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.     

Incremental dynamic analysis of steel frames equipped with NiTi shape memory alloy braces

This paper determines the seismic performance of four‐storey concentrically braced frames equipped with either steel buckling‐restrained braces or buckling‐restrained superelastic shape memory alloy (SMA) braces through incremental dynamic analysis. The incremental dynamic analysis technique is used to examine the behaviour of four‐storey braced frames with four different bracing configurations (including diagonal, split‐X, chevron‐V and inverted‐V) under 20 different ground motion records. The study reveals a satisfactory performance at the design intensity level for both types of braced frames. The results show that the SMA braces lead to a uniform distribution of inelastic response over the height of the buildings, as well as mitigating seismic response in terms of maximum inter‐storey drift and residual roof displacement. By comparing the responses of SMA and buckling‐restrained braced frames under higher intensities of earthquake loading, it is found that the SMA braces can be more beneficial especially under severe ground motion excitations. 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.     

Fundamental period of irregular moment‐resisting steel frame structures

The determination of the fundamental period of vibration of a structure is essential to earthquake design. Current code equations such as American Society of Civil Engineers Standard (ASCE) 7‐10 provide formulas for the approximate period of moment‐resisting frames (MRFs), which are dependent only on the height of the structure or number of stories. Such a formulation is overly conservative and unable to account for structures with geometric irregularities. This study investigated the fundamental periods of MRFs with varying geometric irregularities. The fundamental periods based on vibration theory for each example were compared with empirical equations, including current code equations as well as equations proposed in recent literature. New equations are proposed for the fundamental periods of MRFs, which take into account vertical and horizontal irregularities. Through statistical comparison, it was found that a three‐variable power model that is able to account for irregularities resulted in a better fit to the Rayleigh data than equations that were dependent on height only. The proposed equations were validated through a comparison of available measured period data for MRFs. They will allow design engineers to quickly estimate the fundamental period of MRF structures by taking into account irregularities. Copyright © 2013 John Wiley & Sons, Ltd.     

Multihazard fragility assessment of steel‐concrete composite frame structures with buckling‐restrained braces subjected to combined earthquake and wind

Engineering structures may inevitably be subjected to multiple natural hazards (such as earthquakes and winds) during their life cycles. This paper presents an efficient multihazard fragility methodology based on the structural demand models. The approach is applied to two steel‐concrete composite frame structures (SCCFSs), with and without buckling‐restrained braces (BRBs), aiming to evaluate the effect of BRBs on controlling the structural responses and fragilities under the combined earthquake and wind loads. In total, 120 earthquake records are selected, and 120 sets of wind drag force time histories are simulated by considering the spatial variation along the height of the exemplar building. The combined “earthquake–wind” events are stochastically assembled, in which the intensities of these two hazards are modeled using the Monte Carlo simulation. The OpenSees platform is employed to calculate the dynamic responses of the SCCFSs with and without BRBs under simultaneous earthquake and wind loads. The goodness of fits of the first‐, second‐, and third‐order polynomial in predicting the structural demand are evaluated, and the optimal polynomial is employed to generate the multihazard fragility surfaces at different damage states. The numerical results indicate that the structural responses and fragilities under the combined earthquake and wind are higher than those under an individual hazard, while the influencing extent varies with the relative intensities of these two hazards. The impact of multiple hazards and the control effect of BRBs on the structural responses and fragilities are systematically quantified and discussed in details.     

Fundamental period of masonry infilled moment‐resisting steel frame buildings

It is known to be a necessary practice to study the effect of infill walls on fundamental period and revise the equations included in the current building codes. In this research, the fundamental period of vibration, by modeling an infill wall as a compression strut in three‐dimensional numerical models of moment‐resisting steel frames, was studied. Areas studied in the present research were structures of symmetric infill wall arrangements wherein the effects of various infill percentage, various moduli of elasticity of masonry wall materials, and different numbers of spans were investigated. The mean fundamental period obtained by the formula provided in FEMA450 was 1.3 times greater than the value resulted from the numerical modeling of infilled moment‐resisting steel frames. Moreover, the corresponding coefficient of determination, R², of the curve fitted to numerically obtained periods was found to be 0.65 when the fundamental period of the infilled frames was only estimated based on the height of buildings, indicating a great deal of variation within the results. A new formula was proposed to estimate the fundamental period based on the structure height, infill wall percentage, and modulus of elasticity, at an acceptable level of accuracy so that the R² coefficient was determined to be 0.99 for the results of the proposed formula.     

Design and seismic response of tall chevron panel buckling‐restrained braced steel frames

The numerical analysis of the seismic performance for tall chevron panel buckling‐restrained braced steel frames (PBRBFs) under small and strong earthquake excitations has been carried out to investigate a capacity design procedure for chevron PBRBFs and to examine the effects of axial strength distribution of braces along the height of buildings, vertical supports of braces for the braced beams and the overstrength of braces on the seismic response of PBRBFs. It revealed that the chevron braces that remained elastic can actually provide the vertical supports for the braced beams. Under severe earthquake excitations, the vertical supports deteriorated greatly after braces yielding. The PBRBFs designed by omitting vertical supports of braces for the braced beams and considering the overstrength of braces exhibited superior performance with smaller plastic deformations for braced beams and reduction in ductility demands for panel buckling‐restrained braces (PBRBs) as compared with the others. The distribution of yielding for PBRBs in 10‐story buildings verified that the participation from the higher modes is not very remarkable and that the capacity design based on the first‐mode response can be considered for multistory PBRBFs. Moreover, on the basis of the analysis results of the 30‐story PBRBF, the participation of the higher modes should be taken into account for high‐rise PBRBFs. Copyright © 2011 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.     

Experimental investigation on buckling‐restrained braces using mortar‐filled steel tubes with steel lining channels

A new type of buckling‐restrained braces (BRBs) using mortar‐filled steel tubes with steel lining channels is proposed to reduce the frictional force between the core member and the restraining member. After covering the core plate with the unbonding material, steel lining channels are set on the surface along the longitudinal direction, thus transforming the contact property between the core plate and the restraining member from steel–mortar interface to steel–steel interface. First, comparative cyclic tests are conducted on two new BRBs with steel lining channels and one conventional BRB. All the specimens exhibit stable hysteretic performance without visible global or local instability prior to failure. It is confirmed that the steel lining channels can improve the interface evenness between the core plate and the restraining member, reduce the compression strength adjustment factor, and improve the low‐cycle fatigue behavior and energy dissipation capacity of the BRB. Furthermore, the analytical model for the local stability of the restraining tube is proposed when the core plate buckles about the strong axis, and the corresponding practical design criteria are provided. The influence of various core plate layouts on the local stability of the restraining tube is finally investigated.     

基于形状记忆合金阻尼器的混凝土框架结构减振分析

针对形状记忆合金具有形状记忆效应、超弹性效应和高阻尼特性,利用其超弹性制成的形状记忆合金阻尼器性能优良的耗能减振装置,可以有效地控制结构的动力响应.本文采用ANSYS通用有限元软件,分别建立无阻尼器混凝土框架结构和在相应框架结构各层角点处加形状记忆合金阻尼器两种不同的模型,进行模态分析,再数值模拟加载地震波载荷作用下结构的瞬态动力响应.模态分析结果表明,形状记忆合金阻尼器作为一种记忆和超弹性材料对混凝土框架结构具有良好的减振效果,与理论结果一致.在加阻尼器的单元上,固有频率增大,刚度增大,整体框架结构刚度矩阵减小,同时,通过调整阻尼器的位置,可以明显改变刚度大小,说明阻尼器具有明显的耗能减振效应.瞬态分析计算结果表明,加形状记忆合金阻尼器模型各层位移时程曲线都有不同程度的减小,振幅也有不同程度的衰减;其中位移最大值比无阻尼器模型减小了42%,振幅衰减了31%.但跟其他粘弹性阻尼器比较,形状记忆合金阻尼器作为一种优质高价材料,在混凝土框架结构中的减振效果无明显优势.通过对计算结果的分析,预计该阻尼器适用于大变形钢结构体系.     

Effect of gusset connection configurations on frame–gusset interaction in steel buckling‐restrained braced frame

Although buckling restrained braces (BRBs) are commonly applied in seismic buildings to mitigate structural damage, their performance was often limited by rupture of the corner gusset connections due to additional frame action. This issue may be resolved by alternative gusset connections to mitigate the frame–gusset interaction. In this study, commonly used procedures for design of the traditional gusset connection are reviewed, followed by a case study on the effect of frame action on the structural behavior of these gusset connections in steel frames with BRBs. Inspired by these analysis, two different strategies, aiming at releasing frame–gusset shear interaction using sliding gusset connection or reducing normal interaction using dual gusset plates, are tried to mitigate the frame action effects. Finite element analysis is conducted on steel frame subassemblages with/without BRBs to examine the effect of different gusset connections on the structural behavior of these framing systems. It shows that the sliding gusset connection shows beneficial effect in reducing the frame action, having much smaller stress responses on the gusset interfaces, as well as smaller shear force and plastic responses on the framing system. Thus, it becomes a promising gusset connection for improved seismic performance of the steel framing system with brace‐type dampers.     

采用马氏体镍钛形状记忆合金螺杆的钢框架梁柱节点滞回性能试验研究

为研究采用马氏体镍钛形状记忆合金螺杆的钢框架梁柱节点力学性能和耗能能力,对马氏体镍钛形状记忆合金材料进行了单调拉伸和形状记忆效应试验,对采用马氏体镍钛形状记忆合金螺杆的梁柱节点进行了拟静力试验和加热修复后的再次试验。研究结果表明：镍钛合金具有较大的可回复应变,从而能够适应节点不同程度损伤状态的性能修复要求;采用马氏体镍钛形状记忆合金螺杆的钢框架梁柱节点具有稳定的滞回性能和耗能能力;节点具有后期强化效应,能够一定程度上弥补或消除节点部位受损元件引起的节点性能退化;镍钛合金独特的形状记忆效应和优越的疲劳性能确保了镍钛形状记忆合金螺杆的可重复使用和复原节点性能的能力;螺杆应变过大将导致形状记忆效应衰退,加热修复后将产生残余变形。最后,通过与角钢连接和钢螺杆连接的钢框架梁柱节点的性能对比分析,发现采用马氏体镍钛形状记忆合金螺杆的钢框架梁柱节点具有更好的延性和能量耗散能力。     

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.     

形状记忆合金补强损伤钢板疲劳性能研究

钢结构在长期服役过程中,受荷载和环境作用,可能发生结构损伤及性能退化。形状记忆合金（shape memory alloy,SMA）因其特殊的形状记忆效应,可以为钢结构施加预应力,提升结构疲劳性能。以形状记忆合金补强的含损伤钢板为分析对象,采用线弹性断裂力学对补强后产生的钢板疲劳性能进行理论分析。与已有研究中的试验结果相比,钢板的疲劳寿命与试验结果最大相差不超过10%。同时,采用理论模型展开参数分析,研究初始损伤程度和SMA受温度作用产生的恢复应力对补强体系的影响。结果表明：对于不同的初始损伤程度,SMA均能够发挥很好的补强作用;经SMA补强后,钢板的疲劳寿命最大可提高362%;当SMA产生的恢复应力大于200 MPa时,可降低加载时钢板实际承担的应力幅,延缓裂纹扩展。     

Wind‐induced vibration control of tall buildings using hybrid buckling‐restrained braces

A buckling‐restrained brace (BRB) is a system with excellent earthquake‐proof performance, but it does not dissipate energies caused by the load from weak earthquakes or winds. A hybrid BRB (H‐BRB), which improved the performance of the BRB, is a type of composite damper system consisting of a BRB and a viscoelastic damper. To explain the wind‐induced vibration control performance of H‐BRB, a 40‐story steel building was designed and used as an analysis model in this study, on the basis of the damping ratio from a structural performance test, using normal steel braces, BRB and H‐BRB. In addition, to evaluate the optimal location of H‐BRB, a time‐history analysis of four models was conducted in the study. For such time‐history analysis, wind‐load data in a 10‐year recurrence interval, which were calculated from the wind tunnel test, were used. The result of the time‐history analysis showed that H‐BRB is effective in improving both the lateral stiffness and serviceability of a building using the existing BRB. It also confirmed that it is most effective to position H‐BRBs mainly on the lower stories. Copyright © 2012 John Wiley & Sons, Ltd.     

基于超弹性形状记忆合金的钢结构抗震研究进展

提高城市强震后恢复韧力(resilience)是新一代结构抗震研究的重点,而结构自复位则是实现功能可恢复的主要方式之一。随着材料科学的发展,形状记忆合金(SMA)在高性能结构设计中得到应用。以钢结构被动抗震领域为对象,从SMA基本元件、构件、结构体系三个层面进行阐述,对最新研究进展、机遇与挑战进行分析与总结。元件层面,对SMA丝材、棒材、绞线、螺旋弹簧、碟形弹簧和环形弹簧的基本特性和应用潜力进行了阐述,分析其承载力、变形、自复位及耗能能力等;构件层面,总结了SMA阻尼器、支撑、梁柱节点以及隔震装置的构造形式、工作机理与力学性态;在此基础上,分析了SMA支撑钢框架与SMA节点钢框架的体系构造、动力响应规律、耗能机制、自复位机制和变形协调特征,以及目前研究存在的不足,以期为SMA在结构抗震领域进一步的实际应用提供参考。     

Subassemblage test of buckling‐restrained braces with H‐shaped steel core

In this study, a subassemblage test was performed using buckling‐restrained braces with an H‐shaped core element, which have been proven in a previous uniaxial component test to have good performance. The loading protocol prescribed the quasi‐static cyclic pattern with stepwise incremental displacement amplitude. Two different end connections (bolted connection and pin connection) and two different buckling‐restrained mechanisms (concrete‐filled tube and hollow steel tube) were examined as the test parameters. The performance of the specimen was evaluated by comparing the test results with the recommended provisions for buckling‐restrained braces. The test results showed that the compression strength capacity of buckling‐restrained brace (BRB) with in‐filled concrete increased by about 10% compared with BRB without in‐filled concrete. According to test result at same story drift of 2D_bm, structural performance of pin connection specimen without bolt slippage is superior to bolted connection specimen. Also, bolted connection specimens showed similar performance for total energy dissipation and cumulative plastic ductility, regardless of the connection types and the existence of concrete filling. Copyright © 2014 John Wiley & Sons, Ltd.     

Seismic retrofit of a high‐rise steel moment‐resisting frame using fluid viscous dampers

The Tall Building Initiative project of Pacific Earthquake Engineering Research Center has been expanded to investigate the seismic performance and possible retrofit of existing tall buildings. A candidate 35‐story steel building with representative details from the early 1970s was analyzed following several guidelines, which revealed a wide range of potential inadequacies. Thus, a two‐level retrofit approach was examined that focused on achieving the collapse prevention limit state under the major basic safety earthquake (BSE‐2E) hazard level prescribed by ASCE 41. This paper focused on a Level‐2 retrofit that used fluid viscous dampers to augment Level‐1 retrofits. For this approach, feasible damper locations and overall effective damping ratios were first evaluated through a series of preliminary studies, and then a two‐phase design method was used to refine the distribution and mechanical properties of the dampers. Thorough assessments of the refined design were carried out following several design guidelines, including ASCE 41, FEMA 351, and FEMA P‐58. The results indicated that the proposed retrofit method of using fluid viscous dampers could achieve the retrofit goal and provide a cost‐effective means of improving the structural behavior and reducing economic losses in a major seismic event.     

Behavior of moment‐resisting tall steel structures exposed to a vertically traveling post‐earthquake fire

An investigation is performed on a 10‐story moment‐resisting steel structure designed to the Life Safety level of performance of the Federal Emergency Management Agency (FEMA) 356 code by exposing it to post‐earthquake fire (PEF). The fire curve is accounted for using the natural fire method, and the fire is subjected to the floors vertically in three different scenarios: (a) fire initiated from the first floor, (b) fire initiated from the fourth floor and (c) fire initiated from the seventh floor. A delay of 5 minutes and 25 minutes are considered for spreading the fire between the floors. To make a comparison between the results, a concurrent fire is also considered for the fire analysis. The results indicate that the PEF resistance of the frame exposed to the concurrent fire and the 5 minutes delay is much lower than that with a delay of 25 minutes. The results also show that subjecting the frame to a delayed fire of 25 minutes leads to the collapse of the frame during cooling phase, whereas in the other scenarios, the frame collapses during heating phase. As a result, more considerations need to be implemented in the codes on top of that for the PEF itself and that is the appropriate rate of spread of fire between floors. Copyright © 2013 John Wiley & Sons, Ltd.     

Seismic performance of a reinforced concrete frame equipped with a double‐stage yield buckling restrained brace

In this paper, a double‐stage yield buckling restrained brace (DYB) is proposed to prevent soft story collapse in structures subjected to strong earthquakes. The DYB consists of two conventional buckling restrained braces (BRBs) with different yield forces: a large BRB and a small BRB. The deformation of the small BRB has an upper threshold value, controlled by a special mechanical mechanism. Once the force acting on the DYB exceeds the yield force of the small BRB, the small BRB yields and the deformation concentrates on the small BRB. When the deformation of the small BRB reaches the threshold value, the small BRB stops deforming. If the force of the DYB continues to increase and exceeds the yield force of the large BRB, the large BRB yields and most of the deformation takes place in the large BRB. In this way, the DYB achieves a double‐stage yield mechanism. To demonstrate the effectiveness of the DYB, a model of a six‐story reinforced concrete frame equipped with DYBs was constructed using the finite element software ABAQUS, and its seismic performance was analyzed. The double‐stage yield mechanism of the DYB was simulated by a gap element. To investigate the effect of DYBs on the seismic performance of the structure, four different models were built: an unbraced frame, frame with DYB, frame with small BRB, and frame with large BRB. The results of the pushover and time‐series analyses showed that the DYB effectively controlled the deformation pattern of the structures, and prevented weak story collapse.