BRBF response modification factor

In this paper, overstrength, ductility and response modification factor of Buckling Restrained Braced frames were evaluated. To do so, buildings with various stories and different bracing configuration including diagonal, split X, chevron (V and Inverted V) bracings were considered. Static pushover analysis, nonlinear incremental dynamic analysis and linear dynamic analysis have been performed using Opensees software. The effects of some parameters influencing response modification factor, including the height of the building and the type of bracing system, were investigated. In this article seismic response modification factor for each of bracing systems has been determined separately and tentative values of 8.35 and 12 has been suggested for ultimate limit state and allowable stress design methods.     

Analytical investigation of response modification (behaviour) factor,R, for reinforced concrete frames rehabilitated by steel chevron bracing

Steel bracing of reinforced concrete (RC) frames has received noticeable attention in recent years as a retrofitting measure to increase the shear capacity of the existing RC buildings. In order to evaluate the seismic behaviour of steel-braced RC frames, some key response parameters, including the ductility and the overstrength factors, should first be determined. These two parameters are incorporated in structural design through a force reduction or a response modification factor. In this paper, the ductility and the overstrength factors as well as the response modification factor (or seismic behaviour factor) for steel chevron-braced RC frames have been evaluated by performing inelastic pushover analyses of brace-frame systems of different heights and configurations. The effects of some parameters influencing the value of behaviour factor, including the height of the frame and share of bracing system from the applied lateral load have been investigated. It is found that the latter parameter has a more localised effect on the R values and its influence does not warrant generalisation at this stage. However, the height of this type of lateral load-resisting system has a profound effect on the R factor, as it directly affects the ductility capacity of the dual system. Finally, based on the findings presented in the article, tentative R values have been proposed for steel chevron-braced moment-resisting RC frame dual systems for different ductility demands and compared with different type of bracing systems.     

Comparative study of special and ordinary braced frames

The collapse probability of ductile and non‐ductile concentrically braced frames was investigated using nonlinear dynamic response analysis. Two buildings with three and nine stories located in Boston and Los Angeles, respectively, were designed and subjected to ground motions from the areas. In Boston area, three‐story and nine‐story buildings were designed as ordinary concentrically braced frame with response modification reduction factor R equal to 3 1/4 to be considered as non‐ductile structural systems; comparatively, in Los Angeles area, three‐story and nine‐story buildings were designed as special concentrically braced frame with response modification reduction factor R equal to 6 to be considered as ductile structural systems. In order to evaluate the performance of ductile and non‐ductile concentrically braced frames in moderate and severe seismic regions, ATC‐63 would be used as reference to assess the seismic behaviors. Evaluation approach recommended by ATC‐63 was adopted, and hundreds of nonlinear dynamic analyses were performed. Through alternating the scale factors of designated ground motions, median of structural collapse intensity was presented for each structure. By observing the results of statistical performance assessment, the seismic performance of the systems was evaluated, and some observations are made based on the study. Copyright © 2012 John Wiley & Sons, Ltd.     

Buckling-restrained braced frame connection performance

Large-scale experimental studies of buckling-restrained braced frames (BRBFs) have shown that although they display good overall seismic performance, they may have limitations due to connection failure modes that do not allow the braces to realize their full ductility capacity. These experimental results motivate further investigation of BRBF connection behavior. In this study, nonlinear finite element models are used to study BRBF beam–column–brace connections. The models focus on a one-story subassembly extracted from a previously-tested, four-story BRBF. After the baseline finite element analysis results are verified with experimental data, parametric studies varying the connection configuration are used to assess the key factors influencing performance. Connection configuration is shown to have a significant impact on global system response and localized connection demands.     

Multi-level enhancement of structural behavior of bracing systems with coupling beams at floor level

Despite good rigidity, braced frames have weak nonlinear behavior and inadequate distribution of ductility in stories, which cause significant structural damage. In this research, a seismic resistant system called coupled concentrically braced frame (CCBF) is developed to enhance the performance of braced frames by coupling them with a beam. In this case, the coupling beams are the primary source for ductility of the system, and after their yielding in more severe earthquakes, the structure continues to benefit from the ductility of the braces as the secondary source; therefore, the system has two-level behavior caused by different probable seismic excitations. In this case, in addition to maintaining the stiffness of the two concentrically braced frames, the coupling beams resist against the movement of the braced frames, and as a result, the stiffness of the system is increased. Therefore, lighter elements can be used to resist lateral loads. Linear and nonlinear analyses of CCBF, and its comparison with other braced frames, indicate that participation of the coupling beams provides an adequate stiffness and ductility. These frames have more stable nonlinear behavior than conventional ones and continue their nonlinear behavior even after fracture of coupling beams in severe earthquakes.     

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%.     

Ductility reduction factor and collapse mechanism evaluation of a new steel knee braced frame

In this study, the suitability of a new structural system called the knee braced frames (KBFs) is investigated for seismic resistant steel structures. In these structural systems, ends of beams are connected to columns by hinges (simple connection) instead of rigid connections, and ends of knee braced elements are connected to columns and beams by hinges as well. In the present paper, in addition to a comparison between elastic behaviour and elastic fundamental natural period, the ductility reduction factor and the type of collapse mechanism in steel KBFs and steel moment resisting frames (MRFs) are compared. The study revealed that the stiffness of steel buildings can be increased considerably by applying knee braced elements and the effects of knee braced elements are highly dependent on knee braced configuration. By applying the pushover analysis, it was observed that the type of collapse mechanism of KBFs is very similar to the mechanism of MRFs. Furthermore in most cases, the ductility reduction factor, R_μ, obtained from steel KBFs is greater than the ductility reduction factor obtained for steel MRFs. Based on the similarity between type of collapse mechanism and the proximity of ductility reduction coefficients of the KBFs and MRFs systems, it can be concluded that the new steel knee braced frame systems can be categorised as steel MRFs with rigid connections.     

Equivalent viscous damping in direct displacement‐based design of steel braced reinforced concrete frames

Performance‐based design method, particularly direct displacement‐based design (DDBD) method, has been widely used for seismic design of structures. Estimation of equivalent viscous damping factor used to characterize the substitute structure for different structural systems is a dominant parameter in this design methodology. In this paper, results of experimental and numerical investigations performed for estimating the equivalent viscous damping in DDBD procedure of two lateral resistance systems, moment frames and braced moment frames, are presented. For these investigations, cyclic loading tests are conducted on scaled moment resisting frames with and without bracing. The experimental results are also used to calibrate full‐scale numerical models. A numerical investigation is then conducted on a set of analytical moment resisting frames with and without bracing. The equivalent viscous damping and ductility of each analytical model are calculated from hysteretic responses. On the basis of analytical results, new equations are proposed for equivalent viscous damping as a function of ductility for reinforced concrete and steel braced reinforced concrete frames. As a result, the new equation is used in direct displacement‐based design of a steel braced reinforced concrete frame. Copyright © 2012 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.     

连于人字形防屈曲支撑的横梁抗震设计探讨

通过3个算例,对采用人字形无粘结内藏钢板支撑剪力墙(即人字形防屈曲支撑)的防屈曲支撑框架结构的抗震性能进行对比分析,以探讨支撑屈服后超强和被撑梁跨中竖向支点作用的有无对支撑跨横梁抗震性能的影响。结果表明,3个算例的框架梁在多遇地震下均能保持弹性,且被撑梁的最大竖向挠度均出现在撑点两侧。罕遇地震下,3个结构中支撑屈服后被撑梁的最大竖向挠度均出现在撑点位置。考虑超强但不考虑支点作用设计的结构中框架梁塑性发展程度较小,而不计超强但考虑支点作用设计的被撑梁塑性发展严重,且横梁的挠度较大,导致层间的两根支撑因承受较大的竖向力而使拉、压支撑的轴向应力-应变曲线明显不对称,不利于支撑受力。     

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.     

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。     

钢框架抗震改造的支撑系统研究

评估了采用不同支撑系统改造的抗弯钢框架的抗震性能。共采用3种结构形式:中心支撑框架、防屈曲支撑框架、巨型支撑框架。设计了一横向刚度不足的9层钢框架,满足规范对高地震灾害区域结构的侧移要求。用中心支撑、防屈曲支撑和巨型支撑改造框架。进行非弹性时程分析,评估地震作用下的结构性能。以局部变形(杆件转角)和整体变形(层间及屋顶侧移)为参数,比较改造框架非弹性性能的不同。结果表明:巨型支撑框架是最有效率的支撑系统,其最大层间侧移比抗弯框架低70%,比中心支撑框架低50%。侧移的减小量与地震特性有关,尤其是频率。防屈曲支撑的抗震性能仅稍优于巨型支撑框架,但其总质量更大。巨型支撑框架的杆件和节点用钢量比同心支撑框架低20%,既可降低费用又具有抗震优势。     

Evaluating response modification factors of TADAS frames

The current paper tries to evaluate overstrength, ductility and response modification factors in special moment resisting frames with TADAS (triangular-plate added damping and stiffness) devices. For that matter, multi-story buildings were considered during the course of study. Further, OpenSees Software was applied to perform the static pushover analysis, the nonlinear incremental dynamic analysis as well as the linear dynamic analysis. In this research, seismic response modification factor for special moment resisting frames (SMRFs) with TADAS devices (T-SMRFs) and without them has been determined separately. The results showed that the response modification factors for T-SMRFs were higher than the SMRFs ones. It was also found that the number of stories of buildings has had greater effect on the response modification factors.     

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。     

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.     

钢框架抗震改造的支撑系统研究

评估了不同支撑系统改造的抗弯钢框架的抗震性能。采用3种结构形式:中心支撑框架、防屈曲支撑框架和巨型支撑框架。设计了一横向刚度不足的9层钢框架,满足规范对高地震灾害区域结构的侧移要求。用中心支撑、防屈曲支撑和巨型支撑改造框架,进行非弹性时程分析,评估地震作用下的结构性能。以局部变形(杆件转角)和整体变形(层间及屋顶侧移)为参数,比较改造框架非弹性性能的不同。结果表明:巨型支撑框架是最有效率的支撑系统,其最大层间侧移比抗弯框架低70%,比中心支撑框架低50%。侧移的减小量与地震特性有关,尤其是频率。防屈曲支撑的抗震性能仅稍优于巨型支撑框架,但其总质量更大。巨型支撑框架的杆件和节点用钢量比中心支撑框架低20%,既降低了费用又体现了抗震优势。     

Plastic design of CB-frames with reduced section solution for bracing members

The seismic behaviour of concentrically braced frames (CBFs) designed according to the current European provisions is unsatisfactory due to the premature out-of-plane buckling of columns. For this reason, a new design methodology, based on a rigorous application of “capacity design” criteria has been recently proposed. In addition, aiming at a reduction of the plastic out of plane deformations of gusset plates due to brace buckling and at the prevention of sudden impact load affecting connections at the end of the straightening phase, Eurocode 8 requires the limitation of the brace slenderness. This limitation leads to the oversizing of diagonals and, consequently, of beams and columns. Therefore, to avoid this problem a new design strategy for bracing members is suggested: the Reduced Section Solution (RSS). It allows the calibration of the diagonal yielding resistance, leaving the brace slenderness practically unchanged.The results of dynamic inelastic analyses carried out with reference to braced frames designed according to the proposed procedure, both with and without RSS, are compared with those obtained with reference to the same structural schemes designed according to Eurocode 8. The obtained results show that the proposed design approaches are able to assure a significant improvement of the seismic performance.     

Bracing systems for seismic retrofitting of steel frames

The present study assesses the seismic performance of steel moment resisting frames (MRFs) retrofitted with different bracing systems. Three structural configurations were utilized: special concentrically braces (SCBFs), buckling-restrained braces (BRBFs) and mega-braces (MBFs). A 9-storey steel perimeter MRF was designed with lateral stiffness insufficient to satisfy code drift limitations in zones with high seismic hazard. The frame was then retrofitted with SCBFs, BRBFs and MBFs. Inelastic time-history analyses were carried out to assess the structural performance under earthquake ground motions. Local (member rotations) and global (interstorey and roof drifts) deformations were employed to compare the inelastic response of the retrofitted frames. It is shown that MBFs are the most cost-effective bracing systems. Maximum storey drifts of MBFs are 70% lower than MRFs and about 50% lower than SCBFs. The lateral drift reductions are, however, function of the characteristics of earthquake ground motions, especially frequency content. Configurations with buckling-restrained mega-braces possess seismic performance marginally superior to MBFs despite their greater weight. The amount of steel for structural elements and their connections in configurations with mega-braces is 20% lower than in SCBFs. This reduces the cost of construction and renders MBFs attractive for seismic retrofitting applications.     

支撑形式对钢框架结构抗震性能影响

钢结构框架类型,按其抗侧力体系可以分为:纯框架(UBF)、中心支撑框架(CBF)和偏心支撑框架(EBF)。纯框架具有较好的延性,刚度较小,往往满足了抗震变形要求;支撑框架刚度较大,具有两道纯抗震防线,适应于抗震设防较高的地区。通过有限元软件ANSYS对6层框架进行动力分析,比较这三种框架的抗震性能。由分析可知,支撑框架能较好地控制结构变形。