Proposal of a probabilistic model for multi-hazard risk assessment of structures in seismic zones subjected to blast for the limit state of collapse

It is desirable to verify the structural performance based on a multi-hazard approach, taking into account the critical actions the structure in question could be subjected to during its lifetime. This study presents a proposal for a probabilistic model for multi-hazard risk associated with the limit state of collapse for a reinforced concrete (RC) structure subjected to blast threats in the presence of seismic risk. The annual risk of structural collapse is calculated taking into account both the collapse caused by an earthquake event and the blast-induced progressive collapse. The blast fragility is calculated using a simulation procedure for generating possible blast configurations, and verifying the structural stability under gravity loading of the damaged structure, using a kinematic plastic limit analysis. As a case study, the blast and seismic fragilities of a generic four-storey RC building located in seismic zone are calculated and implemented in the framework of a multi-hazard procedure, leading to the evaluation of the annual risk of collapse.     

爆炸荷载作用下钢框架结构连续倒塌分析

传统数值方法模拟建筑结构在爆炸荷载作用下的结构响应和连续倒塌时,具有计算模型复杂、计算量大的特点,实际应用价值不大。基于将爆炸荷载作用下结构响应分析分两步进行的数值模拟方法,利用非线性显式动力分析软件AUTODYN的Remap技术模拟爆炸波在空气中的传播过程,利用压强测点记录结构构件表面的爆炸压强时程曲线;建立结构精细化有限元模型,并将上一步记录的爆炸压强时程曲线施加于结构构件,利用LS-DYNA显式求解器分析结构在爆炸荷载作用下的动态响应和倒塌过程。将该方法应用于某钢框架结构在爆炸荷载作用下的动态响应和连续倒塌分析。结果表明：钢框架结构具有较好的抗爆性能,在发生1000kg TNT当量及以下规模的室外爆炸时,主体结构能够保证安全;在发生1500kgTNT当量及以上大规模爆炸时,发生次梁塌落等局部破坏,亦可能发生结构连续倒塌。     

建筑结构抗倒塌分析方法

建筑结构的倒塌可分为两种模式:建筑物侧向增量倒塌与竖向连续倒塌。本文论述了建筑物结构失效模式的如何辨识以及侧向倒塌能力点确定准则的相关研究进展。介绍了现有的连续倒塌研究方法,尤其是基于备用荷载路径的竖向非线性动力分析方法。为建筑结构整体抗震可靠度研究以及结构抗震性能评定与设计提供参考。     

成都市规划展览馆辅楼抗连续倒塌评估(Ⅱ)#br# ——基于IDA的弹塑性动力分析

成都市规划展览馆辅楼因建筑功能需要,具有大跨度、大悬挑、竖向抗侧力构件不连续、楼板局部不连续等结构特点。作为城市重要的大型公共建筑,需要评估其地震作用下的抗连续倒塌能力。首先,采用有限元软件Midas Gen建立有限元模型,并基于改进的IK骨架模型,模拟结构在地震作用下的非线性行为;其次,根据结构所在场地类型,选取20条地震动记录,并采用增量动力分析的方法对结构进行三向地震激励下的弹塑性动力分析,在此基础上,定义基于性能的结构倒塌判别准则,分别以结构最大层间位移角和梁端转角为结构损伤指标,根据增量动力分析结果对结构进行易损性分析;最后,根据结构整体的倒塌安全储备能力,评估结构的抗倒塌性能。研究结果表明：在7度罕遇地震作用下,结构发生侧向增量倒塌的概率为0.14%,发生竖向连续倒塌的概率为10.82%;在7度多遇地震和设防地震作用下,结构发生侧向增量倒塌和竖向连续倒塌的概率基本为0。同时,该结构的水平破坏倒塌安全储备系数为2.708,竖向破坏的倒塌安全储备系数为1.469。评估结果表明：该结构在地震作用下具有较好的抗连续倒塌性能。     

动力备用荷载路径法的力学机理分析

备用荷载路径法是结构连续性倒塌分析中最常用的方法.根据备用荷载路径法的力学机理,建立了中间底柱破坏后固支梁的等效结构动力计算模型,分析了框架结构中常见的柱瞬时失效情况下梁的动力响应,得到了梁的位移时程曲线以及静载作用下梁的挠度曲线.对比分析表明,瞬间移除梁的中柱后梁的挠度可以达到静力作用挠度的2倍.研究表明,结构连续性倒塌分析时瞬时移除构件后梁的动力效应不可忽略,动力条件下的备用荷载路径法为框架结构连续性倒塌等效静力分析中荷载的确定提供了依据.     

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.     

Experimental investigation of asymmetrical reinforced concrete spatial frame substructures against progressive collapse under different column removal scenarios

The progressive collapse resistance design approach is generally applied in the context of a “column‐removal” scenario to assess the structural vulnerability to progressive collapse. To obtain a better understanding of the complex progressive collapse resistance of 3D asymmetrical column‐beam‐slab systems, five one‐third scale 2 × 2 bay asymmetrical reinforced concrete (RC) spatial frame substructure specimens were tested to analyze their collapse mechanisms under five different column removal scenarios, namely, an interior column removal scenario (INT), an exterior column removal scenario in the asymmetrical direction (EXT‐X), an exterior column removal scenario in the symmetrical direction (EXT‐Y), a corner column removal scenario at the long bay (COR‐L), and a corner column removal scenario at the short bay (COR‐S), which are among the most critical scenarios for analyzing structural resistance against progressive collapse. The test results showed that INT had the highest progressive collapse resistance capacity among the scenario substructures and exhibited two progressive collapse‐resisting mechanism stages: a primary mechanism stage (beam and compressive membrane mechanism) under small deformations and a secondary mechanism stage (catenary and tensile membrane mechanism) under large deformations in both the X‐direction and the Y‐direction. In EXT‐X and EXT‐Y, the collapse resistance was mainly provided by the double‐span beam at both the primary mechanism stage and the secondary mechanism stage. In COR‐L and COR‐S, the tensile membrane mechanism could not be mobilized, as the single‐span beams in both the X‐direction and the Y‐direction behaved like cantilevers. Additionally, the asymmetrical span design reduced the resistance of the structure against progressive collapse.     

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.     

钢框架梁柱节点在结构连续倒塌中的性能分析

自英国Ronan Point公寓楼发生连续倒塌以来,各国学者开始对结构的连续倒塌进行研究,但新型抗震节点对结构抗连续倒塌性能的作用尚缺乏研究.利用非线性有限元方法,对3种型式的梁柱节点在结构的某一柱子失效后的性能进行了弹塑性分析.结果表明:在未考虑节点焊接缺陷的条件下,新型节点的延性较好,可以在结构发生局部破坏时不发生脆性破坏.     

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.     

建筑结构倒塌过程模拟与防倒塌设计

针对结构性能的数值模拟方法进行了综合分析,基于离散单元法提出了结构倒塌分析的理论模型。通过试验研究了混凝土块体间碰撞的力学行为,并结合扩展的数值分析建立了混凝土块体在不同碰撞形式下的计算模型。针对建筑物内的局部爆炸作用,在分析构件反应的基础上采用离散单元法对结构平面及空间倒塌过程进行了数值模拟。建立了适用于任意加载路径的材料弹簧力-位移本构关系,据此分析了地震作用下钢筋混凝土框架结构以及砌体结构空间倒塌过程。采用基于OpenGL的图形建模技术,实现了结构倒塌过程数值模拟结果的三维可视化。模拟结果与振动台模型试验结果及工程实测结果比较表明,所采用的离散单元法适合结构大变形阶段的分析。为提高计算效率和精度,有必要采用多尺度的模拟分析。另外,对不同荷载及作用下建筑结构抗倒塌设计方法进行了总结。并指出结构参数对抗连续倒塌性能的影响、结构倒塌机理、实用的防连续倒塌设计方法以及地震作用下结构防倒塌定量设计方法等方面尚待深入研究,以便建立相应的防倒塌设计规范。图15参21     

地震作用下钢框架结构抗竖向连续倒塌可靠度分析

为了对偶然荷载作用下结构的抗连续性倒塌进行可靠度研究,采用OpenSEES软件建立钢框架连续倒塌分析数值模型,考虑到钢框架结构材料和荷载的不确定性,使用拉丁超立方抽样方法生成钢框架结构随机样本,并采用随机Pushover算法对钢框架结构进行分析,计算得到X、Y两个方向不同地震水平下各钢柱承载能力及变形能力的可靠度指标,根据抗震可靠度相关理论确定目标可靠度指标,通过比较各柱的可靠度指标和目标可靠度,对结构在地震作用下最可能失效构件进行识别。基于所识别的最可能失效构件,结合所生成的100组钢框架结构随机样本,采用拆除构件法对钢框架结构在单柱失效和多柱失效等工况下进行抗连续倒塌IDA分析,得到随机IDA分析曲线。通过结构连续倒塌极限状态方程,计算得到损伤结构发生连续倒塌的概率及连续倒塌条件可靠度指标。采用基于风险的结构连续倒塌概率表达式,分析地震作用下钢框架结构发生局部破坏后的连续倒塌全概率可靠度,为准确评价钢框架结构在地震作用下的抗连续倒塌能力提供依据。     

多层框架结构在其地下室内部爆炸冲击下的连续倒塌机理研究

为了研究框架结构在地下室内部爆炸时的连续倒塌特性,运用有限元显式动力分析软件LS-DYNA建立了一个含有1层地下室、4层地上结构的钢筋混凝土框架结构分离式模型;考虑钢筋和混凝土共节点的连接方式,采用三阶段模拟法将不同炸药作用位置时结构的连续倒塌模式进行对比,分析梁、板、柱等主要受力构件的具体破坏方式;阐释结构在地下室内部爆炸后的不同倒塌破坏模式。结果表明:地下室内部爆炸发生后地上一层破坏严重,在地下结构柱附近爆炸使得结构节点处破坏严重,从而导致结构的连续倒塌。     

基于性能设计方法在超限高层抗连续倒塌设计应用

我国抗震设计规范提出"小震不坏、中震可修、大震不倒"的设防目标,其中,"中震可修、大震不倒"的第二、第三水准设防目标,主要是采用经验调整系数及构造加强措施来实现,缺乏定量的计算分析。本文应用基于性能抗震设计的方法,采用静力非线性有限元倒塌计算原理,分析框架结构在大震作用下的破坏机理,对结构"大震不倒"进行定量分析,确保结构实现抗倒塌的设计目标。并结合一个超限高层实例,验证倒塌计算结果与振动台实验结果的相符性。     

钢框架结构连续倒塌的能量流

给出多层建筑物基于能量的连续倒塌评估方法,从能量流的角度分析钢框架结构的连续倒塌。该文引入基于能量的构件评估方法,并与传统的荷载-变形方法进行比较;讨论了能量流分析法在解释地震事件中的优势。从整体看来,只有当动能通过结构全部耗散时,建筑物才不会倒塌,达到稳态构型。否则,残余动能会导致结构继续倒塌。传统建筑物中,动能通过转化为结构构件的变形能而耗散,构件在失稳前能够消耗一定的能量。与地震作用相比,动力荷载作用下柱的变形能能够更好地作为稳定性指标。能量流分析法可用于典型钢结构的倒塌评估。     

Performance assessment of innovative seismic resilient steel knee braced frame

Buckling restrained knee braced truss moment frame (BRKBTMF) is a novel and innovative steel structural system that utilizes the advantages of long-span trusses and dedicated structural fuses for seismic applications. Steel trusses are very economical and effective in spanning large distance. However, conventional steel trusses are typically not suitable for seismic application, due to its lack of ductility and poor energy dissipation capacity. BRKBTMF utilizes buckling restrained braces (BRBs) as the designated structural fuses to dissipate the sudden surge of earthquake energy. This allows the BRKBTMF to economically and efficiently create large span structural systems for seismic applications. In this paper, a prototype BRKBTMF office building located in Berkeley, California, USA, was designed using performance-based plastic design procedure. The seismic performance of the prototype building was assessed using the state-of-the-art finite element software, OpenSees. Detailed BRB hysteresis and advanced element removal technique was implemented. The modeling approach allows the simulation for the force-deformation response of the BRB and the force redistribution within the system after the BRBs fracture. The developed finite element model was analyzed using incremental dynamic analysis approach to quantify the seismic performance of BRKBTMF. The results show BRKBTMF has excellent seismic performance with well controlled structural responses and resistance against collapse. In addition, life cycle repair cost of BRKBTMF was assessed using the next-generation performance-based earthquake engineering framework. The results confirm that BRKBTMF can effectively control the structural and non-structural component damages and minimize the repair costs of the structure under different ranges of earthquake shaking intensities. This studies conclude that BRKBTMF is a viable and effective seismic force resisting system.     

局部爆炸作用下混凝土框架结构抗连续倒塌设计

采用替代路径法对按我国规范设计的钢筋混凝土框架结构进行局部爆炸作用下抗连续倒塌设计验算和再设计.竖向构件移除位置、荷载组合、构件失效准则以及结构破坏范围限值均参考美国<结构抗连续倒塌设计>(UFC4-023-03)中的规定,改进了其中的构件塑性铰模型,采用条分法得到了纯弯和压弯构件塑性铰的荷载-变形全过程,以合理描述构件受力性能.研究发现,常规设计难以保证结构在局部爆炸作用下不发生连续倒塌.随后进行了参数研究,以认识重要参数对结构抗连续倒塌性能的影响.结果表明,减少柱距后结构抗连续倒塌性能提高明显,而提高结构抗震设防烈度的方法效果不明显,增加结构层数则会削弱结构的抗连续倒塌能力.在此基础上给出了三个抗连续倒塌设计方案并进行了经济性比较.比较表明,同时提高结构的冗余度和构件的承载力具有较好的经济性.     

Robustness assessment of planar steel frames caused by failure of a side column under localized fire

Progressive collapse of a building structure under fire is a disaster that may cause heavy casualties and serious economic loss. However, there is a lack of codified method to assess fire‐induced progressive collapse of building structures. A global–local analysis method (GLAM) has recently been proposed by the authors and their colleagues to assess progressive collapse of steel buildings under localized fire, and its application on fire scenarios that causes one inner column to fail has been verified. This paper extends the application of GLAM to fire scenarios that causes a side column to fail in a planar steel frame. The predictions of the GLAM were validated against the results obtained from nonlinear dynamic analysis of the whole frame model. Besides, effects of location of the heated column at different storeys and load level of the frame were also studied. The results show that GLAM gives the same collapse predictions to the case studies with detailed nonlinear dynamic analysis. The differences between the critical load obtained from GLAM and that provided by the nonlinear dynamic analysis is within 7%. Therefore, GLAM has good applicability on robustness assessment of planar steel frames caused by failure of a side column under localized fire.     

Retrofit of RC frames against progressive collapse using prestressing tendons

This study investigates the effect of prestressing tendons on the progressive collapse performance of a 6‐ and 20‐story reinforced concrete model structures. According to nonlinear static and dynamic analysis results, the analysis model structures turned out to be vulnerable to progressive collapse caused by sudden loss of a first story column. However, the RC structures reinforced by external prestressing tendons along floor girders showed stable behavior against progressive collapse. The retrofit effect increased as the initial tension and cross‐sectional area of tendons increased. The incremental dynamic analyses showed that the seismic performance of the model structure was also enhanced after the retrofit using tendons. Based on analysis results, it was concluded that the retrofit of existing buildings using prestressing tendons could be effective for increasing both progressive collapse resisting capacity and seismic performance of RC framed structures. Copyright © 2011 John Wiley & Sons, Ltd.