Peak displacement patterns for the performance-based seismic design of steel eccentrically braced frames

Performance-based seismic design(PBSD) aims to assess structures at different damage states. Since damage can be directly associated to displacements, seismic design with consideration of displacement seems to be logical. In this study, simple formulae to estimate the peak floor displacement patterns of eccentrically braced frames(EBFs) at different performance levels subjected to earthquake ground motions are proposed. These formulae are applicable in a PBSD and especially in direct displacement-based design(DDBD). Parametric study is conducted on a group of 30 EBFs under a set of 15 far field and near field accelerograms which they scaled to different amplitudes to adapt various performance levels. The results of thousands of nonlinear dynamic analyses of EBFs have been post-processed by nonlinear regression analysis in order to recognize the major parameters that influence the peak displacement pattern of these frames. Results show that suggested displacement patterns have relatively good agreement with those acquired by an exact nonlinear dynamic analysis.

Steel rings as seismic fuses for enhancing ductility of cross braced frames

A new remedy is proposed in this study to increase the ductility of cross-braced frames to a level comparable with ductile moment frames.The suggested system consists of one or two concentric steel rings installed in the cross-braced bay vertically.The steel rings are designed such that they fail in bending sooner than failure of the braces in compression.Then the rings act as seismic fuses with multiple bending plastic hinges.Using nonlinear static analysis,it is shown that the proposed system can be designed to behave like cross-braced frames with regard to stiffness and strength,and like special moment frames with regard to ductility.Seismic design factors for the proposed system are recommended based on nonlinear pushover and cyclic analysis studies.

Optimization of design parameters for controlled rocking steel braced dual-frames

A controlled rocking concentrically steel braced frame(CR-CSBF)is introduced as an alternative to conventional methods to prevent major structural damage during large earthquakes.It is equipped with elastic post-tensioned(PT)cables and replaceable devices or fuses to provide overturning resistance and dissipate energy,respectively.Although CR-CSBFs are not officially legalized in globally valid codes for new buildings,it is expected to be presented in them in the near future.The main goal of this study is to determine the optimal design parameters consist of the yield strength and modulus of elasticity of the fuse,the initial force of the PT cable,and the gravity load on the rocking column,considering different heights of the frame,spanning ratios and ground motion types for dual-configuration CR-CSBF.Nonlinear time-history analyses are performed in OpenSees.This study aims to define the optimal input variables as effective design parameters of CR-CSBFs by comparing four seismic responses consisting of story drift,roof displacement,roof acceleration and base shear,and also using the Euclidean metric optimization method.Despite the previous research,this study is innovative and first of its kind.The results demonstrate that the optimal design parameters are variable for various conditions.

Behavior of ductile steel X-braced RC frames in seismic zones

A satisfactory ductile performance of moment-resisting reinforced concrete concentric braced frame structures (RC-MRCBFs) is not warranted by only following the provisions proposed in Mexico’s Federal District Code (MFDC-04). The nonlinear behavior of low to medium rise ductile RC-MRCBFs using steel X-bracing susceptible to buckling is evaluated in this study. The height of the studied structures ranges from 4 to 20 stories and they were located for design in the lakebed zone of Mexico City. The design of RC-MRCBFs was carried out considering variable contribution of the two main lines of defense of the dual system (RC columns and steel braces). In order to observe the principal elements responsible for dissipating the earthquake input energy, yielding mappings for diff erent load-steps were obtained using both nonlinear static and dynamic analyses. Some design parameters currently proposed in MFDC-04 as global ductility capacities, overstrength reduction factors and story drifts corresponding to diff erent limit states were assessed as a function of both the considered shear strength and slenderness ratios for the studied RC-MRCBFs using pushover analyses. Additionally, envelopes of response maxima of dynamic parameters were obtained from the story and global hysteresis curves. Finally, a brief discussion regarding residual drifts, residual drift ratios, mappings of residual deformations in steel braces and residual rotations in RC beams and columns is presented. From the analysis of the obtained results, it is concluded that when a suitable design criterion is considered, good structural behavior of RC-MRCBFs with steel-X bracing can be obtained. It is also observed that the shear strength balance has an impact in the height-wise distribution of residual drifts, and an important “shake-down” eff ect is obtained for all cases. There is a need to improve design parameters currently proposed in MFDC to promote an adequate seismic performance of RC-MRCBFs.

Shaking table test and numerical analysis of a 1:12 scale model of a special concentrically braced steel frame with pinned connections

This paper describes shaking table tests of a 1：12 scale model of a special concentrically braced steel frame with pinned connections, which was fabricated according to a one-bay braced frame selected from a typical main factory building of a large thermal power plant. In order to investigate the seismic performance of this type of structure, several ground motion accelerations with different levels for seismic intensity Ⅷ, based on the Chinese Code for Seismic Design of Buildings, were selected to excite the model. The results show that the design methods of the members and the connections are adequate and that the structural system will perform well in regions of high seismicity. In addition to the tests, numerical simulations were also conducted and the results showed good agreement with the test results. Thus, the numerical model is shown to be accurate and the beam element can be used to model this structural system.

Seismic response of high-rise steel framed buildings with Chevron-braced designed according to Venezuelan codes

The object of this study is to determine the seismic response of regular high-rise steel buildings with chevron-braced frames. Mechanics models of three buildings of 14, 18 and 20 stories are studied, all of them with similar geometric characteristics in plant and elevation. These models are realized using prescriptions and parameters from venezuelan design codes. The seismic action is carry on through varius synthetic design spectrum compatible accelerograms defined by the seismic codes in this study, with three levels of intensity corresponding to three specific Limit States. Dynamic analysis is used to compute parameters of ductility, over strength and maximum displacements. From these results it can be concluded that chevron-braced frames presented a good overall performance and non V-braced frames show greater damage due to dynamic actions, validating non linear dynamic analysis as a very powerful tool to seismic-resistance design and chevron-braced frames as a very useful choice in improving the response of tall steel structures. since this lateral bracing system is absent from Venezuelan seismic codes.

Effectiveness of two conventional methods for seismic retrofit of steel and RC moment resisting frames based on damage control criteria

This study investigates the efficiency of two types of rehabilitation methods based on economic justification that can lead to logical decision making between the retrofitting schemes. Among various rehabilitation methods, concentric chevron bracing（CCB） and cylindrical friction damper（CFD） were selected. The performance assessment procedure of the frames is divided into two distinct phases. First, the limit state probabilities of the structures before and after rehabilitation are investigated. In the second phase, the seismic risk of structures in terms of life safety and financial losses（decision variables） using the recently published FEMA P58 methodology is evaluated. The results show that the proposed retrofitting methods improve the serviceability and life safety performance levels of steel and RC structures at different rates when subjected to earthquake loads. Moreover, these procedures reveal that financial losses are greatly decreased, and were more tangible by the application of CFD rather than using CCB. Although using both retrofitting methods reduced damage state probabilities, incorporation of a site-specific seismic hazard curve to evaluate mean annual occurrence frequency at the collapse prevention limit state caused unexpected results to be obtained. Contrary to CFD, the collapse probability of the structures retrofitted with CCB increased when compared with the primary structures.

Determination Method on Initial Pretension of Cables in Steel Mega Frame and Prestressed Composite Bracing Structure

Determining initial pretension values of pre-stressed cables is one of the key problems for a steel mega frame and pre-stressed composite bracing structure.Through the mechanical analysis of the composite bracing under vertical loading,the critical factors deciding the initial pretention value were found.According to these factors,a rule for the initial pretension value was put forward.The determination equations were acquired based on the principle of force equilibrium at nodes.The numerical results indicate that the internal force disequilibrium in composite bracings resulted from symmetrical load can be eliminated only in a symmetrical way,so that initial pretention values are decided only by vertical loads.The influencing coefficient leveling method,taking into account interactions between story and story,is accurate and feasible.

四川大学博物馆结构设计重难点分析研究

四川大学博物馆采用钢框架结构体系,并设置屈曲约束支撑改善结构的抗震性能。介绍了四川大学博物馆的结构设计参数及抗震性能目标。针对结构设计中的重难点,开展了屈曲约束支撑的工作机理分析、框架柱计算长度与穿层柱分析、东侧大悬挑空间的相关分析。利用反应谱分析及时程分析探究了屈曲约束支撑工作机理,分析结果表明,屈曲约束支撑能够有效改善钢结构产生塑性铰的状况,起到保护主体钢框架的作用。针对东侧大悬挑空间的功能要求,设计采用了屋顶桁架结合钢拉杆吊挂东侧大楼梯的结构形式,通过结构防连续倒塌分析、楼板应力分析、楼盖舒适度分析、关键节点有限元分析等确保该部位结构设计安全合理。对整体结构受力机理和薄弱部位进行了研究,提出了重点部位的设计原则和加强措施。

钢支撑-钢筋混凝土掉层框架结构抗震性能试验研究

为改善坡地钢筋混凝土(RC)掉层框架结构抗侧刚度分布的不均匀性,设计了一栋在上接地层设置钢支撑的RC掉层框架结构,按1/4比例提取其顺坡向中榀框架下部5层子结构为试验对象,开展钢支撑-RC掉层框架拟静力试验,观测试件的破坏过程,并对比RC掉层框架试验所得试件的破坏形态,分析试件滞回性能、延性和刚度退化等抗震性能指标。结果表明:试件的最终破坏是以第4层柱底混凝土压溃,梁端混凝土剥落、底部钢筋断裂,顶层柱顶混凝土剥落为标志。与RC掉层框架试验结果相比,钢支撑-RC掉层框架试件上接地柱的破坏明显减轻,柱端破坏分布也更为均匀,避免了掉层框架结构“半层破坏模式”的出现,改善了结构的耗能和抗地震倒塌能力。但试件上接地层相邻上一层的变形和破坏程度较大,应对该楼层的柱端进行适当的抗震加强;设置钢支撑后,试件的滞回曲线较为饱满,并具有良好的延性(正、负向加载的位移延性系数分别可达5.13、5.69),正负向加载的刚度退化曲线也趋于对称。

基于多水准的高强钢框架-D形偏心支撑结构层剪力分布研究

高强钢框架GD形偏心支撑结构中耗能梁段采用Q355钢,非耗能构件(即框架梁和柱)采用高强度钢材,形成一种新型结构体系.耗能梁段在结构遭遇罕遇地震时充分发挥塑性变形耗能,保护高强钢框架处于弹性受力状态,高强钢框架GD形偏心支撑结构由于在塑性状态下结构刚度发生改变,其层剪力分布模式不再符合基于强度的设计理论,因此,本文研究了高强钢框架GD形偏心支撑结构在多水准地震动状态下的层剪力分布模式.本文设计了四种不同层数(4层,8层,12层和16层)和三种不同耗能梁段长度(900mm,1000mm和1100mm)的高强钢框架GD形偏心支撑结构,输入40条近场脉冲地震和40条远场地震记录,得到结构在多遇地震、设防地震、罕遇地震以及极罕遇地震下的层剪力分布模式,并对罕遇地震下的层剪力分布模式进行参数标定,获取塑性状态下结构的弹塑性侧向力分布状态.

自复位支撑钢框架抗震性能评估与损伤演化分析

为研究自复位耗能(SCED)支撑失效前后支撑钢框架结构的抗震韧性及损伤演化规律,设计了一单榀三层自复位支撑钢框架(SCBSF)并对其进行精细化有限元模拟,探究了支撑第二刚度和摩擦力在考虑支撑失效时对结构宏观响应和关键构件损伤状态的影响规律。结果表明,在地震作用下,SCED支撑作为SCBSF第一道抗震防线,激活工作后为结构提供了稳定的耗能与卓越的复位能力,相比屈曲约束支撑(BRB)框架残余变形角最大减小了84.9%,显著降低结构震后修复成本;支撑达到最大行程破坏失效后,结构层间变形加剧,框架梁和柱塑性耗能占比增加了92.33%,主体结构损伤值增加了48.45%,结构的抗震性能与韧性水平明显降低。参数分析结果表明,提升支撑的第二刚度和摩擦力能有效降低结构响应,但具有较大第二刚度和摩擦力的支撑框架仍抵抗不了强震作用时,可能导致更多的支撑失效破坏,结构损伤加剧。因此,设计SCED支撑时,需要适当提高支撑第二刚度和摩擦力,建议第二刚度取值为第一刚度的7/50~4/25,支撑摩擦力与预压力比值取为1~1.2;提升支撑第二刚度相比提升摩擦力对结构损伤值控制效果更好,随着地震动强度增大,摩擦力与第二刚度对结构抗震性能的影响逐渐减小。

中心支撑半刚性连接钢框架抗震性能

为研究单边螺栓与T型钢连接的中心支撑钢框架的抗震性能,对1个1∶2缩尺的单跨单层的钢框架进行拟静力加载试验。采用ABAQUS有限元软件,建立子精细化非线性有限元模型,在验证子有限元模型的可靠性与精度基础上,通过改变支撑的孔径尺寸、壁厚以及形状等变量对框架整体抗震性能进行参数化分析。结果表明:有限元模拟结果与试验结果吻合度良好,可用于半刚性连接钢框架在循环荷载下的性能研究。随着支撑孔径和壁厚的增加,框架的滞回曲线更加饱满,刚度、耗能能力和延性皆有明显提升。方形截面支撑的抗震性能对比圆截面更加优异。梁柱节点的半钢性连接方式较刚性连接方式抗震性能更加优异。

多层剪切型支撑钢框架整体稳定性的解析算法研究

剪切型支撑钢框架整体稳定计算需考虑钢框架与剪切型支撑之间的相互作用,还需要考虑同层柱间的相互支援以及层与层之间的支援作用,规范计算长度系数法无法考虑这些因素可能会造成不合理设计。提出了一种计算多层剪切型支撑钢框架整体稳定性的解析算法,首先分析了支撑刚度与结构临界承载力之间的关系,推导了层临界支撑刚度计算公式,接着利用弹簧-摇摆柱力学模型推导了任意支撑下框架柱临界刚度比系数计算公式,然后将各楼层的有效抗侧刚度及荷载刚度进行楼层间的组装,将求解剪切型支撑钢框架的临界承载力转化为求解结构的楼层有效抗侧刚度,最后基于轴力权重加权平均的方法推导了可直接求解多层剪切型支撑钢框架临界承载力的计算公式,该公式能够判断结构的薄弱层,可以定量地计算楼层之间相互支援程度,有效地弥补规范尚无法求解弱支撑钢框架柱计算长度系数的不足。

某新型半导体显示产业钢厂房备料区抗震性能分析

以某新型半导体显示产业钢结构厂房备料区为工程背景,建立了厂房钢框架-支撑结构的非线性有限元模型,分别对多遇地震和罕遇地震下的结构抗震性能进行了分析。结果表明,结构整体指标均满足设计规范要求,第3层和第6层的层间位移角较大;罕遇地震下构件损伤主要集中在支撑,而结构底层角柱和中部楼层的边跨梁有轻微损伤;钢支撑能够有效提高结构刚度,减小梁柱的损伤。

广州南沙国际金融论坛永久会址主体结构设计

广州南沙国际金融论坛永久会址属于大型复杂会议类建筑,具有大底盘双塔、钢屋盖连体、大开洞、竖向质心偏移、大跨结构等多项不规则项。介绍了结构重力体系和抗侧体系的选型思路,主体结构选用混凝土框架+屈曲约束支撑体系,外表皮采用空间钢结构。重点分析了单塔与整体结构的差异以及钢屋盖对下部双塔的影响。通过弹塑性分析,对大震作用下各个时程点的损伤水平进行评估,对大开洞的楼板进行了地震及温度作用下的楼板应力分析;对大跨桁架进行了抗连续倒塌分析。结果表明结构可满足大震不倒的要求,支撑承载力能充分发挥,大底盘结构具有良好的基座支撑特性。

中原文旅城欢乐世界一期雪世界高区结构设计

中原文旅城欢乐世界一期雪世界高区结构体系采用巨型钢框架,存在结构扭转不规则、楼板缺失、大悬挑、竖向构件间断及钢桁架平台为重载大跨钢结构等不规则情况和设计难点。设计中采用基于性能的抗震设计方法,根据构件的重要程度采用不同的性能目标。4个巨型框架柱为支撑框架结构,其支撑采用屈曲约束支撑,使结构在大震作用下形成良好的屈服机制和耗能能力。对结构进行小震反应谱法弹性分析、弹性时程分析、中震弹性分析以及大震弹塑性分析。结果表明,结构整体和构件的抗震性能均能达到设计的预期目标;巨型钢框架结构体系较好地适应了本工程的实际情况,可供类似项目在选择结构体系时参考。

连梁可更换的Y型偏心支撑组合框架力学性能试验研究

为研究连梁可更换的Y型偏心支撑组合框架(简称“Y型V-EBCF”)的力学性能,设计制作了一榀Y型V-EBCF试件,其中可更换式连梁由低屈服点钢加工并通过高强螺栓与周边构件相连。对Y型V-ECBF试件和拆卸了连梁后的试件分别进行了拟静力试验,分析了试件的失效模式、荷载-位移曲线和残余位移角,试验结果表明:在连梁失效前,结构具有稳定的滞回性能和良好的耗能能力;当拆卸失效连梁后,结构在组合框架的不平衡内力下恢复至原位,残余变形角为0.002 rad,小于规范限值(0.005 rad)及最大可更换残余位移角限值(0.0023 rad),且余下试件再加载时的滞回曲线依旧饱满稳定,表明Y型V-ECBF体系具有良好的震后恢复能力;采用现行AISC360-16规范相关公式可以准确计算低屈服点钢连梁的承载力,但超强系数建议取3.0以防止周边构件的破坏;Y型V-ECBF体系的刚度和承载力等于偏心支撑中连梁和组合框架的刚度与承载力之和。此外,Y型V-EBCF中的连梁会引起框架梁跨中承担额外的弯矩,导致跨中混凝土板的开裂,该文给出了计算跨中混凝土板裂缝宽度的公式。

基于遗传算法的钢框架-BRB结构优化

将钢框架—BRB结构优化设计分为小震优化阶段和大震调整阶段。小震优化阶段以造价最低为优化目标,引入拓扑变量来表示BRB的布设位置,将BRB的位置、等效截面面积和框架的截面作为设计变量,在同时满足梁柱构件的强度和稳定性约束、BRB不屈服约束及层间位移约束的前提下,通过遗传算法对钢框架—BRB结构进行整体优化;大震调整阶段对不符合大震层间位移约束的结构采用准则法调整构件截面。进行了较为全面的钢框架—BRB结构优化设计,在满足规范要求的前提下大幅降低结构造价,提高了结构的经济性。

某异形钢结构的消能减震设计

以一栋异形高层钢结构工程为例,为控制结构构件尺寸,减小地震作用下结构的层间位移,提高结构的抗震性能,故引入消能减震技术。采用黏滞阻尼器进行消能减震设计,分别运用YJK和Perform3D软件进行了多遇地震、设防地震和罕遇地震分析。结果表明:多遇地震作用下,减震结构的基底剪力和层间位移角均有不同程度的减小,减震效果明显。设防地震和罕遇地震作用下,黏滞阻尼器耗能占比较大,梁柱构件得到了较好的保护,各项指标满足减震性能目标。