Forward directivity near‐fault and far‐fault ground motion effects on the behavior of reinforced concrete wall tall buildings with one and more plastic hinges

Near‐fault (NF) ground motion having forward directivity and far‐fault (FF) earthquakes can generate different responses on tall reinforced concrete cantilever walls. In this paper, the behavior of the core wall buildings were examined by performing nonlinear time history analyses on 20‐story, 30‐story and 40‐story fiber element models. The concept of one, two, three and extended plastic hinge in the core walls subjected to the NF motions having forward directivity (pulse‐like) and FF motion was studied by carrying out inelastic dynamic analysis. At the upper levels of the walls, NF pulse‐like ground motions can produce considerably larger curvature ductility, inter‐story drift and displacement demands as compared with the FF motions. A new approach was proposed to obtain the moment demand and reinforcement required to balance the curvature ductility demand along the height of a core wall. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

Vulnerability of steel moment‐resisting frames under effects of forward directivity

In near‐fault regions, forward directivity causes long‐period pulse‐like motions with high amplitude and short duration perpendicular to the fault surface. Pulse‐like motions have important roles in forming the distribution of damages over the structure height. Recent studies indicate that the number of spans influences the demand distribution over the moment frame's height. Considering the destruction of the buildings near causative fault in Bam earthquake, Iran (2003) demonstrates that most damages are concentrated in the ground floor of moment frames. Hence, in this study, forward directivity effect on vulnerability distribution of steel moment‐resisting frames with a few number of spans has been studied by nonlinear dynamic analysis of five structural models with different heights under 20 earthquake records. Related to frames height, results showed that 70% to 90% of forward directivity effects are accumulated in lower one‐third or half of model's height. Also, in near field of fault, growing rate of ductility demand at lower parts of model's height is two times higher than that of far‐fault regions. In addition, it was observed that ductility capacity in lower half of low‐rise or one‐third of high‐rise models has a key role in stability of moment frames under near‐fault pulse‐like motions. Copyright © 2014 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.     

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.     

钢筋混凝土框架结构地震易损性分析

本文以一榀7层的钢筋混凝土框架结构为分析对象,采用SAP2000建立有限元模型,选取非线性Nlink单元来模拟梁和柱两端的塑性角,其中柱铰单元采用纤维PMM铰,梁铰单元设置弯矩铰。分别指定最大层间位移角和最大顶点位移角作为结构地震需求参数,并定义了相应的破坏状态。输入8条地震动记录进行结构地震易损性分析,得到了相应的易损性曲线。该方法可为结构的抗震性能评价提供参考。     

Considering rupture directivity effects,which structures should be named ‘long‐period buildings’?

Recorded accelerograms in the regions near active faults may have specific characteristics that inclusion of their effects on the response of structures is necessary. Of particular importance are permanent displacement, i.e. fling‐step, rupture directivity pulses and high‐frequency content. Several researchers have focused on the effects of rupture directivity pulses on response of structures. They have shown that long‐period structures are severely affected by these types of excitations. However, in near‐fault regions, the question ‘which building structures are long period?’ has not been clearly and quantitatively answered. In this paper, responses of 10‐, 20‐, 30‐ and 40‐story steel structures designed based on Uniform Building Code 1997 regulations are investigated under artificial pulses produced by directivity effects. It is shown that, considering rupture directivity effects, a long‐period structure is the one that has a first‐mode period–to–pulse period ratio greater than about 0.44. Furthermore, the effects of variations in the period of the near‐fault velocity pulses on the characteristics of inelastic response of structures are examined. Consequently, analysis of the structures experiencing actual near‐fault records indicates that the pattern of response spectrum obtained from artificial pulses presents the behavior of the structure under real near‐fault earthquakes rather accurately. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

Wavelet‐Based Method for Generating Nonstationary Artificial Pulse‐Like Near‐Fault Ground Motions

A method to generate a suite of artificial near‐fault ground motion time histories for specified earthquakes is presented. A wavelet‐based nonstationary (WB‐NS) model has been employed to effectively capture the time‐varying frequency content of a particular acceleration record and continuous wavelet transform has been used to simulate the largest velocity pulse. Furthermore, an iterative procedure using discrete wavelet transform is utilized to modify an earthquake ground motion and generate energy‐compatible ground motion. Eventually, the artificial near‐fault accelerogram is achieved via the superposition of a coherent extracted velocity pulse with a random acceleration record corresponding to a WB‐NS model and multiplied by a time‐modulating envelope function. The effectiveness of the method is demonstrated by comparing the spectral response and Arias intensity curves of the simulated accelerograms with those of the real records.     

Active Tendon Control of Torsionally Irregular Structures under Near‐Fault Ground Motion Excitation

In this study, torsionally irregular single‐story and multistory structures under the effect of near‐fault ground motion excitation were controlled by active tendons. Near‐fault ground motions contain two impulsive characters. These impulsive characters are the directivity effect perpendicular to fault and the flint step parallel to fault. The structural models were simulated under bidirectional earthquake records superimposed with impulsive motions to examine the response of active control under near‐fault effects. Also, the structures were analyzed only under the effect of bidirectional impulsive pulses. The control signals were obtained by Proportional–Integral–Derivative (PID) type controllers and the parameters of the controllers were obtained by using a numerical algorithm depending on time domain analyses. Time delay effect was also considered for active control system. Different cases of orientation of active tendons were examined and the results of the single‐story structure were compared with another control strategy using frequency domain responses in the optimization process. As a conclusion, the control concept is significantly effective on reducing maximum responses in translational and rotational directions and obtaining a steady‐state response.     

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.     

Effectiveness of modified pushover analysis procedure for the estimation of seismic demands of buildings subjected to near‐fault earthquakes having forward directivity

Near‐fault ground motions with long‐period pulses have been identified as being critical in the design of structures. These motions, which have caused severe damage in recent disastrous earthquakes, are characterized by a short‐duration impulsive motion that transmits large amounts of energy into the structures at the beginning of the earthquake. In nearly all of the past near‐fault earthquakes, significant higher mode contributions have been evident, resulting in the migration of dynamic demands (i.e., drifts) from the lower to the upper stories. Due to this, the static nonlinear pushover analysis (PA) (which utilizes a load pattern proportional to the shape of the fundamental mode of vibration) may not produce accurate results when used in the analysis of structures subjected to near‐fault ground motions. The objective of this paper was to improve the accuracy of the pushover method in these situations by introducing a new load pattern into the common pushover procedure. Several PAs are performed for six existing reinforced concrete buildings that possess a variety of natural periods. Then, a comparison is made between the PA results (with four new load patterns) and those of FEMA‐356 with reference to nonlinear dynamic time‐history analyses. The comparison shows that, generally, the proposed pushover method yields better results than all FEMA‐356 PA procedures for all investigated response quantities, and is a closer match to the nonlinear time‐history responses. In general, the method is able to reproduce the essential response features providing a reasonable measure of the likely contribution of higher modes in all phases of the response. Copyright © 2009 John Wiley & Sons, Ltd.     

A new control algorithm to protect nonlinear base‐isolated structures against earthquakes

Currently, nonlinear base isolation systems are widely used in the construction of earthquake resistant structures. However, they are found to be vulnerable in near‐fault regions as a result of long‐period pulses that may exist in near‐source ground motions. Various control strategies including passive, active and semi‐active control systems have been studied in order to handle this issue. In this study, a semi‐active control algorithm based on the different performance levels anticipated from an isolated building during different levels of ground shaking was developed. The proposed performance‐based algorithm is based on a modified version of the well‐known semi‐active skyhook control algorithm. A series of analyses were performed on the base‐isolated benchmark building, suggested by the American Society of Civil Engineers committee, subject to seven pairs of scaled ground‐motion records. The results proved that the new control algorithm is successful in improving structural and nonstructural performance of isolated buildings under near‐fault earthquakes. Copyright © 2012 John Wiley & Sons, Ltd.     

浅论强震动下钢筋混凝土桥墩的残余抗剪强度

讨论了钢筋混凝土桥墩在强震动作用下的剪切机理 ,提出强震动作用下混凝土桥墩塑性铰区截面残余抗剪强度的概念。在评述了国外主要地震国家现行的规范公式的基础上 ,提出对国内现行规范关于强震动作用下钢筋混凝土桥墩抗剪验算的改进建议。     

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.     

Shake table tests of steel embedded composite columns under moderate near-fault motion

Shaking table tests were conducted on steel-embedded composite columns with core structural steel on near fault ground motion (NFGM) to investigate their seismic behavior. Four composite columns and one reinforced concrete (RC) column with diameters of 400 mm and heights of 1400 mm were tested on a shake table, and one specimen was studied with a pseudodynamic test. A sliding steel frame, called the inertial force rig, was designed and implemented to completely support the inertial mass of the column and the shaking table system. The composite column with a higher steel ratio required more volumetric confinement steel in the plastic hinge region. For the steel embedded composite columns with an axial steel ratio of less than 2%, The same amount of confining steel as in the normal reinforced concrete columns was sufficient to obtain the required ductility for the composite columns with low steel ratio. Composite columns with an embedded steel tube at the center showed better displacement ductility than RC columns with similar steel ratios, but showed smaller energy absorption capacity. The effect of the lap splice in a composite column was not significant because the spliced reinforcing bars had a lower area than RC columns with a 50% lap splice. The shake table test specimen showed much less ductility than the pseudo-dynamic test specimen and more energy dissipation capability, while the maximum capacity of the composite column was the same for the different test methods.     

Earthquake responses of RC moment frames subjected to near‐fault ground motions

There are three objectives in this paper. The first objective is to compare the dynamic behaviour of a reinforced concrete building structure subjected to near‐fault and far‐field ground motions. A twelve‐storey and a five‐storey reinforced concrete building with moment resisting frames were selected in this study. The Chi‐Chi earthquake was selected as a first set in this study to test near‐fault earthquake characteristics. Further, another earthquake record of an event at the same site was selected to test the far‐field earthquake characteristics for comparison. Through nonlinear time history analyses, the results show that the near‐fault earthquake results in much more damage than the far‐field earthquake. The second objective of this paper is to compare the predictions for ductility demand by the nonlinear time history analyses with those obtained by the pushover analysis procedure. The third objective is to explore the parameters that will more significantly affect the the building structure's dynamic response characteristics of base shear reduction and displacement amplification. Copyright © 2001 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.     

Fuzzy‐Valued Intensity Measures for Near‐Fault Pulse‐Like Ground Motions

Two fuzzy‐valued (FV) structure‐specific intensity measures (IMs), one based on squared spectral velocity and the other on inelastic spectral displacement, are presented to characterize near‐fault pulse‐like ground motions for performance‐based seismic design and assessment of concrete frame structures. The first IM is designed through fuzzying structural fundamental period to account for the period shift effect due to stiffness degradation, whereas the second IM is developed to take into account higher mode contribution in high‐rise buildings by employing a fuzzy combination of the first two or three modes for the lateral loading pattern in pushover analysis. A benchmark study of three example reinforced concrete frame structures shows that for moderate‐ to medium‐period structures, both of the proposed IMs improve prediction accuracy in comparison with the existing IMs. For short‐period structures, the FV inelastic spectral displacement is the best.     

Conditions that give rise to unusually large long period ground motions

Several ground motion conditions that are of little concern for conventional structures can be quite important for structures having natural periods longer than 1 s. Firstly, a large long-period pulse of motion, due to the effect of rupture propagation (directivity), is observed in the direction normal to the fault at strong motion stations located close to the fault. The fault-normal motions are about twice as large as the fault-parallel motions for periods longer than about 1 s. The importance of this effect, especially in strike-slip earthquakes, has probably been underestimated in existing ground motion attenuation relations because until very recently most of the near-fault strong motion data from large earthquakes have come from thrust earthquakes, for which directivity effects are usually less pronounced. Near-fault recordings of the 1971 San Fernando, 1979 Imperial Valley, 1989 Loma Prieta, and 1992 Landers, California, earthquakes show large directivity effects. We describe the systematic characteristics of the directivity effect derived from empirical analysis of recorded data and from synthetic seismogram modeling of these data. Secondly, the trapping and amplification of long period waves by sedimentary basins can generate amplitudes that are significantly larger than those calculated from simple ID models of site resonance. Using a ID model with a plane wave incident from below, it is not possible to trap energy in the near-surface sediment layers. However, for 2D and 3D basin models, waves become trapped as they enter the thickening edge of a basin, and partially escape as they reach the thinning edge of the basin. We show evidence from both recorded data and synthetic seismogram modeling of the San Fernando and Loma Prieta earthquakes that the surface waves are confined to the basin and are not recorded at rock sites adjacent to the basin. This implies that the long period ground motion response at sites within these basins is not adequately represented by the conventional approach of vertically propagating an adjacent rock recording through a ID soil column.