Modeling of lightly reinforced concrete walls subjected to near‐fault and far‐field earthquake ground motions

Reinforced concrete bearing walls with low vertical reinforcement ratios of less than 0·2% are referred to as lightly reinforced walls. Recently, Eurocode 8 and the French code PS 92 adopted a special design concept for lightly reinforced concrete walls based on the multifuse principle favouring rupture occurrence at several storeys. This design leads to lower reinforcement ratios with their optimized distribution allowing wide cracks to take place with large energy dissipation potential. In addition, the vertical displacement of the mass results in energy transformation from kinematic to potential. The objective of the investigation is to analytically predict the response of such lightly reinforced walls when subjected to near‐fault and far‐field ground motion records up to failure to establish the load‐carrying capacity and ductility of the walls. A wall was modeled using six‐node two‐dimensional panel elements. The panel elements have lumped flexural/axial plasticity at their top and bottom fibre sections. The response of the wall was evaluated in terms of pushover, spectral, displacement‐based, and time history analyses. The model and the response data were verified against available measurements from a test program conducted using a shake table. The comparison indicated that the model closely represented the behaviour observed in the test. Copyright © 2007 John Wiley & Sons, Ltd.     

Plastic hinge length of reinforced concrete columns subjected to both far‐fault and near‐fault ground motions having forward directivity

In a strong earthquake, a standard reinforced concrete (RC) column may develop plastic deformations in regions often termed as plastic hinge regions. A plastic hinge is basically an energy dampening device that dampens energy through the plastic rotation of a rigid column connection, which triggers redistribution of bending moments. The formation of a plastic hinge in an RC column in regions that experience inelastic actions depends on the characteristics of the earthquakes as well as the column details. Recordings from recent earthquakes have provided evidence that ground motions in the near field of a rupturing fault can contain a large energy or ‘directivity’ pulse. A directivity pulse occurs when the propagation of the fault proceeds at nearly the same rate as the shear wave velocity. This pulse is seen in the forward direction of the rupture and can cause considerable damage during an earthquake, especially to structures with natural periods that are close to those of the pulse. In the present paper, 1316 inelastic time‐history analyses have been performed to predict the nonlinear behaviour of RC columns under both far‐fault and near‐fault ground motions. The effects of axial load, height over depth ratio and amount of longitudinal reinforcement, as well as different characteristics of earthquakes, were evaluated analytically by finite element methods and the results were compared with corresponding experimental data. Based on the results, simple expressions were proposed that can be used to estimate plastic hinge length of RC columns subjected to both far‐fault and near‐fault earthquakes that contain a forward‐directivity effect. Copyright © 2011 John Wiley & Sons, Ltd.     

Seismic response of mega buckling‐restrained braces subjected to fling‐step and forward‐directivity near‐fault ground motions

Special characteristics of earthquakes in the near‐fault regions caused failures for many modern‐engineered structures. Fling‐step and forward‐directivity are the main consequences of these earthquakes. High‐amplitude pulses at the beginning of the seismograph have been obviously presented in forward‐directivity sites. These pulses have high amount of seismic energy released in a very short time and caused higher demands for engineering structures. Fling‐step is generally characterized by a unidirectional large‐amplitude velocity pulse and a monotonic step in the displacement time history. These monotonic steps cause residual ground displacements that are associated with rupture mechanism. In this paper, the seismic performance of steel buckling‐restrained braced frames with mega configuration under near‐source excitation was investigated. Fourteen near‐fault records with forward‐directivity and fling‐step characteristics and seven far‐faults have been selected. Nonlinear time‐history analyses of 4‐story, 8‐story, 12‐story and 15‐story frames have been performed using OpenSees software. After comparing the results, it is shown that, for all frames subjected to the selected records, the maximum demand occurred in lower floors, and higher modes were not triggered. Near‐fault records imposed higher demands on the structures. The results for near‐fault records with fling‐step were very dispersed, and in some cases, these records were more damaging than others. Copyright © 2014 John Wiley & Sons, Ltd.     

Effectiveness of sliding isolators with variable curvature in near‐fault ground motions

Recent studies have revealed that a sliding isolator with variable curvature (SIVC) can mitigate the resonance phenomenon likely to occur in seismic response of a conventional friction pendulum system (FPS) isolator due to its constant isolation frequency. The present study simulates four SIVC isolators and an FPS to find the optimum range of initial isolation period and coefficient of friction and employ them in comparing the effectiveness of SIVC in different peak ground acceleration (PGA) scales of near‐fault earthquakes. Velocity‐dependent coefficient of friction and modified viscoplasticity model have been used to simulate nonlinear friction force of the isolators. Results indicate identical performance of all SIVC isolators in PGA scales up to 0.4 g. When subjected to PGA scales from 0.4 g to 1.0 g, polynomial friction pendulum isolator (PFPI) and variable curvature friction pendulum system (VCFPS) reduce base displacement greatly, while conical friction pendulum isolator (CFPI) and variable frequency pendulum isolator (VFPI) show amplified responses. However, in mitigating structural acceleration, performance of CFPI and VFPI, unlike PFPI and VCFPS, which perform poorly, is excellent. Thus, in a strong near‐fault earthquake, PFPI and VCFPS or CFPI and VFPI can be chosen based on whether reduction of base displacement or super‐structural acceleration is the main concern of designer, respectively. Copyright © 2015 John Wiley & Sons, Ltd.     

Reliable fragility functions for seismic collapse assessment of reinforced concrete special moment resisting frame structures under near‐fault ground motions

A collapse fragility function shows how the probability of collapse of a structure increases with increasing ground motion intensity measure (IM). To have a more reliable fragility function, an IM should be applied that is efficient and sufficient with respect to ground motion parameters such as magnitude (M) and source‐to‐site distance (R). Typically, pulse‐like near‐fault ground motions are known by the presence of a velocity pulse, and the period of this pulse (T_p) affects the structural response. The present study investigates the application of different scalar and vector‐valued IMs to obtain reliable seismic collapse fragility functions for reinforced concrete special moment resisting frames (RC SMRFs) under near‐fault ground motions. The efficiency and sufficiency of the IMs as the desirable features of an optimal IM are investigated, and it is shown that seismic collapse assessments by using most of the IMs are biased with respect to T_p. The results show that (Sa(T₁), Sa(T₁)/DSI) has high efficiency and sufficiency with respect to M, R, T_p, and scale factor for collapse capacity prediction of RC SMRFs. Moreover, the multiobjective particle swarm optimization algorithm is applied to improve the efficiency and sufficiency of some advanced scalar IMs, and an optimal scalar IM is proposed.     

Effects of hysteretic model on seismic demands: consideration of near‐fault ground motions

This paper summarizes the results of a study that is to evaluate the structural response attributes of near‐fault ground motion. Ground motion recordings from the Chi‐Chi earthquake are used as inputs to the structural system. An improved nonlinear hysteretic model, based on the experimental study, was used to calculate the response of the single degree‐of‐freedom inelastic system. Comparison of the results of analysis with traditional elastic–perfect plastic mode calculations was made. Discussions on the inelastic design spectrum, particularly the code‐specified base shear coefficients, using the improved nonlinear hysteretic model incorporated with the near‐fault input ground motion are made. Copyright © 2002 John Wiley & Sons, Ltd.     

Estimation of seismic demands of steel frames subjected to near‐fault earthquakes having forward directivity and comparing with pushover analysis results

High statistics of damages in modern structures (buildings structured based on new codes) exposed to near‐fault earthquake illustrates the necessity of more studies on this kind of earthquake effects on the structures. A specification of near‐fault earthquakes is the directivity effects. Existing records of near‐fault quakes containing directivity effects including records of Iran and abroad were modified and used for linear time history analysis of three steel moment frames (5, 8 and 12 story frames), and the results were compared with nonlinear time history analysis and pushover analysis of far‐fault quakes in this paper. The results showed that these records (near fault) motivate high modes of the structure, and especially for the 12‐story structure, high response was detected, but none of these results made the frames collapse. By comparing nonlinear dynamic analysis (time history) with nonlinear static analysis (pushover), it was concluded that various lateral load patterns in pushover cannot cover the time history result needs. Load distribution pattern based on the first vibration mode covers these demands in the lower floors, but in higher floors, this method is not applicable. Copyright © 2012 John Wiley & Sons, Ltd.     

Efficiency of active systems in controlling base‐isolated buildings subjected to near‐fault earthquakes

It has been pointed out that base‐isolated structures may be vulnerable during near‐fault earthquakes and special considerations are required in the design of isolated structures in near‐fault areas. This paper investigates the efficiency of active control systems in reducing the responses of base‐isolated structures with various isolation parameters. The design of hybrid control systems using base isolation and active systems are optimized in order to accomplish different design purposes. Also for some cases, equivalent passive control systems are introduced which result in comparable responses with respect to hybrid control systems. Copyright © 2009 John Wiley & Sons, Ltd.     

Earthquake performance assessment of concrete gravity dams subjected to spatially varying seismic ground motions

This paper investigates the earthquake performance of concrete gravity dams under spatially variable seismic excitations. A nonlinear finite element model is developed and validated using shake table experimental results. The model is then subjected to spatially varying earthquake ground motions incorporating the wave passage effect, with values for apparent propagation velocities consistent with the source-site geometry and the shear wave velocity in the foundation rock. The evaluation reveals that different response patterns occur when spatially non-uniform and uniform seismic ground motions are applied as input excitations to the model, because spatially non-uniform excitations induce the quasi-static response, whereas uniform excitations do not, and, in addition, the dynamic response caused by different input motions varies. Notably, spatially non-uniform excitations produce larger opening at the heel of the dam and severer slipping at its toe; this latter observation can have a significant effect on the global equilibrium and stability of the dam during an earthquake.     

近断层脉冲型地震作用下防屈曲支撑-钢筋混凝土框架结构的抗震性能

具有向前方向性效应和滑冲效应的近断层脉冲型地震动对建筑结构的破坏已受到工程界的广泛关注。为了解设置防屈曲支撑（BRB）的混凝土框架在近断层脉冲型地震动激励下的抗震性能,采用基于能量平衡的塑性设计方法完成了3个V形BRB支撑的RC框架结构的抗震设计。分别选取具有向前方向性效应和滑冲效应的脉冲型以及非脉冲型三组共36条近断层地震动,对结构进行罕遇地震作用下的非线性动力分析,研究了结构的最大层间位移角、最大顶点加速度、最大顶点位移和BRB的轴向性能;分析和评估了结构在3条典型地震动激励下的地震响应。结果表明：近断层脉冲型地震动比非脉冲型地震动对结构会产生更大的地震响应,且响应显著集中于速度脉冲时刻;BRB能充分发挥其耗能特性,提高RC框架结构体系的抗震性能。     

One-dimensional wave equation analyses for pile responses subjected to seismic horizontal ground motions

This paper presents a numerical one-dimensional wave equation analysis technique for piles and pile groups subjected to seismic horizontal ground motions in liquefiable zones. The so-called Earthquake Wave Equation Analysis for Piles (EQWEAP) procedure is introduced for piles subjected to horizontal earthquake excitations. Disregarding the effects of kinematic soil–pile interaction, the seismic responses of piles can be obtained by approximating the free-field ground response analysis, the ultimate earth pressure model, and the ground displacement profiles. The nonlinearities of the concrete piles were modeled using the approximate tri-linear moment–curvature relationships. A case study and application concerns were presented. Although the analysis is in one dimension, it is found to be effective and able to provide a rapid estimation in foundation design when seismic pile behaviors are of interest. The advantages of this analysis are the time efficiency of the seismic design of pile foundations and the relative simplicity of the analysis. In addition, it suggests alternative modeling for the dynamic analysis adopting the commonly known static models and/or methods.     

Energy dissipation of tall core‐wall structures with multi‐plastic hinges subjected to forward directivity near‐fault and far‐fault earthquakes

In this study, different energy components in the tall reinforced concrete core‐wall buildings with numerous plastic hinges over the height are investigated using nonlinear time history analysis. The effect of near‐fault and far‐fault earthquakes is compared. The idea of one‐plastic, two‐plastic, three‐plastic and whole‐plastic hinge approaches along the core wall is examined. The input energy, inelastic, damping, kinetic and elastic strain energy during the earthquakes are studied. The results show that a large energy quantity transfers to the structure at the arrival time of the near‐fault motion pulse. Inelastic energy distribution over the height shows a considerable amount of inelastic energy dissipation occurring at the base and above the mid‐height of the walls. Copyright © 2016 John Wiley & Sons, Ltd.     

Seismic collapse analysis of high‐rise reinforced concrete frames under long‐period ground motions

Structural damages associated with buckling of longitudinal reinforcing steel and crushing of concrete induce strength and stiffness degradation in reinforced concrete (RC) beams and columns. This paper presents a numerical investigation on earthquake‐induced damages and collapse of typical high‐rise RC buildings model incorporating strength degradation (SD) effects. In a simple finite‐element analysis program with the generalized stress fiber discretization, hysteretic constitutive models primarily dominate the inelastic behavior. Buckling of reinforcing steel and crushing of confined concrete are taken into accounted to the stress–strain relationship of fiber elements. The SD effect in components with small hoop ratio tends to amplify the seismic responses high‐rise RC moment‐resisting frames when the intensity of ground motions exceeds the design level. Buckling of steel rebar and crushing of concrete should be fully considered together with the P‐Δ effect for collapse simulations.     

RC框架基于典型相关分析的地震动多元破坏势评估

为了更加全面地评估地震动对结构的潜在破坏势,考虑多个地震动强度参数和多个结构损伤参数对地震动破坏势的影响,提出一种基于典型相关分析的地震动多元破坏势评估方法。为说明所提方法,以具有不同高度和不同抗震设防水平的16个钢筋混凝土框架结构为研究对象,采用100条真实地震动记录作为地震输入。将与地震动加速度、速度和位移相关的8个地震动强度参数进行对数线性组合,构造成地震动多元强度参数的典型变量。将结构最大层间位移角、整体损伤指数、最大楼层加速度进行对数线性组合,构造成结构多元损伤参数的典型变量。通过使地震动强度参数的典型变量和结构损伤参数的典型变量的相关系数达到最大,获得可以较好评估地震动破坏势的复合地震动参数。研究结果表明：相比于单一地震动参数,复合地震动参数可以更好地评估地震动的破坏势,其与结构损伤的相关性也更强。     

Earthquake response control of a super high‐rise structure subjected to long‐period ground motions via a novel viscous damped system with multilever mechanism

A novel viscous damped system and its principles are proposed in the paper. It is a novel viscous damped system with multilever mechanism that can improve the energy dissipation capacity of conventional viscous dampers. In order to compare the damping effects of the novel viscous damper with that of the conventional viscous damper, a shaking table test of a three‐story steel frame structure is performed. Testing results indicate that the novel viscous damped system is more efficient. The elastic time‐history analysis of a super high‐rise frame‐core tube structure is studied under the frequently occurring earthquake. Dynamic loads take two groups of ground motions with different period characteristics into account. Main response values such as base shear, interstory drift, and acceleration factor under long‐period ground motions are apparently larger than the seismic results due to standard ground motions. Responses between the undamped structure and the damped structure with conventional viscous dampers or the latest products are compared. It is concluded that the proposed viscous damped system can perform more effectively in reducing high‐rise structural responses subject to long‐period ground motions.     

Stochastic dynamic response of masonry minarets subjected to random blast and earthquake‐induced ground motions

This paper presents the stochastic seismic response analysis of masonry minarets subjected to random underground blast and earthquake‐induced ground motions by using a three‐dimensional finite element model. The random blast and earthquake‐induced ground motions are represented by the power spectral density function and applied to each support point of the three‐dimensional finite element model of the masonry minaret system. This research conducted a parametric study to estimate the effects of the blast‐induced ground motion on the stochastic response of the minaret. Therefore, the analyses were carried out for the different values of the charge weight and the distance from the charge centre. In addition, in order to investigate the effect of earthquake‐induced ground motion on the stochastic response of the masonry minaret, three different soil conditions—soft, medium and firm soils—are considered in the analyses. Finally, it is noted that underground blast and earthquake effects cause the stochastic behaviour of minaret to change considerably. Copyright © 2009 John Wiley & Sons, Ltd.     

Seismic performance of steel mega braced frames equipped with shape‐memory alloy braces under near‐fault earthquakes

This paper has two main objectives. The first objective is to compare the dynamic behavior of mega shape‐memory alloy (SMA) braced frames subjected to far‐fault and near‐fault ground motions. Therefore, four mega SMA braced frames with various stories located in the vicinity of an active fault were considered. Fourteen near‐fault records with two well‐known characteristics of these records, i.e. forward directivity and fling step, were selected to test near‐fault earthquake characteristics. Furthermore, other seven far‐field records were selected for comparison. Through the nonlinear dynamic analyses, the results showed that for high‐rise frames, the near‐fault earthquakes resulted in more demands than the far‐field, but for low‐rise frames, far‐fault records imposed more demands. It was also found that mega configuration and SMA stiffening at large strains played key roles in seismic vibration control of frames. The second objective of this paper is to study the superior performance of SMA braces over the buckling restrained braces by exploiting the super‐elastic characteristic of the SMA. Identical buildings equipped with buckling restrained braces were also studied for comparison purposes. The results revealed the excellent performance of SMA braces under near‐fault records by reducing both interstory drift and residual displacement of the top floor. Copyright © 2015 John Wiley & Sons, Ltd.     

Performance evaluation of different types of steel moment resisting frames subjected to strong ground motion through incremental dynamic analysis

Comparative studies on seismic performance for various types of steel moment resisting frames subjected to near field and far field earthquakes are performed through Incremental dynamic analysis (IDA) method in this study. Near field earthquake has a pulse like effects on the structures. It imports immediate force in very short duration to buildings. Therefore, destructive effects of surge energy are not negligible. Four intensity indices are used, namely, peak acceleration (PGA), spectral acceleration at the structure??s first-mode period (S_a(T₁, 5%)), spectral acceleration at the structure??s nth effective-mode period (S_a(T_n, 5%)) and the Spectral velocity at the structure??s first-mode period (S_v(T₁, 5%)). Numerical results illustrate that the intensity measure parameters related to ground velocity and the higher mode-related parameters present better correlation with the seismic responses of near source ground motion for given systems. The higher mode-related parameters are more suitable for tall systems subjected to near field earthquakes. Moreover, the chosen parameters S_a(T_n, 5%) and S_v(T₁, 5%) of near-fault impulsive ground motions enhance the performance of intensity measure of corresponding conventional parameters, i.e. S_a(T₁, 5%). A comparison for the special and intermediate steel moment resisting frames is made as regard to performance using IDA method. A more efficient performance is observed for the special moment resisting frames compare to intermediate ones.     

Empirical approaches to estimate the nonlinear dynamic responses of earth-core rockfill dams subjected to earthquake ground motions

This study investigates three aspects of the dynamic response of earth-core rockfill dams (ECRDs) under earthquake loadings, including induced shear strain, increased fundamental period, and shear modulus reduction. A database of the recorded ground motions of ECRD cases in Japan is analyzed to carry out this study. The required response parameters as mentioned above are extracted by analyzing the acceleration time histories recorded at the dams’ crests and foundations. Subsequently, statistical analyses are performed to achieve the objectives of this study. A graph is developed describing the change in the dam’s fundamental period with the induced shear strain, and it is observed that the increase of the dam’s period is dependent on the increase in the shear strain levels. A relationship is established to estimate the anticipated levels of shear strain from the intensity measure (IM) of the earthquake signals. In the proposed predictive relationship, a new IM is used that can adequately characterize the severity of an earthquake. Finally, a curve and a range are suggested for the average shear modulus degradation of the ECRD’s core materials. The presented empirical graphs and relationships in this study are valuable tools to obtain an appropriate perception of dams’ nonlinear behaviour under strong earthquake excitations.     

Assessment of RC columns subjected to horizontal and vertical ground motions recorded during the 2009 L’Aquila (Italy) earthquake

In the aftermath of recent earthquakes there has been substantial field evidence demonstrating that the collapse of several existing structures was caused by the effects of the vertical component of seismic ground motions. Such field evidence has not yet been supported by comprehensive analytical assessment and experimental tests. The present work focuses on preliminary analyses of the seismic response of reinforced concrete (RC) members subjected to horizontal (HGMs) and vertical (VGMs) ground motions recorded during the 2009 L’Aquila (Italy) earthquake. Normalised axial loads in beam-columns as well as the peak ground acceleration ratios between horizontal and vertical ground accelerations are emphasised as they are considered parameters of paramount importance for the assessment of structural components and systems subjected to combined horizontal and vertical ground motions (HVGMs).Results of extensive parametric nonlinear dynamic analyses carried out on simplified structural models are discussed in detail. The sample models comprise cantilever RC columns and a two-storey, two-bay plane frame designed for gravity loads. The structural response quantities for the performed analyses are expressed in terms of axial loads, axial deformations, bending moment-axial load interaction and shear demand/capacity ratios. It is found that the variation of axial loads is significant in columns under HVGMs, especially in compression. For values of normalised axial loads (ν) corresponding to actual RC columns in framed building structures, e.g., normalised axial load ν>0.10, the average increase of the compression load ranges between 174% (ν=0.20) and 59% (ν=0.50). For high values of normalised axial loads the computed axial load-bending moment pairs lie beyond the threshold interaction curves and, in turn, the RC members may fail. The shear demand-to-supply ratio is also detrimentally affected by the high fluctuations of axial loads in the columns. Net tensile forces were computed for columns with low-to-moderate axial gravity preload. In multi-storey framed buildings, the response of central columns is significantly affected by the HVGMs. Reliable seismic performance assessment of framed systems requires that combined HGMs and VGMs should be accounted for in the analyses. Further experimental and numerical research is needed to formulate efficient mechanical models to evaluate the shear capacity of structural members of existing RC framed buildings under earthquake loading.