Horizontal bracing to enhance progressive collapse resistance of steel moment frames

In this paper, the effect of horizontal bracing on enhancing the resistance of steel moment frames against progressive collapse is investigated. Previously designed 6 bay by 3 bay 18‐story steel frame prototype building with 6 m bay span (namely, unbraced frame), which was susceptible to progressive collapse, is retrofitted by four types of horizontal bracing systems on the perimeter of the topmost story and analyzed using 3D nonlinear dynamic method. Six different cross‐sections for each bracing system type are considered, and the capacity curves for each model are obtained. Three column removal circumstances, namely, Edge Short Column, First Edge Long Column, and Edge Long Column are considered in this paper. The results imply that horizontal bracing would increase the resistance of moment frames against progressive collapse. However, one of the bracing types in which axial compressive force is created in braces is not appropriate for retrofitting.     

Estimating the cross-sectional area of inverted-V braces required for mitigating the progressive collapse of Steel Intermediate Moment Resisting Frames

The use of inverted-V braces is one of the retrofitting strategies engineers employ to mitigate progressive collapse in steel intermediate moment-resisting frames (SIMFs). However, the required cross-sectional area of the bracing member is not only dependent on the structural configurations but also on the seismicity of the region. In this study, a new equation is proposed to estimate the cross-sectional area of inverted-V braces required to mitigate failure progression in SIMFs that are subject to abrupt loss of an exterior column in the first storey. In the proposed equation, the area of beam element in the affected span and the seismicity of the region are considered. For this purpose, six SIMF buildings are designed, considering variation in the seismic base shear and the number of storeys. Then, the structural response of these SIMFs when subjected to an exterior column loss is studied. Using dynamic pushdown analyses, failure load factors of these frames are determined. Then, inverted-V braces of five different sizes are added to the studied frames. Results provide a mathematical correlation between the base shear coefficients, the size of structural elements and the associated retrofitting effect. This approximate equation is then validated by analysing two extra case study structures.     

Progressive collapse‐resisting capacity of modular mega‐frame buildings

In this study, the progressive collapse‐resisting capacity of modular mega‐frame structures consisting of a few identical subsystems was investigated based on column‐loss scenario. Four types of mega‐frame structures were designed as basic analysis model structures. According to pushdown analysis results, the mega‐frame structure with four corner columns did not satisfy the design guidelines for progressive collapse regardless of the number of subsystems when one of the first‐story mega‐columns was removed. To enhance the resistance against progressive collapse, we redesigned the basic model structure with four mega‐columns by adding additional floor trusses in the transfer floors, adding moment‐resisting frames at the perimeter and adding vertical mega‐bracing. The pushdown analysis results showed that the schemes with additional mega‐braces were most effective in increasing the progressive collapse‐resisting capacity of mega‐frame buildings. Copyright © 2011 John Wiley & Sons, Ltd.     

Collapse safety margin and seismic loss assessment of RC frames with equal material cost

With the premise of equal material cost, a collapse safety margin‐based collapse resistance optimization strategy for passively controlled reinforced concrete (RC) frames is proposed based on seismic fragility analysis, collapse safety margin analysis, and seismic hazard loss assessment. The efficiency of introducing buckling restrained braces or lead–rubber bearings on the performance of RC frames is studied by so‐called collapse margin ratio (CMR) suggested by FEMA P695 and the modified rigidity‐to‐gravity ratio (RGR). The proposed strategy is developed from the case study on 4 low‐rise and medium‐rise RC frames and then verified on a high‐rise RC frame. The study indicates that lead–rubber bearings can cause a significant improvement at all damage levels. The contribution of buckling restrained braces to structural stiffness and collapse resistance can be maximized when they are located in potential weak stories determined through inelastic time history analysis. CMR exhibits a better linear relation with the minimum modified RGR. Increasing the equivalent story lateral stiffness and the minimum modified RGR simultaneously can give rise to a significant improvement in seismic capacity, especially CMR. Base isolation is proved to be desirable not only for improving the collapse safety margin of RC frames significantly but also for reducing seismic hazard loss.     

The influence of joints and composite floor slabs on effective tying of steel structures in preventing progressive collapse

The event of the terrorist attack on 11th September 2001 in the USA has attracted increasing attention of researchers and engineers on progressive collapse of structures. It has gradually become a general practice for engineers to consider progressive collapse resistance in their design. In this paper, progressive collapse of steel frames with composite floor slabs is simulated by the finite element method. The numerical results are compared with test results. The influence of the joints and the concrete slabs on the effective tying of steel beams is investigated through parametric studies. From the analysis, methods of preventing progressive collapse that can be considered in design and when retrofitting existing structures are proposed. The results show that retrofitting a structure with pre-stressed steel cables and an increase of crack resistance in the concrete near joints can effectively improve effective tying of a structure, which results in an enhanced structural capacity in preventing progressive collapse.     

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.     

Assessment of eccentrically braced frames strength against progressive collapse

One of the most important and effective factors of structural strength against the risk of progressive collapse is the type of lateral load bearing system of a building. In this research, strength of dual steel moment frames equipped with a variety of eccentric bracings against progressive collapse was evaluated by using nonlinear static alternate path method. 6-floored building samples were designed with steel frame using a dual steel moment system together with 3 different types of bracing, including inverted eccentrically V-shaped bracing (chevron bracing), eccentrically V-shaped bracing and eccentrically X-shaped bracing, each with two different kinds of arrangement of bracings in the structural plan, in form of alternate and neighbor. The effects of sudden removal of columns on different floors of these buildings were examined. These studies showed that dual steel moment frames equipped with eccentric bracings generally exhibited desirable strength against progressive collapse. A change in the type of bracing resulted in significant changes in the system capacity in the progressive collapse. Among the different types of braces assessed, chevron type eccentrically brace showed higher strength against progressive collapse. Also, that alternate arrangement of bracings in structure plan demonstrated better performance than neighboring arrangement.     

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.     

消能支撑加固底部框架砌体房屋的弹塑性地震反应分析

本文采用弹塑性时程分析方法,对底部框架多层砌体房屋在不同烈度罕遇地震作用下进行了抗震性能评估;研究了在底部框架内增设消能外包混凝土无粘结钢支撑对房屋抗震性能的影响;对增设消能支撑加固方案和增设抗震墙加固方案进行了对比分析。分析结果表明,在底层框架砌体房屋的底层增设消能支撑可显著降低底层框架的最大层间位移,控制结构的塑性损伤,提高结构的抗震性能。增设消能支撑加固方案与增设抗震墙加固方案相比,结构加固更合理、更易实施。     

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.     

Effects of eccentric steel bracing systems on seismic fragility curves of mid-rise R/C buildings: A case study

In this study, the seismic reliability of a mid-rise reinforced concrete (R/C) building retrofitted using eccentric steel braces is investigated through fragility analysis. As a case study, a six storey mid-rise R/C building was selected. The design of selected sample building was made with reference to 1975 version of the Turkish Seismic Code. The effectiveness of using different types of eccentric steel braces in retrofitting the building was examined. The effect of distributing the steel bracing over the height of the R/C frame on the seismic performance of the retrofitted building was studied. For the strengthening of the original structure, D, K, and V type eccentric bracing systems were utilized and each of these bracing systems was applied with four different spatial distributions in the structure. For fragility analysis, the study employed a set of 200 generated earthquake acceleration records compatible with the elastic code design spectrum. Nonlinear time history analysis was used to analyze the structures subjected to this set of earthquake accelerations generated in terms of peak ground accelerations (PGA), whilst monitoring four performance limit states. The fragility curves were developed in terms of PGA for these limit states; namely: slight, moderate, major, and collapse with lognormal distribution assumption. The improvement of seismic reliability achieved through the use of D, K, and V type eccentric braces was evaluated by comparing the median values of the fragility curves of the existing building before and after retrofits. As a result of this study, the improvement in seismic performance of this type of mid-rise R/C building resulting from retrofits by different types of eccentric steel braces was obtained by formulation of the fragility reduction.     

基于易损性的RC空间框架填充墙结构连续倒塌能力分析

为研究填充墙对RC空间框架抗竖向连续倒塌能力的影响,对一栋6层双跨RC框架结构采用OpenSees有限元软件进行了建模,利用集中塑性的beamWithHinges单元模拟填充墙的性能。考虑到结构的不确定性,在有限元模型的基础上采用改进的拉丁超立方体抽样法获得了性能不同的100个纯框架结构和100个框架填充墙结构,并采用PDA方法和竖向IDA方法对取得的样本进行了易损性分析和对比。分析结果表明：填充墙提高了框架结构的抗倒塌能力,发生连续倒塌概率相同的情况下,框架填充墙结构的承载能力要比纯框架结构的高10%~20%。此外,两种分析方法对纯框架结构的抗连续倒塌能力分析差别较小,而对于带填充墙的框架结构,PDA方法计算结果则偏于保守。     

后加交叉钢支撑对RC框架结构的影响研究

为揭示后加交叉钢支撑对RC框架结构的影响规律,考虑结构层数、支撑截面尺寸及支撑与框架结构的连接方式三种影响因素,采用ETABS建立结构模型,对加钢支撑后RC框架结构的周期、刚度及基础反力及梁柱内力进行了定量分析。结果表明,后加交叉钢支撑对不同结构层数的RC框架的影响规律基本相同,支撑截面尺寸对支撑所在的结构单元的构件内力有较大影响,支撑与框架结构刚接或铰接连接的计算结果基本相同。此外,后加钢支撑承担了大部分的水平剪力,钢支撑对重力荷载或地震作用下的基础反力、地震作用下的柱轴力、梁弯矩及剪力,以及重力荷载作用下的梁轴力均有较大的影响。     

框架结构抗地震倒塌能力的研究——汶川地震极震区几个框架结构震害案例分析

介绍了汶川地震中极震区几组相同场地条件下倒塌与未倒塌框架结构的震害案例,并分别采用弹塑性时程分析方法、推覆分析方法和基于IDA的结构倒塌储备系数分析方法,对其中2个典型框架结构的抗地震倒塌能力进行了分析研究。在此基础上,结合国外关于结构抗地震倒塌计算方法及其相关研究,分析了影响结构抗倒塌能力的主要影响因素和评价指标。研究结果表明,保证结构的整体承载力储备和变形能力,增加结构的冗余度和整体性,采取有效措施使结构形成合理的屈服机制,充分利用填充墙使框架结构形成双重抗震防线,可显著提高框架结构的抗倒塌能力。最后提出了结构抗地震倒塌需进一步研究的问题。     

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.     

考虑全填充墙作用的钢筋混凝土框架抗连续倒塌性能分析

为探究钢筋混凝土(RC)全填充墙框架在边中柱失效情况下的抗连续倒塌性能及其承载力计算方法,基于已有试验和有限元程序OpenSees建立宏观有限元数值模型展开研究。数值模型中的梁柱采用基于力的纤维梁单元模拟,填充墙则转化为等效斜撑并用桁架单元进行表征。填充墙宏观模型涉及等效斜撑的宽度、数量和相应材料属性的确定,为此对比了不同等效斜撑模型的适用性,确定了一种连续倒塌工况下全填充墙的宏观模拟方案。进而利用验证的数值模型,揭示全填充墙框架防倒塌的荷载传递机制,并研究了层数和填充墙砌体抗压强度对抗倒塌性能的影响。结果表明：全填充墙框架荷载主要通过墙体对角传递且全填充墙会与周围框架形成一种桁架机制;随填充墙砌体抗压强度降低,结构抗力峰值呈下降趋势。最后,以填充墙和周边框架竖向承载刚度比为基本参数,建立了通过求得填充墙和框架刚度以及纯框架理论弯曲承载力,便可快速评估规则RC填充墙框架防倒塌能力的回归模型。     

Seismic performance of moment resisting steel frame subjected to earthquake excitations

This study presents static and dynamic assessments on the steel structures. Pushover analysis (POA) and incremental dynamic analysis (IDA) were run on moment resisting steel frames. The IDA study involves successive scaling and application of each accelerogram followed by assessment of the maximum response. Steel frames are subjected to nonlinear inelastic time history analysis for 14 different scaled ground motions, 7 near field and 7 far field. The results obtained from POA on the 3, 6 and 9 storey steel frames show consistent results for both uniform and triangular lateral loading. Uniform loading shows that the steel frames exhibits higher base shear than the triangular loading. The IDA results show that the far field ground motions has caused all steel frame design within the research to collapse while near field ground motion only caused some steel frames to collapse. The POA can be used to estimate the performance-based-seismic-design (PBSD) limit states of the steel frames with consistency while the IDA seems to be quite inconsistent. It is concluded that the POA can be consistently used to estimate the limit states of steel frames while limit state estimations from IDA requires carefully selected ground motions with considerations of important parameters.     

Progressive collapse performance analysis of precast reinforced concrete structures

In this paper, the progressive collapse performance analysis of precast reinforced concrete (RC) structures is performed. A numerical simulation framework for precast RC structures is developed on the basis of the OpenSEES software, where the fiber frame element is used for beam and column type members and Joint2D element is used for the beam‐to‐column connections. The conjugated material models are then introduced, and a min–max failure criterion is imposed on the original models to reflect the steel fracture and concrete crushing when the structure is undergoing progressive collapse. In addition, to overcome the computational difficulties arisen from progressive collapse behavior, two enhanced nonlinear solutions , that is, the consistent quasi‐Newton algorithm and the explicit KR‐α algorithm, are employed, respectively, for static and dynamic analysis. A 10‐storey prototype precast RC structures is designed to verify the developed numerical framework, and the progressive collapse resisting mechanism of the structures is investigated through both static pushdown analysis and dynamic column‐removal analysis. Finally, influences of some typical parameters in precast RC structures on their progressive collapse performance are studied.     

丙类与乙类设防RC框架结构抗地震倒塌能力对比

按照GB 50011-2001《建筑抗震设计规范》6～8.5度的丙类与乙类设防要求分别设计了10个典型RC框架结构算例,采用基于动力增量时程分析(IDA)地震倒塌易损性分析方法得到了各算例结构在遭遇罕遇地震和特大地震时的倒塌率,据此对不同设防烈度的丙类和乙类RC框架结构的抗地震倒塌能力进行了评价分析。研究指出,结构的抗倒塌能力需根据结构的绝对抗震能力和相对地震动强度来进行评价。分析结果表明,对于6～7.5度抗震设防的结构,因其绝对抗震能力偏低,抗震设防烈度仅由丙类提高到乙类,其在遭遇特大地震时的抗地震倒塌能力仍较差,需采取进一步的措施增强其抗震能力。     

Inelastic seismic response of steel bracing members

A survey of past experimental studies on the inelastic response of diagonal steel bracing members subjected to cyclic inelastic loading was carried out to collect data for the seismic design of concentrically braced steel frames for which a ductile response is required under earthquakes. The parameters that were examined are the buckling strength of the bracing members, the brace post-buckling compressive resistance at various ductility levels, the brace maximum tensile strength including strain hardening effects, and the lateral deformations of the braces upon buckling. Equations are proposed for each of these parameters. In addition, the maximum ductility that can be achieved by rectangular hollow bracing members is examined.