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.     

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.     

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.     

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.     

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.     

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.     

Torsional analysis of multi‐storey building structures under horizontal load

New closed‐form formulae are presented for the torsional analysis of asymmetrical multi‐storey buildings braced by moment‐resisting (and/or braced) frames, (coupled) shear walls and cores. The analysis is based on an analogy between the bending and torsion of structural systems. A closed‐form solution is presented for the rotation of the building. The torsional behaviour is defined by three distinctive phenomena: warping torsion, Saint‐Venant torsion and the interaction between the two basic modes. Accordingly, the formula for the maximum rotation of the building consists of three parts: the warping rotation is characterized by the warping stiffness of the bracing system, St Venant rotation is associated with the St Venant stiffness of the building and the third part is responsible for the interaction. It is demonstrated that the interaction between the warping and St Venant modes is always beneficial, as it reduces the rotation of the structure. It is shown how the proposed formula for torsion can be used for the determination of the maximum deflection of multi‐storey asymmetrical building structures. The results of a comprehensive accuracy analysis demonstrate the validity of the method. A worked example is given to show the ease of use of the procedure. Copyright © 2010 John Wiley & Sons, Ltd.     

结构大变形的质点动力学分析

针对传统有限元在解决结构大变形时存在的问题,提出了一种新的解决办法——质点动力学方法,该方法把结构离散成为质点和与之相连的结构单元,质点遵循牛顿第二运动定律,由此求解质点的位移,从而得到结构的变形位移。     

Collapse behavior evaluation of asymmetric buildings subjected to bi‐directional ground motion

This paper discusses the collapse behavior of low‐rise plan‐asymmetric buildings under bi‐directional horizontal ground motions and utilizing strength and stiffness degrading nonlinear models. For this purpose, three‐dimensional three‐story and six‐story reinforced concrete frame buildings with uni‐directional mass eccentricities equal to 0% (symmetrical), 10%, 20% and 30% are subjected to nonlinear static (pushover) as well as incremental dynamic analyses using a set of far‐field two‐component ground motions and their performance are assessed on the basis of the safety margin against collapse and its probability of occurrence. Comparison of the collapse margin ratios as well as the fragility curves demonstrates significant reduction of the collapse‐level ground motion intensity with increasing eccentricity in plan. Results also indicate that current seismic design parameters including the response modification (R), overstrength (Ω) and ductility (μ) factors are not appropriate for buildings with high levels of plan eccentricity. Buildings with high values of plan eccentricity do not meet the design target life safety performance level on the basis of the calculated probability of collapse and safety margin against collapse. It appears that re‐evaluation of their design parameters is necessary. Copyright © 2015 John Wiley & Sons, Ltd.     

非规则多层建筑楼板变形动力性能分析

应用子结构方法，将楼板视为剪切弯曲深梁，用二次位移函数和一次转角函数描述楼面运动，从而建立了平面非规则房屋结构动力反应的简化计算模型。同时用时程分析方法计算分析了T型和U型平面非规则结构的楼板变形动力特性。     

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.     

大跨度桁架几何大变形结构分析的一种数值方法

基于ANSYS平台的二次开发,针对桁架结构几何大变形分析问题提出了一种有效的数值算法。主要思路为:采用两步交替迭代逐步逼近,使平衡状态与变形状态协调、统一,建立并求出变形后的平衡方程及其解;也就是说,首先由已知杆件内力建立计算节点位移的连续方程并求解,然后由已知节点位移建立计算杆件内力的平衡方程并求解,通过多次迭代求得平衡状态与变形状态协调统一的非线性大变形结构分析的精确解。本文方法在整个求解过程中仅需做一次结构分析,此在几何大变形桁架结构的优化设计中更突显其优点。本文方法对扁桁架、大跨度桁架尤其有用。通过编制相应的ANSYS二次开发程序,并由数值算例验证了本文方法的可行性、计算精确和计算效率。     

Vertical deformation analysis of a high‐rise building with high‐position connections

In this paper, a high‐rise frame‐core tube structure with strengthened stories and high‐position connections, a new landmark building in Wuhan, whose height is 238.6 m, is selected as an example. Construction simulation analysis is carried out by the finite element analysis software ETABS to study the vertical deformation and deformation difference of vertical members under the gravity load, taking the influence of construction processes and shrinkage and creep of concrete into consideration. The results show that there is a significant difference between the vertical deformation of the twin‐tower model with connections and that of the single‐tower model. Some engineering countermeasures are put forward to reduce the vertical deformation difference of the twin‐tower connected structure.     

An area‐based intensity measure for incremental dynamic analysis under two‐dimensional ground motion input

Incremental dynamic analysis (IDA) is a useful method in performance‐based earthquake engineering. IDA curves combine the intensity measure (IM) of ground motions with structural responses (as measured by engineering demand parameter) from nonlinear dynamic analysis. However, the curves display large record‐to‐record variability. And various IMs can lead to different results. Therefore, it is important to find a desirable IM to reduce the discreteness of the IDA results. So far, the studies on IM for IDA have been carried out by many scholars from scalar‐valued to vector‐valued, but few were based on 2‐dimensional ground motion input. To make the analysis more reasonable and practical as well as investigate the desirable IM under 2‐dimensional ground motion input, incremental dynamic analyses when ground motions are inputted in 2 directions should be investigated. In this paper, 2 combinational types of area‐based IM incorporating the influence of ground motion record components in secondary directions were proposed. To investigate the applicability, efficiency and desirable combinational form of the area‐based IM under 2‐dimensional ground motion input, incremental dynamic analysis were carried out using 2 reinforced concrete frames. Then the efficiency of the IMs was measured by the residual sum of squares and R². It is concluded that the area‐based IM with a combination by the square root of the sum of the squares (SRSS) method is the most efficient for IDA under 2‐dimensional ground motion input. The methods and conclusions will provide significant reference for studying IMs under 2‐dimensional ground motion input. Further research will focus on the applicability of the area‐based IM for tall buildings whose higher modes need to be considered.     

近场地震动对隔震结构的影响分析

运用脉冲分量与宽频分量叠加合成地震波的方法,来研究近场地震动的作用特性.对一5层框架隔震结构进行加载仿真分析,结果表明:脉冲分量较之宽频分量更能使结构响应产生放大作用;同时,脉冲分量的周期及成分占有量均为影响结构响应的重要因素.     

Fuzzy‐sliding mode control of nonlinear smart base‐isolated building under earthquake excitation

In this paper, the effectiveness of the fuzzy sliding mode control strategy on three‐dimensional benchmark building with smart base isolation under seismic excitation has been examined. One of the appropriate control theories for such this nonlinear system is the sliding mode control theory; discontinuous sliding mode theory has weakness such as chattering phenomena. In this paper, we used a combination of fuzzy logic and sliding mode theory for the deletion of this defect. The proposed control theory has been scrutinized by applying on lately developed nonlinear three‐dimensional base‐isolated benchmark building. This building because of the three‐dimensional nature, coalescing of lateral and torsional responses, continuity of responses of the superstructure, and base is modeled with three degrees of freedom on every floor. In this building eight actuators assigned only at the base level and in the two directions (x, y). In other words, 16 actuators are located underneath the structure. Furthermore, the base isolation system has been modeled by considering lateral coupled equations for a better examination of the performance of the system. The results indicate that reduction of control performance is remarkable. Also, utilizing proposed control theory can decrease the responses of building in two main directions and, particularly, in the rotational degree of freedom.     

Automated classification of near‐fault acceleration pulses using wavelet packets

This study proposes a new algorithm for automatically classifying two types of velocity‐pulses that are integral of a distinct acceleration pulse (acc‐pulse) or a succession of high‐frequency one‐sided acceleration spikes (non‐acc‐pulse). For achieving this, wavelet packet transform is used to filter the high‐frequency content and to extract the coherent velocity‐pulse. Then, the pulse period is unequivocally derived through the peak point method. Following the determination of the pulse‐starting (t_s) and pulse‐ending (t_e) time instants in the velocity time‐history, a local acceleration time‐history truncated by t_s and t_e is obtained. The maximum relative energy of the pulse between two adjacent zero crossings is then employed as indicator for distinguishing the two types of velocity‐pulses. The criteria for identifying acc‐pulses and non‐acc‐pulses are calibrated using a training data set of manually classified ground motions from the Next Generation Attenuation West 2 project. Finally, significance of such a classification between velocity‐pulses of different characteristics is assessed through the comparison of elastic acceleration response spectra of the two categories of pulse‐like records.