Approximate limit analysis of three-dimensional multi-storey frames under seismic actions

The introduction of seismic codes compels the engineer to check the resistance of structures when they are subjected to horizontal forces. If an existing frame was built when seismic codes were not in use, then the columns will obviously appear as the weakest point of the whole structure. In this case, therefore, sway mechanisms are the usual failure mechanisms under the action of seismic horizontal forces. Accordingly, it would appear that an ad hoc limit analysis, which allows the lateral load capacity to be quickly checked, would be appropriate.

The proposed method is based on the hypothesis that the limit curve of the shearing stresses of each column can be approximated by an ellipse. This hypothesis allows simplifications to be introduced which make the method feasible for implementation on a small personal computer.     

New nonlinear anti‐seismic steel device for the increasing the seismic capacity of multi‐storey reinforced concrete frames

In order to increase the seismic capacity of multi‐storey planar reinforced concrete (r/c) frames, a new metal frictional device, which the capacity of a restricted rotation around the horizontal axis perpendicular to the vertical frame plane, is presented. The proposed steel device is joined to the four joints of the vertical floor span of the frame via four diagonal steel dual‐hinge bars. During the above restricted rotation, frictional forces develop, due to a suitable synthetic material that is inserted into the rotational frictional connections. When the horizontal relative floor displacement, between two floors, exceeds a desired specific value, then the proposed device locks and the diagonal steel bar becomes fully activated to tension, adding significant additional strength to the frame. This device is installed in a vertical floor span, but it is worth noting that the devices can be placed at floor spans that are not on the same vertical line. The nonlinear numerical (static/dynamic) analyses carried out in the present article, shows that the proposed devices contribute in increase of the lateral stiffness of the frame, the lateral strength of the frame in the inelastic area and the absorption of the inserted seismic energy. Furthermore, this device protects the diagonal steel bars from the buckling or from premature failure of compression. In addition, the proposed device is designed in a way that allows it to operate effectively under large horizontal relative floor displacements due to the cyclic dynamic loads, and can be used instead of the structural r/c walls. Copyright © 2010 John Wiley & Sons, Ltd.     

Design of unbraced multi-storey steel frames

Simple approximate methods for analysis of unbraced frames have played a part in encouraging the use of steel in construction. These methods have been supported by research, to verify and to extend their scope. This paper questions their continued use and further development. It is appropriate to do this at the present time when documentation and software are needed to introduce the Eurocodes. Approximate methods still have a role, in initial design. For unbraced frames early consideration needs to be given to limiting sway in service conditions. A very simple formulation is presented, to enable section sizes to be determined to satisfy this criterion.     

多层砖混结构顶层裂缝探究

通过对多层砖混结构的观察 ,分析了顶层墙体变形、裂缝及薄弱的表现形式、规律、特点和原因 ,从理论到实践 ,从设计到施工提出控变抗裂措施。     

Stability of multi-storey unbraced steel frames subjected to variable loading

Proposed in this paper is an approach of evaluating the elastic buckling loads for multi-storey unbraced steel frames subjected to variable loading or non-proportional loading. In the case of variable loading, the conventional assumption of proportional loading is abandoned, and different load patterns may cause the frame to buckle at different levels of critical loads. In light of the use of the storey-based buckling concept to characterize the lateral sway buckling of unbraced framed structures, the problems of determining the lower and upper bounds among all of the frame buckling loads associated with different load patterns are presented as a pair of minimization and maximization problems subjected to elastic stability constraints. The problems take into account the semi-rigid behaviour of beam-to-column connections and the lateral stiffness reduction of columns due to the presence of an axial compressive load. The minimization and maximization problems are then solved by a linear programming method; thus, the lower and upper bounds of the frame buckling loads subjected to variable loading are obtained. Parametrical studies on the influence of the connection rigidity to the lower and upper bounds of critical loads and the comparisons to the conventional proportional loading are also presented in this paper.     

高位转换超限高层结构抗震分析

针对一高位转换超高层超限建筑进行抗震计算及分析,分析中采用SATWE和MIDAS两种软件对结构进行小震计算,并补充弹性时程分析,使用PUSH&EPDA计算罕遇地震下的弹塑性分析,找出结构关键及薄弱部位,确定合理的性能目标,采取相应的计算或加强措施,分析结果表明,结构的工作状态和性能均能达到设计的预期性能目标和规范要求。     

多层钢框架偏心支撑的抗震性能试验研究

偏心支撑钢框架结构是一种比较理想的多层钢结构抗侧力体系,刚度大,受力性能合理。为了研究偏心支撑钢框架在地震作用下的滞回性能,为多层钢框架偏心支撑结构体系设计提供试验依据,选用耗能梁长度为可变参数,完成了2个不同构造形式的弯曲型耗能梁-偏心支撑框架在水平低周循环荷载下的破坏试验,分析了框架的承载能力及变形特征,研究了框架的破坏模式。研究表明,偏心支撑框架在弹性阶段内具有良好的变形能力,屈服荷载和框架刚度随弯曲型耗能梁长度增加呈下降趋势;偏心支撑框架能够控制框架破坏模式,地震作用下框架在耗能梁段屈服后屈曲破坏,破坏发生在耗能梁端部翼缘和支撑与柱脚连接的节点板位置,降低了梁柱节点受力。     

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。     

采用隔震支座的加层框架结构地震反应研究

以一栋高烈度区钢筋混凝土框架结构为原型,对其顶部增加结构层,提出了分别采用两种不同材料(钢和钢筋混凝土)进行传统直接加层和隔震加层四个不同的加层方案,通过工程数值模拟分析,对比研究了它们在地震作用下的响应,以评断隔震加层结构的减震效果及相互之间的减震规律。     

浅谈多层砖混结构房屋的抗震设计

从合理选择建筑物平、立面形状,合理布置纵墙和横墙,有效设置房屋圈梁和构造柱等方面论述了多层砖混结构房屋的抗震设计要点,并提出相关质量保证措施,从而保证建筑质量,以最大限度地减少灾害给人类造成的损失。     

多层砌体结构窗下墙破坏模式试验研究

为验证多层砌体结构开洞墙体窗下墙破坏模式震害现象,设计缩尺比例为1:3的三层三跨砌体结构开洞墙体试件,对其进行拟静力低周往复加载试验。分析多层砌体墙体窗下墙破坏模式的破坏特征,研究整体墙及各层墙体的滞回特性、骨架曲线、刚度退化规律、耗能特性、延性与变形特征等抗震性能。试验结果表明：发生窗下墙破坏模式的开洞墙体破坏过程分为裂缝出现、变形发展与强度退化三个阶段;窗下墙作为第一道抗震防线可提前参与整体结构耗能、降低墙体刚度退化速率、增加整体墙的延性和变形能力,其破坏模式相对窗间墙破坏模式更为合理。研究结果揭示了发生窗下墙破坏模式墙体的抗震性能与变形特征,为实现砌体结构合理破坏机制提供依据。     

Influence of the link overstrength factor on the seismic behaviour of eccentrically braced frames

The paper investigates the influence of the link overstrength factor on the seismic behaviour of eccentrically braced frames designed in accordance with capacity design principles. To this end, systems characterised by different level number and link length are designed according to the force-based approach, by either static or modal analysis. In order to correctly evaluate the ability of structures to develop global collapse mechanisms characterized by remarkable exploitation of the deformation capacity of links, incremental dynamic analyses are carried out with reference to either deterministic or random values of yield strength. While the first analyses are intended to lead to the conventional evaluation of the seismic response of EBFs, those referred to random values of yield strength are carried out to achieve a more correct verification of the capacity design principles and a deep comprehension of the sensitivity of the structural response to real strength distributions. On the basis of such results, the study also permits a critical analysis of the values of the behaviour factor generally proposed by building codes with reference to this structural typology.     

Collapse analysis of steel moment frames with various seismic connections

Seismic connections with high ductile capacity are generally considered to be effective for resisting seismic loads. However, additional studies are still needed to evaluate the performance of seismic connections during progressive collapse. In this study the progressive collapse resisting capacity of the Reduced Beam Section (RBS), Welded Cover Plated Flange (WCPF), and Welded Unreinforced Flange-Welded Web (WUF-W) connections, which are seismic connections recommended by the FEMA/SAC project, was investigated. For progressive collapse analysis, two types of steel moment frame buildings were considered; one designed for high-seismic load and the other designed for moderate-seismic load. The vertical displacement at the point of column removal and the plastic hinge rotation at beam ends were checked by using an alternative load path method proposed in the guidelines. The analysis results showed that the performance of the Cover Plate connection turned out to be the most effective in resisting progressive collapse, especially in structures located in moderate-seismic regions.     

The seismic behaviour of steel moment-resisting frames with stiffening braces

The present paper deals with the seismic behaviour of steel structures which are designed in the attempt of exploiting the dual characteristics of moment resisting frames (MRFs) and concentrically braced frames (CBFs) as lateral force resisting systems. Three prototype frames are studied within the context of Eurocode 8 (EC8) provisions; these are MRFs which are traditionally designed, against ultimate seismic actions (ULS), without checking serviceability limit state rules (SLS-interstory drift limits). To fulfill these requirements concentrical braces are inserted in the frames, which are not considered in the collapse resistance of the structural system. The non-linear behaviour of these systems under ultimate seismic environment is studied and compared to the one of the unbraced MRFs. The influence of second order effects is also investigated.     

Estimation of the Park-Ang damage index for planar multi-storey frames using equivalent single-degree systems

A detailed characterization of potential structural damage is essential to performance-based seismic design. The Park-Ang damage index is selected in this work as the seismic damage measure, since it is one of the most realistic measures of structural damage. Response spectra constitute the most common tool used for characterizing the seismic hazard at a site, and these spectra represent the demand on single-degree oscillators. To use these spectra for estimating the Park-Ang damage index demand on an MDOF system, three equivalent single-degree system-based approximate schemes are proposed. These schemes are tested on three moment resisting frames under several ground motion scenarios. The effectiveness of an equivalent system scheme is measured by comparing with the estimates from a nonlinear response-history analysis of the MDOF model. These schemes are tested for both global and storey-level damage indices. Variation of the non-dimensional parameter β is also considered in these case studies. Overall, all the three schemes are found to be effective with varying degrees of accuracy. The proposed methods are recommended for damage-based seismic design and performance evaluation of structures because these schemes can use response spectra for demand estimation and reduce computation cost.     

一级抗震预应力型钢混凝土空腹桁架转换层框架结构三道防线抗震设防设计方法

基于"强化转换层及其下部、相对弱化转换层上部"的抗震设计理念,提出了一级抗震大跨度预应力型钢混凝土(PSRC)空腹桁架转换层框架结构"三道防线抗震设防"的设计目标,明确了三道防线的具体划分方法、各防线内力调整措施初步方案以及主要抗震性能的量化控制指标.然后,以两榀不同上抬层数的8度(0.2g)一级抗震的两跨PSRC空腹...     

增层改造中黏性土地基承载力的确定

根据工程经验，分析了增层改造中黏性土地基在长期静载作用下因固结而出现的承载力提高，并提出了计算方法，为设计前期预估实际地基承载力提供了理论计算依据，可供设计人员参考．     

乡镇多层砌体房屋抗震设防问题探讨

本文剖析了乡镇多层砌体房屋在结构抗震设计方面存在的问题,并针对这类房屋存在的问题提出了改进措施和应注意事项。     

Linear and nonlinear seismic response of a 52‐storey steel frame building

This paper presents the results of a study on the seismic behaviour of a well‐instrumented 52‐storey steel frame building in Los Angeles, California. This building has been subjected to ground motions from several earthquakes among which the records obtained during the 1991 Sierra Madre earthquake and the 1994 Northridge earthquake were selected for this study. Detailed time and frequency domain analyses of the recorded motions from these two earthquakes were conducted to determine the dynamic characteristics of the structure. This information was used to calibrate a three dimensional dynamic computer model of the building. Nonlinear dynamic computer analyses were then employed to investigate the response of the structure during severe ground shaking. The results of this study showed that by performing a linear three‐dimensional analysis, the response of the building during past earthquakes can be reproduced with confidence. The results also show that because of the torsional response of this high‐rise building is not negligible, two‐dimensional analysis is not feasible for reliably predicting its nonlinear response during earthquakes. By further performing a nonlinear three‐dimensional analysis, the state and sequence of damage could also be predicted. The study also included an investigation of the effectiveness of pushover analysis for predicting the nonlinear behaviour of the building. This type of analysis has the deficiency of excluding the participation of higher modes, which is obvious for high‐rise buildings, especially for shaking from near‐field type ground motions. Improvements to the pushover analysis for such a type of shaking were explored. Copyright © 2000 John Wiley & Sons, Ltd.     

两边连接钢板剪力墙-装配式钢管混凝土框架结构#br# 抗震性能研究

圆形或方形钢管混凝土柱与钢梁通过单边高强螺栓和端板组成框架,薄钢板剪力墙与钢梁采用两边连接方式,形成钢板剪力墙-装配式钢管混凝土框架结构。为获悉两边连接钢板剪力墙-装配式钢管混凝土框架的抗震性能和破坏机理,对2榀2层单跨钢板剪力墙-装配式钢管混凝土框架试件进行水平低周反复荷载试验,研究柱截面类型、墙梁连接方式和半刚性节点类型对该体系破坏形式和抗震性能的影响。在试验过程中观察结构的破坏特征和发展,分析滞回曲线、骨架曲线、刚度退化规律、延性、耗能能力以及主要构件的应变规律。研究结果表明,两边连接钢板剪力墙-装配式钢管混凝土框架结构具有良好的滞回性能和耗能能力,其弹性刚度比纯装配式钢管混凝土框架结构提高了163.8%~249.4%,水平极限承载力提高了41.0%~97.1%。采用OpenSees程序建立钢板剪力墙-装配式钢管混凝土框架的非线性分析模型,通过试验结果验证计算模型的准确性,为该结构体系的理论分析提供基础。文章研究结果将有助于钢板剪力墙-装配式钢管混凝土框架结构的应用和推广。