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.     

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.     

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.     

Recent research on eccentrically braced frames

Recent research developments on eccentrically braced steel frames for seismic design are reviewed. The emphasis is placed on the design of links, which are short sections of beams between columns and braces, and similar elements at eccentric joints. The review includes some highlights of the latest experiments with one-third scale models employing different eccentric bracing schemes, an updated classification of links, and special design requirements for different types of links. Some results are given on recent cyclic tests of full-size links.     

人字支撑复合结构的合理耗能机制选择分析

通过对柔性梁柱加"人字"支撑形成的一榀新型复合结构进行静力弹塑性(Pushover)分析,从理论上研究该复合结构的抗震性能,满足按9度多遇地震的抗震能力和罕遇地震的抗倒塌验算。其耗能机制为"混合机制",塑性铰出现的位置及顺序为:支撑、梁端、柱下端、柱上端。为了得到更合理的抗震耗能机制,对影响复合结构耗能机制的主要因素、底层柱刚度和配筋、底层及二层支撑的配筋进行分析,由此优化出满足"强柱、中梁、弱支撑"多道防线抗震性能要求的合理耗能机制。计算结果表明,人字支撑复合结构的水平侧移表现为剪切变形。     

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.     

消能摇摆钢框架结构抗震性能的影响因素与设计方法

消能摇摆钢框架结构包含主体钢框架结构、摇摆结构和耗能阻尼器三部分。刚度较大的摇摆结构可以使主体钢框架在地震作用下发生均匀的层间变形,抑制薄弱层产生。布设于摇摆结构底部的阻尼器,能够耗散地震动能量,减小整体结构的地震反应,提高结构的抗震性能。文中对消能摇摆钢框架结构抗震性能的影响因素进行参数分析,并基于我国建筑抗震设计规范的原则提出了抗震设计方法。根据消能摇摆钢框架结构的受力机理,提出简化分析模型,通过弹塑性地震反应分析,验证简化模型的有效性。基于简化分析模型对无量纲参数进行参数分析,根据各参数的影响规律得到无量纲参数的建议范围。结合我国“三阶段”抗震设防要求,提出消能摇摆钢框架结构的设计方法,并结合算例进行验证。研究表明,消能摇摆钢框架结构抗震性能良好,设计合理的摇摆结构与阻尼器能够抑制钢框架的薄弱层、减小结构的地震反应。     

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。     

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%.     

Use of steel bracing in reinforced concrete frames

In this paper the use of steel bracing in concrete-framed structures is investigated. The investigation is carried out through a series of tests conducted on a number of model frames. The object of the tests was to determine the degree of effectiveness of different diagonal bracing arrangements to increase the in-plane shear strength of the concrete frame and to observe the relative behaviour of tension and compression braces. The important question of the proper connections between the steel braces and the concrete frame is also considered. The test results indicate a considerable increase in the in-plane strength of the frame due to steel bracing. As an overall conclusion it is noted that, with proper connection between the brace and the frame, the steel bracing could be a viable alternative or supplement to shear walls in concrete framed buildings in seismic areas.     

异形柱用于8度抗震设防建筑研究

通过混凝土异形柱框架和异形柱框架 -剪力墙结构的弹塑性时程分析以及总结异形柱和异形柱框架的部分试验成果 ,研究了异形柱用于 8度抗震设防地区房屋建筑的可能性 ,提出了 8度抗震设防建筑采用异形柱的设计建议     

多高层框架-弯剪型支撑结构的稳定性研究

在多高层框架-支撑结构中,支撑体系通常有剪切型、弯曲型和弯剪型之分,三类支撑之间没有一个定量的区分准则。本文在变截面、变轴力弯剪型支撑柱稳定分析的基础上,提出了三类支撑的定量划分准则。对弯剪型支撑-框架结构的稳定性进行了研究,得到了使被支撑框架发生无侧移弹性失稳的支撑临界刚度的简单式,它可以作为结构稳定性设计中判定框架失稳模式的标准。同时给出了支撑刚度较小时框架临界荷载于支撑刚度的简单关系,可用于弱支撑框架的稳定性计算。研究结果对于高层、超高层建筑结构的稳定计算具有重要意义。     

Topology optimization and seismic collapse assessment of shape memory alloy (SMA)-braced frames: Effectiveness of Fe-based SMAs

This paper presents a seismic topology optimization study of steel braced frames with shape memory alloy (SMA) braces. Optimal SMA-braced frames (SMA-BFs) with either Fe-based SMA or NiTi braces are determined in a performance-based seismic design context. The topology optimization is performed on 5- and 10-story SMA-BFs considering the placement, length, and cross-sectional area of SMA bracing members. Geometric, strength, and performance-based design constraints are considered in the optimization. The seismic response and collapse safety of topologically optimal SMA-BFs are assessed according to the FEMA P695 methodology. A comparative study on the optimal SMA-BFs is also presented in terms of total relative cost, collapse capacity, and peak and residual story drift. The results demonstrate that Fe-based SMA-BFs exhibit higher collapse capacity and more uniform distribution of lateral displacement over the frame height while being more cost-effective than NiTi braced frames. In addition to a lower unit price compared to NiTi, Fe-based SMAs reduce SMA material usage. In frames with Fe-based SMA braces, the SMA usage is reduced by up to 80%. The results highlight the need for using SMAs with larger recoverable strains.     

钢框架作为柱间支撑在门式刚架结构中的应用

因使用功能的需要,轻钢厂房内常常出现不允许设置柱间支撑的情况。研究利用由钢柱、钢梁组成的钢框架在平面内具有刚度较大、不易变形的特点,在局部适当跨间设置钢框架作为柱间支撑来代替普通柱间支撑使用,为结构单元提供必须的纵向刚度。选取几种常用的支撑体系进行受力分析、对比,论证钢框架支撑在轻钢结构厂房中使用的适用性、合理性、经济性,为类似工程提供参考。     

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.     

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.     

屈曲约束支撑结构性能的数值模型

介绍一种耗能屈曲约束支撑循环结构性能的数值模型,该支撑常用来替代传统的轴向支撑作为框架结构和其他结构中的抗震构件。这些支撑通常由1根细长的插入短粗套管中的钢芯组成,套管是预先设置的,以防止其受压时屈曲。套管由砂浆或钢构成,在钢芯和套管间设置滑移面以阻止剪应力过度传递。钢芯的性能、砂浆套管的性能和界面的滑移性能分别通过塑性破坏模型、各向同性的破坏模型和罚函数模型来描述。按照给出的公式通过ABAQUS软件完成了这3个模型的建模。在许多典型但简单的工况下,该算法被证实可以模拟屈曲约束支撑的循环性能。模型模拟结果与试验结果吻合较好,证实了其正确性。对初步得到的结论进行讨论并指出进一步研究的方向。     

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.     

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。     

附加减震钢框架的加固混凝土框架结构抗震性能试验研究

为提高既有混凝土框架抗震性能,在不显著增加既有结构构件受力的同时,在混凝土框架外部增设钢框架并设置屈曲约束支撑。设计并制作1榀纯混凝土框架和2榀设置附加减震钢框架的混凝土框架,通过低周往复加载试验,研究其开裂和破坏状态、滞回曲线、骨架曲线、刚度退化、等效阻尼比以及钢筋混凝土梁、附加钢框架的应变发展等。试验结果表明：增设附加减震框架后,结构破坏机制更加合理,屈曲约束支撑耗能性能稳定,加固后结构的滞回曲线饱满;采用外部附加钢框架加固钢筋混凝土框架,可提高既有混凝土框架结构的受剪承载能力至3倍以上,既有结构、外部附加钢框架和屈曲约束支撑可协同工作,在设防目标下可避免混凝土柱发生压剪脆性破坏;最后对附加减震框架改进连接构造设计提出了建议。