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.     

考虑楼梯的框架结构连续倒塌分析

利用有限元软件SAP2000的动力非线性计算方法对考虑楼梯与不考虑楼梯两种情况下的框架进行连续倒塌计算,对比分析了楼梯位置处两处框架柱失效时框架的抗连续倒塌能力。分析结果表明:当楼梯部位的柱失效时,考虑楼梯与不考虑楼梯对框架的连续倒塌计算影响很大。考虑楼梯后,模型失效点处的位移和框架梁的内力减小,部分柱内力增大;楼梯构件受力改变,可能会因此失效,但有利于实现荷载的多路径传递,从而对框架的抗连续倒塌有利。     

Influence of seismicity level and height of the building on progressive collapse resistance of steel frames

Influences of building height and seismicity level on progressive collapse resistance of buildings are investigated in this paper. For the height, 4‐story, 8‐story and 12‐story steel special moment resisting frames are focused. The obtained results indicate that taller buildings are safer against progressive collapse. To study the influence of seismicity level, different four‐story structures having special moment resisting frame systems are designed for different levels of seismicity, namely, very high, high, moderate and low. The structures are evaluated, using nonlinear dynamic method and two main scenarios of the codes, including sudden removal of a corner and a middle column in the first floor. Some graphs are presented for progressive collapse resistance of the structures, depending on their seismic base shears. It is shown that the structures designed for greater seismic base shears are more resistant against progressive collapse. Copyright © 2016 John Wiley & Sons, Ltd.     

Progressive collapse resisting capacity of tilted building structures

In this study, the progressive collapse resisting capacities of tilted buildings are evaluated on the basis of arbitrary column removal scenario. As analysis model structures both regular and tilted moment‐resisting frames, structures with outrigger trusses, and tubular/diagrid structures are designed, their progressive collapse resisting capacities are evaluated by nonlinear static and dynamic analyses. It turns out that the tilting of the structures requires increased steel tonnage due to the increased p‐delta effect. In addition in the tilted structures the plastic hinges are more widely distributed throughout the bays and stories when a column is removed from a side or a corner of the structures. With the analysis results, it is concluded that the tilted building structures, once they are properly designed to satisfy a given design code, may have at least an equivalent resisting capacity for progressive collapse caused by sudden loss of a column. Copyright © 2012 John Wiley & Sons, Ltd.     

Seismic collapse assessment of reinforced concrete moment frames using endurance time analysis

In this paper, seismic collapse of reinforced concrete moment frames is assessed using endurance time (ET) analysis. A set of 30 frames that incorporate deterioration of concrete components is used for this assessment. Application of ET method for collapse assessment of structures is explained, and its accuracy for this purpose is evaluated by comparing its results with incremental dynamic analysis results. Input motions for ET analysis are generated based on ASCE7‐05 design spectrum, and also accelerograms used for incremental dynamic analysis are spectrally matched to the same spectrum. Distribution of different engineering demand parameters over frames height and their values at collapse occurrence are compared for two methods. Results show that spectral accelerations in which collapse occurs in both analyses are very similar for most of the frames, and ET method can appropriately predict the collapse mechanisms of the structures especially for taller frames. Accuracy of ET method in collapse assessment of reinforced concrete moment frames is satisfactory, and this method can be used as a good estimator for study of collapse mechanisms with much less computational effort. Copyright © 2014 John Wiley & Sons, Ltd.     

Seismic performance of MR frames protected by viscous or hysteretic dampers

This study concerns the behaviour of steel frames protected by different anti‐seismic devices (dampers). Typical hysteretic and viscous dampers are arranged in three steel moment‐resisting frames (MRFs) having different dynamical features but are designed to accomplish determined performance objectives. The proposed devices are selected following an iterative procedure based on the use of a suitable damage functional, which has been applied to control the behaviour of the protected structures under a specific seismic record. The outcomes obtained by implementing incremental dynamic analyses, carried out on the basis of seven historical records characterized by different features, allow to analyse the improvement of the structural performance due to the considered dampers and, therefore, to provide design information about their employment. The comparison of results is carried out taking into account the dampers capacity to protect the structures from damage, the inter‐storey drifts, the residual deformations and the possible amplification effects. In conclusion, the equivalent behaviour factors for each damper type are given, with the aim of providing useful design parameters for the implementation of simplified conventional linear analyses. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

Simplified nonlinear progressive collapse analysis of welded steel moment frames

In this study, two nonlinear analysis methods are proposed that can be used for a simplified but accurate evaluation of progressive collapse potential in welded steel moment frames. To this end, the load-resisting mechanism of the column-removed double-span beams in welded steel moment frames was first investigated based on material and geometric nonlinear parametric finite element analysis. A simplified tri-linear model for the vertical resistance versus chord rotation relationship of the double-span beams was developed. The application of the developed model to energy-based nonlinear static progressive collapse analysis was then proposed. The relationship between the gravity loading and the maximum dynamic chord rotation or the concept of collapse spectrum was also established for a quick assessment of the maximum deformation demands.     

Progressive collapse analysis of seismically designed steel braced frames

The progressive collapse resistance of seismically designed steel braced frames is investigated using validated computational simulation models. Two types of braced systems are considered: namely, special concentrically braced frames and eccentrically braced frames. The study is conducted on previously designed 10-story prototype buildings by applying the alternate path method. In this methodology, critical columns and adjacent braces, if present, are instantaneously removed from an analysis model and the ability of the model to successfully absorb member loss is investigated. Member removal in this manner is intended to represent a situation where an extreme event or abnormal load destroys the member. The simulation results show that while both systems benefit from placement of the seismically designed frames on the perimeter of the building, the eccentrically braced frame is less vulnerable to progressive collapse than the special concentrically braced frame. Improvement in behavior is due to improved system and member layouts in the former compared to the latter rather than the use of more stringent seismic detailing.     

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 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.     

Progressive collapse analysis of an RC building with exterior partially infilled walls

Exterior partially infilled walls of RC condominium buildings are often monolithically cast with beams and/or columns in Taiwan. In practice, they are regarded as non‐structural elements in usual structural design, and only their weight is conservatively considered. In this study, effects of three common types of exterior partially infilled walls on the progressive collapse potential of an RC building are investigated. Linear and nonlinear static analyses under different column‐loss scenarios are carried out. Analysis results indicate that influence of the partially infilled walls differs from each type. Changes in the demand‐to‐capacity ratios indicate that without considering the walls, the moment demand of beams may be overestimated. However, their shear demand may be underestimated, especially with the panel‐type walls. Meanwhile, they may increase the collapse resistance of the building frame under column loss but with decreased ductility capacity. From the structural aspect, with a constant opening rate of 60%, the wing‐type wall is a better option than the parapet‐ and panel‐type walls. The panel‐type wall appears to be the worst choice since shear failure of their connected beam members may be induced. Copyright © 2011 John Wiley & Sons, Ltd.     

非线性粘滞阻尼器减震结构参数研究

结合工程实例，对多层钢筋混凝土框架结构进行了分析，考虑非线性粘滞阻尼器斜撑的施加对结构地震反应的影响，并根据计算结果研究了支撑刚度对阻尼器减震效果的影响，所得结果可为粘滞阻尼器支撑结构的设计提供依据。     

Loss-of-stability induced progressive collapse modes in 3D steel moment frames

This paper deals with the progressive collapse analysis of a tall steel frame following the removal of a corner column according to the alternate load path approach. Several analysis techniques are considered (eigenvalue, material nonlinearities, material and geometric nonlinearities), as well as 2D and 3D modelling of the structural system. It is determined that the collapse mechanism is a loss-of-stability-induced one that can be identified by combining a 3D structural model with an analysis involving both material and geometric nonlinearities. The progressive collapse analysis reveals that after the initial removal of a corner column, its two adjacent columns fail from elastic flexural-torsional buckling at a load lower than the design load. The failure of these two columns is immediately followed by the failure of the next two adjacent columns from elastic flexural–torsional buckling. After the failure of these five columns, the entire structure collapses without the occurrence of any significant plastification. The main contribution is the identification of buckling-induced collapse mechanisms in steel frames involving sequential buckling of multiple columns. This is a type of failure mechanism that has not received appropriate attention because it practically never occurs in properly designed structures without the accidental loss of a column.     

The modified dynamic‐based pushover analysis of steel moment resisting frames

A modified dynamic‐based pushover (MDP) analysis is proposed to properly consider the effects of higher modes and the nonlinear behavior of the structural systems. For this purpose, first, a dynamic‐based story force distribution (DSFD) load pattern is constructed using a linear dynamic analysis, either time history (THA) or response spectrum (RSA). Performing an initial pushover analysis with the DSFD load pattern, a nonlinearity modification factor (NMF) is calculated to modify the DSFD load pattern. The envelope of the peak responses of the structure obtained from 2 pushover analyses with the modified DSFD load pattern as well as the code suggested first mode load pattern are considered as the final demand parameters of the structural system. The efficiency of the proposed MDP procedure is investigated using the results of nonlinear THA besides some existing pushover procedures. For this purpose, the 2‐dimensional 9‐, 15‐, and 20‐story, SAC steel frame building models are considered for parametric studies using OpenSees program. The results indicate that the proposed MDP‐THA and MDP‐RSA methods can significantly improve the performance of the pushover analysis. Considering the accuracy and calculation efforts, the MDP‐RSA method is strongly suggested as an efficient and applicable method to estimate the nonlinear response demands of steel moment resisting frames.     

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.     

黏滞阻尼器有效刚度对结构的影响及其取值方法研究

目前,在进行消能减震结构分析时,通常忽略黏滞阻尼器的刚度,影响了结构受力与响应的准确性。为此,阐述黏滞阻尼器刚度特性及其对结构的影响,给出一种适用于时程分析的有效刚度迭代取值方法。对设置黏滞阻尼器消能减震模型、附加有效阻尼比等效模型和附加有效阻尼比与有效刚度等效模型的三组模型进行分析,研究黏滞阻尼器有效刚度对结构受力与响应的影响。结果表明:采用附加有效阻尼比与有效刚度等效模型分析得到的结构楼层剪力与层间位移角的变化趋势基本和采用消能减震模型分析的结果一致;采用附加有效阻尼比等效模型与采用消能减震模型分析的结果则相差较大;时程分析中采用提出的有效刚度迭代取值方法是可行的,建议黏滞阻尼器消能减震结构设计时考虑黏滞阻尼器有效刚度的影响。     

Influence of infill configurations on seismic responses of steel self‐centering moment resisting frames

Steel self‐centering moment resisting frames (SC‐MRFs) have been validated experimentally as resilient structural systems, mainly highlighting the minimized residual drift responses but are prone to suffering high‐mode effects. In this paper, the influence of infill configurations on seismic responses of steel SC‐MRFs was first analyzed. A comparison of the previous experimental results was conducted to investigate the effect of infills on the residual drift of steel frames. In the numerical simulation, the infills were modeled as the equivalent strut diagonals, and the force–displacement of the infills was modeled using the combination of Elastic‐No Tension Material and Hysteretic Material offered by the OpenSees program. The seismic analyses of 3‐ and 9‐story SC‐MRFs with and without infills were carried out to analyze the effects of infills on the residual drift responses and high‐mode contribution under the selected ground motions. Finally, the different infill types and infill irregularities on the seismic responses were investigated to obtain general conclusions. The plastic deformations of columns and infills are also compared in the different cases of infill configurations. The results reveal that all infilled cases experience reduced peak‐story drift and force demands at the upper stories.     

虹桥综合交通枢纽结构连续倒塌分析研究

为减少局部构件破坏引起结构连续倒塌的可能性,对虹桥综合交通枢纽东交通广场B3结构单元进行了3个单柱失效和1个双柱失效情况下的结构连续倒塌分析。采用有限元程序LS-DYNA对结构进行非线性动力分析,构件破坏判断标准参考美国国防部规范UFC4-023-03的要求。研究结果表明：在不同位置单柱失效情况下,该框架结构发生了局部破坏,但破坏限制在去除构件周围的一定范围内;支撑的存在可改变破坏路径;在双柱失效情况下,结构破坏范围明显加大,但最终破坏仍限制在局部范围,不会造成整体结构的连续坍塌。