Column Removal Research Articles

Uncertainty analysis on progressive collapse of RC frame structures under dynamic column removal scenarios

The progressive collapse of structures often causes serious casualties and property loss. The intrinsic characteristic of progressive collapse is large-scale domino-like structural damage caused by accidental local failure induced by uncertainty factors. Therefore, it is vital to conduct uncertainty analyses on the progressive collapse of reinforced concrete (RC) frame structures under dynamic loads. In this study, to better quantify the progressive collapse resistance of RC frame structures, progressive collapse fragility analyses and sensitivity analyses were conducted at both the component and structural levels, taking into consideration the uncertainties in the structural design parameters, including structural loads, geometries, material properties, etc. At the component level, an energy-based simplified analysis method was proposed for the quick assessment of the progressive collapse vulnerability of RC beam-column substructures. At the structural level, a fragility curve cluster was adopted for evaluating the progressive collapse resistance of RC frame structures. A typical 3-story, 4 × 3 span RC frame structure designed according to Chinese codes was evaluated by the progressive collapse fragility curve cluster. It is found that the failure probability of the RC frame may increase by more than 30% after considering the uncertainty factors. Based on the fragility curve cluster, the robustness and reliability of both the RC frame and its components were quantified. And a safety index is proposed for judging whether a structure/component requires a progressive collapse redesign. Furthermore, the sensitivity analysis results indicate that the load effects, strength of the reinforcement steel, and the reinforcement area in a beam significantly affect the structural progressive collapse resistance. The research outcomes can provide references for quantifying the progressive collapse resistance and improving the progressive collapse design of RC frame structures.

Investigation of the Occurrence of Progressive Collapse in High-Rise Steel Buildings with Different Braced Configurations

Abstract The progressive collapse phenomenon refers to a chain of damages in a structure where all or a large part of the structure is destroyed by an initial local collapse in it, which can lead to very disastrous results. Therefore, the prevention of progressive collapse has become a necessary action in the design and analysis of buildings and it is vital to investigate this topic more accurately. This study aims to present a proposed pattern in the configuration of braces at the height of a high-rise steel building for reducing the probability of progressive collapse. In this regard, the vertical displacement of 18-story structure with four scenarios of column removal and five concentric bracing patterns including V, Inverted V, X, discontinuous X-bracing at height, and a combination of Xbracing in the side spans and discontinuous X-bracing at height in the middle spans are investigated and compared. In this study, the Alternative Path Method (APM) is used based on the GSA guideline for the analysis of progressive collapse. The results of this research showed that the use of X-bracing in the side spans and discontinuous X-bracing in the middle spans in nonlinear static and dynamic analyses performed better in reducing the probability of progressive collapse than other bracing configurations. Finally, it is recommended to use discontinuous X-bracing at the height that would place the bracings in one direction and providing alternative paths for force transferring in the structure.

Unconventional microbial mechanisms for the key factors influencing inorganic nitrogen removal in stormwater bioretention columns

Bioretention systems are environmentally friendly measures to control the amount of water and pollutants in urban stormwater runoff, and their treatment performance for inorganic N strongly depends on various microbial processes. However, microbial responses to variations of N mass reduction in bioretention systems are complex and poorly understood, which is not conducive to management designs. In the present study, a series of bioretention columns were established to monitor their fate performance for inorganic N (NH4+and NO3−) by using different configurations and by dosing with simulated stormwater events. The results showed that NH4+ was efficiently oxidized to NO3−, mainly by ammonia- and nitrite-oxidizing bacteria in the oxic media, regardless of the configurations of the bioretention systems or stormwater conditions. In contrast, NO3− removal pathways varied greatly in different columns. The presence of vegetation efficiently improved NO3−mass reduction through root assimilation and enhancement of microbial NO3− reduction in the rhizosphere. The construction of an organic-rich saturation zone can make the redox potential too low for heterotrophic denitrification to occur, so as to ensure high NO3− mass reduction mainly via stimulating chemolithotrophic NO3− reduction coupled with oxidation of reductive sulfur compounds derived from the bio-reduction of sulfate. In contrast, in the organic-poor saturation zone, multiple oligotrophic NO3− reduction pathways may be responsible for the high NO3− mass reduction. These findings highlight the necessity of considering the variation of N bio-transformation pathways for inorganic N removal in the configuration of bioretention systems.

Linear static progressive collapse analysis of RC structures

When there is a sudden loss of load carrying element in a structure, the structure will choose alternate load paths until equilibrium is reached. In this process, elements overloaded in the alternate load paths fails, leading to a large damage disproportionate to the initial event which is referred to as Progressive Collapse (PC). Fire, gas explosions, terrorist attacks, car crashes, and improper design and construction are all common causes of progressive collapse. As a result, it is important to research and examine the effects of this phenomena on structures, as well as to reconstruct the structure to be resistant to it. Government buildings, hotels, offices and residential buildings require different spacings and may require large spans. Though lot of research work has done on PC resistance of Reinforced Concrete (RC) structures to explore that research, in this study, RC buildings with 4 m, 6 m and 8 m clear span between columns have taken and linear static analysis (LSA) has done for different column removal conditions. PC resistance is measured in terms of vertical joint displacement (VJD), chord rotation (CR) and demand capacity ratio (DCR). The VJD and CR results are discussed and compared with each other. The calculated CR values and DCR values are compared with the limiting value as mentioned in GSA and DoD. The tendency to PC is more for 8 m span model is more than the other models with 6 m and 4 m.

Effects of loading regimes on the structural behavior of RC beam-column sub-assemblages against disproportionate collapse

The majority of previous quasi-static tests on disproportionate collapse simulated the column removal through applying concentrated load/displacement on the top of the removed column until failure. However, uniformly distributed service load always exists on the frames. Therefore, to reflect the actual load condition more accurately, uniformly distributed load should be applied along the beams first. Then, the temporary support is gradually removed to simulate the process of column removal. By this way, the beams may undergo only a small deflection as the dynamic effect is neglected. Thus, a subsequent concentrated loading process is employed to evaluate the behavior of the beams at the ultimate stage, which may be reached if the dynamic effect is considered. Such a loading process is named sequential loading regime. To evaluate the effects of loading regimes on the behavior of reinforced concrete (RC) frames under a middle column removal scenario, two series of half-scale RC beam-column sub-assemblages were tested in this study. It is found that the conventional concentrated loading regime could accurately estimate the yield strength and the compressive arch action capacity of the RC beam-column sub-assemblages, but it may over-estimate the catenary action (CA) capacity and the deformation capacity. Moreover, although the concentrated loading regime is convenient and able to demonstrate the load transfer mechanisms of the sub-assemblages against disproportionate collapse, it may mistakenly identify the locations of critical sections. Furthermore, based on the failure modes and local strain gauge results, analytical models were proposed for predicting the CA capacity of the tested specimens under two loading regimes. Results suggest that the analytical models could predict the CA capacity well.

Dynamic response of RC beam-slab substructures following instantaneous removal of columns

High fidelity solid-element-based numerical modeling is employed in this paper to study the dynamic response of reinforced concrete (RC) beam-slab substructures following instantaneous removal of columns. After validation of the numerical model against experimental results, incremental dynamic analysis is conducted to obtain the dynamic structural resistance to progressive collapse. The numerical results for development of internal forces and distribution of stress and strain are used to illustrate the load redistribution and load transfer mechanisms of RC beam-slab substructures subjected to sudden loss of columns. The numerical model is further employed to explore parameters affecting the dynamic response and structural resistance of beam-slab substructures under sudden column removal scenario. The parameters studied include damping ratio, slab thickness, location of removed columns, multi-column removal sequence and interval. The numerical result shows that the dynamic progressive collapse resistance of the beam-slab substructure is much smaller than the quasi-static test result due to a dynamic increase factor (DIF) of 1.14. Moreover, a minimum slab thickness of 1/45 of span length is found vital for the improvement of progressive collapse resistance of RC beam-slab substructures through developing compressive membrane action (CMA) and tensile membrane action (TMA) as well as enhancing the development of compressive arch action (CAA) and tensile centenary action (TCA) of beams. When the damping ratio is smaller than 5%, the increase of damping ratio can significantly benefit the structural resistance against progressive collapse by producing smaller deflection and milder vibration. The additional loss of the corner column besides the penultimate perimeter column is found to form the most hazardous situation among the double-column removal scenarios. In contrast to the location of removed columns, the column removal sequence and interval tend to little affect the maximum deflection developed and consequently the progressive collapse resistance of substructures.

Progressive collapse mechanisms of post-tensioned reinforced concrete frames considering effect of infill walls

This study numerically investigates progressive collapse mechanisms of post-tensioned RC frames under column removal scenarios by using the finite element method. The primary objective of this study is to investigate the influences of infill walls on mechanical behavior of post-tensioned (PT) frames against progressive collapse. Three experimental conventional reinforced concrete (RC) frames are modified into PT reinforced concrete frames: PT bare frame, PT full infilled frame and PT partial infilled frame. The numerical study was performed and the applied vertical load, horizontal displacement, failure process, load redistribution mechanisms, and re-centering capacity of PT frames are investigated under middle column pushdown scenario. Parametric analyses are conducted to study influences of material properties, arrangement of PT tendons, and magnitude of PT forces. The results show that masonry infill walls enhance the resisting capacity; however, change the failure modes and reduce the re-centering capacity of the PT frames against progressive collapse. The presence of the opening reduces the contribution of the infill walls. The PT bare frame have better resilience performance compared with the conventional bare frame. The bed joints, strength of masonry bricks, arrangement of PT tendons and magnitude of PT force greatly influence the progressive collapse resisting capacity of PT frames. Finally, a preliminary design method is proposed to predict the progressive collapse resisting capacity of PT frames with good accuracy and efficiency.

Study of progressive collapse distribution in reinforced concrete buildings due to simultaneous effects of earthquake loads and edge column removal

Study of progressive collapse distribution in reinforced concrete buildings due to simultaneous effects of earthquake loads and edge column removal

Progressive collapse resistance of exterior reinforced concrete frames and simplified method for catenary action

This paper investigated the progressive collapse resistance of exterior reinforced concrete frames under middle column removal scenarios. Five half-scale exterior frames comprising of a double-span beam, a middle beam-column joint and two side columns were tested under a concentrated point load on the middle joint. Different joint links and reinforcement ratios in beams and column sizes were used in frames. The resistances, deformation capacities and failure modes of frames were experimentally obtained. Test results showed that catenary action in bridging beams resulted in flexural failure of side columns, and thus the load capacity of frames depended highly on the moment capacity of column sections. A simplified method was also proposed based on the moment capacity of column sections and the axial force-bending moment interaction diagram of beams. The axial tension force in beams and the associated catenary action capacity were calculated by using the proposed method. Comparisons with test data suggest that the simplified method could predict the load capacity and tension force in beams under catenary action with reasonably good accuracy.

Examining progressive collapse robustness of a high-rise reinforced concrete building

To examine high-rise building robustness to sudden column removal, progressive collapse studies of a 13-story flat-slab reinforced concrete (RC) core-wall building system were completed in this study. Two buildings on the University of Nebraska-Lincoln (UNL) campus, Cather and Pound Halls, were demolished in December of 2017. UNL researchers were given unique access to the building sites to assess their response before, during, and after the event. Prior to demolition, accelerometers were placed throughout both buildings, and ambient measurements were conducted to establish natural frequencies and mode shapes. The response of both buildings during controlled demolition was recorded and analyzed using time histories from the accelerometers. Controlled demolition initiated with sequential blast detonation at selected column sections that resulted in progressive collapse. A detailed 3D nonlinear numerical model of Pound Hall was developed using LS-DYNA to mimic the progressive collapse. The accuracy of the building model was evaluated via comparisons between (i) simulated and measured building ambient frequencies and modes; and (ii) displacement and video recorded during the full-scale demolition. The validated model was then used to further investigate damage scenario effects on the robustness of high-rise RC buildings caused by sudden removal of columns at various locations on the ground floor. The study results can be used to develop and calibrate the nonlinear numerical model for analyzing high-rise building progressive collapse behavior and can help provide information that may improve new and existing flat-slab RC core-wall building robustness against progressive collapse.

Scenarij postupnog rušenja čeličnih konstrukcija s nepravilnostima po visini

Sudden removal of load-bearing elements such as columns in engineering structures, and lack of sufficient capacity to withstand the overload caused by removal of these elements can cause damage and Progressive Collapse (PC) in structures. Therefore, the effect of sudden column removal and structural capacity against PC scenarios in medium and high-rise buildings is investigated in this study. The irregularity in height has a great influence on lateral behaviour of structures and it affects the design of cross-sections. Various sudden column removal scenarios are investigated in this research for steel structures with and without irregularity in height. To assess the effects of sudden column removal, the Alternate load Path Method (APM) and Nonlinear Dynamic Analysis (NDA) are utilized. In addition, a Nonlinear Static Analysis (NSA) is performed to investigate the capacity of structures against the PC phenomenon. Using OpenSees software, 10-, 15- and 20-storey structures with three distinct irregularity types are analysed during four different column removal scenarios. The results are presented in the form of static and dynamic nonlinear curves. The results indicate that making geometric irregularity in height in the sudden column removal scenario can cause the reduction of capacity and growth of the structural response in comparison to the structure with regularity in height. Moreover, the capacity of structures increases and the dynamic response declines by increasing the number of elements in the structures.

Influence of welding deformation on anti-collapse performance of single-layer reticulated shell structures with prestressed cables

In this paper, the ABAQUS finite element software is used to model and analyze the single-layer reticulated shell structure. And the method of removing the constraints of key components is used to analyze the anti-continuous collapse performance of this reticulated shell structure under the action of geometric defects and welding deformation. Welding deformation is mainly caused by welding and prestressed cable tensioning of single-layer reticulated shell. The results show that the removal of the key columns under the reticulated shell will lead to the continuous collapse of the upper part, which leads to the continuous collapse of the whole structure, and the welding deformation can make the continuous collapse of the whole reticulated shell more obvious. Thus determining the influence of geometric defects and temperature shrinkage deformation on the anti-continuous collapse performance.

Performance of Steel Frames with Different Web Bolt Number and Arrangements in a Column-Loss Scenario

When a steel frame is subjected to an abnormal load, a sudden local failure due to the loss of a load carrying member may result in progressive collapse of the entire structure. In this study, experiments and numerical simulations were performed to investigate the anti-collapse performance of double half-span assemblies (a column connected with two half-span beams) with welded unreinforced flange–bolted web (WUF-B) connections in a column removal scenario. Three types of bolted webs—two bolts in one row, three bolts in one row, and four bolts in two rows—were considered. For the post-peak response, the connections exhibited a significant resistance recovery owing to the catenary action. The bolt-bearing areas on the beam web were prone to fracture owing to excessive compression, and the failure modes of the web varied significantly with the bolt arrangement. The test results demonstrate that increasing the number of bolt rows is a fairly effective approach for enhancing the anti-collapse performance of the WUF-B connections. Numerical models were used to further investigate the effects of the bolt arrangement on the anti-collapse performance of the WUF-B connections. The results indicate that the bolt arrangement can directly affect the response of the connections after the first peak load, especially in the catenary phase.

Evaluating the effect of Buckling-Restrained Braces in Steel Buildings against Progressive failure using Different Simulation Strategies

Ignoring the primary damage to structural components due to blast load or fire is the alternate load path (APM) method's weakness in progressive failure analysis. The new technique used in this study examines the structure's more realistic responses by considering the initial cause of the failure. Also, buckling-restrained braces (BRBs) are applied to diminish the potential for progressive failure in braced steel buildings. Variables include the type of primary local loading (APM, blast loading, and heat caused by fire), the position of column removal in the plan (inner and outer frame), the type of brace (BRB and CB), and the number of stories (3, 5, and 8 stories). The buildings were simulated using ABAQUS. The results showed that BRBs in steel buildings under blast load, compared to conventional braces, reduce the potential of progressive failure. The use of BRBs provides much more energy absorption than conventional bracing systems due to brace buckling prevention.

Progressive collapse resistance of reinforced concrete frame structures subjected to column removal scenario

The progressive collapse of a building occurs due to an unusual loading condition or when there is a sudden loss of the primary structural element. This could lead to a partial collapse or entire collapse of the building due to the failure of one or more structural elements. This study investigates the progressive collapse demand of the RC building by linear static analysis and vertical displacement responses using non-linear dynamic analysis. ETABS software is used for corner column removal, edge column removal, penultimate column removal and internal column removal scenarios. The RC buildings considered in this investigation are G + 6, G + 8 and G + 10 storey. The results showed that progressive collapse demand value of the bottom end of adjacent columns were found to be safe for all damage cases. However, the top end of the adjacent columns was found to be unsafe. The most critical damage case was found to be the penultimate column removal case that possessed highest PCD values. Finally, the vertical displacement of time history results is represented graphically and later the results of time history analysis is compared with linear static analysis results.

Energy-based criteria for assessment of box-section steel columns against progressive collapse

Recent studies show that the evaluation of structures in the energy domain has attracted some researchers’ attention in the field of progressive collapse. Column buckling and failure energy were introduced as the energy-based criteria and can be obtained from the single-column displacement controlled analysis for progressive collapse assessment of steel structures. Accordingly, the structural performance following the columns removal scenarios can be evaluated by comparing the column’s internal energy to the energy criteria. In this paper, three-dimensional numerical simulations are conducted to investigate the buckling and post-buckling behavior of the box-section steel columns and derive the energy criteria. In addition to the simplified single-column model, to evaluate the role of beams on the derivation of the energy criteria, beams with different sections are explicitly considered in the finite element models according to the column location. The results show that the lack of explicit modeling of beams in the analysis, depending on the column’s permanent load, can lead to conservative and non-conservative results in deriving the energy-based criteria for progressive collapse assessment of steel structures.

Modeling damaged precast structural system when its robustness check

Within the framework of traditional approaches to checking for resistance of reinforced concrete buildings and structures to the progressive collapse development, membrane (chain) forces in a damaged structural system are calculated separately, without considering its non-linear bending behavior during the formation of the plastic hinges and without checking the possibility of achieving large deflections.The authors propose an approach to modelling a nonlinear quasi-static reaction of a damaged structural system in an accidental design situation. This approach considers non-linear bending and the resistance of reserved horizontal ties, considering their ultimate ductility. The authors verified the proposed approach based on the results of experimental studies by others researchers.An example of the application of the proposed approach in assessing the robustness of a structural system made of precast concrete with a sudden removal of the central column is considered. In accordance with the provisions of the energy approach, an analysis is made of the contribution of individual resistance mechanisms to the total quasi-static and dynamic resistance of the damaged structural system.We show that the proposed calculation model adequately describes the behavior of a damaged structural system in an accidental design situation, and therefore to carry out parametric studies and check the robustness of building structures.

Effect of loading methods on progressive collapse behavior of RC beam-slab substructures under corner column removal scenario

In this paper, high-fidelity finite element (FE) models were developed to investigate the behavior of reinforced concrete (RC) beam-slab substructures to resist progressive collapse under a corner column removal scenario. The numerical models were validated by test results. Then, the validated FE models were employed to investigate the structural behavior under different loading methods, including concentrated loading (CL) and uniformly distributed loading (UDL) methods. Moreover, multi-story frames were built to capture the load redistribution behavior of substructures at different floors under different loading methods. The results indicated that the loading methods affect overall structural responses, load transfer mechanisms, and failure modes. It was demonstrated that the Vierendeel action could not be ignored for multi-story frames to resist progressive collapse caused by the loss of a corner column scenario. More significant Vierendeel action was developed in the structure subjected to CL method than that subjected to UDL method. It was also found that the load transfer mechanisms developed in the top story and bottom story for a multi-story frame are pretty different from those in the middle stories. The bottom story has the most remarkable load resisting capacity.

Robustness-oriented optimal design for reinforced concrete frames considering the large uncertainty of progressive collapse threats

Design optimization of building structures under progressive collapse threats is a challenging problem. This is not only because the progressive collapse is a system failure phenomenon involving complicated material and geometrical nonlinearities, but also because the extremely low probability and the much larger uncertainty associated with the initiating events that trigger such a failure could render most risk reduction strategies economically unjustified, should the conventional risk-neutral decision making approach be followed. Facing these challenges, this paper presents an innovative two-stage optimization method for progressive collapse design of reinforced concrete frame structures. The first, tactical stage involves a bi-objective optimization that maximizes the structural robustness while minimizing the total additional reinforcement beyond the conventional design. This stage of optimization results in a Pareto efficiency frontier between the increase in longitudinal reinforcement and structural robustness, which is measured by a new risk-based robustness index. At each efficiency point, the optimal allocation of reinforcement across the beams affected by a column removal scenario is determined. The second, strategic stage includes a descriptive decision making module based on the cumulative prospect theory (CPT) in order to characterize the bounded rationality of a decision maker when facing large uncertainty. Based on an empirically calibrated CPT model, the study finds that the threshold for the probability of initiating event below which no additional investment is justified for progressive collapse design is pushed to the lower end of the empirical incidence rate. This is completely in contrast to previous research results based on the risk-neutral argument. Using a multi-storey plane frame for illustration, the study also clearly shows that the Alternate Path Method following the linear static procedure is not efficient from a purely engineering perspective. When the decision maker’s risk attitude is considered, the APM design is found to be unduly conservative.

Experimental investigation on progressive collapse performance of prestressed precast concrete frames with dry joints

The prestressed precast concrete frame with dry joints is a novel structural system which has been proved to perform excellently under seismic loads in many researches, but its progressive collapse resistant mechanism need to be investigated further due to the discontinuity of longitudinal bars at joint interface. In this paper, quasi-static pushdown tests were conducted to investigate progressive collapse behavior of three half-scaled plane frame substructures with two stories and two spans under first-story middle column removal scenario. The specimens, including one cast-in-place concrete frame substructure and two prestressed precast concrete frame substructures, were designed according to the same design loads. The test results showed that these two types of frame structures could supply similar vertical resistances in compressive arch action (CAA) stage, while the prestressed precast structure presented a higher bearing capacity in tensile catenary action (TCA) stage, and the load-carrying ability was enhanced with the increase of steel strands area. However, the prestressed precast frame had a smaller failure displacement, which showed a lower ductility. Compared to the cast-in-place concrete frame, the prestressed precast concrete frame showed a distinct failure mode, in which there were almost no obvious cracks in beams, and no plastic hinges formed at beam ends, but a wide through crack appeared at the joint interface. The tension of steel strands decreased the CAA effect in beams, and speeded up the transformation from CAA stage to TCA stage during the progressive collapse process. This phenomenon became prompt with the increase of steel strands area. In terms of the same frame specimen, the peak value of axial compression force of beam in the second story was lower than that in the first story due to distinction of boundary conditions at beam ends in the two stories. However, the strength of steel strands of beams in both stories was fully utilized under large deformation due to a reliable tensile restrained stiffness at beam ends, leading to a similar peak tensile force at TCA stage.