Sequential Ground Motions Research Articles

Structural Damage Prediction of a Reinforced Concrete Frame under Single and Multiple Seismic Events Using Machine Learning Algorithms

Advanced machine learning algorithms have the potential to be successfully applied to many areas of system modelling. In the present study, the capability of ten machine learning algorithms to predict the structural damage of an 8-storey reinforced concrete frame building subjected to single and successive ground motions is examined. From this point of view, the initial damage state of the structural system, as well as 16 well-known ground motion intensity measures, are adopted as the features of the machine-learning algorithms that aim to predict the structural damage after each seismic event. The structural analyses are performed considering both real and artificial ground motion sequences, while the structural damage is expressed in terms of two overall damage indices. The comparative study results in the most efficient damage index, as well as the most promising machine learning algorithm in predicting the structural response of a reinforced concrete building under single or multiple seismic events. Finally, the configured methodology is deployed in a user-friendly web application.

Experimental and Numerical Investigation of a Dissipative Connection for the Seismic Retrofit of Precast RC Industrial Sheds

Past earthquakes have highlighted the seismic vulnerability of prefabricated industrial sheds typical of past Italian building practices. Such buildings typically exhibited rigid collapse mechanisms due to the absence of rigid links between columns, beams, and roof elements. This study aims at presenting the experimental and numerical assessment of a novel dissipative connection system (DCS) designed to improve the seismic performance of prefabricated sheds. The device, which is placed on the top of columns, exploits the movement of a rigid slider on a sloped surface to dissipate seismic energy and control the lateral displacement of the beam, and to provide a recentering effect at the end of the earthquake. The backbone curve of the DCS, and the effect of vertical load, sliding velocity, and number of cycles were assessed in experimental tests conducted on a scaled prototype, according to a test protocol designed accounting for similarity requirements. In the second part of the study, non-linear dynamic analyses were performed on a finite element model of a portal frame implementing, at beam-column joints, either the DCS or a pure friction connection. The results highlighted the effectiveness of the DCS in controlling beam-to-column displacements, reducing shear forces on the top of columns, and limiting residual displacements that can accrue during ground motion sequences.

An enhanced sequential ground motion selection for risk assessment using a Bayesian updating approach

In the context of probabilistic prediction of seismic demand parameters, this research presents a novel method for sequential ground motion selection based on the Bayesian updating approach. Unlike traditional methods of ground motion selection, which rely on spectral shape or intensity measures, the ground motions in this approach are chosen directly through a probabilistic evaluation of the structural responses. In this method, the probability density function produced by a limited number of nonlinear analyses closely resembles the curve produced by a large number of nonlinear analyses. To accomplish this, a large number of linear analysis results obtained in a short period of time are utilized to form the initial belief in a Bayesian model. Subsequently, in a two-way interaction, the results of fresh nonlinear analysis can be introduced sequentially to improve the predictive power of the model while concurrently selecting new ground motions. For this purpose, three case study buildings, including an 8-, 12-, and 20-story structure, were studied at three hazard levels. The efficiency of the proposed approach was investigated for sequential ground motion selection, and its effect on the probability density function of the peak floor acceleration and maximum story drift were studied. The proposed method is highly independent of the hazard level or the height of the building, and it may be used in almost every situation with significant improvement. This method of selecting ground motions is fairly efficient in terms of computing. The results show a significant improvement, especially at higher demand values that are more important in risk assessment. Significant improvements in peak floor acceleration results are also observed, which are notoriously difficult to evaluate.

Multiple-Input Convolutional Neural Network Model for Large-Scale Seismic Damage Assessment of Reinforced Concrete Frame Buildings

This study introduces a multiple-input convolutional neural network (MI-CNN) model for the seismic damage assessment of regional buildings. First, ground motion sequences together with building attribute data are adopted as inputs of the proposed MI-CNN model. Second, the prediction accuracy of MI-CNN model is discussed comprehensively for different scenarios. The overall prediction accuracy is 79.7%, and the prediction accuracies for all scenarios are above 77%, indicating a good prediction performance of the proposed method. The computation efficiency of the proposed method is 340 times faster than that of the nonlinear multi-degree-of-freedom shear model using time history analysis. Third, a case study is conducted for reinforced concrete (RC) frame buildings in Shenzhen city, and two seismic scenarios (i.e., M6.5 and M7.5) are studied for the area. The simulation results of the area indicate a good agreement between the MI-CNN model and the benchmark model. The outcomes of this study are expected to provide a useful reference for timely emergency response and disaster relief after earthquakes.

Seismic performance of RC frames under sequential ground motion

Seismic performance of RC frames under sequential ground motion

Damage assessment of single-layer reticulated domes subjected to mainshock-aftershock sequences based on structural damage factor

Long-span infrastructures are usually taken over for public shelters, in which single-layer reticulated dome is a kind of popular structure. But there was less research for the damage assessment of single-layer reticulated domes subjected to mainshock-aftershock sequences. How to accurately assess the damage by mainshock and the stability redundancy for aftershock becomes an important problem. In order to explore the effects on the structural damage, this paper processes the dynamic time-history analyses for single-layer reticulated domes subjected to mainshock-aftershock sequences. The finite element model of Kiewitt8 single-layer spherical reticulated shell structure is established in ANSYS. Ten pairs of ground motion sequences in horizontal direction are generated with modulating amplitude to simulate the rare and extreme earthquakes. The results demonstrated that the structure even suffered from the extreme earthquake mainshock still has loading capacity and structural safety with the local plastic damage. But the effect on structural damage of aftershock ground motion with greater earthquake intensity cannot be ignored for single-layer reticulated domes. A structural damage assessment method is utilized with combining the damage factor by performing static stability analysis twice. It can effectively reflect the remaining loading capacity and structural safety factor for the practical seismic engineering applications.

A Park-Ang damage index-based framework for post-mainshock structural safety assessment

Following an earthquake event, the safety of continued occupancy in terms of whether a mainshock-damaged building could withstand upcoming strong aftershocks is a key question for stakeholders. To answer this question, the residual collapse capacity of mainshock-damaged buildings should be evaluated, and in this way, an accurate assessment of the structural damage inflicted during a mainshock-aftershock sequence is of great importance. Recent studies have shown that energy-based damage measures, which calculate the dissipated energy due to cyclic-plastic deformations, can more accurately reflect the secondary damage induced by aftershocks than deformation-based damage measures. Park-Ang damage index is a combined seismic damage measure that integrates the damage caused by excessive deformations with the damage caused by the cumulative hysteretic energy dissipation. This study presents a framework for assessing the post-mainshock structural safety, in which the quantification of the damage accumulated during a mainshock and the determination of the remaining capacity to sustain strong aftershocks are made based on the Park-Ang damage index. First, residual collapse capacity diagrams are produced by performing incremental dynamic analysis under a suite of selected sequential ground motions. Next, an average diagram is made to find the maximum safe damage index, so that values above this threshold would represent the buildings that are deemed to be unsafe. The proposed framework is applied to a reference four-story steel moment-frame building to investigate its collapse potential and evaluates its structural safety in a hypothetical earthquake scenario. The results of the case study indicate that the proposed framework can effectively help structural engineers to decide on whether a mainshock-damaged building is safe to reoccupy and communicate the final decision in a simple diagram to stakeholders.

Seismic vulnerability assessment of ageing reinforced concrete structures under real mainshock-aftershock ground motions

Ageing structures located in moderate to high seismicity regions are exposed to multiple natural stressors during their lifetime. Large earthquake events coupled with environmental aggressive agents increase the progressive failure probability of these structures. The accumulated damage during the main earthquake event might be exacerbated by its following aftershocks. This might result in catastrophic failure of these structures, and consequently, result in several socio-economic losses. Taking such progressive deterioration mechanism into consideration, the current study presents a framework to assess the vulnerability of ageing Reinforced Concrete (RC) frames subject to real Mainshock-Aftershock (MS-AS) ground motion sequences. Employing an advanced fibre-based finite element modelling technique, the nonlinear static and dynamic behaviour of a case-study RC frame with various ages is simulated under 48 real MS-AS record pairs. Quantifying corrosion-variant damage states, the age-specific fragility curves are developed for the considered structure under both single MS events and MS-AS sequences. It was found that the severely corroded RC frames are most likely to collapse before the second event comes up. Moreover, results show that the PGA ratio of AS to MS plays a critical role in seismic vulnerability assessment of highly corrosion-damaged RC frames.

Effect of sequential earthquakes on evaluation of non-linear response of 3D RC MRFs

Most of the existing seismic codes for RC buildings consider only a scenario earthquake for analysis, often characterized by the response spectrum at the specified location. However, any real earthquake event often involves occurrences of multiple earthquakes within a few hours or days, possessing similar or even higher energy than the first earthquake. This critically impairs the rehabilitation measures thereby resulting in the accumulation of structural damages for subsequent earthquakes after the first earthquake. Also, the existing seismic provisions account for the non-linear response of an RC building frame implicitly by specifying a constant response modification factor (R) in a linear elastic design. However, the 'R' specified does not address the changes in structural configurations of RC moment-resisting frames (RC MRFs) viz., building height, number of bays present, bay width, irregularities arising out of mass and stiffness changes, etc. resulting in changed dynamic characteristics of the structural system. Hence, there is an imperative need to assess the seismic performance under sequential earthquake ground motions, considering the adequacy of code-specified 'R' in the representation of dynamic characteristics of RC buildings. Therefore, the present research is focused on the evaluation of the non-linear response of medium-rise 3D RC MRFs with and without vertical irregularities under bi-directional sequential earthquake ground motions using non-linear dynamic analysis. It is evident from the results that collapse probability increases, and 'R' reduces significantly for various RC MRFs subjected to sequential earthquakes, pronouncing the vulnerability and inadequacy of estimation of design base shear by code-specified 'R' under sequential earthquakes.

Procedure for Ranking Ground Motion Records Based on the Destructive Capacity Parameter

The relative input energy of a ground motion is used to quantitatively estimate the destructive capacity of ground motion to a structure, which reflects the deformation capacity and energy dissipation capacity of a structure. The destructive capacity of a ground motion on a structure is known to be relevant to the structural period. Because ranking the destructive capacity of ground motion at all periods is not practical, rationally dividing period ranges should be performed before ranking ground motions according to their destructive capacity and a new method to divide period ranges based on the relative input energy is proposed. According to the correlation coefficient of the relative input energy at adjacent structural periods, the structural periods from 0.2 s to 10 s are divided into several representational period ranges, which include a recommended period within each period range to represent the whole range. Only the ground motion ranking sequence at the recommended period is used in this period range. The rationality of the ranking sequence of ground motions based on destructive capacity is verified by a series of structural seismic response analyses. The results of this paper are compared with those of existing methods, and the results show that the relative input energy index is able to better quantitatively describe the destructive capacity of ground motion.

Performance of sheet pile to mitigate liquefaction-induced lateral spreading of loose soil layer under the embankment

Liquefaction-induced lateral spreading of the foundation soil severely damages the embankment resting on it. Installation of sheet pile near the toe of the embankment is a widely used technique to mitigate lateral spreading of the foundation soil and the associated damage to the embankment. However, several factors, such as the thickness of the liquefiable foundation, width of sheet pile, and softening of the bearing stratum, that may affect the performance of the sheet pile as seismic retrofit against lateral spreading has not been well studied yet. Therefore, a series of dynamic centrifuge experiments are carried out to study the applicability of sheet pile in various thickness of liquefiable foundation under moderate to strong sequential ground motions. It is found that the sheet pile effectively mitigates the lateral spreading of a thin as well as thick liquefiable foundation. Besides, the sheet pile having the same width of the target area performs equally as a wider sheet pile in mitigating channel-ward ground movement. The experimental results also show that the liquefaction-induced softening of the dense bearing stratum immediately below the loose foundation significantly influences the performance of the sheet pile.

Seismic performance of bridge with unbonded posttensioned self-centering segmented concrete-filled steel-tube columns: An underwater shaking table test

As one of the prefabricated structures, precast segmented bridges can be used to shorten construction time, reduce traffic delays, improve quality control, and minimize environmental impact. Despite the numerous advantages of segmented pre-fabricated rapid assembly technology, it is not widely used in high seismic hazard zones nor applied to complex water-abundant situations because of the lack of knowledge on its seismic performance. Through experimental and numerical studies, researchers have studied the seismic performance of such type of column under quasi-static testing. The behaviors of the column or bridge under desired strong ground motions (GMs) have been rarely investigated. This paper presents the major findings of an underwater shaking table testing program conducted on a 1/4 scaled single-span bridge with novel self-centering segmented concrete-filled steel tube columns. This study focuses on the performance of the bridge subjected to different GMs under 4 water conditions, which are with water at a height of 1.2 m, 1.5 m, 1.8 m, and without water. The bridge model incorporated two post-tensioned precast self-centering segmented concrete-filled steel tube (PSCFST) single columns with fixed and sliding supports connected to a single span steel box-girder superstructure. The internal unbonded PT tendons are installed at the center of the segment section, and external energy dissipation (ED) bars are installed on the outside of the bottom segment. The bridge model was then subjected to a sequence of natural and artificial GMs with different amplitudes. The well-designed PSCFST bridge demonstrated a desirable seismic performance, and no significant damage was observed. Concerning the overall response of the structure, the maximum drift ratio reached 2.14%. Moreover, a maximum residual drift of 0.09% was observed after the tests, which indicates that the test model demonstrated a desirable self-centering capability after a sequence of GMs. The cumulative energy dissipation and equivalent viscous damping ratio of the bridge were calculated. A comparison is made for the seismic performance of the bridge especially in terms of the displacement and acceleration of the whole structure under the cases of with and without water.

Random field model of sequential ground motions

This paper proposes a random field model of sequential ground motions, which considers the spatial correlation between the mainshock and aftershock from a stochastic view. First, the correlation between the mainshock and aftershock under the physical “source–path–local site” mechanism is explained. Based on the mechanism, the point-source model, and uniform-isotropic-medium model, a random single-point model for sequential ground motion with eight basic parameters is presented. Second, the random field model is developed from the random single-point model. More than 1000 pairs of sequential ground motions are used to identify and analyze statistically the basic parameters in the model. Moreover, the probability distribution of each parameter is presented based on copula functions. The results show that the spatial correlations of sequential ground motions can be effectively simulated based on the proposed random field model. Furthermore, it is possible to realistically reproduce the attenuation and time lag of ground motions at a local site.

Cross-aisle seismic performance of selective storage racks

A series of single-axis shaking table tests were conducted on three full-scale selective storage racks in the cross-aisle direction. The uplifting and rocking behaviour of the racks was examined under three baseplate types: ductile, heavy duty, and unanchored. Each rack was subjected to a sequence of ground motions of increasing intensity up to failure, with a total of 29 tests conducted. At 1.5 times the respective design level ground motions, the heavy duty baseplates caused a foundation failure while the unanchored rack failed by overturning. The rack with ductile baseplates survived all tests up to 2.3 times the design level. For a given ground motion, the unanchored rack upright always had the smallest peak axial load. However, the unanchored rack had much larger sways under the Northbridge and Kobe ground motions. The NZS 1170.5 equivalent static method design loading was found to be overly conservative for racks with ductile and heavy duty baseplates, of which the upright design axial forces were better predicted using the refined equivalent static method.

Nonlinear dynamic analysis and damage evaluation of hydraulic arched tunnels under mainshock–aftershock ground motion sequences

The significant life engineering failure and financial toll caused by mainshock–aftershock (MSAS) ground motion sequences have highlighted the importance of focusing on aftershocks. The current seismic design of hydraulic arched tunnels generally involves a dynamic analysis that assumes that a single seismic excitation or multiple single seismic excitations are applied. However, this assumption may underestimate the seismic response of hydraulic arched tunnels. The focus of this study is to evaluate the effect of both the mainshock excitation-induced nonlinear response and the MSAS excitation-induced nonlinear response of hydraulic arched tunnels. For this purpose, a numerical model is built in the commercial software ABAQUS, and it considers the fluid-structure-rock interaction system. Twenty-one as-recorded MSAS ground motion sequences are selected, and the seismic wave input method is verified before a nonlinear dynamic analysis is conducted. According to the damage patterns of tunnels under Wenchuan seismic excitation, the lining local damage index (LLDI) and the lining global damage index (LGDI) are established to assess the effect of single mainshock excitations and MSAS excitations on hydraulic arched tunnels. The results reveal that MSAS excitations can cause relatively severe cumulative damage and have a negative impact on the nonlinear dynamic response of the hydraulic arched tunnel compared to single mainshock excitations. Moreover, the aftershocks not only lead to an increase in the LGDI of the hydraulic arched tunnel at the end of the MSAS excitations but also enhance the LLDI of the arch section and sidewall section. In contrast, the aftershocks induce no significant change in the LLDI of the bottom section of the hydraulic arched tunnel. Thus, MSAS excitations should be considered in the seismic design of hydraulic arched tunnels.

Seismic response of partially saturated soils beneath shallow foundations under sequential ground motions

Induced partial saturation is an innovative soil improvement technique intended to mitigate earthquake-induced liquefaction. Historical records indicate that successive earthquakes may occur in high seismic areas. Therefore, a comprehensive understanding of the response of partially saturated soils to sequential ground motions is of great significance for the rational design and execution of this method in engineering practice. In this study, a series of dynamic centrifuge experiments were conducted to investigate the impacts of sequential ground motions on the behavior of partially saturated soils beneath shallow foundations. Two different shallow foundation models with a bearing pressure of 135 kPa and 50 kPa were examined. Three seismic simulations, in order of increasing amplitudes, were sequentially applied to loosely-packed partially saturated sand models prepared with air injection technique. The assessment of the test results indicated that shallow foundations resting on saturated models of loose sand did suffer excessive settlements with each event, producing a large embedment of the foundation. However, much smaller settlements were recorded for partially saturated ground, and the level of the foundation embedment remained limited in this case. The deformation vector fields also indicated that different displacement mechanisms were observed for each successive event.

Centrifuge model test and numerical interpretation of seismic responses of a partially submerged deposit slope

Abstract Partially submerged deposit slopes are often encountered in practical engineering applications. However, studies on evaluating their stability under seismic loading are still rare. In order to understand the seismic behavior of partially submerged deposit slopes, centrifuge shaking table model tests (50g) were employed. The responses of horizontal accelerations, accumulated excess pore pressures, deformation mode, and failure mode of the partially submerged deposit slope model were analyzed. In dynamic centrifuge model tests, EQ5 shaking event was applied numerically. The results indicated that in the saturated zone of the deposit slope, liquefaction did not occur, and the measured horizontal accelerations near the water table were amplified as a layer-magnification effect. It was also shown that the liquefaction-resistance of the deposit slope increased under multiple sequential ground motions, and the deformation depth of the deposit slope induced by earthquake increased gradually with increasing dynamic load amplitude. Except for the excessive crest settlement generated by strong shaking, an additional vertical permanent displacement was initiated at the slope crest due to the dissipation of excess pore pressure under seismic loading. The result of particle image velocimetry (PIV) analysis showed that an obvious internal arc-slip was generated around the water table of the partially submerged deposit slope under seismic loading.

Expected seismic fragility of code-conforming RC moment resisting frames under twin seismic events

Seismic design codes require buildings to be designed only for one seismic event, neglecting the possibility of multiple earthquake events. However considerable damage and loss is observed due to aftershocks where buildings have been affected by the first event. The purpose of this research is to introduce fragility curves of code-conforming reinforced concrete moment resisting frames (RCMRFs) under aftershock. Three RC moment resisting frames of 4, 8 and 15 stories, are loaded and designed based on the latest three editions of the Iranian code of seismic design for buildings referred to as Standard No. 2800 (STD-2800). The nine designed frames are then numerically simulated in OpenSees software to perform nonlinear dynamic analysis. Twenty natural ground motion sequences, each including two seismic events, were selected from previous studies for the purpose of nonlinear incremental dynamic analysis. Maximum inter-story drift was employed as a damage index, to capture performance of the RC frames under the sequences. The accepted performance for the collapse level was taken from Iranian instructions for seismic rehabilitation of existing buildings, PBO-Publication No. 360, 2007, which is similar to ASCE 41-13. Seismic fragility values are calculated for the buildings in the second event after being damaged in different first event intensity scenarios. Assuming a log-normal distribution for the failure probability function, corresponding values of mean, μ, and standard deviation, β, for each case are calculated and discussed. Results indicate how the probability of failure in the seismic fragility curves may differ as a result of the effects of a first event scenario. It is also shown how updating the design criteria as well as the height of frames can affect fragility in a second event.

Study of the correlations between main shocks and aftershocks and aftershock synthesis method

To consider the influence of aftershocks in engineering design, the correlations between main shocks and aftershocks should be examined, and an aftershock simulation method with main shock ground motions needs to be developed. In this study, the data on the sequences of main shock-aftershock ground motions and other related parameters were collected. Using these data, correlations between the magnitude, frequency, duration and energy of the main shock-aftershock ground motions were investigated. The results showed that the magnitude of the aftershock can be larger than that of the main shock. The shapes of the Fourier amplitude spectra of main shocks and aftershocks were similar; however, the predominant frequency and high-frequency components of the aftershock tended to be larger. Considering the magnitude difference between the main shock and the aftershock, the correlation of durations was explored. Additionally, a new concept, the duration ratio, was defined to describe the concentration of seismic energy release, and main shock energy was strongly positively correlated with the energy attenuated during the main shock-aftershock sequence. Finally, based on these results regarding correlation, an aftershock synthesis using recorded main shock ground motions was constructed with the trigonometric series method for seismic design, and some examples are given to analyze the rationality of this synthetic method.

Seismic loss and damage in light‐frame wood buildings from sequences of induced earthquakes

SummaryActivities related to oil and gas production, especially deep disposal of wastewater, have led to sequences of induced earthquakes in the central United States. This study aims to quantify damage to and seismic losses for light‐frame wood buildings when subjected to sequences of induced, small to moderate magnitude, events. To conduct this investigation, one‐ and two‐story multifamily wood frame buildings are designed, and their seismic response dynamically simulated using three‐dimensional nonlinear models, subjected to ground motion sequences recorded in induced events. Damage is quantified through seismic losses, which are estimated using the FEMA P‐58 methodology. Results show that at levels of shaking experienced in recent earthquakes, minor damage, consisting of cracking of interior finishes and nonstructural damage to plumbing and heating, ventilation, and air conditioning systems, is expected, which is consistent with observed damage in these events. The study also examines how expected losses and building fragility will accumulate and/or change over a sequence of earthquakes. Results indicate that damage quantified in terms of absorbed hysteretic energy tended to accumulate over the sequences; this damage corresponds to elongation or widening of cracks. However, fragility is not significantly altered by damage in a preceding event, meaning structures are not becoming more vulnerable due to existing damage. In addition, sequences of events do not change losses if the building is only repaired once at the end of the sequence, as the worsening of damage does not alter repair actions. If repairs are conducted after each event, though, total seismic losses can increase greatly from the sequence.