Progressive collapse‐resisting capacity of modular mega‐frame buildings

In this study, the progressive collapse‐resisting capacity of modular mega‐frame structures consisting of a few identical subsystems was investigated based on column‐loss scenario. Four types of mega‐frame structures were designed as basic analysis model structures. According to pushdown analysis results, the mega‐frame structure with four corner columns did not satisfy the design guidelines for progressive collapse regardless of the number of subsystems when one of the first‐story mega‐columns was removed. To enhance the resistance against progressive collapse, we redesigned the basic model structure with four mega‐columns by adding additional floor trusses in the transfer floors, adding moment‐resisting frames at the perimeter and adding vertical mega‐bracing. The pushdown analysis results showed that the schemes with additional mega‐braces were most effective in increasing the progressive collapse‐resisting capacity of mega‐frame buildings. Copyright © 2011 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.     

Behavior and design of reinforced concrete frames retrofitted with steel bracing against progressive collapse

Steel bracing is able to improve progressive collapse resistance of reinforced concrete (RC) frames, but the bracing design is typically based on seismic retrofitting or lateral stability. There is no approach for design of steel bracing against progressive collapse. To this end, a retrofitting approach with steel braces is proposed based on analysis of macro finite element (FE) models with fiber beam elements. The FE models were initially validated through the experimental results of a braced frame and then used to investigate the effects of pertinent parameters on the progressive collapse resistance of planar frames. The results suggest the braces should be placed at the top story. Thereafter, macro FE models are built to investigate the dynamic responses of the three‐dimensional prototype RC frames under different column removal scenarios (CRS) and show the necessity of retrofitting. Accordingly, the design approach of steel bracing is proposed with incremental dynamic analysis (IDA) and assuming independent contribution of braces and frames to resistance. Finally, the fragility analysis of the frames under a corner‐penultimate‐exterior CRS is conducted through IDA and Monte Carlo simulation, and the results confirm the validity of the proposed design approach for retrofitting RC frames.     

Seismic fragility analysis of concrete encased frame‐reinforced concrete core tube hybrid structure based on quasi‐static cyclic test

Concrete‐encased frame‐core tube hybrid structural system has been widely employed in high‐rise buildings. This paper intends to analyze the seismic fragility of this structural system under ground motion excitation. The quasistatic cyclic test on a 1/5‐scaled, 10‐story three‐bay specimen is introduced. Fiber‐based finite element model is developed and integrated with numerical techniques that would be able to simulate the nonlinear response based on the OpenSees program. As the model is verified by the experimental data, a series of incremental dynamic analyses (IDAs) considering different frame‐tube stiffness ratios are carried out. IDA curves are drawn to describe each structural performance state. Fragility curves and probabilistic demand models are proposed for quantifying failure probability. The collapse margin ratio is employed to evaluate the collapse probability. The result shows that the collapse probability under rare earthquake still meets the requirement of Applied Technology Committee‐63 Report. The hybrid structure is proved to perform superior collapse resistance ability. The proper increase in the stiffness of core tube can reduce the collapse probability and enhance the collapse resistance capacity.     

Seismic collapse analysis of high‐rise reinforced concrete frames under long‐period ground motions

Structural damages associated with buckling of longitudinal reinforcing steel and crushing of concrete induce strength and stiffness degradation in reinforced concrete (RC) beams and columns. This paper presents a numerical investigation on earthquake‐induced damages and collapse of typical high‐rise RC buildings model incorporating strength degradation (SD) effects. In a simple finite‐element analysis program with the generalized stress fiber discretization, hysteretic constitutive models primarily dominate the inelastic behavior. Buckling of reinforcing steel and crushing of confined concrete are taken into accounted to the stress–strain relationship of fiber elements. The SD effect in components with small hoop ratio tends to amplify the seismic responses high‐rise RC moment‐resisting frames when the intensity of ground motions exceeds the design level. Buckling of steel rebar and crushing of concrete should be fully considered together with the P‐Δ effect for collapse simulations.     

爆炸荷载下钢筋混凝土框架结构连续倒塌分析方法比较

运用显式动力有限元分析软件LS-DYNA建立了一个四层两跨钢筋混凝土框架结构的有限元模型,并对其施加爆炸荷载,在爆炸荷载仅引起结构关键柱失效的情况下,研究了不同比例距离爆炸荷载作用下钢筋混凝土结构的动力响应及倒塌过程;同时采用美国国防部2009年出台的建筑物抗连续倒塌设计规范UFC 4-023-03中推荐使用的替代传力路径法对相同的四层钢筋混凝土框架结构进行了非线性动力连续倒塌分析。通过比较研究上述数值分析结果,揭示了UFC 4-023-03规范推荐使用的替代传力路径分析方法的不足,进而对考虑爆炸荷载作用的结构抗连续倒塌分析设计方法的改进提出了设想。研究结果表明:通过引入爆炸荷载放大系数来考虑爆炸荷载作用的影响具有更广的适用范围。     

采用逐步增量弹塑性时程方法对RC框架结构推覆分析侧力模式的研究

以两个普通六层和十层钢筋混凝土框架结构为例,采用基于纤维模型的逐步增量弹塑性时程方法得到的层间剪力-位移关系曲线,与不同侧力模式的推覆分析结果进行了对比,研究了推覆分析结果的可靠程度。通过与不同场地的大量地震记录的弹塑性时程计算结果进行比较分析,建议采用多种合理的侧力模式进行推覆分析,对结构不同楼层的抗震性能进行全面的评价,即均布侧力模式适合于底部楼层的评价;考虑高度影响分布力模式适合于结构中部楼层的评价;而对结构上部楼层特别是顶层,可以采用SRSS侧力模式或规范侧力模式进行评价。分析研究表明,采用本文建议的侧力模式进行推覆分析,可在统计意义上可对钢筋混凝土规则框架结构的抗震能力作出偏于安全的估计。     

Assessment of sliding‐rubber isolator effect in progressive collapse of bridges under two scenarios

In this study, performances of 2 types of bridges, with and without seismic isolation, are addressed in 2 damage analysis scenarios, where, in the first, the side column and, in the second, the middle column are removed from the bridge piers. The performance was assessed using nonlinear dynamic analysis, and the time history and maximum structural responses were evaluated. Initially, sliding‐rubber isolators were designed according to AASHTO guide specifications, and then the bridges were modeled in OpenSees software package. Additionally, the coefficient of friction for the isolator was considered as a variable due to sudden removal of the columns and the consequent changes in the sliding velocity and axial forces. The results indicate that use of seismic isolation systems caused an increase in all maximum structural responses except that of the base shear. Considering the frictional performance of the isolators, slides in the deck are not caused by yielding of seismic isolators, and the reason for permanent displacements of the deck may be attributed to bridge instabilities in the first scenario. However, decrease in the horizontal stiffness results in increased maximum permanent displacement. In the first scenario, uplift of the deck occurred in the case of isolated bridge.     

Application of system reliability‐based assessment for collapse fragility of braced moment‐resisting frames

This paper presents a system reliability‐based framework for collapse fragility assessment of steel braced moment‐resisting frames (BMRFs). The conditional failure of intermediate events is calculated, considering two important features in the design of BMRFs: (i) different failure scenarios (FSs) with multiple sequences of components failure formation and (ii) structural reliability analysis based on the failure propagation from components to system. The system collapse reliability‐based assessment of BMRFs is developed with an efficient algorithm using the Monte Carlo simulation procedure incorporated into a nonlinear finite element (FE) analysis program. An appropriate nonlinear FE model of such systems is demonstrated, and the probability of various predefined components' failure over the most likely FSs in the presence of both epistemic and inherent uncertainties is calculated. Then, a system‐simulated reliability index (SSRI) is computed by lower and upper bounds in the probability of BMRF system collapse. Finally, fragility curves based on the SSRI is compared with the ones from incremental dynamic analysis, and later, the outcomes from multiple FSs are compared with the codified main collapse criterion. For the BMRFs analyzed herein, it is shown that the existing allowable story drift for the collapse limit state is conservative, and a new criterion is appraised. Copyright © 2015 John Wiley & Sons, Ltd.     

Comparative reliability of two 24‐story braced buildings: traditional versus innovative

The seismic reliability of two 24‐story buildings that have the same geometry and structural layout was evaluated and compared. The structural system of the first building consists of ductile steel braces and composite moment‐resisting frames (traditional building). The structural system of the second building consists of nonductile flexible steel frames stiffened through a system of buckling‐restrained braces (innovative building). Whereas the former was designed according to the Mexico City Building Code, the latter was designed according to a displacement‐based methodology. Both buildings were assumed to be located at the same site in the lake zone of Mexico City. The study shows that in spite of being considerably lighter, the innovative building exhibits higher levels of reliability. Copyright © 2011 John Wiley & Sons, Ltd.     

Stability analysis of large‐span steel truss structure under local cooling from fire extinguishing system

In case of a fire in large‐span steel structure buildings, the firefighters usually use water to cool the steelwork. However, during the cooling by the spraying water, the collapse of these structures is unclear. In this paper, a large‐span center is taken as an example to study the overall mechanical response process under local cooling conditions in fire. The Fire Dynamics Simulator has been used to investigate the temperature field distribution rule of large‐span buildings under fire. Using the coupled thermal‐mechanical method to study the overall mechanical response process of steel structure under local abrupt drop in temperature, displacement and stress variations of the large‐span steel structure under local cooling have been analyzed. Results of this study show that the temperature distribution in large‐span structures is nonuniform in fire. Using spraying water to cool the structural members in fire can cause the structural stress field redistributed that may lead to further damage to the structure.     

An area‐based intensity measure for incremental dynamic analysis under two‐dimensional ground motion input

Incremental dynamic analysis (IDA) is a useful method in performance‐based earthquake engineering. IDA curves combine the intensity measure (IM) of ground motions with structural responses (as measured by engineering demand parameter) from nonlinear dynamic analysis. However, the curves display large record‐to‐record variability. And various IMs can lead to different results. Therefore, it is important to find a desirable IM to reduce the discreteness of the IDA results. So far, the studies on IM for IDA have been carried out by many scholars from scalar‐valued to vector‐valued, but few were based on 2‐dimensional ground motion input. To make the analysis more reasonable and practical as well as investigate the desirable IM under 2‐dimensional ground motion input, incremental dynamic analyses when ground motions are inputted in 2 directions should be investigated. In this paper, 2 combinational types of area‐based IM incorporating the influence of ground motion record components in secondary directions were proposed. To investigate the applicability, efficiency and desirable combinational form of the area‐based IM under 2‐dimensional ground motion input, incremental dynamic analysis were carried out using 2 reinforced concrete frames. Then the efficiency of the IMs was measured by the residual sum of squares and R². It is concluded that the area‐based IM with a combination by the square root of the sum of the squares (SRSS) method is the most efficient for IDA under 2‐dimensional ground motion input. The methods and conclusions will provide significant reference for studying IMs under 2‐dimensional ground motion input. Further research will focus on the applicability of the area‐based IM for tall buildings whose higher modes need to be considered.     

Performance‐based evaluation of tall buildings using advanced intensity measures (case study: 30‐story steel structure with framed‐tube system)

Performance assessment of high‐rise buildings has attracted peculiar attention among engineers. Care should be taken once higher‐mode effects are to be incorporated into analyses and designs. Recently, performance‐based evaluations have been widely used by designers to meet the required target capacities of engineering projects. A common tool to perform such studies is incremental dynamic analysis (IDA), which has been utilized for first‐mode‐dominant ordinary structures, whereas taller buildings demand other considerations to be made so that a thorough assessment of the structural response can be achieved. In this paper, performance‐based studies have been carried out for a sample 30‐story tall building, which takes advantage of tubular frame as lateral‐load‐resisting system. IDA is performed subsequently to quantify the structural response against a wide‐range of seismic loadings. Advanced intensity measures (IMs) are applied to optimize the capacity assessments resulting from multitude of non‐linear time‐history analyses. Finally, performance‐based evaluations have been carried out to provide a thorough assessment of target capacities that are normally advised by widely accepted codes. Results are also compared with regular short buildings where higher‐mode effects do not contribute significantly to structural response. Copyright © 2012 John Wiley & Sons, Ltd.     

Life‐cycle cost assessment of mid‐rise and high‐rise steel and steel–reinforced concrete composite minimum cost building designs

The traditional trial‐and‐error design approach is inefficient to determine an economical design satisfying also the safety criteria. Structural design optimization, on the other hand, provides a numerical procedure that can replace the traditional design approach with an automated one. The objective of this work is to propose a performance‐based seismic design procedure, formulated as a structural design optimization problem, for designing steel and steel–reinforced concrete composite buildings subject to interstorey drift limitations. For this purpose, eight test examples are considered, in particular four steel and four steel–reinforced concrete composite buildings are optimally designed with minimum initial cost. Life‐cycle cost analysis (LCCA) is considered as a reliable tool for measuring the damage cost due to future earthquakes that will occur during the design life of a structure. In this study, LCCA is employed for assessing the optimum designs obtained for steel and steel–reinforced concrete composite design practices. Copyright © 2011 John Wiley & Sons, Ltd.     

Seismic partitioned fragility analysis for high‐rise RC chimney considering multidimensional ground motion

In order to identify the vulnerable parts and areas of the high‐rise reinforced concrete chimney, this paper presents an effective method, which called partitioned fragility analysis. One 240‐m‐high reinforced concrete chimney was selected as the practical project, and its analytical model was created with ABAQUS software. The selected high‐rise chimney structure was divided into 17 parts, and then the damage probability of each part in different damage states was obtained with the fragility analysis considering multidimensional ground motions. Twenty ground motion records were taken from the Next Generation Attenuation database as the input motions, and the peak ground acceleration was selected as the intensity measure. The response of the chimney structure under multidimensional ground motions was obtained based on incremental dynamic analysis. The maximum strains of concrete and steel bars were defined as the damage limit states of the chimney structure. The fragility curves and surfaces obtained from this analysis showed that the vulnerable areas of the chimney structure appear at 0–20 m, 90–130 m, and 150–200 m along the height of the chimney respectively. Based analytical results, these vulnerable parts can be retrofitted to enhance the seismic resistance of existing chimney structures. And the partitioned fragility analysis method can also be used to improve the design of new chimney structures.     

A displacement‐based adaptive pushover method considering modal sign reversal for RC frame structures

Nonlinear static procedures are favored tools for practical applications in the structural engineering profession. However, some limitations are associated with them, including their deficiencies to properly reflect higher modal effects and inertial seismic forces fluctuations in their responses. Some different adaptive pushover methods intended to improve these limitations have been proposed in the literature, but each one has come out with a special deficiency. In this study, based on the concepts of the displacement‐based adaptive pushover, a new dual‐run procedure method called Improved DAP (IDAP) has been developed, aiming to improve higher modal and sign reversal consideration of pushover methods. The seismic scope of this study has been focused on near‐fault regions. Four concrete SMRF with different heights have been employed for the evaluations. The results of the proposed method in terms of capacity curves, interstory and shear profiles are compared with those of the IDA method. Results indicate that the ability of the new method in reproducing seismic story forces and capacity curves, as well as interstory drifts, has been improved in comparison with its primitive counterpart. Copyright © 2015 John Wiley & Sons, Ltd.     

Outrigger system analysis and design under time‐dependent actions for super‐tall steel buildings

The outrigger system has been widely adopted as an efficient structural lateral‐load resisting system for super‐tall buildings in recent years. Although the outrigger system has many structural advantages, it has a significant defect due to differential shortening, which cannot be neglected. Due to the shrinkage and creep of concrete, as well as the differential settlement of foundation, the shortening of the structural member is an important time‐dependent issue, which leads to additional forces in the outriggers after the lock‐in of the outriggers. As a result, it will increase the size of the structural member cross section in the design. In a real project, engineers can delay the lock‐in time of the outrigger system to release the additional forces caused by the differential shortening during the construction phase. The time‐dependent actions, such as the column shortening and the differential settlement of the foundation, were estimated. A mega frame steel structure was employed to illustrate the analysis and design of the outrigger under the time‐dependent actions. Furthermore, a simple optimal method, considering the structural stability and overall stiffness, was proposed to optimize the construction sequence of the outrigger system.     

Analytical model and evaluation of maximum crack width for unbonded PSRC frame beam under short‐term service load

Prestressed steel reinforced concrete (PSRC) beam members have the advantages of both ordinary prestressed concrete and SRC members and are usually applied to the structures with large span or heavy load. They are often designed to crack under service load. In this paper, the service‐load behavior is studied based on the experimental results of four unbonded PSRC frame beam specimens. The cracking behavior, deflection, and strains in tensile reinforcement during service‐load stage are described in detail. A computer program for a simple macroelement analysis approach, based on conventional matrix‐displacement method, is written to predict the response of unbonded PSRC frame beam members under service load. The calculation results by this method agree well with the observed experimental results. Moreover, an approach based on two enacted Chinese codes, one for ordinary concrete members (GB50010‐2010) and another for steel‐concrete composite members (JGJ138‐2001), is provided to calculate the short‐term maximum crack width of PSRC beam members. By comparing with the test results, it implies that this approach can be applied to the evaluation of short‐term maximum crack width.