are carried out to study the effects of mainshock-aftershock earthquakes. It is indicated that strong aftershock may aggravate the potential cumulative damage and take a significant influence on the seismic performance of the nuclear island building (NIB). Mainshock and aftershock also influence the floor acceleration response spectrum in horizontal directions significantly. Therefore, to assess the safety of AP1000 NIB, aftershock should be considered in the seismic design of strong mainshock with strong aftershock, especially for the isolated NPPB.

COMPORTAMIENTO DE EDIFICIOS DE ACERO CON CONTRAVENTEOS EXCÉNTRICOS ANTE SECUENCIAS SÍSMICAS

En este artículo se presentan los resultados de un estudio analítico enfocado a evaluar el comportamiento sísmico de edificios de acero estructurados a base de marcos y contraventeos excéntricos (MCE). Para tal fin, se modelaron dos MCE de 4 y 8 niveles mediante la plataforma computacional OpenSees. En particular, se modeló el comportamiento histerético de los eslabones en los contraventeos excéntricos considerando un criterio de falla, así como la zona del panel en las uniones viga-columna. Los MCE en estudio fueron sometidos a un conjunto de secuencias sísmicas evento principal-réplicas artificiales, las cuales son representativas de secuencias registradas en terreno blando de la Ciudad de México. Los resultados de está investigación indican que los eslabones pueden fallar ante la acción de réplicas intensas, lo cual conduce a un incremento de sus demandas de distorsión máxima de entrepiso respecto a las originadas por el evento principal. Ante esta situación, se observó que las vigas y columnas cercanas al eslabón que falla exhiben un comportamiento inelástico, lo cual es contrario a su filosofía de diseño. Finalmente, se aprecio que los MCE exhiben una concentración de distorsión máxima de entrepiso en los pisos inferiores dado que los eslabones no tienen un comportamiento histerético uniforme en la altura, lo cual desaprovecha su capacidad de disipación de energía.

Probabilistic seismic demand assessment of self-centering concrete frames under mainshock-aftershock excitations

This paper investigates the effect of aftershocks on the seismic performance of self-centering (SC) prestressed concrete frames using the probabilistic seismic demand analysis methodology. For this purpose, a 4-story SC concrete frame and a conventional reinforced concrete (RC) frame are designed and numerically analyzed through nonlinear dynamic analyses based on a set of as-recorded mainshock-aftershock seismic sequences. The peak and residual story drifts are selected as the demand parameters. The probabilistic seismic demand models of the SC and RC frames are compared, and the SC frame is found to have less dispersion of peak and residual story drifts. The results of drift demand hazard analyses reveal that the SC frame experiences lower peak story drift hazards and significantly reduced residual story drift hazards than the RC frame when subjected to the mainshocks only or the mainshock-aftershock sequences, which demonstrates the advantages of the SC frame over the RC frame. For both the SC and RC frames, the influence of as-recorded aftershocks on the drift demand hazards is small. It is shown that artificial aftershocks can produce notably increased drift demand hazards of the RC frame, while the incremental effect of artificial aftershocks on the drift demand hazards of the SC frame is much smaller. It is also found that aftershock polarity does not influence the drift demand hazards of both the SC and RC frames.

Seismic performance investigation of high CFRDs subjected to mainshock-aftershock sequences

In this paper, 24 as-recorded mainshock-aftershock seismic sequences are selected to analyse the seismic performance of a 200-m-high concrete face rockfill dam (CFRD) subjected to mainshock-aftershock seismic sequences by a series of elastic-plastic analyses, based on three damage measures (DMs), displacement, shear strain and damage index (DI). The dynamic responses of CFRD subjected to single mainshocks and single aftershocks are also obtained for comparison. Evidently, CFRD will experience additional damage or destruction while experiencing an aftershock, which cannot be ignored in the corresponding seismic design codes.

The capacity loss of a RCC building under mainshock-aftershock seismic sequences

Reinforced concrete containment (RCC) building has long been considered as the last barrier for keeping the radiation from leaking into the environment. It is important to quantify the performance of these structures and facilities considering extreme conditions. However, the preceding research on evaluating nuclear power plant (NPP) structures, particularly considering mainshock-aftershock seismic sequences, is deficient. Therefore, this manuscript serves to investigate the seismic fragility of a typical RCC building subjected to mainshock-aftershock seismic sequences. The implementation of the fragility assessment has been performed based on the incremental dynamic analysis (IDA) method. A lumped mass RCC model considering the tri-linear skeleton curve and the maximum point-oriented hysteretic rule is employed for IDA analyses. The results indicate that the seismic capacity of the RCC building would be overestimated without taking into account the mainshock-aftershock effects. It is also found that the seismic capacity of the RCC building decreases with the increase of the relative intensity of aftershock ground motions to mainshock ground motions. In addition, the effects of artificial mainshockaftershock ground motions generated from the repeated and randomized approaches and the polarity of the aftershock with respect to the mainshock on the evaluation of the RCC are also researched, respectively.

Seismic Fragility Analysis of Multispan Reinforced Concrete Bridges Using Mainshock-Aftershock Sequences

Although the knowledge and technology of performance-based earthquake engineering have rapidly advanced in the past several decades, current seismic design codes simply ignore the effect of aftershocks on the performance of structures. Thus, the present paper investigated the effect of aftershocks on seismic responses of multispan reinforced concrete (RC) bridges using the fragility-based numerical approach. For that purpose, a continuous girder RC bridge class containing 8 bridges was selected based on the statistical analysis of the existing RC bridges in China. 75 recorded mainshock-aftershock seismic sequences from 10 well-known earthquakes were selected in this study. In order to account for the uncertainty of modeling parameters, uniform design method was applied as the sampling method for generating the samples for fragility analysis. Fragility curves were then developed using nonlinear time-history analysis in terms of the peak curvature of pier column and displacement of bearings. Finally, the system fragility curves were derived by implementing Monte Carlo simulation on multinormal distribution of two components. From the results of this investigation, it was found that, for the RC continuous bridges, the influence of aftershocks can be harmful to both bridge components and system, which increases both the component fragility of the displacement of bearings and seismic curvature of pier sections and system fragility.

Damage demand assessment of mainshock-damaged concrete gravity dams subjected to aftershocks

In China, the current seismic codes specify a single earthquake event as the design seismic load for concrete gravity dams. However, a large mainshock usually triggers numerous aftershocks in a short period. This paper assesses the effects of aftershocks on concrete gravity dam–reservoir–foundation systems and provides a quantitative description of the damage demands prior to and following the aftershocks. For this purpose, a set of 20 as-recorded mainshock–aftershock seismic sequences is considered in this study. The correlation between the ground motion characteristics of the as-recorded mainshocks and those of the aftershocks is examined. In order to identify the influence of the ground motion characteristics of aftershocks on the damage demands of the mainshock-damaged dams, the nonlinear behavior of the concrete gravity dams that are subjected to single seismic events and typical as-recorded seismic sequences is compared in terms of the structural damage, displacement response, and damage dissipated energy. A series of nonlinear dynamic analyses is performed to quantify the influence of aftershocks, which are selected by using different methods, on the damage demands of concrete gravity dam–reservoir–foundation systems in terms of the local and global damage indices. The results show that the aftershocks lead to an increase in the damage demands of the dam at the end of the seismic sequence when the concrete gravity dam is already damaged during the first individual seismic event and has not been repaired. In addition, the results also reveal that the repeated seismic sequences tend to underestimate the level of damage demands.

Seismic Performance of a Concrete Highway Tunnel under Earthquake Sequence Using a Concrete Damage Plasticity Model

The aim of this paper was to investigate the dynamic response of a concrete tunnel subjected to mainshock–aftershock seismic sequence. Three components of the registered seismic saquence were applied as seismic excitation acting in three directions. A three-dimensional FE model of a tunnel section (600 m long) was prepared with the ABAQUS software. The soil layers interacting with the tunnel lining were also taken into consideration. To represent the inelastic behavior of the concrete material under the earthquakes, a concrete damage plasticity model (CDP) was assumed a constitutive model for the concrete. The analysis proved that strongly nonlinear behaviour of the concrete lining of the tunnel was observed under the sequence of seismic events. The plastic strains as well the tensile damage (cracking) were noticed in some zones of the concrete lining after the first and the second event. The crack patterns were in good agreement with damages observed on concrete tunnels during real earthquakes. The results indicate that aftershocks can enlarge zones affected by irreversible strains or cause damage evolution. The analysis also revealed that aftershocks, even being much weaker than main events, may result in additional loss of concrete material strength while performing in mainshock-aftershock seismic sequences and striking a structure which is already degraded by a mainshock.

Dynamic behaviour of a concrete building under a mainshock–aftershock seismic sequence with a concrete damage plasticity material model

ABSTRACTThe aim of this paper was to investigate the dynamic response of a concrete structure subjected to a mainshock–aftershock seismic sequence. In the dynamic analysis, three components of the registered mainshock and aftershock were taken into account. The peak ground accelerations of about 0.5 g were assumed for both shocks. A one-storey shed was modelled with the ABAQUS software to represent a large concrete structure under the repeated earthquakes. For proper characterization of concrete structure behaviour under the sequence of shocks, a concrete damage plasticity model was assumed as a constitutive model of concrete. The obtained results indicate that aftershocks can have considerable effect on dynamic behaviour of concrete structures in terms of enlarging zones affected by irreversible strains or additional damage evolution. The analysis revealed that aftershocks, which are usually not as strong as mainshocks, may result even in total loss of concrete material strength while performing in mainshock–aftershock seismic sequences.

Damage accumulation of a base-isolated RCC building under mainshock-aftershock seismic sequences

Strong aftershocks have the potential to induce greater structural damage for Nuclear Power Plant (NPP) structures. In order to ensure the safety and integrity of Reinforced Concrete Containment (RCC) buildings, the base isolation technique is proposed to reduce the effects of earthquake shaking on the building and avoid damage of the structure under earthquakes. However, none of the studies have focused on evaluating the seismic performance of RCC buildings with isolation for mainshock-aftershock seismic sequences. The aim of this paper is to thoroughly investigate the structural damage of RCC buildings with Lead Rubber Bearing (LRB) isolators under seismic sequences. A suite of twenty-eight artificial sequential ground motions is developed to investigate the influence of considering aftershocks on the performance of RCC buildings with isolators for both design-basis and beyond-designbasis earthquake shaking. The results reveal that strong aftershocks would induce greater damage for the RCC without isolators under beyond-design-basis earthquake. While, base isolation can protect the RCC building under seismic sequences being damaged even for beyond-design-basis earthquake shaking. The IDA results of base-isolated RCC demonstrate that isolators also increase the structural reserve capacity significantly under both mainshocks and seismic sequences.

Seismic analyses of a RCC building under mainshock–aftershock seismic sequences

A large mainshock may trigger numerous aftershocks within a short period, and nuclear power plant (NPP) structures have the probability to be exposed to mainshock–aftershock seismic sequences. However, the researchers focused on seismic analyses of reinforced concrete containment (RCC) buildings under only mainshocks. The aim of this paper is to thoroughly investigate the dynamic responses of a RCC building under mainshock–aftershock seismic sequences. For that purpose, 10 as-recorded mainshock–aftershock seismic sequences with two horizontal components are considered in this study, and a typical three-dimensional RCC model subjected to the selected as-recorded seismic sequences is established. Peak ground accelerations (PGAs) of mainshocks equal to 0.3g (safe shutdown earthquake load-SSE load) are considered in this paper. The results indicate that aftershocks have a significant effect on the responses of the RCC in terms of maximum top accelerations, maximum top displacements and accumulated damage. Furthermore, in order to preserve the RCC from large damage under repeated earthquakes, local damage and global damage indices are suggested as limitations under only mainshocks.

Aftershock seismic assessment taking into account postmainshock residual drifts

SummaryThis paper introduces and evaluates a methodology for the aftershock seismic assessment of buildings taking explicitly into account residual drift demands after the mainshock (i.e., postmainshock residual interstory drifts,RIDRo). The methodology is applied to a testbed four‐story steel moment‐resisting building designed with modern seismic design provisions when subjected to a set of near‐fault mainshock–aftershock seismic sequences that induce five levels ofRIDRo. Once the postmainshock residual drift is induced to the building model, a postmainshock incremental dynamic analysis is performed under each aftershock to obtain its collapse capacity and its capacity associated to demolition (i.e., the capacity to reach or exceed a 2% residual drift). The effect of additional sources of stiffness and strength (i.e., interior gravity frames and slab contribution) and the polarity of the aftershocks are examined in this study. Results of this investigation show that the collapse potential under aftershocks strongly depends on the modeling approach (i.e., the aftershock collapse potential is modified when additional sources of lateral stiffness and strength are included in the analytical model). Furthermore, it is demonstrated that the aftershock capacity associated to demolition (i.e., the aftershock collapse capacity associated to a residual interstory drift that leads to an imminent demolition) is lower than that of the aftershock collapse capacity, which mean that this parameter should be a better measure of the building residual capacity against aftershocks. Copyright © 2014 John Wiley & Sons, Ltd.