Effect of retrofit strategies on mitigating progressive collapse of steel frame structures

In this study, the effect of three retrofit strategies on enhancing the response of existing steel moment resisting frames designed for gravity loads is investigated using Alternate Path Methods (APM) recommended in the General Services Administration (GSA) and the Department of Defense (DoD) guidelines for resisting progressive collapse. The response is evaluated using 3-D nonlinear dynamic analysis. The studied models represent 6-bay by 3-bay 18-storey steel frames that are damaged by being subjected to six scenarios of sudden removal of one column in the ground floor. Four buildings with bay spans of 5.0 m, 6.0 m, 7.5 m, and 9.0 m were studied. The response of the damaged frames is evaluated when retrofitted using three approaches, namely, increasing the strength of the beams, increasing the stiffness of the beams, and increasing both strength and stiffness of the beams.The objective of this paper is to assess effectiveness of the studied retrofit strategies by evaluating the enhancement in three performance indicators which are chord rotation, tie forces, and displacement ductility demand for the beams of the studied building after being retrofitted.     

子结构模型对钢管混凝土组合框架抗连续性倒塌能力的影响

为提高钢管混凝土结构抗连续倒塌分析效率,采用子结构的静力试验和有限元方法进行研究,但是子结构模型的选取对分析结果影响较大。为此,利用ABAQUS显式模块对不同子结构模型的钢管混凝土组合框架进行抗连续倒塌性能分析。在验证材料本构和建模方法合理的基础上,建立单柱双半跨、三柱双跨、四柱三跨和五柱四跨平面子结构模型,以分析不同子结构模型对钢管混凝土平面框架抗连续倒塌性能的影响;对比单柱四半跨、五柱四跨和2×2跨的钢管混凝土空间子结构模型的抗连续倒塌性能,并分析混凝土楼板和层数对其影响。结果表明：不同子结构模型的选取决定了不同的边界约束条件,进而对钢管混凝土组合框架抗连续倒塌性能影响较大;单柱双半跨平面子结构和单柱四半跨空间子结构的边界强约束使其前期荷载较大,但整体变形能力较差;由于失效柱相邻柱的约束增强,四柱三跨、五柱四跨平面子结构的极限弦转角和峰值荷载较三柱双跨子结构明显提高,其中有楼板平面子结构的提高更明显;由于角柱的约束和楼板整体性的提高,有楼板单层2×2跨空间子结构的极限弦转角和峰值荷载均高于五柱四跨空间子结构,但无楼板单层2×2跨和五柱四跨空间子结构相差不大;无论楼板存在与否,随着层数的增加,空间子结构模型的极限弦转角和峰值荷载均大幅提高。     

Assessment of progressive collapse-resisting capacity of steel moment frames

In this study the progressive collapse-resisting capacity of steel moment resisting frames was investigated using alternate path methods recommended in the GSA and DoD guidelines. The linear static and nonlinear dynamic analysis procedures were carried out for comparison. It was observed that, compared with the linear analysis results, the nonlinear dynamic analysis provided larger structural responses and the results varied more significantly depending on the variables such as applied load, location of column removal, or number of building story. However the linear procedure provided more conservative decision for progressive collapse potential of model structures. As the nonlinear dynamic analysis for progressive collapse analysis does not require modeling of complicated hysteretic behavior, it may be used as more precise and practical tool for evaluation of progressive collapse potential of building structures.     

Influence of Semi‐Rigid Connections and Local Joint Damage on Progressive Collapse of Steel Frameworks

Abstract: Semi‐rigidly connected steel frames are more vulnerable than rigidly connected frames to resist progressive collapse due to abnormal loading events. This article extends the threat‐independent method for progressive‐failure analysis of rigid frames to analysis accounting for semi‐rigid connections. The influence of joint damage caused by disengagement of member(s) is also considered in the analysis, and the degree of damage is modeled by a health index. A compound element model is employed to include the contributions of nonlinear behavior of beam‐to‐column connections, connection and member‐end damage, member inelasticity, member shear deformation, and geometrical nonlinearity to structural response. Four beam collapse modes are illustrated for the progressive collapse analysis associated with debris loading generated when disengaged structural components fall onto lower parts of the structure. The impact effect is taken into account for the quasi‐static nonlinear analysis by utilizing an impact amplification factor according to GSA and DoD guidelines. Any progressive collapse occurring thereafter involves a series of collapse events associated with topological changes of the frame. The analysis procedure is illustrated for the progressive collapse behavior of two planar steel frames. The results demonstrate that the proposed method is potentially an effective tool for the progressive collapse analysis of semi‐rigid steel frames under abnormal loading events.     

Horizontal bracing to enhance progressive collapse resistance of steel moment frames

In this paper, the effect of horizontal bracing on enhancing the resistance of steel moment frames against progressive collapse is investigated. Previously designed 6 bay by 3 bay 18‐story steel frame prototype building with 6 m bay span (namely, unbraced frame), which was susceptible to progressive collapse, is retrofitted by four types of horizontal bracing systems on the perimeter of the topmost story and analyzed using 3D nonlinear dynamic method. Six different cross‐sections for each bracing system type are considered, and the capacity curves for each model are obtained. Three column removal circumstances, namely, Edge Short Column, First Edge Long Column, and Edge Long Column are considered in this paper. The results imply that horizontal bracing would increase the resistance of moment frames against progressive collapse. However, one of the bracing types in which axial compressive force is created in braces is not appropriate for retrofitting.     

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.     

Progressive collapse design of seismic steel frames using structural optimization

This paper uses structural optimization techniques to cost-effectively design seismic steel moment frames with enhanced resistance to progressive collapse, which is triggered by the sudden removal of critical columns. The potential for progressive collapse is assessed using the alternate path method with each of the three analysis procedures (i.e., linear static, nonlinear static, and nonlinear dynamic), as provided in the United States Department of Defense United Facilities Criteria (UFC) Design of Buildings to Resist Progressive Collapse. As a numerical example, member sizes of a two-dimensional, nine-story, three-bay regular steel immediate moment frame are optimally determined such that the total steel weight is minimized while the design satisfies both AISC seismic provisions and UFC progressive collapse requirements. Optimization results for the example frame reveal that the traditional minimum weight seismic design, which does not explicitly consider progressive collapse, fails to meet the UFC alternate path criteria associated with any analysis procedure. Progressive collapse design optimization using the linear static procedure produces the most conservative and consequently heaviest design against progressive collapse. In contrast, the more accurate nonlinear static and dynamic procedures lead to more economical designs with UFC-acceptable resistance to progressive collapse, at the expenses of considerable modeling and computing efforts.     

Assessment of eccentrically braced frames strength against progressive collapse

One of the most important and effective factors of structural strength against the risk of progressive collapse is the type of lateral load bearing system of a building. In this research, strength of dual steel moment frames equipped with a variety of eccentric bracings against progressive collapse was evaluated by using nonlinear static alternate path method. 6-floored building samples were designed with steel frame using a dual steel moment system together with 3 different types of bracing, including inverted eccentrically V-shaped bracing (chevron bracing), eccentrically V-shaped bracing and eccentrically X-shaped bracing, each with two different kinds of arrangement of bracings in the structural plan, in form of alternate and neighbor. The effects of sudden removal of columns on different floors of these buildings were examined. These studies showed that dual steel moment frames equipped with eccentric bracings generally exhibited desirable strength against progressive collapse. A change in the type of bracing resulted in significant changes in the system capacity in the progressive collapse. Among the different types of braces assessed, chevron type eccentrically brace showed higher strength against progressive collapse. Also, that alternate arrangement of bracings in structure plan demonstrated better performance than neighboring arrangement.     

Evaluation of progressive collapse potential of steel moment frames considering catenary action

This study investigated the effect of catenary action on the progressive collapse potential of steel moment framed structures. Non‐linear static and dynamic analyses of three‐ and six‐story model structures with and without bracing were carried out following the alternate path method recommended by the General Services Administration 2003. According to the non‐linear static push‐down analysis results, the contribution of catenary action and the progressive collapse potential of structures increased as the number of story and the number of bay increased. The effect of catenary action increased significantly in braced frames, in which the movement of beam–column joints were fully restrained until the tensile capacity of beams located both sides of the removed column reached their maximum values. The non‐linear dynamic analyses showed that the maximum deflection caused by sudden removal of a column decreased when the catenary action was taken into account. Copyright © 2008 John Wiley & Sons, Ltd.     

Progressive collapse resisting capacity of moment frames with viscous dampers

In this paper, the effect of viscous dampers on reducing progressive collapse potential of steel moment frames was evaluated by nonlinear dynamic analysis. Parametric studies were conducted first to evaluate the effects of dampers installed in a steel beam‐column subassembly with varying natural period and yield strength on the reduction of progressive collapse potential. Then 15‐story moment‐resisting frames with three different span lengths were designed with and without viscous dampers, and the effect of viscous dampers was investigated by nonlinear dynamic analysis. According to the parametric study, the vertical displacement generally decreased as the damping ratio of the system increased, and the dampers were effective in both the elastic and the elasto‐plastic systems. It was also observed that the effect of the damper increased as the natural period of the structure increased and the strength ratio decreased. The analysis results of 15‐story analysis model structures showed that the viscous dampers, originally designed to reduce earthquake‐induced vibration, were effective in reducing vertical displacement of the structures caused by sudden removal of a first‐story column, and the effect was more predominant in the structure with longer span length. Copyright © 2011 John Wiley & Sons, Ltd.     

采用优化方法进行钢框架抗连续倒塌设计

采用结构优化方法设计钢框架,提高其对临界柱突然拆除而引起的连续倒塌的抵抗性能。利用美国UFC规范抗连续倒塌设计中建议的线性静力、非线性静力或非线性动力分析方法,采用构件拆除法获得结构抗连续倒塌性能。以某9层3跨规则钢框架平面模型为例,优化构件尺寸使总用钢量最小,并满足AISC抗震规范和UFC规范抗连续倒塌的要求。结果表明:无论采用哪种分析方法,最小用钢量抗震设计都不能明确考虑连续倒塌且不能满足构件拆除法的要求。采用线性静力方法进行抗连续倒塌设计最保守,效果最差。相比而言,若以UFC抗连续倒塌允许值为标准,只要模型和计算过程正确,非线性静力和动力分析能使设计更经济。     

对减轻钢结构逐步破坏的改造方法的效应评估

根据GSA和DoD为预防结构逐步破坏而制订的规范中所推荐的APM方法,对提高重力荷载下,钢结构抗弯能力的三种改造方法的有效性进行了分析。主要采用的是三维非线性动力分析法。分析模型为6×3跨的18层钢结构,这个结构曾由于底层突然移除一根柱而遭受到破坏。对跨长为5,6,7·5,9m的4个建筑进行了分析。分别采用三种方法改造结构并对其进行了评估,即增加梁的强度、增加梁的刚度以及同时增加梁的强度与刚度。通过分析改造后梁的转角、约束力和位移延性这三个性能指标的增强效应,来评估相应改造方法的有效性。     

HRB500级钢筋混凝土框架梁开裂性能试验研究

为了研究HRB500级钢筋作为受力主筋的混凝土框架梁开裂性能,对3榀1层2跨的混凝土框架结构进行静力试验,量测了在跨中集中荷载作用下梁端与跨中截面的裂缝宽度、位移、转角以及混凝土和钢筋应变。结果表明：框架梁端裂缝宽度实测值一般大于采用现行混凝土结构设计规范公式的计算值。研究了框架梁端裂缝的开展机理,基于黏结-滑移理论,提出把裂缝处钢筋与混凝土之间的滑移分为在节点内滑移和在梁端内滑移两部分,建立框架梁抵抗负弯矩的纵向钢筋在节点区的黏结-滑移模型和相应的最大裂缝宽度计算式,并根据对试验结果的统计分析,校准了黏结-滑移模型的参数。框架梁端最大负弯矩作用下的裂缝宽度计算式可供工程实践和规范修订参考。     

焊接箱形截面梁抗震性能影响因素研究

为考察板件宽厚比对焊接箱形截面梁抗震性能的影响,对中国、美国、日本和欧洲的钢结构设计标准中的相关规定进行了比较,结果表明各国规范对于梁板件宽厚比限值的规定总体上具有较好的一致性。采用钢材循环加载本构,建立了多尺度非线性有限元计算模型。提出了刚性竖杆 箱形梁加载方式,模拟水平地震、重力荷载与轴向压力对箱形截面框架梁的作用。有限元分析结果表明,在设计常用的板件宽厚比范围内,箱形截面梁的弹性屈曲荷载均显著高于其屈服荷载。在水平往复荷载作用下,随着板件宽厚比减小,箱形截面梁极限变形角与延性系数随之增大,抗弯刚度降低速率变缓,塑性耗能能力显著增强。当满足一级抗震宽厚比要求时,焊接箱形截面梁的梁端截面转角约为1/30。承受轴压作用时梁刚度退化很快,变形能力减弱。当轴压比不大于0.2、满足一级抗震宽厚比要求时,梁端截面转角约为1/50。跨高比对梁承载力影响不大,但变形能力可以大幅度提高。横向荷载对梁抗震性能的影响显著,随着静载比（重力荷载代表值与屈服弯矩之比）增大,骨架曲线逐渐发生平移,抗弯刚度降低,耗能性能减弱。当地震弯矩与静力弯矩方向相同时,箱形截面梁承载力显著降低,静载比0.8时极限变形角可减小约50%;当地震弯矩与静力弯矩方向相反时,梁虽然承载力稍有提高,但极限变形角略有减小。     

Investigation of ductility in hybrid and high strength steel beams

Compactness and lateral support configuration provisions for design of steel beams are formulated so as to ensure that the resulting beam exhibits adequate ductility. It appears from the current research that slenderness limitations are not valid for beams made of the high strength steel grades. In this paper an attempt is made to study on influence of flange and web slenderness as well as lateral support spacing of homogenous and hybrid welded I-sections made of high strength steel on member ductility. For this purpose an experimentally verified nonlinear numerical analysis of the local and overall stability was performed. These beams are subjected to constant moment loading a new theoretical method is proposed to calculate the rotation capacity for this loading type. A comparative study was carried out between this method and numerical study results to ensure the accuracy of proposed method. In this research realistic material behavior and residual stresses were adopted in finite element models. Results have shown that using the high strength steel in cross sections subjected to bending has a significant effect on flexural behavior of these members. Meanwhile, in present study, interaction between the flange and web slenderness ratios was evaluated in accordance to AISC criteria for compact sections.     

Experimental study of lateral-torsional buckling behavior of stainless steel welded I-section beams

Investigations on lateral-torsional buckling behavior of stainless steel welded I-section beams were carried out through fourpoint bending tests. The Chinese austenitic stainless steel adopted in this study contains approximately 17% of Cr, which could be graded as EN 1.4401. The non-linear stress-strain relationship of the material, the residual stress distribution of the section and the ultimate moment capacity of the welded I-section beams were obtained from the experiments, and they were compared with those results predicted by analytical models recommended by several researchers. Besides, a new model is proposed to predict the residual stress distribution of the stainless steel welded I-section beams.     

Performance of bolted angle connections in progressive collapse of steel frames

This paper is mainly focused on the behaviour of bolted top‐seat angle connections with double web angles, categorized as partially restrained connections, in progressive collapse of semi‐rigid steel frames due to sudden column loss. The main characteristics of this type of loading are declared and the effect of the imposed boundary conditions on the motivated elements is depicted. To study the performance of frame connections under these conditions, refined nonlinear finite element modelling technique is used. The models are created based on the previous experimental studies and their accuracy is examined through a comparison to the results of these tests. New models are created under conditions of the progressive collapse and the behaviour of the connections is studied. The study showed that due to the applied boundary conditions, the connections behave stiffer and show a higher moment capacity. Based on the results of the numerical models, equations are presented to estimate the moment‐rotation response of the studied connections under progressive collapse conditions based on the beam and connection mechanical and geometrical properties and the connection rotation. Copyright © 2009 John Wiley & Sons, Ltd.     

Response of a multi-storey steel composite building with concentric bracing under consecutive column removal scenarios

In this paper, a 3-dimensional finite element modelling technique developed by the author was used to analyse the progressive collapse of multi-storey buildings with composite steel frames. The nonlinear dynamic analysis procedure was performed to examine the behaviour of the building under consecutive column removal scenarios. The response of the building was studied in detail and the measures to mitigate progressive collapse in future designs were also recommended.     

钢-混凝土组合框架梁的稳定设计方法

在已有的钢-混凝土组合梁和钢框架理论研究基础上,分析了影响组合梁负弯矩区转动能力的主要因素,建立了组合梁连续和钢柱连续两种组合框架结构体系的节点塑性极限转角的求解方法,提出了相应的计算公式及一种简化的组合框架梁稳定设计方法,并得出了组合梁负弯矩极限调幅系数的计算方法.结果表明,该设计方法可以使组合框架塑性铰处的内力充分重分布,使梁柱连接的转动刚度合理,也能使组合梁的设计达到充分利用组合效应及节点延性的目的.     

The influence of joints and composite floor slabs on effective tying of steel structures in preventing progressive collapse

The event of the terrorist attack on 11th September 2001 in the USA has attracted increasing attention of researchers and engineers on progressive collapse of structures. It has gradually become a general practice for engineers to consider progressive collapse resistance in their design. In this paper, progressive collapse of steel frames with composite floor slabs is simulated by the finite element method. The numerical results are compared with test results. The influence of the joints and the concrete slabs on the effective tying of steel beams is investigated through parametric studies. From the analysis, methods of preventing progressive collapse that can be considered in design and when retrofitting existing structures are proposed. The results show that retrofitting a structure with pre-stressed steel cables and an increase of crack resistance in the concrete near joints can effectively improve effective tying of a structure, which results in an enhanced structural capacity in preventing progressive collapse.