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 steel braced frames with shape memory alloy braces

In this paper, the seismic performance of steel frames equipped with superelastic SMA braces was investigated. To do so, buildings with various stories and different bracing configurations including diagonal, split X, chevron (V and inverted V) bracings were considered. Nonlinear time history analyses of steel braced frames equipped with SMA subjected to three ground motion records have been performed using OpenSees software. To evaluate the possibility of adopting this innovative bracing system and its efficiency, the dynamic responses of frames with SMA braces were compared to the ones with buckling restrained braces. After comparing the results, one can conclude that using an SMA element is an effective way to improve the dynamic response of structures subjected to earthquake excitations. Implementing the SMA braces can lead to a reduction in residual roof displacement and peak inter-story drift compare to the buckling restrained braced frames.     

人字形延性中心支撑钢框架的抗震设计与分析

讨论美国钢结构抗震设计规程中人字形延性中心支撑钢框架的抗震设计和分析。在强震作用下的基本设计理念为:钢支撑框架中的塑性变形集中于支撑上,通过支撑受拉屈服和受压屈曲来消耗能量,其他结构构件需保持弹性。结合1栋9层支撑钢框架,阐述美国钢结构抗震设计规程的相关具体规定,并得到设计建议:支撑截面的选择受到局部宽厚比的限制;框架梁截面的尺寸由强度控制,主要考虑由于支撑受拉屈服和受压屈曲所形成的竖向不平衡力的作用;框架柱截面的尺寸由考虑所有受压支撑移去时的结构模型控制。     

考虑预应力筋断裂失效的自复位支撑钢框架结构抗震性能与风险评估

为研究自复位支撑中预应力筋发生断裂失效对结构整体抗震性能的影响,以形状记忆合金(SMA)拉索自复位支撑结构为研究对象。通过理论分析典型单核和双核自复位支撑的失效机理,发现双核自复位支撑在提升变形能力的同时存在瞬时连锁失效风险。为此,设计了不考虑预应力筋失效的理想自复位支撑框架、考虑预应力筋失效的单核、双核自复位支撑框架以及SMA自复位支撑框架等4个原型结构模型。考虑预应力筋断裂失效的随机性分布特征,在远场和近场地震作用下对框架进行非线性时程分析;进一步采用IDA方法对框架进行倒塌及残余变形易损性分析,并结合场地特征进行风险评估。分析结果表明：考虑预应力筋的断裂失效会显著增加框架倒塌的风险,在服役期风险评估中,其倒塌概率是理想情况下的5倍左右,残余变形超越概率最高为理想情况下的6倍;SMA自复位支撑具有更大的变形能力以及额外的耗能能力,在抗倒塌和抑制残余变形方面表现更为良好。     

Seismic performance of Y‐type eccentrically braced frames combined with high‐strength steel based on performance‐based seismic design

In the Y‐type eccentrically braced frame structures, the links as fuses are generally located outside the beams; the links can be easily repairable or replaceable after earthquake without obvious damage in the slab and beam. The non‐dissipative member (beams, braces, and columns) in the Y‐type eccentrically braced frames are overestimated designed to ensure adequate plastic deformation of links with dissipating sufficient energy. However, the traditionally code design not only wastes steel but also limits the application of eccentrically braced frames. In this paper, Y‐type eccentrically braced steel frames with high‐strength steel is proposed; links and braces are fabricated with Q345 steel (the nominal yield stress is 345 MPa); the beams and columns are fabricated with high‐strength steel. The usage of high‐strength steel effectively decreases the cross sections of structural members as well as reduces the construction cost. The performance‐based seismic design of eccentrically braced frames was proposed to achieve the ideal failure mode and the same objective. Based on this method, four groups Y‐type eccentrically braced frames of 5‐story, 10‐story, 15‐story, and 20‐story models with ideal failure modes were designed, and each group includes Y‐type eccentrically braced frames with ordinary steel and Y‐type eccentrically braced frames with high‐strength steel. Nonlinear pushover and nonlinear dynamic analyses were performed on all prototypes, and the near‐fault and far‐fault ground motions are considered. The bearing capacity, lateral stiffness, story drift, link rotations, and failure modes were compared. The results indicated that Y‐type eccentrically braced frames with high‐strength steel have a similar bearing capacity to ordinary steel; however, the lateral stiffness of Y‐type eccentrically braced frames with high‐strength steel is smaller. Similar failure modes and story drift distribution of the prototype structures designed using the performance‐based seismic design method are performed under rare earthquake conditions.     

Stochastic optimisation of buckling restrained braced frames under seismic loading

A stochastic optimisation procedure is proposed for the design of low- and mid-rise buckling restrained braced frames subject to seismic loading. The seismic excitation is represented as a zero-mean nonstationary filtered white noise. The Bouc–Wen model is chosen to represent the hysteretic behaviour of the buckling restrained braces. The equivalent linearisation method is employed to determine the second-order statistics of structural responses from the non-linear system. Three seismic intensity levels are considered in this study, which are associated to earthquakes with different probability of occurrence during the building’s lifecycle. It was observed that the optimal design that minimises the maximum ductility demand produces a more uniform distribution of energy dissipation and avoids soft-storey mechanisms; therefore, this design objective is considered to be a more reasonable optimisation objective for the design of buckling restrained braced frames. For higher rise structures, buckling restrained braces may experience over-dimensioning in the top stories, which means that dissipation will not occur. Thus, an upper bound constraint for the stiffness design of the buckling restrained braces is taken into account.     

Optimization of link member of eccentrically braced frames for maximum energy dissipation

An optimization method is presented for design of an eccentrically braced frame (EBF), which is used as a passive control device for seismic design of building frames. The link member between the connections of beams and braces of EBF is reinforced with stiffeners in order to improve its stiffness and plastic deformation capacity. We present a method for optimizing the locations and thicknesses of the stiffeners of the link member. The optimal solutions are found using a heuristic approach called tabu search. The objective function is the plastic dissipated energy before failure. The deformation of the link member under static cyclic loads is simulated using a finite element analysis software package. It is demonstrated in the numerical examples that the dissipated energy can be increased through optimization within small number of analyses.     

Behavior and design of reinforced concrete frames retrofitted with steel bracing against progressive collapse

Steel bracing is able to improve progressive collapse resistance of reinforced concrete (RC) frames, but the bracing design is typically based on seismic retrofitting or lateral stability. There is no approach for design of steel bracing against progressive collapse. To this end, a retrofitting approach with steel braces is proposed based on analysis of macro finite element (FE) models with fiber beam elements. The FE models were initially validated through the experimental results of a braced frame and then used to investigate the effects of pertinent parameters on the progressive collapse resistance of planar frames. The results suggest the braces should be placed at the top story. Thereafter, macro FE models are built to investigate the dynamic responses of the three‐dimensional prototype RC frames under different column removal scenarios (CRS) and show the necessity of retrofitting. Accordingly, the design approach of steel bracing is proposed with incremental dynamic analysis (IDA) and assuming independent contribution of braces and frames to resistance. Finally, the fragility analysis of the frames under a corner‐penultimate‐exterior CRS is conducted through IDA and Monte Carlo simulation, and the results confirm the validity of the proposed design approach for retrofitting RC frames.     

带支撑胶合木框架抗震性能试验研究

针对胶合木框架侧向位移不易满足抗震要求这一问题,研究了增设人字形胶合木支撑和铝合金屈曲约束支撑的带支撑胶合木框架的抗震性能。对纯胶合木梁柱框架和3个增设支撑胶合木框架进行了低周反复加载试验,分析了4个胶合木框架试件的水平承载力、耗能能力、刚度退化、转角变形和木支撑应变。结果表明：增设人字形木支撑和铝合金屈曲约束支撑均可以显著提高胶合木框架的承载力、耗能能力和刚度;支撑端部连接形式对胶合木框架的抗震性能有一定影响;增设支撑的3个胶合木框架试件均在支撑或支撑连接处发生破坏,胶合木框架主体并未发生明显损伤,两类支撑均很好地起到了第一道抗震防线的作用,保证了主体框架的安全。胶合木框架数值模拟和木支撑截面尺寸参数分析结果表明,经柱截面尺寸修正后的有限元模型针对框架抗侧刚度和承载力具有较好的预测精度。     

Seismic collapse margin of steel frame structures with symmetrically placed concentric braces

This paper focuses on the seismic collapse assessment conducted on the steel frame structures with symmetrically placed concentric braces using collapse margin ratio (CMR). The main issue with concentrically braced frames (CBFs) is the existence of a vast number of possible combinations in their topology. Therefore, selecting the most efficient group of retrofit schemes (RS), regarding the overall performance index (PI) of the structure requires a simple and quick method of brace manipulation, specifically, removal and/or addition, and a series of basic symmetrical principles in the retrofitting process. However, identifying the optimal RS for new or existent structures requires an evaluation of the optimal CMR value and the retrofitting cost as well. The influence of both cross-section and configuration type of the bracing system provides new insights into the collapse margin. Finally, this paper points out new parameters that should not be neglected in the retrofit design of bracing systems for steel frame structures.     

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.     

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.     

优化偏心支撑框架的连接件以获得最大耗能能力

给出偏心支撑框架(EBF)的优化设计方法,将其作为被动控制手段应用于建筑结构的抗震设计中。在EBF梁与支撑的连接件处设置加劲肋以提高其刚度和塑性变形能力。给出连接件加劲肋位置及厚度的优化设计方法,采用Tabu搜索算法求解最优解,其目标函数为破坏前的塑性能量耗散。采用有限元软件计算静态循环荷载作用下连接件的变形,结果显示,通过微单元优化可以提高连接件的耗能能力。     

Progressive collapse analysis of seismically designed steel braced frames

The progressive collapse resistance of seismically designed steel braced frames is investigated using validated computational simulation models. Two types of braced systems are considered: namely, special concentrically braced frames and eccentrically braced frames. The study is conducted on previously designed 10-story prototype buildings by applying the alternate path method. In this methodology, critical columns and adjacent braces, if present, are instantaneously removed from an analysis model and the ability of the model to successfully absorb member loss is investigated. Member removal in this manner is intended to represent a situation where an extreme event or abnormal load destroys the member. The simulation results show that while both systems benefit from placement of the seismically designed frames on the perimeter of the building, the eccentrically braced frame is less vulnerable to progressive collapse than the special concentrically braced frame. Improvement in behavior is due to improved system and member layouts in the former compared to the latter rather than the use of more stringent seismic detailing.     

Seismic response of stainless steel braced frames

The present paper investigates the feasibility of the application of stainless steel (SS) in the seismic design of braced frames, either concentrically (CBFs) or eccentrically (EBFs) braced. A sample of regular multi-storey CBFs and EBFs was designed in compliance with modern seismic standards based on capacity-design rules. The results of pushover and inelastic response history analyses demonstrate that systems employing SSs exhibit enhanced plastic deformations and excellent energy absorbing capacity with respect to mild steel braced frames. The augmented strain hardening of SS, which is nearly twice that of carbon steels, is beneficial to prevent local buckling in steel members, especially those subjected to high axial compression. The performed analyses also demonstrate that in CBFs with SS braces and columns the increase in overstrength is about 40% with respect to the configuration in mild steel. For EBFs, the use of SS in the diagonals or in braces and links increases the global overstrength of the lateral resisting system by 20%. When the EBFs employ braces and columns in SS the increase can be as high as 50%.     

Progressive collapse resisting capacity of braced frames

In this study, the progressive collapse potential of braced frames was investigated using nonlinear static and dynamic analyses. Eight different bracing types were considered and their performances were compared with those of a special moment‐resisting frame designed with the same design load. According to the pushdown analysis results, most braced frames designed per current design codes satisfied the design guidelines for progressive collapse initiated by loss of a first story interior column; however, most model structures showed brittle failure mode caused by buckling of braces and columns. Among the braced frames considered, the inverted‐V type braced frames showed superior ductile behaviour during progressive collapse. The nonlinear dynamic analysis results showed that all the braced structures remained in stable condition after sudden removal of a column, and their deflections were less than that of the moment‐resisting frame. Copyright © 2009 John Wiley & Sons, Ltd.     

Dual‐steel frame consisting of moment‐resisting frame and shape memory alloy braces subjected to near‐field earthquakes

In this paper, concentric braced frames are combined with moment‐resisting frame (MRF) as a dual system subjected to near‐field (NF) pulse‐like and far‐field ground motions. The braced frame in this system configuration consists of steel buckling‐restrained braces (BRB model), braces with shape memory alloy (SMA model), or combination of BRB and SMA braces (COMBINED model). Some prototype structures of the proposed systems are designed according to the code recommendations. Then, the nonlinear models of the considered structures are developed in SeismoStruct software, and nonlinear time history analysis (NLTHA) is implemented. NLTHA is performed subjected to earthquake record sets at maximum considered earthquake (MCE) and design base earthquake (DBE) levels, and responses of the systems are investigated and compared with each other. Among the examined models, the SMA and COMBINED models exceed the CP level subjected to NF‐MCE record set. Therefore, more investigation is needed for using short‐segment SMA braces in the dual‐steel frames in NF area.     

长耗能梁-偏心支撑机制对中心支撑钢框架结构抗震性能的影响

为研究地震作用下人字形中心支撑钢框架结构因支撑的部分失效导致的长耗能梁-偏心支撑机制对结构抗震性能的影响,基于ABAQUS建立了6层人字形中心支撑钢框架结构数值模型,开展了增量动力分析与易损性分析,对比了考虑与不考虑长耗能梁-偏心支撑机制时结构抗倒塌性能的差异,分析了结构的损伤演化过程。结果表明:长耗能梁-偏心机制改变了结构的失效过程,抑制了薄弱层的产生与发展,对结构抗震性能具有显著影响,算例结构的倒塌富余度提高20%以上; 梁的抗弯刚度对长耗能梁-偏心支撑机制具有一定影响,较大的刚度将不利于该机制的形成,降低了结构的抗倒塌能力; 在人字形中心支撑钢框架结构体系的设计与分析中,宜考虑长耗能梁-偏心支撑机制,否则将低估结构的抗倒塌性能。     

Analytical expressions for preliminary design of dissipative bracing systems in steel frames

In this paper a direct displacement-based design (DDBD) method for seismic design of steel frames equipped with dissipative braces is proposed. Attention is focused on concentric braced steel frames with pinned beam-to-column joints in which the bracing system (with viscoelastic or elastoplastic dissipative devices) is the main seismic resistant component. The proposed design method uses an equivalent continuous model where flexural deformability and shear deformability are related respectively to columns and diagonals of the bracing system. In this way, analytical expressions of the required flexural and shear stiffness distributions are obtained. These expressions are quite simple and can be conveniently used in preliminary design of dissipative diagonal braces and columns. Examples are shown for steel frames with dissipative braces based on elastomeric dampers (viscoelastic devices) and steel frames with buckling-restrained braces (elastoplastic devices). Results of time history analyses are illustrated and discussed in order to evaluate the effectiveness of the proposed DDBD procedure.     

Dissipative (INERD) Verbindungen für Stahltragwerke in Erdbebengebieten

Dissipative (INERD) connections for seismic resistant steel braced frames. Innovative dissipative (INERD) connections were developed for seismic resistant steel braced frames. The dissipative zones in such frames are the connections, while the other parts of the structure are protected against inelastic deformations and instability phenomena. The braces and the columns are connected with eye‐bars and a pin running through them, which is the dissipative element. Experimental and theoretical investigations were performed to study the monotonic and cyclic behavior of the connections. Simple rules were developed for the design of the connection and the adjacent elements.