Bracing configuration and seismic performance of reinforced concrete frame with brace

Brace has been used to improve structural seismic performance in reinforced concrete (hereafter called RC) frame extensively. However, the reasonable bracing configurations were not highlighted. And the brace buckling was not considered. Bracing configurations and seismic performance of RC frame with brace are studied in this paper. First, optimized bracing configurations of entire structures and 1‐story 1‐span structures are derived by topology optimization using truss‐like material model. Next, the structure models are simulated in OpenSees. Moreover, different bracing configurations are employed. Last, seismic performance and seismic response of RC frame structures with brace are discussed using static nonlinear analysis and dynamic time–history analysis, and brace buckling is considered. The numerical results indicate that more details for manufacturing or constructing can be presented by bracing configuration using truss‐like model. Further, the increase of structural strength and stiffness is more advisable in bracing configuration using topology optimization (hereafter called TPB), which corresponds to optimized result using entire structure than other configurations. The optimized bracing configuration based on 1‐story 1‐span structure is also beneficial within the prescribed conditions. Moreover, the brace buckling can decrease both strength and stiffness, and the buckling can also aggravate the damage extent than the brace without buckling.     

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.     

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.     

Displacement-restraint bracing for seismic retrofit of steel moment frames

This paper presents a seismic retrofit method using wire rope (cable) bracing for steel moment-resisting frames. The retrofitted frame using the proposed bracing system exhibits ductile behavior and maintains seismic energy dissipation capacity to the same extent as the original bare frame. The bracing member does not act for small and medium vibration amplitudes. For large vibration amplitudes, the bracing member acts and restrains unacceptably large story drift. This retrofit method prevents an increase in the column compression force resulting from the brace action. Cyclic loading test results of the portal frames reveal fundamental characteristics of the proposed bracing system. Seismic response analyses are also conducted for the three-story moment-resisting frames. The effectiveness of the retrofit method is discussed in light of these test and analysis results.     

Seismic Study of Buckling Restrained Brace System without Concrete Infill

Bracing is the one of the best-known means of seismic retrofitting. Buckling restrained brace (BRB) is a certain type of brace with great efficiency against lateral loading. This paper presents the results of a finite element analysis on a BRB in which casing has no concrete infill. The core segment of this brace is similar to the conventional BRB, but it has a different buckling restraining system. The aim of this paper was to perform a parametric seismic study on the effect of a gap and also the effect of friction between the core and the casing and to evaluate the buckling behavior of these braces in response to changes in the initial shape of the bracing system. The results show that the flexural stiffness of the casing system, regardless of size of the gap, can significantly affect the buckling behavior of bracing.     

Seismic performance of a reinforced concrete frame equipped with a double‐stage yield buckling restrained brace

In this paper, a double‐stage yield buckling restrained brace (DYB) is proposed to prevent soft story collapse in structures subjected to strong earthquakes. The DYB consists of two conventional buckling restrained braces (BRBs) with different yield forces: a large BRB and a small BRB. The deformation of the small BRB has an upper threshold value, controlled by a special mechanical mechanism. Once the force acting on the DYB exceeds the yield force of the small BRB, the small BRB yields and the deformation concentrates on the small BRB. When the deformation of the small BRB reaches the threshold value, the small BRB stops deforming. If the force of the DYB continues to increase and exceeds the yield force of the large BRB, the large BRB yields and most of the deformation takes place in the large BRB. In this way, the DYB achieves a double‐stage yield mechanism. To demonstrate the effectiveness of the DYB, a model of a six‐story reinforced concrete frame equipped with DYBs was constructed using the finite element software ABAQUS, and its seismic performance was analyzed. The double‐stage yield mechanism of the DYB was simulated by a gap element. To investigate the effect of DYBs on the seismic performance of the structure, four different models were built: an unbraced frame, frame with DYB, frame with small BRB, and frame with large BRB. The results of the pushover and time‐series analyses showed that the DYB effectively controlled the deformation pattern of the structures, and prevented weak story collapse.     

两边连接屈曲约束钢板剪力墙受力机理与等效支撑模型

采用理论分析和有限元方法,针对两边连接屈曲约束钢板剪力墙的受力机理和传力规律进行研究。提出了钢板墙边缘约束区的概念并确定了边缘约束区的宽度,分析了钢板墙的屈服形状、钢板墙内各部分应力流的分布规律和钢板墙与梁连接处的受力特点等。在此基础上提出了两边连接屈曲约束钢板剪力墙等效支撑模型,对不同尺寸、不同层数的框架 屈曲约束钢板剪力墙结构和框架 等效支撑结构在水平荷载作用下的力学性能进行分析,并对两种结构的荷载 位移曲线进行了对比。分析表明,所提出的等效支撑模型在结构刚度和承载力方面具有较好的准确性,无论是单调加载还是反复加载均能准确地模拟两边连接屈曲约束钢板剪力墙结构的受力行为。     

Experimental investigation of behavior of steel frames with y-shaped concentric bracing

Concentric bracings composed of three members arranged in y-shaped geometry have been traditionally used to provide openings in braced bays. However, using common single gusset plates in y-braced frames leads to single curvature flexure and out of plane buckling of braces accompanied by low hysteretic energy dissipation. In order to explore and improve the behavior of y-braced frames, a research program including experimental tests was conducted at BHRC¹ structural engineering laboratory. Specimens presented in this paper include four full-scale frames with y-bracings of different geometries and cross sections. Quasi-static cyclic loading was increasingly applied until yielding and failure occurred in the specimens. The results show that out-of-plane buckling with single curvature in braces can be replaced by in-plane, double curvature buckling through appropriate detailing of cross sections and connections. These sections have larger radius of gyration for out of plane buckling of bracing members. Hysteretic energy dissipation and damping of y-bracing are increased due to inelastic flexural deformation of brace members. Energy dissipation capacity of y-braced frames with new details is comparable with the traditional X bracing. Based on these findings, two-bay y-braced frames were designed to carry the same lateral load as X-braced frames. The seismic performance of these frames was compared using nonlinear static procedures and found to be similar.     

Experimental and numerical studies on ribbed bracing system

Ribbed bracing system (RBS) is an innovative structural system designed to eliminate the buckling of braces and enhance the behavior of structures under seismic loads. In this study, a collaborative performance of 2 RBS devices bracing a frame was assessed numerically and experimentally. In the numerical phase, the collaboration of ribbed braces at various stages of reversal loading was elaborated mathematically and was used for representing the system using finite element modeling. In the next phase, the numerically observed behavior was validated experimentally by conducting cyclic quasistatic tests of the proposed configurations. Two alternative RBS configurations—called completely closed (CC) and improved centering (IC)—were considered in this regard. Various characteristics of CC‐ and IC‐RBS configurations were evaluated using validated methods, after which they were compared against configurations of concentrically braced frames and buckling‐restrained braces. The stiffness and the energy absorption capacity of CC‐RBS configuration were shown to exceed all the considered bracing types. IC‐RBS, on the other hand, showed the lowest residual drift. The CC‐RBS configuration was also shown to have excellent performance under moderate loadings experienced under service level.     

Prevention of story drift amplification in a 20‐story steel frame structure by tension‐rod displacement–restraint bracing

This paper proposes an application of tension‐rod displacement–restraint bracing to prevent story drift amplification in tall steel moment frames. Seismic response analyses of a 20‐story bare steel frame are performed first, revealing that story drift amplification occurs in the upper and lower stories at different times. Characteristics observed for the seismic response of the bare frame suggest the efficacy of the delay action of bracing. Subsequently, seismic response analyses of the 20‐story braced frame with tension‐rod displacement–restraint bracings reveals that the increment of the column axial force by addition of bracing is reduced dramatically by the delay action of bracing. The story rotation angles within partial stories where the story drift amplification occurs in the bare frame are also reduced efficiently by the displacement–restraint bracing. The delay action of bracing influences the floor response acceleration and the residual displacement. Finally, parametric analysis results indicate an appropriate value of the story rotation angle at which the brace action starts.     

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.     

Subassemblage test of buckling‐restrained braces with H‐shaped steel core

In this study, a subassemblage test was performed using buckling‐restrained braces with an H‐shaped core element, which have been proven in a previous uniaxial component test to have good performance. The loading protocol prescribed the quasi‐static cyclic pattern with stepwise incremental displacement amplitude. Two different end connections (bolted connection and pin connection) and two different buckling‐restrained mechanisms (concrete‐filled tube and hollow steel tube) were examined as the test parameters. The performance of the specimen was evaluated by comparing the test results with the recommended provisions for buckling‐restrained braces. The test results showed that the compression strength capacity of buckling‐restrained brace (BRB) with in‐filled concrete increased by about 10% compared with BRB without in‐filled concrete. According to test result at same story drift of 2D_bm, structural performance of pin connection specimen without bolt slippage is superior to bolted connection specimen. Also, bolted connection specimens showed similar performance for total energy dissipation and cumulative plastic ductility, regardless of the connection types and the existence of concrete filling. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

Drift-based and dual-parameter fragility curves for concentrically braced frames in seismic regions

This paper discusses the development of drift-based and dual-parameter fragility curves for steel braces as part of concentrically braced frames designed in seismic regions. The experimental results from 24 different research programs are compiled into a database for this effort. Drift-based fragility curves are developed for three damage states of steel braces subjected to cyclic loading associated with brace flexural buckling, local buckling and brace strength loss due to fracture. The effects of material variability, brace cross sectional shape and loading protocol on the drift-based fragility curves are investigated. The effect of global and local slenderness ratios on the fracture ductility of various shapes of steel braces is examined through dual-parameter fragility curves that relate these geometric ratios with the expected story drift ratios that each of the three pre-described damage states occur. The proposed fragility curves can be employed for rapid assessment of the seismic vulnerability of concentrically braced frames.     

双阶屈服屈曲约束支撑框架小震参数分析

为了弥补常规屈曲约束支撑在多遇地震作用下处于弹性状态,不能发挥消能减震作用的不足,提出了一种将金属套管阻尼器与屈曲约束支撑组合形成的双阶屈服屈曲约束支撑,经试验验证其具有良好、稳定的小震及中大震下的滞回特性。在小震作用下,金属套管阻尼器屈服消能,屈曲约束芯板保持弹性承载。借助有限元软件ETABS建立了一系列双阶屈服屈曲约束支撑框架模型,通过改变支撑与框架刚度比、阻尼器与芯板的轴向刚度关系以及套管阻尼器的屈服比例,对各模型进行小震作用下的动力弹塑性分析,将各模型基底剪力和最大层间位移角与相应的常规屈曲约束支撑框架的分析结果进行对比。结果表明:双阶屈服屈曲约束支撑与支撑芯板的轴向弹性刚度比取2左右,阻尼器屈服比例取0.3左右时,可取得较好的减震效果; 双阶屈服屈曲约束支撑的参数取值改变,对降低结构地震响应的影响趋势不因支撑与框架刚度比不同而改变; 当支撑刚度贡献较大时,相较常规屈曲约束支撑,双阶屈服屈曲约束支撑的设置能降低结构的层间位移角,若要同时降低基底剪力,阻尼器屈服比例不宜高于0.3。     

多层屈曲约束斜撑钢框架弹塑性分析研究

以多层屈曲约束斜撑钢框架为主要研究对象,斜撑框架强度CB以及屈曲约束斜撑的水平力分担率β为主要研究参数,拟设计6个建筑模型,对各模型进行了基于塑性铰理论的二阶非线性时程分析,考察了强震作用下各模型各层的层间位移、层剪力、屈曲约束斜撑的塑性能量分布等。结果表明,各模型的强度CB值从0.3增大到0.5或屈曲约束斜撑的水平力分担率β值从30%增大到90%时,各模型的各层最大层间位移反应值中的弯曲变形反应值越大,这种情况上层部位更明显。随着斜撑框架强度CB值或屈曲约束斜撑的水平力分担率β值的增加,各模型的各层屈曲约束斜撑分担的剪力和各层最大层剪力比值也变大。     

自复位摩擦耗能支撑框架的抗震性能分析

自复位摩擦耗能支撑(SCFED)是一种兼具耗能能力与自定心能力的新型支撑。本文将SCFED应用于一9层Benchmark钢框架,采用OpenSEES有限元软件,进行了非线性静力推覆分析(Pushover)和非线性动力时程分析,通过与屈曲约束支撑(BRB)框架结构的对比,研究了SCFED框架结构的抗震性能。结果表明:SCFED框架的最大层间位移角较BRB框架小,且沿楼层高度分布更为均匀;BRB框架平均残余层间位移角约为最大位移角的10%~20%,而SCFED框架的残余位移角仅为最大位移角的1%~2%,表明SCFED能有效控制结构的残余变形;由于SCFED在自复位时的刚度转换较为剧烈,导致SCFED框架的层加速度比BRB框架大。     

Performance evaluation and strengthening of concrete structures with composite bracing members

Steel bracings, in different geometrical forms, are commonly used in steel and concrete structures. The lateral stiffness of structures with concentric bracings depends on the buckling capacity of compressive bracings; in turn, this buckling phenomenon leads to a decrease in the energy dissipation capacity. Composite bracings, composed of steel cores encased in concrete, can increase their capacities. In this paper, the behavior of composite bracings is studied in two parts. At first, a number of braced frames are selected and their behaviors under cyclic loading are studied. Then, using the data obtained from the first part, two existing concrete structures, a three story and a nine story building, are selected and strengthened against seismic loadings by both the conventional concentric steel and the latter composite bracing systems. The behaviors of these structures are then studied by the push-over method and the results for the two types of bracings are compared.     

Seismic response of mega buckling‐restrained braces subjected to fling‐step and forward‐directivity near‐fault ground motions

Special characteristics of earthquakes in the near‐fault regions caused failures for many modern‐engineered structures. Fling‐step and forward‐directivity are the main consequences of these earthquakes. High‐amplitude pulses at the beginning of the seismograph have been obviously presented in forward‐directivity sites. These pulses have high amount of seismic energy released in a very short time and caused higher demands for engineering structures. Fling‐step is generally characterized by a unidirectional large‐amplitude velocity pulse and a monotonic step in the displacement time history. These monotonic steps cause residual ground displacements that are associated with rupture mechanism. In this paper, the seismic performance of steel buckling‐restrained braced frames with mega configuration under near‐source excitation was investigated. Fourteen near‐fault records with forward‐directivity and fling‐step characteristics and seven far‐faults have been selected. Nonlinear time‐history analyses of 4‐story, 8‐story, 12‐story and 15‐story frames have been performed using OpenSees software. After comparing the results, it is shown that, for all frames subjected to the selected records, the maximum demand occurred in lower floors, and higher modes were not triggered. Near‐fault records imposed higher demands on the structures. The results for near‐fault records with fling‐step were very dispersed, and in some cases, these records were more damaging than others. Copyright © 2014 John Wiley & Sons, Ltd.     

横梁加强型人字撑结构的弹塑性滞回性能

人字形支撑是多高层钢结构建筑的一种常用支撑形式。在侧向荷载作用下,由于受压支撑发生屈曲,承载能力快速下降,支撑架的横梁会受到拉压支撑间的竖向不平衡力的作用,JGJ 99—98《高层民用建筑钢结构技术规程》规定需加强支撑架横梁来承受这一不平衡力的作用。静力弹塑性推覆分析的结果显示,在加载后期横梁受拉支撑屈服荷载对加强型人字撑结构抗侧能力有很大的贡献。但是,在强烈地震作用下,由于每一支撑均处于拉压交替的受力状态,因此支撑架的受力性能要复杂得多。通过对横梁加强型支撑结构在往复荷载和地震荷载作用下的抗侧性能进行研究,以更好地了解这种支撑结构的抗震性能。