Quantification of seismic performance factors of steel diagrid system

This study is carried out to evaluate the response modification, overstrength, and deflection amplification factors for steel diagrid system. To quantify these factors, the rational procedure introduced in Federal Emergency Management Agency (FEMA) P695, which is based on low probability of structural collapse and encompasses nonlinear static and dynamic analyses, is used. To this end, a series of diagrid structures with various slopes of external braces are designed. Nonlinear static analyses are performed to obtain system overstrength and period‐based ductility. Incremental dynamic analyses are then performed to assess collapse margin ratio (CMR) of the archetypes. The effects of spectral shape of different ground motions are considered in CMR values to obtain an adjusted CMR (ACMR), for each archetype. The values of calculated ACMRs are compared with the acceptable values of FEMA P695 to assess reliability of presumed seismic performance factors of diagrid systems. The results show that values of calculated ACMRs for steel diagrid systems designed using the response modification factor equal to 4.50 are acceptable. It is also observed that as the slope of the peripheral diagonal elements increases, the overstrength and the collapse margin ratios decrease, and the ductility of system increases.     

Seismic performance evaluation of diagrid system buildings

In this study, the seismic performance of typical diagrid structures was investigated. To this end, 36‐storey diagrid structures with various slopes of external braces were designed and their seismic responses were evaluated using nonlinear static and dynamic analyses. A tubular structure and a diagrid structure with buckling‐restrained braces were also designed with the same design loads, and their seismic performances were compared with those of the diagrid structures. According to the analysis results, the diagrid structures showed higher overstrength with smaller ductility compared with the tubular structure. It was also observed that as the slope of braces increased the shear lag effect increased and the lateral strength decreased. Both the strength and ductility of diagrid structures increased significantly when the diagonal members were replaced by buckling‐restrained braces. Copyright © 2010 John Wiley & Sons, Ltd.     

Progressive collapse behavior of rotor‐type diagrid buildings

In this study, the progressive collapse‐resisting capacities of axi‐symmetric or rotor‐type diagrid structural system buildings were evaluated based on arbitrary column removal scenario. For analysis models, 33‐story buildings with cylindrical, convex, concave and gourd shapes were designed, and their nonlinear static and dynamic analysis results were compared. The effect of design variables such as the number of total stories, slope of diagrids and the location of removed members was also investigated. According to the analysis results, the rotor‐type diagrid structures showed sufficient progressive collapse‐resisting capacity regardless of the differences in shapes when a couple of diagrids were removed from the first story. The design parameter such as building height and the slope of the diagrids did not affect the results significantly as long as they were designed to meet the current design code. Copyright © 2012 John Wiley & Sons, Ltd.     

Behaviour Factor of code-designed steel moment-resisting frames

Current seismic codes are based on force-controlled design or capacity design, using the base shear concept. The most important parameter in this approach is the response modification factor, also called behaviour factor, which is used to design the structure at the ultimate limit state by taking into account its capacity to dissipate energy by means of plastic deformations. In this paper overstrength, redundancy and ductility response modification factors of steel moment resisting frames are evaluated. In order to cover a wide range of structural characteristics, 12 steel moment-resisting frames (6 regular and 6 irregular in elevation) have been designed and analysed. Both static pushover analyses and nonlinear incremental dynamic analyses have been performed. The investigation focuses on the effects of some parameters influencing the responsemodification factor, including the regularity, the number of spans and the number of storeys. As a conclusion, a local ductility criterion has been proposed to improve the provisions given in the Italian seismic code.     

Empirical evaluation of ductility factors for the special steel moment‐resisting frames in view of soil condition

It is well known that the response modification factor (R) takes into account the ductility, over‐strength, redundancy and damping of structural systems. The ductility factor has played an important role in seismic design, as it is a key component of R. In this study, the ductility factors (R_μ,MDOF) of special steel moment‐resisting frames are calculated by multiplying the ductility factor of single degree of freedom (SDOF) systems (R_μ,SDOF) with the multi‐degree of freedom (MDOF) modification factors (R_M). The ductility factors (R_μ,SDOF) of SDOF systems are computed from non‐linear dynamic analysis undergoing different levels of displacement ductility demands and periods when subjected to a large number of recorded earthquake ground motions. To compute the R_μ,SDOF, a group of 1,860 ground motions recorded from 47 earthquakes were considered. R_M factors are proposed to account for the MDOF systems, based on previous studies. A total of 108 prototype steel frames were designed to investigate the ductility factors, considering design parameters such as the number of stories (4, 8 and 16), framing systems (perimeter frames and distributed frames), failure mechanisms (strong column‐weak beam and weak column‐strong beam), soil profiles (S_A, S_C and S_E in Uniform Building Code 1997) and seismic zone factors (Z = 0·075, 0·2, and 0·4 in UBC 1997). The effects of these design parameters on the R_μ,MDOF of special steel‐moment‐resisting frames were investigated. Copyright © 2009 John Wiley & Sons, Ltd.     

Analytical investigation of response modification (behaviour) factor,R, for reinforced concrete frames rehabilitated by steel chevron bracing

Steel bracing of reinforced concrete (RC) frames has received noticeable attention in recent years as a retrofitting measure to increase the shear capacity of the existing RC buildings. In order to evaluate the seismic behaviour of steel-braced RC frames, some key response parameters, including the ductility and the overstrength factors, should first be determined. These two parameters are incorporated in structural design through a force reduction or a response modification factor. In this paper, the ductility and the overstrength factors as well as the response modification factor (or seismic behaviour factor) for steel chevron-braced RC frames have been evaluated by performing inelastic pushover analyses of brace-frame systems of different heights and configurations. The effects of some parameters influencing the value of behaviour factor, including the height of the frame and share of bracing system from the applied lateral load have been investigated. It is found that the latter parameter has a more localised effect on the R values and its influence does not warrant generalisation at this stage. However, the height of this type of lateral load-resisting system has a profound effect on the R factor, as it directly affects the ductility capacity of the dual system. Finally, based on the findings presented in the article, tentative R values have been proposed for steel chevron-braced moment-resisting RC frame dual systems for different ductility demands and compared with different type of bracing systems.     

抗弯钢框架结构影响系数研究

结构影响系数主要包括延性折减系数和超强系数。给出了按我国抗震规范设计的几个典型抗弯钢框架结构,采用弹塑性时程分析方法确定了结构的整体能力曲线。考虑了结构层数、跨数等对结构延性和超强能力的影响,给出了在典型地震波作用下的各结构的结构影响系数值。建议在我国的结构抗震设计中,抗弯钢框架的结构影响系数最小值可取4。     

Overstrength and force reduction factors of multistorey reinforced‐concrete buildings

This paper addresses the issue of horizontal overstrength in modern code‐designed reinforced‐concrete (RC) buildings. The relationship between the lateral capacity, the design force reduction factor, the ductility level and the overstrength factor are investigated. The lateral capacity and the overstrength factor are estimated by means of inelastic static pushover as well as time‐history collapse analysis for 12 buildings of various characteristics representing a wide range of contemporary RC buildings. The importance of employing the elongated periods of structures to obtain the design forces is emphasized. Predicting this period from free vibration analysis by employing ‘effective’ flexural stiffnesses is investigated. A direct relationship between the force reduction factor used in design and the lateral capacity of structures is confirmed in this study. Moreover, conservative overstrength of medium and low period RC buildings designed according to Eurocode 8 is proposed. Finally, the implication of the force reduction factor on the commonly utilized overstrength definition is highlighted. Advantages of using an additional measure of response alongside the overstrength factor are emphasized. This is the ratio between the overstrength factor and the force reduction factor and is termed the inherent overstrength (Ω _i). The suggested measure provides more meaningful results of reserve strength and structural response than overstrength and force reduction factors. Copyright © 2002 John Wiley & Sons, Ltd.     

Response modification factor for steel moment-resisting frames by different pushover analysis methods

The earthquake loads imposed to the structures are generally much more than what they are designed for. This reduction of design loads by seismic codes is through the application of response modification factor (R-factor). During moderate to severe earthquakes, structures usually behave inelastically, and therefore inelastic analysis is required for design. Inelastic dynamic analysis is time consuming and interpretation of its results demands high level of expertise. Pushover analysis, recently commonly used, is however, a simple way of estimating inelastic response of structures. Despite its capabilities, conventional pushover analysis (CPA) does not account for higher mode effects and member stiffness changes. Adaptive pushover analysis (APA) method however, overcomes these drawbacks. This research deals with derivation and comparison of some seismic demand parameters such as ductility based reduction factor, R_μ, overstrength factor, Ω, and in particular, response modification factor, R, from capacity curves obtained from different methods of APA and CPA. Three steel moment-resisting frames of 3, 9 and 20 stories adopted from SAC steel project are analyzed. In pushover analyses for each frame, eight different constant as well as adaptive lateral load patterns are used. Among the main conclusions drawn is that the maximum relative difference for response modification factors was about 16% obtained by the methods of conventional and adaptive pushover analyses.     

Proposing the hexagrid system as a new structural system for tall buildings

In order to improve the efficiency of tube‐type structures in tall buildings, a new structural system, called hexagrid, is introduced in this paper. In comparison with diagrid system, it consists of multiple hexagonal grids on the face of the building. In this research, a set of structures using diagrid system having four various diagonal angles and hexagrid system were designed on a strength and stiffness‐based approach for buildings with 30, 50, 70 and 90 stories to withstand wind load. The impact of different geometric configurations of structural members on the maximum lateral displacement and architectural performance in both diagrid and hexagrid systems is compared. The stiffness sensitivity using a similar interior bracing system in both systems is also discussed. In this study, the seismic performance of a 30‐story diagrid structure and a hexagrid structure was evaluated using nonlinear static and dynamic analyses. According to the results, the hexagrid system has a better architectural view and more ductility and stiffness sensitivity, which are about three times than that of the diagrid system. And finally, in comparison with the diagrid system, the hexagrid system has enough potential to push the height limit. The guidelines discussed here are for architectural and structural engineers to improve freehand design. Copyright © 2012 John Wiley & Sons, Ltd.     

Diagrid: An innovative,sustainable, and efficient structural system

In recent years, diagrid structures have received increasing attention among both designers and researchers of tall buildings for creating one‐of‐a‐kind signature structures. This paper presents a state‐of‐the‐art review of diagrid structures. First, various diagrid configurations, main factors affecting their behaviors, and related design parameters and approaches are discussed. Then, diagrid applications for free‐form steel and concrete structures are introduced showing the diagrid applicability for complex structures followed by recent advances in structural design of diagrid connections, diagrid nonlinear behavior, and structural control of diagrids. Recent studies about a new variation of tubular and diagrid systems, hexagrids, are discussed briefly. Finally, the diagrid potential in design of sustainable buildings is delineated.     

Inelastic performance of cold-formed steel strap braced walls

The inelastic performance of sixteen 2.44 m×2.44 m cold-formed steel strap braced walls was evaluated experimentally. The performance was affected by the holddown detail, which in many cases did not allow the test specimens to reach or maintain a yield capacity and severely diminished the overall system ductility. “Test-based” R_d×R_o values of 3.65, 2.11 and 1.72 indicate the low ductility levels, which were not adequate to warrant the use of a seismic response modification coefficient of R=4.0 in design. Capacity design of the SFRS elements must account for the overstrength of the strap material.     

非均匀弯曲下钢梁的极限承载性能

钢梁的极限承载性能极大地影响着钢框架的整体性能。主要的响应参数是转动能力和极限抗弯承载力。前者来自地震作用下通过结构的整体能量耗散性能获得的对局部延性的需求,而后者则需要在抗震设计中成功运用等级标准来获取抗弯强度。因此,对于抗震设计的应用来说,根据钢构件的延性和强度来对其进行双重分类是最合适的方法。目前,现代的国际设计规范是根据型钢的分类来对结构进行塑性设计和抗震设计,但有所误导地着重以局部屈曲为主要的应变-弱化效应。即使科技文献中提供了预测非均匀弯曲下钢构件的极限承载性能的各种方法,仍应对其进行进一步研究,因为最终结果和各种横断面的形状会受到高参数的影响。因此,在科技文献中介绍了一种新的试验方案,其能通过挑选过的﹑合适的﹑局部长细比不同的试验样本进行数据集成,从而处理单调和循环加载下许多不同的横截面类型(H型截面,工字型截面,方管形管截面,矩形管截面)。最后对得到的数据结果进行研究讨论。     

New approach to evaluate the response modification factors for steel moment resisting frames

The design force levels currently specified by most seismic codes are calculated by dividing the base shear for elastic response by the response modification factor (R). This is based on the fact that the structures possess significant reserve strength, redundancy, damping and capacity to dissipate energy. This paper proposed the evaluation methodology and procedure of the response modification factors for steel moment resisting frames. The response modification factors are evaluated by multiplying ductility factor (R _μ) for SDOF systems, MDOF modification factor (R _M ) and strength factor (R _S ) together. The proposed rules were applied to existing steel moment resisting frames. The nonlinear static pushover analysis was performed to estimate the ductility (R _μ), MDOF modification (R _M ) and strength factors (R _S ). The results showed that the response modification factors (R) have different values with various design parameters such as design base shear coefficient (V/W), failure mechanism, framing system and number of stories.     

不同设防烈度下RC框架结构的抗侧向倒塌能力

地震作用下建筑结构的抗侧向倒塌能力是抗震性能评价的基础。选取4个结构整体性能参数作为结构抗侧向倒塌能力评价指标,分别为结构强屈比、超强系数、延性系数和延展系数。按中国现行规范设计了12个RC框架结构,考虑侧向力分布形式和设防烈度的影响,采用Pushover方法对结构进行计算,并根据能力曲线和结构整体性能参数对结构抗侧向倒塌能力进行评价。结果表明:结构整体性能参数能从强度储备和变形能力两个方面对结构抗侧向倒塌能力进行分析;随着设防烈度和结构高度的提高,侧向力分布形式对结构抗侧向倒塌能力的影响增大;设防烈度对结构强屈比和结构延展系数的影响较小,对结构超强系数和结构延性系数的影响较大;随着设防烈度的提高,结构超强系数减小,而结构延性系数增大。     

Effects of soil‐structure interaction and lateral design load pattern on performance‐based plastic design of steel moment resisting frames

The effects soil‐structure interaction (SSI) and lateral design load‐pattern are investigated on the seismic response of steel moment‐resisting frames (SMRFs) designed with a performance‐based plastic design (PBPD) method through a comprehensive analytical study on a series of 4‐, 8‐, 12‐, 14‐, and 16‐story models. The cone model is adopted to simulate SSI effects. A set of 20 strong earthquake records are used to examine the effects of different design parameters including fundamental period, design load‐pattern, target ductility, and base flexibility. It is shown that the lateral design load pattern can considerably affect the inelastic strength demands of SSI systems. The best design load patterns are then identified for the selected frames. Although SSI effects are usually ignored in the design of conventional structures, the results indicate that SSI can considerably influence the seismic performance of SMRFs. By increasing the base flexibility, the ductility demand in lower story levels decreases and the maximum demand shifts to the higher stories. The strength reduction factor of SMRFs also reduces by increasing the SSI effects, which implies the fixed‐base assumption may lead to underestimated designs for SSI systems. To address this issue, new ductility‐dependent strength reduction factors are proposed for multistory SMRFs with flexible base conditions.     

带可更换低屈服点耗能梁段-端板连接的#br# 钢框筒结构抗震性能试验研究

为研究带可更换低屈服点耗能梁段 端板连接的钢框筒结构(SFTS-RSLs)抗震性能和震后可更换能力,以耗能梁段长度和楼板组合效应为研究变量,设计3个2/3缩尺的单层单跨SFTS-RSLs子结构平面试件。框筒柱和裙梁采用Q460高强钢,耗能梁段采用低屈服点钢LYP225。通过水平低周往复加载试验对结构的破坏模式、刚度、承载力、耗能能力、延性、可更换能力以及耗能梁段塑性转角与超强系数进行研究。试验结果表明：试件滞回曲线饱满,延性高,具有稳定、良好的耗能能力和塑性变形能力;耗能梁段的破坏模式主要为翼缘严重屈曲且翼缘 端板焊缝撕裂或腹板撕裂;耗能梁段超强系数均值约为1.95,极限塑性转角超过0.18rad,远大于AISC 341-16规定的塑性转角限值0.08rad;楼板组合效应对结构承载力、耗能能力、延性、可更换能力、耗能梁段塑性转角和超强系数影响不大,对结构的弹性刚度影响显著;减小耗能梁段长度能够提高结构承载力、抗侧刚度、耗能梁段塑性转角和超强系数,但会降低结构的耗能能力和延性;加载过程中,结构的塑性变形与损伤集中在耗能梁段,框筒柱和裙梁处于弹性状态,有利于结构震后修复与正常使用功能的快速恢复。     

Evaluation of the AISI S213 seismic design procedures through testing of strap braced cold-formed steel walls

The North American standard for the design of lateral systems constructed of cold-formed steel (CFS) (AISI S213), which is published by the American Iron and Steel Institute (AISI), was updated in 2007. Included in this update were changes to the provisions related to strap braced walls. The 2007 version of AISI S213 includes requirements for the brace material and the use of capacity design principles; as well, the engineer is directed towards using welded connections to avoid possible net section fracture of the braces. Screw-connected braces may be used if shown to perform in a ductile fashion. The research described herein was undertaken to evaluate typical weld and screw-connected single-storey strap braced wall configurations with respect to their ability to resist lateral in-plane loads in the inelastic range of behaviour; that is, the results were used to verify the new strap braced wall seismic design provisions in AISI S213. A total of 44 tension-only X-braced walls ranging in size from 610×2440 mm² to 2440×2440 mm² (aspect ratios from 4:1 to 1:1), designed and detailed following a capacity-based approach, were tested under lateral loading using monotonic and reversed cyclic protocols. The strap braces were expected to undergo gross cross-section yielding along their length, while the other elements in the seismic force resisting system (SFRS) were selected to be able to carry the probable brace capacity. A summary of the test program is provided, including design approach, failure modes and ductility measures. The scope of the research also included the determination of “test-based” seismic force modification factors based on the measured ductility and overstrength of the test walls for comparison with the R-values recommended in AISI S213.     

Evaluation of behavior factor of tunnel‐form concrete building structures using Applied Technology Council 63 methodology

Tunnel‐form construction system is used for buildings made up of slabs and walls. This system has represented fast construction technique, low cost and time saving in the construction period. However, there are no special requirements for seismic design of this type of buildings, and some provisions of the concrete bearing wall system are used. None of the seismic codes addresses the R factor for tunnel‐form buildings directly, and some doubts remained about prediction of the fundamental period. Meanwhile the force‐based methods, which depended on R factor and fundamental period, are still used for seismic design of this type of buildings. There is a need for evaluating the R factor for tunnel‐form buildings. The methodology was developed by the Applied Technology Council (ATC) entitled ATC‐63 Project for quantifying building system performance and response parameters for use in seismic design. Here, this methodology is used to estimate more reliable R factor for tunnel‐form buildings. An experimental program was carried out to decrease the uncertainty of finite element modeling and using it as a part of the methodology. A database was created, and some analyses were performed. The results of analysis were summarized to propose more accurate R factor for tunnel‐form buildings. Copyright © 2011 John Wiley & Sons, Ltd.     

压弯钢板延性系数及面向抗震设计的截面分类研究

对简支边界条件的压弯矩形钢板进行了考虑几何和材料非线性、残余应力、初始几何缺陷的非线性分析,得到弯矩-边缘纤维压应变曲线和弯矩-曲率曲线,按照两种曲线上延性系数的定义确定不同通用宽厚比下板件的延性系数,拟合了压弯板延性系数计算式。分析表明,按曲率定义延性小于按边缘纤维压应变定义的延性。另外根据面向抗震设计的钢构件截面分类的方法,利用结构延性需求与截面延性需求的关系和得到的截面延性需求的计算式,区分结构影响系数中是否包含超强系数,对各类截面板件的宽厚比进行分界,并拟合了宽厚比分界与结构影响系数的相关计算式。