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.     

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.     

基于屈服点谱法的中心支撑钢框架抗震性态评估

屈服点谱(Yield Point Spectra,YPS)是以位移-加速度表述的反应谱形式。YPS可以用于对现有结构进行抗震评估,确定结构在给定地震作用下的峰值位移和延性。本文按照我国设计规范分别设计了6层、9层、12层3个人字形中心支撑钢框架结构,利用YPS对3个结构进行非线性静力分析,得到结构在设防地震和罕遇地震下的峰值位移和层间位移角,并与SAP2000动力时程分析得到的结果进行对比,评估人字形中心支撑钢框架在设防地震和罕遇地震下的抗震性态,评价了YPS方法用于中心支撑钢框架抗震性态评估的可靠性。     

Assessment of current nonlinear static procedures for seismic evaluation of BRBF buildings

Nonlinear static procedures (NSPs) are now standard in engineering practice to estimate seismic demands in the design and evaluation of buildings. This paper aims to investigate comparatively the bias and accuracy of modal, improved modal pushover analysis (MPA, IMPA) and mass proportional pushover (MPP) procedures when they are applied to buckling-restrained braced frame (BRBF) buildings which have become a favorable lateral-force resisting system for earthquake resistant buildings. Three-, 6-, 10-, and 14-storey concentrically BRBF buildings were analyzed due to two sets of strong ground motions having 2% and 10% probability of being exceeded in 50 years. The assessment is based on comparing seismic displacement demands such as target roof displacements, peak floor/roof displacements and inter-storey drifts. The NSP estimates are compared to results from nonlinear response history analysis (NL-RHA). The response statistics presented show that the MPP procedure tends to inaccurately estimate seismic demands of lower stories of tall buildings considered in this study while MPA and IMPA procedures provide reasonably accurate results in estimating maximum inter-storey drift over all stories of studied BRBF systems.     

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.     

Lateral cyclic behavior of zipper braced frames-considering connection details

During Northridge earthquake in USA in 1994, a variety of failures occurred in welded steel connections. Studying these structural failures has led to development of more reliable moment resisting connections and new ways of using braced frames as seismic load resisting systems. This article investigates through numerical simulations, the lateral capacity and seismic behavior of two of these newly-thought braced frames, zipper braced frames and suspended zipper braced frames. The overall seismic behavior of these frames is investigated through displacement-based pushover analyses considering the effect of connection elements such as gusset plate and shear tab. To study the efficiency of these two types of concentrically braced frames, a numerical investigation on their behaviors for low-, mid- and high-rise buildings was conducted. Three zipper braced frames and three suspended zipper braced frames with different number of stories have been modeled using OpenSEES software. For each simulation, frame maximum strength, maximum drift capacity, and weight are determined and compared with each other. It is concluded that connection modeling has significant effects on the lateral behavior of these frames. Furthermore, the suspended zipper braced frames show higher ductility when compared with the ductility of zipper braced frames. Finally, the suspended zipper braced frames are recommended to be used in high-rise buildings, however, for the lowand mid-rise buildings it is recommended to use zipper frames due to economic efficacy.     

采用逐步增量弹塑性时程方法对RC框架结构推覆分析侧力模式的研究

以两个普通六层和十层钢筋混凝土框架结构为例,采用基于纤维模型的逐步增量弹塑性时程方法得到的层间剪力-位移关系曲线,与不同侧力模式的推覆分析结果进行了对比,研究了推覆分析结果的可靠程度。通过与不同场地的大量地震记录的弹塑性时程计算结果进行比较分析,建议采用多种合理的侧力模式进行推覆分析,对结构不同楼层的抗震性能进行全面的评价,即均布侧力模式适合于底部楼层的评价;考虑高度影响分布力模式适合于结构中部楼层的评价;而对结构上部楼层特别是顶层,可以采用SRSS侧力模式或规范侧力模式进行评价。分析研究表明,采用本文建议的侧力模式进行推覆分析,可在统计意义上可对钢筋混凝土规则框架结构的抗震能力作出偏于安全的估计。     

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。     

高层支撑钢框架弹塑性地震反应简化分析模型

本文首先将高层支撑钢框架结构分解成纯框架和纯支撑体系两部分,并与一列受载铰接刚性杆并联,以考虑几何非线性的影响;然后将框架部分简化为半刚架,将支撑体系部分简化为铰接桁架,以此进行结构弹塑性地震反应分析.在前人试验和理论研究成果的基础上,本文还提出了能准确反映钢支撑主要滞回特征的支撑恢复力模型,便于工程实用.通过算例对比分析,表明采用本文提出的高层支撑钢框架简化模型进行弹塑性地震反应分析,具有计算精度高、计算自由度少、计算时间省的优点,是一种有效的近似方法.     

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.     

On the applicability of pushover analysis for seismic evaluation of medium‐ and high‐rise buildings

The assumption that the dynamic performance of structures is mainly determined from the corresponding single‐degree‐of‐freedom system in pushover analysis is generally valid for low‐rise structures, where the structural behaviour is dominated by the first vibration mode. However, higher modes of medium‐ and high‐rise structures will have significant effect on the dynamic characteristics. In this paper, the applicability of pushover analysis for seismic evaluation of medium‐to‐high‐rise shear‐wall structures is investigated. The displacements and internal forces of shear wall structures with different heights are determined by nonlinear response history analysis, where the shear walls are considered as multi‐degree‐of‐freedom systems and modelled by fibre elements. The results of the analysis are compared with those from the pushover procedure. It is shown that pushover analysis generally underestimates inter‐storey drifts and rotations, in particular those at upper storeys of buildings, and overestimates the peak roof displacement at inelastic deformation stage. It is shown that neglecting higher mode effects in the analysis will significantly underestimate the shear force and overturning moment. It is suggested that pushover analysis may not be suitable for analysing high‐rise shear‐wall or wall‐frame structures. New procedures of seismic evaluation for shear‐wall and wall‐frame structures based on nonlinear response history analysis should be developed. Copyright © 2009 John Wiley & Sons, Ltd.     

Incremental dynamic analysis of steel braced frames designed based on the first,second and third editions of the Iranian seismic code (standard no. 2800)

Incremental dynamic analysis (IDA) is an emerging method in structural analysis which allows evaluation of seismic capacity and demand of structures through a series of nonlinear dynamic analyses using multiple scaled ground motion records. Seismic behaviour of concentrically braced frames designed based on the first, second and third revisions of the Iranian seismic code, standard no. 2800, has been evaluated through IDA in the present paper. Besides, a brief comparison is made between seismic behaviour of these frames, frames with different heights and different bracing types. Seismic capacity and limit states of such structures have been reviewed through the paper. The IDA results imply that frames designed with the first edition are seriously vulnerable and fail before reaching the acceleration levels predicted in the code. On the other hand, frames designed with the second and third editions, although behaving better, need partial reinforcement in some cases. Other results of this study show that chevron braced frames behave seismically better than X‐braced ones. Copyright © 2009 John Wiley & Sons, Ltd.     

防屈曲支撑、普通和特殊中心支撑钢框架结构抗震性能分析

基于考虑人字形防屈曲支撑屈服后超强和几乎不再对被撑梁提供竖向支点作用这两个因素,本文提出了采用该种支撑的钢框架结构的设计方法,并分别对采用普通及特殊中心支撑和防屈曲支撑的框架结构的抗震性能进行了对比分析.结果表明,虽然防屈曲和特殊中心支撑框架结构的层间侧移总体上大于普通中心支撑框架结构,但前者的基底剪力却大大低于后者.罕遇地震下,三种结构中的柱子基本保持弹性,普通和特殊中心支撑出现了大幅的平面外失稳,而防屈曲支撑在拉压作用下均进入屈服耗能.三种结构中被撑梁的最大挠度在支撑屈服或失稳前后分别出现在撑点两侧和撑点位置.屈服后的防屈曲支撑几乎不产生对被撑梁竖直向下的不平衡剪力,而失稳后的普通和特殊中心支撑则对被撑梁产生较大的不平衡剪力.     

支撑—异形柱钢框架结构的静力弹塑性分析

简述了Push-over方法的基本原理,并运用ETABS对柱截面积相同、柱形(H形柱,异形柱)不同的支撑—钢框架结构进行弹塑性分析,发现了支撑—异形柱钢框架结构抗震性能优于纯钢框架结构,支撑—异形柱钢框架较支撑—H形柱钢框架弱侧抗侧能力更好等优点。     

The modified dynamic‐based pushover analysis of steel moment resisting frames

A modified dynamic‐based pushover (MDP) analysis is proposed to properly consider the effects of higher modes and the nonlinear behavior of the structural systems. For this purpose, first, a dynamic‐based story force distribution (DSFD) load pattern is constructed using a linear dynamic analysis, either time history (THA) or response spectrum (RSA). Performing an initial pushover analysis with the DSFD load pattern, a nonlinearity modification factor (NMF) is calculated to modify the DSFD load pattern. The envelope of the peak responses of the structure obtained from 2 pushover analyses with the modified DSFD load pattern as well as the code suggested first mode load pattern are considered as the final demand parameters of the structural system. The efficiency of the proposed MDP procedure is investigated using the results of nonlinear THA besides some existing pushover procedures. For this purpose, the 2‐dimensional 9‐, 15‐, and 20‐story, SAC steel frame building models are considered for parametric studies using OpenSees program. The results indicate that the proposed MDP‐THA and MDP‐RSA methods can significantly improve the performance of the pushover analysis. Considering the accuracy and calculation efforts, the MDP‐RSA method is strongly suggested as an efficient and applicable method to estimate the nonlinear response demands of steel moment resisting frames.     

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.     

考虑土-结构相互作用高层框架结构静力弹塑性分析方法的研究

本文应用子结构方法对天然地基上带刚性整体基础 (箱型基础或筏板基础 )的高层框架结构进行了平面静力弹塑性 (Pushover)分析 ,地基阻抗采用拟合数值结果得到的近似公式 ,上部框架结构采用杆系模型。并就其水平荷载模式、目标位移等几个关键问题进行了研究。文末对一栋 12层的钢筋混凝土框架结构进行了静力弹塑性 (Pushover)分析 ,说明了本文建立的考虑土 -结构相互作用高层框架结构静力弹塑性 (Pushover)分析方法的可行性     

高层钢结构支撑方案抗震性能对比分析

高层钢结构中框架—支撑结构是一种常用的结构形式,文中介绍了支撑结构体系的发展情况.以某实际工程为例,说明了普通钢支撑和屈曲约束支撑两种不同支撑对结构弹性性能的影响,通过静力弹塑性推覆分析对比研究了两种方案的弹塑性性能,并对屈曲约束支撑采用偏心布置形式对框架梁的影响进行了探讨.     

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%.     

Cyclic flexural analysis and behavior of beam-column connections with gusset plates

This research investigates the cyclic flexural behavior of double-angle concentrically braced frame beam-column connections using three-dimensional nonlinear finite element analysis. Prior experimental research demonstrated that such connections possess appreciable flexural stiffness, strength, and ductility. The reserve capacity provided by these connections plays a significant role in the seismic behavior of low-ductility concentrically braced frames, so knowledge about the impact of connection parameters on local limit states and global connection performance is needed for employing reserve capacity to design and assess concentrically braced frames. Finite element models were developed and validated against prior experiments with focus on the limit states of failure of the fillet weld between the gusset plate and beam, low-cycle fatigue fracture of the steel angles joining the beam and gusset plate to the column, and bolt fracture. The models were used to evaluate the flexural stiffness, strength, and ductility of braced frame connections with primary attention on the effects of beam depth, angle thickness, and a supplemental seat angle. The finite element analysis demonstrated that increasing beam depth and angle thickness and adding a supplemental seat angle all increased the stiffness and strength of the connection while maintaining deformation capacity. A procedure to estimate the flexural behavior of beam-column connections with gusset plates was developed based on the results of the numerical simulations.