Seismic eccentrically braced frames

This paper provides an overview of the behavior and design of seismic-resistant eccentrically braced frames (EBFs). Basic characteristics of EBFs are first discussed. The important effects of link length on the performance of EBFs are reviewed. The capacity design concept for EGFs is then addressed. The paper addresses several design issues that appear to have been inadequately considered either in current practice or in the emerging seismic code provisions. Some important observations are provided from pseudodynamic tests of large EBFs and experimental studies of EBF subassemblages with link-to-box column connections conducted recently. Future research needs are discussed.     

Seismic behaviour of eccentrically braced frames

In the past, the analysis of the seismic behaviour of eccentrically braced frames designed in fulfilment of capacity design principles has highlighted the significant role of the link overstrength factor. The link overstrength factor is, however, unable to explain many seismic responses because it is defined on the basis of the sole elastic behaviour of structures. To achieve thorough comprehension of the seismic behaviour of eccentrically braced systems, a new parameter, called damage distribution capacity factor, is defined here. The proposed parameter is calculated on the basis of the inelastic structural behaviour and is intended to evaluate the effect of premature yielding of links on the ability of structures to develop significant inelastic behaviour of all links prior to link failure. The paper discusses the distribution of the damage distribution capacity factor in eccentrically braced structures designed in accordance with capacity design principles and the influence of this parameter on the seismic response of structures. Finally, an analytical relation is defined between overstrength factor of links, damage distribution capacity factor and plastic rotation of links to obtain quantitative evaluation of the structural damage of eccentrically braced structures upon first failure of links.     

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%.     

BRBFs的抗震性能分析

应用ANSYS软件对屈曲约束支撑钢框架(BRBFs)和普通支撑钢框架的抗震性能进行有限元数值模拟,分析了两种结构在基本烈度地震作用下和罕遇地震作用下的层位移、顶层加速度及层间相对位移等结构响应,结果表明,在小震作用下两种结构抗震性能均表现良好,但在罕遇地震作用下普通支撑钢框架由于支撑的平面外失稳,导致整个结构刚度退化,而屈曲约束支撑钢框架则能更加有效地控制结构的侧移,降低结构的地震响应。     

偏心支撑钢框架的设计

简述了偏心支撑钢框架结构的工作原理及特点,介绍了偏心支撑钢框架的设计计算方法,其中重点介绍了各杆件的内力计算：耗能梁段设计、非耗能梁段设计、支撑设计和框架柱设计,为工程设计人员提供了指导。     

有支撑钢框架离散型拓扑优化设计

为了研究离散型拓扑优化理论在实际工程中的应用,在遗传算法和渐进结构优化算法的基础上对有支撑钢框架的离散型拓扑优化设计进行了分析.通过引入拓扑变量并修改无效杆件的弹性模量,提出了一个能适用于桁架结构、刚架结构和桁架刚架混合结构的离散型拓扑优化问题统一数学列式.该统一数学列式能解决桁架拓扑优化问题中以截面面积作为设计变量而...     

Experimental investigation of steel frames braced with symmetrical pairs of y-shaped concentric bracings

Concentric bracings composed of three members arranged in y shaped geometry have been traditionally used to provide openings in braced bays. However, simultanous occurance of compression in three braces leads to instability and out of plane buckling of braces accompanied by low hysteretic energy absorption. In order to study the behavior of y-braced frames, a research program including experimental tests was conducted at BHRC† structural engineering laboratory. Quasi-static cyclic loading was applied to specimens including four full-scale two-bay frames with y-bracings of different cross sections and connection types. The bays are braced symmetrically to have a combination of tensile and compressive braces at all loading stages. The results show that out-of-plane buckling with single curvature in braces can be substituted by in plane, double curvature buckling through appropriate detailing of cross sections and connections. Thus, hysteretic energy dissipation of y-bracing is remarkably improved due to spreading of plastic strains in braces. In this paper, seismic performance of y-braced frame specimens and a reference X-braced frame are also assessed by capacity spectrum method.     

Seismic performance of buckling-restrained braced frames with varying beam-column connections

The effect of beam-column connections and brace configurations on the overall seismic response of a medium-rise bucklingrestrained braced frame (BRBF) is analytically evaluated in the present study. Two types of brace configurations (chevron and Double-X) and a combination of the moment-resisting and the non-moment-resisting beam-column connections are considered. A total of five design cases are studied for a seven-story BRBF in which a constant value of response reduction (R) factor equal to 8 is considered in the design. Nonlinear dynamic analyses are carried out for all study frames for an ensemble of forty ground motions representing the DBE and MCE hazard levels. Fragility curves are developed for all study frames considering the interstory drift ratio and residual drift ratio as the damage parameters. Results showed that a higher value of response reduction factor should be adopted in the design of BRBFs for both pinned and rigid beam-column connections. Further, in order to achieve the desired seismic performance of BRBFs, Double-X brace configurations and rigid beam-column connections at the alternate story levels should be used.     

Performance-based plastic design method for buckling-restrained braced frames

This paper presents a performance-based plastic design (PBPD) methodology for the design of buckling-restrained braced frames (BRBFs). The design base shear is obtained based on energy–work balance using pre-selected target drift and yield mechanism. Three low-to-medium rise BRBFs (3-story, 6-story and 9-story) were designed by the proposed methodology and their seismic performance was evaluated through extensive nonlinear time-history analyses using forty ground motions representing the DBE and the MCE hazard levels. Both isotropic and kinematic hardening characteristics of buckling-restrained braces were considered in the modeling of their force–deformation behaviors. All BRBFs considered in this study reached the intended performance objectives in terms of yield mechanisms and target drift levels. Since PBPD is a direct design method, no iterations were carried out to achieve the performance objectives of BRBFs.     

防屈曲支撑钢框架结构中被撑柱抗震设计探讨

通过3个算例,对采用人字形和V字形的无粘结内藏钢板支撑剪力墙(即人字形和V字形防屈曲支撑)的防屈曲支撑钢框架结构的抗震性能进行分析。重点考察大震下,支撑的轴力分布和对被撑柱所受轴力的影响。分析表明,采用结构在一阶振型下的支撑轴力分布来设计被撑柱的做法,适用于多层的防屈曲支撑钢框架结构;而对于高层的防屈曲支撑钢框架结构,高振型影响较显著,上述设计方法对被撑柱的设计较保守,有必要考虑高振型参与下的支撑轴力分布来设计被撑柱。     

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.     

防屈曲支撑框架设计方法研究

采用数值手段,研究了防屈曲支撑框架和普通中心支撑框架在性能上的差异。基于弹塑性时程分析,由大量防屈曲支撑框架算例,建立支撑框架结构在大震下的层间位移角最大值随抗侧刚度比k变化的关系曲线,按照大震不倒的要求,获得k值的合理取值范围。分别对不同k值下支撑框架结构的设计进行研究,得到相应k值下允许设计的最弱框架(即框架设计的临界点),供结构设计参考。     

考虑填充墙影响的防屈曲支撑-钢框架抗震性能分析

按照7度设防标准设计一20层平面钢框架,采用层刚度比方法布置防屈曲支撑,通过计算分析选出合理的刚度比例。采用Wael三撑杆模型、改进的Atkinson和Yan本构关系曲线模拟填充墙,并建立含填充墙的20层防屈曲支撑-钢框架模型。采用增量动力分析(IDA)方法并结合ATC委员会的提出的倒塌储备系数(CMR)评价方法,对防屈曲支撑-钢框架和含有填充墙的防屈曲支撑-钢框架进行了抗震性能对比研究。结果表明,填充墙可以显著提高结构的抗侧刚度,并能增强结构在罕遇地震下的抗倒塌能力,通过合理的布置防屈曲支撑可使7度设防钢框架基本实现8度"大震不倒"的设防目标,同时表明CMR抗震性能评估方法在长周期结构中的应用需进一步研究。     

Special braced stairs versus typical braced frames. New architectural-structural-seismic approach to stair design

This paper presents a new approach to the project of steel buildings, mainly focused on the architectural, structural, and seismic design of stairs. The objective is to design a structural stair system capable of controlling seismic damage and contributing to the bracing system of the building. The article begins with a review of the seismic standard (ATC, FEMA, and EC8) on which the current design criteria for new buildings with stairs are based. The research is based on two spatial building models (A–B) with the same bracing elements but placed differently. Reference Model A follows classical design approaches. It means, stairs are considered nonstructural elements that do not influence the seismic behavior of the building. This structure corresponds to typical braced frames (IV-CBF and EBF) according to EC8. Model B includes a stair system designed to help control the effects of inter-story drifts and inertia forces. In this case, the same bracing elements of Model A were integrated into the stair structure of Model B. A comparative seismic behavior analysis of typically braced frames (A) versus specially braced stairs (B) is presented. The research was based on the static nonlinear (pushover) analysis and the capacity spectrum method (ATC-40) according to the seismic performance levels (FEMA) and damage limitation (EC8). Finally, the braced stairs was verified via nonlinear time-history analysis in order to better capture the structural safety of the evacuation routes and their influence on the behavior of the building. This deterministic analysis of the braced stairs verified satisfactory results compared to reference bracing systems.     

基于增量动力分析的梁贯通式支撑钢框架地震易损性研究

为了得到梁贯通式支撑钢框架节点刚度及承载力设计方法,基于增量动力分析(IDA)方法,研究了节点性能对多层梁贯通式支撑钢框架地震易损性的影响,得到了不同节点刚度和承载力设置下四种模型(刚接、全强度半刚接、半强度半刚接和铰接模型)的易损性曲线,定量评价了各模型超越各极限状态的概率和倒塌储备系数.研究结果表明:无论节点刚度和...     

Optimum seismic design of concentrically braced steel frames: concepts and design procedures

A methodology is presented for optimization of the dynamic response of concentrically braced steel frames subjected to seismic excitation, based on the concept of uniform distribution of deformation. In order to obtain the optimum distribution of structural properties, an iterative optimization procedure has been adopted. In this approach, the structural properties are modified so that inefficient material is gradually shifted from strong to weak areas of a structure. This process is continued until a state of uniform deformation is achieved. It is shown that the seismic performance of such a structure is optimal, and behaves generally better than those designed by conventional methods. In order to avoid onerous analysis of the frame models, an equivalent procedure is introduced for performing the optimization procedure on the modified reduced shear-building model of the frames, which is shown to be accurate enough for design purposes.     

Seismic performance of chevron braced frames with shape memory alloy vertical shear link

Chevron braced steel frames with vertical shear link have reliable displacement dependent dampers with sufficient strength and ductility to dissipate energy of strong earthquakes through inelastic mechanisms. In this study, shape memory alloy with its superelastic behavior is utilized as material of vertical shear link to improve the ductility characteristics of the system and decrease residual displacements in the structures due to its self‐centering capability. Nonlinear time‐history, incremental dynamic analysis, and dynamic pushover analysis techniques are used to investigate the behavior of two different four‐ and eight‐story frames with steel vertical shear link (STVL) and shape memory alloy vertical shear link (SMVL) under various earthquake records. The results show that there is a negligible residual displacement in the structures with SMVL, although considerable residual displacements can be observed in the structures with STVL. For instance, when the eight‐story frame is subjected to the Northridge earthquake, a 12‐cm residual displacement is observed in the structures with STVL, whereas the structures with SMVL might experience just 3‐cm residual displacement.     

Plastic design of eccentrically braced frames, I: Moment-shear interaction

With reference to eccentrically braced frames (EBFs), the issues regarding moment-shear interaction in plastic design are examined. In particular, a procedure for computing the ultimate shear forces and end moments of intermediate links for a given collapse mechanism is presented. The requirements to be fulfilled are the yielding condition, the flow rule, the kinematic compatibility condition depending on the examined collapse mechanism and, finally, the minimisation of the internal work according to the kinematic theorem of plastic collapse.Moreover, through the definition of an equivalent moment, the link is modelled as an element with plastic hinges in simple bending. This is obtained by imposing the equivalence between the internal work developed by the actual link and the internal work corresponding to the simplified theoretical model. Therefore, moment-shear interaction in plastic design can be accounted for by means of formulations which are formally coincident with those commonly used for plastic hinges in simple bending.Finally, the kinematic theorem of plastic collapse is used to define the design requirements to be fulfilled to prevent yielding of the beam part outside of the link and yielding of the bracing element. The additional design rules to be satisfied for failure mode control of multi-storey EBFs are presented in a companion paper.     

Plastic design of eccentrically braced frames, II: Failure mode control

A design methodology aiming at the development of a collapse mechanism of a global type for eccentrically braced frames is presented in this paper. This result is of primary importance in earthquake resistant design, because partial and local failure modes are responsible of the worsening of the energy dissipation capacity leading to an increased risk of collapse under destructive ground motions.The proposed method is based on the assumption that horizontal member sections are known, as they are designed to resist internal actions due to vertical loads and design horizontal forces. Conversely, column and diagonal sections constitute the unknowns of the design problem. In particular, the presented approach also includes the influence of second order effects which are accounted for by means of the concept of mechanism equilibrium curve. The design requirements are derived by means of the kinematic theorem of plastic collapse. Column and diagonal sections are obtained by imposing that the mechanism equilibrium curve corresponding to the global mechanism has to lie below those corresponding to the undesired mechanisms within a displacement range compatible with the local ductility supply. Moreover, the introduction of the equivalent moment concept presented in a companion paper provides the proposed method with the ability to deal with short, intermediate and long links in the same manner.Aiming at the evaluation of the accuracy of the presented procedure, the inelastic performances of eccentrically braced frames designed by means of the proposed method are investigated, by means of static and dynamic non-linear analyses, in terms of collapse mechanism typology, available ductility and energy dissipation capacity.     

Influence of connection design parameters on the seismic performance of braced frames

Special concentrically braced frames (SCBFs) are commonly used lateral-load resisting systems in seismic design. In SCBFs, the braces are connected to the beams and columns by gusset plate connections, and inelastic deformation is developed through tensile yielding and inelastic post-buckling deformation of the brace. Recent experimental research has indicated that the seismic performance of SCBFs can be improved by designing the SCBF gusset plate connections with direct consideration of the seismic deformation demands and by permitting yielding in the gusset plate at select performance levels.Experimental research provides important information needed to improve SCBF behavior, but the high cost of experiments limits this benefit. To extend and better understand the experimental work, a companion analytical study was conducted. In an earlier paper, the inelastic finite element model and analysis procedure were developed and verified through detailed comparison to experimental results. In this paper, the model and analytical procedure extend the experimental results. A parametric study was conducted to examine the influence of the gusset plate and framing elements on the seismic performance of SCBFs and to calibrate and develop improved design models. The impact of the frame details, including the beam-to-column connections, the brace angles, and their inelastic deformation demands, was also explored. The results suggest that proper detailing of the connections can result in a large improvement in the frame performance.