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.     

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.     

Progressive collapse performance of irregular buildings

In this study, the progressive collapse‐resisting capacities of tilted or twisted buildings were evaluated by nonlinear static and dynamic analyses. For analysis models, 30‐storey tilted buildings with braced cores and 30‐storey twisted buildings with reinforced concrete cores were designed, and their performances were compared with those of the regular buildings. According to the analysis results, the progressive collapse potential of the tilted structures varied significantly, depending on the location of the removed column. It was also observed in the tilted structures that the plastic hinges formed not only in the bays from which a column was removed, but also in the nearby bays. Similar results were observed in the analysis of the twisted structures. The progressive collapse potentials of the tilted structure were high when a column was removed from the tilted side. However, the twisted structures considered in this study had progressive collapse potentials not very large compared with those of the corresponding regular structures, mainly because more structural elements were involved in resisting progressive collapse when a structural member was eliminated. Copyright © 2009 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.     

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.     

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.     

Dual‐steel frame consisting of moment‐resisting frame and shape memory alloy braces subjected to near‐field earthquakes

In this paper, concentric braced frames are combined with moment‐resisting frame (MRF) as a dual system subjected to near‐field (NF) pulse‐like and far‐field ground motions. The braced frame in this system configuration consists of steel buckling‐restrained braces (BRB model), braces with shape memory alloy (SMA model), or combination of BRB and SMA braces (COMBINED model). Some prototype structures of the proposed systems are designed according to the code recommendations. Then, the nonlinear models of the considered structures are developed in SeismoStruct software, and nonlinear time history analysis (NLTHA) is implemented. NLTHA is performed subjected to earthquake record sets at maximum considered earthquake (MCE) and design base earthquake (DBE) levels, and responses of the systems are investigated and compared with each other. Among the examined models, the SMA and COMBINED models exceed the CP level subjected to NF‐MCE record set. Therefore, more investigation is needed for using short‐segment SMA braces in the dual‐steel frames in NF area.     

Investigation of progressive collapse in intermediate RC frame structures

A large part of common reinforced concrete (RC) structures are designed as intermediate moment resisting frames. However, current progressive collapse studies have not paid much attention to these frames. In this study, based on the acceptance criteria of the UFC 4‐023‐03 document, the influence of some external and corner column removal cases are evaluated by nonlinear procedures in all storeys of a regular structure. Although this structure considers the acceptance criteria of nonlinear dynamic analyses, nonlinear static analyses require additional reinforcement in beam supports in the top two storeys. Compared to dynamic analyses, it is also concluded that static analyses are more conservative in the estimation of maximum vertical displacement and shear force in beams. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

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.     

Comparative study of special and ordinary braced frames

The collapse probability of ductile and non‐ductile concentrically braced frames was investigated using nonlinear dynamic response analysis. Two buildings with three and nine stories located in Boston and Los Angeles, respectively, were designed and subjected to ground motions from the areas. In Boston area, three‐story and nine‐story buildings were designed as ordinary concentrically braced frame with response modification reduction factor R equal to 3 1/4 to be considered as non‐ductile structural systems; comparatively, in Los Angeles area, three‐story and nine‐story buildings were designed as special concentrically braced frame with response modification reduction factor R equal to 6 to be considered as ductile structural systems. In order to evaluate the performance of ductile and non‐ductile concentrically braced frames in moderate and severe seismic regions, ATC‐63 would be used as reference to assess the seismic behaviors. Evaluation approach recommended by ATC‐63 was adopted, and hundreds of nonlinear dynamic analyses were performed. Through alternating the scale factors of designated ground motions, median of structural collapse intensity was presented for each structure. By observing the results of statistical performance assessment, the seismic performance of the systems was evaluated, and some observations are made based on the study. Copyright © 2012 John Wiley & Sons, Ltd.     

Progressive collapse analysis of concentrically braced frames through EPCA algorithm

This paper introduces a new algorithm, namely the Extended Progressive Collapse Analysis (EPCA) algorithm, whose features, such as potential and capacity of buildings for occurrence of progressive collapse, were investigated. Their failure modes were determined by using pushdown and vertical incremental dynamic analyses. Moreover, by applying this procedure, the element removal impact factor and the most critical locations of such removals were obtained. This algorithm was utilized for progressive collapse analysis of two newly designed concentrically braced frames with different numbers and locations of braced bays in order to quantitatively determine its effect on mitigating progressive collapse. Using this method, the minimum residual capacity and the most critical locations of element loss as well as element removal impact factor for the frames that were studied were determined. Results showed that the frame with two braced bays had more robustness for mitigating progressive collapse, at least to the rate of 17.21% comparing to the frame with three braced bays.     

Progressive collapse performance analysis of precast reinforced concrete structures

In this paper, the progressive collapse performance analysis of precast reinforced concrete (RC) structures is performed. A numerical simulation framework for precast RC structures is developed on the basis of the OpenSEES software, where the fiber frame element is used for beam and column type members and Joint2D element is used for the beam‐to‐column connections. The conjugated material models are then introduced, and a min–max failure criterion is imposed on the original models to reflect the steel fracture and concrete crushing when the structure is undergoing progressive collapse. In addition, to overcome the computational difficulties arisen from progressive collapse behavior, two enhanced nonlinear solutions , that is, the consistent quasi‐Newton algorithm and the explicit KR‐α algorithm, are employed, respectively, for static and dynamic analysis. A 10‐storey prototype precast RC structures is designed to verify the developed numerical framework, and the progressive collapse resisting mechanism of the structures is investigated through both static pushdown analysis and dynamic column‐removal analysis. Finally, influences of some typical parameters in precast RC structures on their progressive collapse performance are studied.     

Equivalent viscous damping in direct displacement‐based design of steel braced reinforced concrete frames

Performance‐based design method, particularly direct displacement‐based design (DDBD) method, has been widely used for seismic design of structures. Estimation of equivalent viscous damping factor used to characterize the substitute structure for different structural systems is a dominant parameter in this design methodology. In this paper, results of experimental and numerical investigations performed for estimating the equivalent viscous damping in DDBD procedure of two lateral resistance systems, moment frames and braced moment frames, are presented. For these investigations, cyclic loading tests are conducted on scaled moment resisting frames with and without bracing. The experimental results are also used to calibrate full‐scale numerical models. A numerical investigation is then conducted on a set of analytical moment resisting frames with and without bracing. The equivalent viscous damping and ductility of each analytical model are calculated from hysteretic responses. On the basis of analytical results, new equations are proposed for equivalent viscous damping as a function of ductility for reinforced concrete and steel braced reinforced concrete frames. As a result, the new equation is used in direct displacement‐based design of a steel braced reinforced concrete frame. Copyright © 2012 John Wiley & Sons, Ltd.     

Seismic performance of Y‐type eccentrically braced frames combined with high‐strength steel based on performance‐based seismic design

In the Y‐type eccentrically braced frame structures, the links as fuses are generally located outside the beams; the links can be easily repairable or replaceable after earthquake without obvious damage in the slab and beam. The non‐dissipative member (beams, braces, and columns) in the Y‐type eccentrically braced frames are overestimated designed to ensure adequate plastic deformation of links with dissipating sufficient energy. However, the traditionally code design not only wastes steel but also limits the application of eccentrically braced frames. In this paper, Y‐type eccentrically braced steel frames with high‐strength steel is proposed; links and braces are fabricated with Q345 steel (the nominal yield stress is 345 MPa); the beams and columns are fabricated with high‐strength steel. The usage of high‐strength steel effectively decreases the cross sections of structural members as well as reduces the construction cost. The performance‐based seismic design of eccentrically braced frames was proposed to achieve the ideal failure mode and the same objective. Based on this method, four groups Y‐type eccentrically braced frames of 5‐story, 10‐story, 15‐story, and 20‐story models with ideal failure modes were designed, and each group includes Y‐type eccentrically braced frames with ordinary steel and Y‐type eccentrically braced frames with high‐strength steel. Nonlinear pushover and nonlinear dynamic analyses were performed on all prototypes, and the near‐fault and far‐fault ground motions are considered. The bearing capacity, lateral stiffness, story drift, link rotations, and failure modes were compared. The results indicated that Y‐type eccentrically braced frames with high‐strength steel have a similar bearing capacity to ordinary steel; however, the lateral stiffness of Y‐type eccentrically braced frames with high‐strength steel is smaller. Similar failure modes and story drift distribution of the prototype structures designed using the performance‐based seismic design method are performed under rare earthquake conditions.     

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.     

Progressive collapse analysis of a steel building with pre‐northridge moment connections

This study is on progressive collapse analysis of a sample steel building using various procedures suggested by the General Service Administration and the Department of Defense. The progressive collapse was accommodated by removing columns as per above guidelines. The analysis methods considered were linear static, linear dynamic, non‐linear static and nonlinear dynamic using SAP2000 structural simulation program. The analysis model was a 3D nine‐storey steel moment resisting building. For linear analysis, rigid moment connections were used, while for nonlinear analysis pre‐Northridge connections were used. The effect of an extreme event was incorporated by removing a column in the analysis model. The static analysis was performed on the model with a column removed subjected to static gravity loadings. The dynamic analysis employed a ramp function to simulate the dynamic effect of the removal of a column. For nonlinear analysis, material nonlinearity was incorporated using strength degrading nonlinear hinges of pre‐Northridge connections as per FEMA 356. The paper documents the results of this analysis with comparison among methods. The potential for collapse and performance levels of the building were determined using the demand capacity ratio and the rotational limit obtained through the analysis. Copyright © 2010 John Wiley & Sons, Ltd.     

Progressive collapse resisting capacity of tube‐type structures

In this study, the progressive collapse potential of tube‐type buildings, such as diagrid and tubular structures, composed of lateral load‐resisting perimeter frames and internal pin‐connected gravity frames, was evaluated by nonlinear static and dynamic analyses. To this end, 36‐ and 54‐storey structures were designed as analysis models and progressive collapse analyses were carried out by removing first‐storey columns. According to the analysis results, the progressive collapse of tube‐type analysis model buildings occurred when perimeter columns corresponding to more than 11% of all member cross‐sectional areas were removed from one side of the structures. When the diagonals located around a corner were removed, the ratio was reduced to 8%. It was observed that the corner columns in the diagrid system helped prevent the propagation of member failure all around the perimeter. Copyright © 2009 John Wiley & Sons, Ltd.     

Moment frames – concentrically braced frames dual systems: analysis of different design criteria

In this paper, a design methodology based on the theory of plastic mechanism control (TPMC) is presented for dual systems combined by moment resisting frames and concentrically braced frames (MRF–CBF dual systems). The study is focused on the design of structures failing in global mode, i.e. whose collapse mechanism is characterised by the yielding of all the tensile diagonals, the buckling of the compressed ones, and the development of plastic hinges at all the beam ends and at the base of first-storey columns. The results of push-over analyses and nonlinear dynamic analyses carried out with reference to MRF–CBF dual systems designed according to the proposed procedure are compared with those obtained with reference to the same structural scheme designed according to Eurocode 8. The advantages obtained in terms of seismic performances are outlined and also economic issues are investigated pointing out the convenience of seismic design based on TPMC.