Sudden Column Removal Research Articles

Resilience of code compliant reinforced concrete buildings to progressive collapse: A numerical analysis investigation

This study investigates the resilience of dual-system reinforced concrete (RC) structures impacted by progressive collapse where the local failure of a primary structural component leads to the sequential collapse of adjoining elements. The study is conducted on buildings designed in the United Arab Emirates in the context of extreme events. Nonlinear dynamic analysis of twenty-seven different building models is performed to evaluate the impact of critical variables, building height, and column spacing on progressive collapse potential under the Unified Facilities Criteria (UFC) regulations. The research methodology aims to study impacts associated with sudden removal of critical columns (corner, edge, and internal) that replicate severe accident scenarios in buildings. The structural responses were assessed based on the ASCE 41 performance design criteria. The formation of plastic hinges was analyzed to determine the ductility and resilience of the structures. The study finds that buildings with removed corner columns exhibit more significant vertical displacements than those with edge or internal column removals. The results also indicate that columns adjacent to the removed ones generally remained elastic and within their structural capacities, showing structural resilience to localized damage. However, the flexural strength of RC flat slabs near removed or adjacent columns demonstrates insufficient flexural capacities. This study underscores the imperative for enhanced design strategies and emphasizes the critical importance of bolstering structural resilience against extreme events. Moreover, it intends to draw the attention of policymakers locally to the need to consider progressive collapse when designing reinforced concrete structures.

Robustness of reinforced concrete frames with elements experiencing bending with torsion

The article presents the results of experimental and theoretical studies of deformation, cracking and fracture of frame structural systems under accidental impacts. The elements of frame structures may exhibit bending with torque as a consequence of the occurrence of an abnormal event. The study provides an energy-based approach to determining internal forces in such structural systems under a scenario in which a column is removed. It also presents a computational model that evaluates the robustness of a reinforced concrete frame of a multi-story building. The model permits the determination of dynamic effects within a structural system, at varying levels of static loads acting within it, and enables the investigation of deformation patterns within the spatial cross-section, including those experiencing bending and torsion. It also allows for the calculation of the robustness parameter of frame systems. In order to validate the proposed computational model, the study employs the outcomes of tests conducted on scale models of reinforced concrete frames with beams subjected to bending and torsion. The experimental frames were loaded in two stages. In the initial stage, the frame structures were subjected to static loading. At the second stage, scaled models were exposed to dynamic loading, which was caused by the sudden removal of columns. A comparison of calculated and experimental results for the investigated structures demonstrates the reliability and efficiency of the proposed calculation model. Consequently, it can be used to calculate the progressive collapse resistance of reinforced concrete frames of multi-storey buildings.

Experimental and numerical investigation on collapse performance of wooden drum-towers in southwest China

The drum-tower belongs to the category of towering structures, and the overall collapse is a common accident in such structures. In order to study the anti-collapse performance of drum-towers, this research used the bottom floor of a wooden drum-tower in the southwest China as a prototype, designing and producing a 1/3 scale structural model. The sudden-column-removal (SCR) method was employed to simulate the structural collapse process, observe collapse characteristics, and analyze the dynamic response of the structure. A finite element model was established, and on the basis of verifying its accuracy, a parameter expansion analysis was conducted. The study indicates that the ductility of wooden drum-towers is relatively poor, and the pulling out of mortise- tenon joints in adjacent components in the failure area is a primary factor for the progressive collapse of the structure. Due to the redistribution of internal forces, the load-bearing status of beams in the failure area changes, experiencing tension, bending moment, and torsion simultaneously, leading to the pulling out, compression, and splitting of mortise-tenon joints. After the failure of these joints, the structure undergoes progressive collapse under the influence of gravity. The dimensions of mortise-tenon joints and the friction coefficient have a significant impact on the anti-collapse performance of the structure. Larger dimensions and friction coefficients result in slower structural damage and more stable structural response. Among these factors, increasing the depth of mortise-tenon joints has the most significant effect.

Robustness assessment of precast concrete connections using component-based modelling

Employing highly optimised precast concrete product-based building solutions increases on-site productivity through elimination of formwork and reduction in propping as well as reducing waste, accidents and embodied carbon. The construction related benefits of precast concrete product-based building solutions are maximised by eliminating structural topping and designing connections between members for ease of assembly. A key challenge in the design of precast concrete buildings is the achievement of robustness under accidental loading. In this paper, sudden column removal is used to assess the robustness of a precast-concrete building system without structural topping. In this case, the development of an alternative load path under sudden column removal relies on the joint response. Joint behaviour is replicated using a component-based design procedure which captures localised failure modes. Robustness is evaluated using a ductility-centred approach and quantified in terms of the pseudo-static resistance. Two types of connection are considered for the provision of continuity at a critical half-lapped joint. The first is a plated connection which was designed initially to meet the tying requirements outlined in Eurocode 2. Under sudden column removal the plated connection’s deformational capacity is limited, which in turn reduces the pseudo-static resistance. An alternative bracketed coupler connection is proposed in which the ductility supply is controlled through debonding of reinforcement. The design concept for the bracketed connection is validated with test results from two full scale sub-assemblies. The experimental results are used to validate a component-based numerical model which is subsequently used to investigate the influence of boundary conditions, and debonding length on the pseudo-static resistance following sudden column loss. The paper shows that the pseudo-static resistance can be significantly enhanced by flexure and compressive membrane action. Consequently, the authors suggest that connection design in precast concrete structures without topping should be based on a realistic assessment of the ability of the structure to develop alternative load paths following instantaneous column removal rather than simplified tying rules.

Sudden column removal in tall buildings with flat slabs supported on core and perimeter columns

This paper focuses on robustness considerations of tall buildings with reinforced concrete flat slabs supported by a central core and one row of perimeter columns. This layout is commonly used in office and residential buildings to reduce storey height and maximise natural daylighting. The core and lateral bracing of tall buildings is generally less sensitive to local failure than the secondary load transfer system provided by the floors and perimeter columns, considering the high vulnerability of perimeter columns to accidental actions and punching of the slab around the supports. This paper analyses sudden corner and edge column removal situations in slab-core-perimeter column systems using dimensions and loading representative of current tall building design and construction, looking at potential punching at the connections and flexural failure in the slab. The dynamic punching model previously developed by the authors, was validated in this work for further cases of punching around edge columns and slabs with low amount of flexural reinforcement. The dynamic punching model is based on the Critical Shear Crack Theory for quasi-static punching and the Ductility-Centred Robustness Assessment method. A simplified level of approximation is proposed for a preliminary dynamic punching check during the conceptual design stage to optimise the position columns and level of flexural and punching reinforcement needed. The approaches proposed are consistent with Model Code 2010 and can be used to arrest the horizontal propagation of failure in the slab. The analyses of the case studies in this work showed that removal of an edge column adjacent to a corner column can be critical and alternative robustness design considerations are needed in this case.

Robustness of a full-scale precast building structure subjected to corner-column failure

Although many analytical and experimental studies have been carried out to date on the progressive collapse and robustness of cast-in-place reinforced concrete (RC) and steel building structures, very few experimental research works have been performed on precast concrete (PC) structures. The small number of publications on these experiments have focused on analysing the behaviour of subassemblies after the sudden loss of an internal column. This paper is the first to describe the construction of a full-scale purpose-built experimental PC building to study the sudden removal of corner columns and the structure’s ability to find alternative load paths (ALPs). The connections between the precast members were designed using existing simplified guidance for robustness. The results obtained from the test using gravity loads corresponding to typical load combinations defined in building codes for accidental design situations, showed a structural response governed by Vierendeel action with a clear contribution from the floor slabs. Load increase factors were obtained from the test results which can be applied by practitioners using simplified linear finite element models to account for non-linear dynamic effects in a simplified manner. This work is expected to form part of a large database of experimental results that are useful for developing advanced numerical simulations and parametric analyses.

Improving structural robustness of steel frame buildings by enhancing floor deck connections

In current composite floors, the strength of the deck connections restraining the profiled steel decks to the floor beams or to the neighboring steel decks are much weaker compared with the sectional strength of the steel deck, which would limit the load-carrying capacity of the composite floors under progressive collapse scenarios. Given this, this study proposed two novel types of enhanced deck connections for improving the load-carrying behavior of the deck-to-beam connection and deck-to-deck connection. Based on a 5-story steel prototype building, the feasibility of the proposed deck connections in improving the progressive collapse resistance was validated by comparing with the common practice of the deck connections via a reduced-order (RO) modeling approach. The structural robustness of the prototype building in the case of sudden column removal was evaluated and discussed. Compared with the commonly used deck connections, the enhanced deck connections could improve the structural robustness of the prototype building by 27%.

Load Distribution after Failure of an RC Column on Moment-Resistant Frames

Progressive collapse can be defined as partially or entirely failure of a structure after failure of an element. Capacity loss of an element creates new load distribution and additional loads for other parts of the structure. To accurately evaluate the progressive collapse of the structure, it is needed to accurate estimation of load distribution after failure. Sudden removal of a column is a widely used approach to evaluate the progressive collapse estimation of the structures. However, in this study, effect of removal of two different columns on the frame are studied under two conditions; a) sudden removal of column, b) progressive loss on the axial capacity of column. To model progressive axial capacity loss, author’s previously developed axial capacity of reinforced concrete column model is used. A 2D two-story two-bay benchmark frame is analyzed under pushdown and quasi-static cyclic loading. Distribution of loads after failure of a column is investigated under four different scenarios.

Survivability of reinforced concrete frames of multi-storey buildings with complex stress elements

Experimental determination of the parameters of the force resistance of reinforced concrete structures aimed at protecting them from emergency beyond design impacts is an important direction in improving the safety of buildings and structures. In this connection, the purpose of the study was an experimental assessment of the deformation parameters in the complexly stressed elements of reinforced concrete frames under special impact in the form of a sudden column removal. Experimental studies were carried out for two frames, one of which was tested when removing the middle column, the second - when removing the extreme. Experimental two-span structures of reinforced concrete frames are designed with three floors in height, reinforcement was made with spatial reinforcing cages that provide resistance to torsion with bending. The results of experimental and theoretical studies of reinforced concrete frame structures under special influences and an assessment of displacements, cracking and destruction of the considered complex-stressed structural elements under such influences are presented. It is established that the type of stress state, the formation and width of crack opening significantly affect the dissipative properties of the structural system.

Reliability-based assessment of progressive collapse in horizontally irregular multi-story concrete buildings

Structures’ resistance against progressive collapse varies depending on their design and taxonomy. However, despite numerous studies addressing the impact of geometric configuration, limited attention is given to the effect of irregularity. This study presents a reliability-based approach to evaluate the effect of horizontal irregularity on the progressive collapse potential of seismically designed concrete buildings. Incremental dynamic analysis is performed on three-dimensional nonlinear finite element models of six prototype buildings with different levels of horizontal irregularity subjected to four column removal scenarios. Probabilistic demand models are then derived to construct fragility curves based on the first-order reliability method. In addition, an analysis of covariance is performed to measure the sensitivity of demand models to column removal scenarios in terms of floor and location. The results show that the studied irregular buildings do not pose a greater risk of progressive collapse than the regular building. In addition, while the probability of collapse under sudden column removal is significantly low for the seismically designed concrete frames, all buildings are expected to experience significant damage under such abnormal loads.

Force Transfer Mechanism and Component-Based Model of Cast-Steel-Stiffened Circular-Tube-Column Frames for Progressive Collapse Analysis

In this paper, the system response, stress redistribution, failure mode, and catenary effect of steel frames with circular tube columns and cast steel stiffener (CSS) joints under a sudden column removal scenario were revealed. Based on this force transfer mechanism analysis, a practical and computationally efficient component-based model considering catenary effects and CSS joint details with a series of springs was established and validated by a detailed solid-element method. By using this component-based model, the proper dynamic response increase factor of the CSS joint frames was investigated. The results show that the great overall stiffness and strength of the CSS limit the deformation of the column front shell. Therefore, the CSS joint frames have superior performance for progressive collapse prevention than the frames using welded joints without stiffeners. In addition, the component-based model is validated to be effective and the dynamic response increase factor of the frames with circular tube columns and CSS joints is smaller than 2.0.

Behaviour of high rise irregular plan steel building in progressive failure due to sudden column removal

Behaviour of high rise irregular plan steel building in progressive failure due to sudden column removal

Numerical Investigation of Beam-Column Connections in Precast Concrete Buildings

The progressive collapse of structures has been the concern of many studies. Efforts have been made to develop methodologies of analysis and design for the mitigation of progressive collapse. As precast structures lack of structural continuity in the load paths, they are more responsive to progressive collapse than the cast-in-situ monolithic reinforced concrete (RC) buildings. In this paper, Analytical study has been conducted to develop a nonlinear finite element FE model using ABAQUS 6.12 software to investigate the behavior of precast non-prestressed reinforced concrete beam-column connection in case of sudden column removal. The FE model consider the nonlinear behavior of concrete and steel. In addition, it takes into account the contact properties between surfaces at joints. A calibration was performed between the numerical and the experimental results, using three half-scale specimens tested under mid column lose, to verify that the FE model could be used for simulating precast beam column connections. There was a good agreement between experimental and numerical results.

Numerical Investigation of Beam-Column Connections in Precast Concrete Buildings

The progressive collapse of structures has been the concern of many studies. Efforts have been made to develop methodologies of analysis and design for the mitigation of progressive collapse. As precast structures lack of structural continuity in the load paths, they are more responsive to progressive collapse than the cast-in-situ monolithic reinforced concrete (RC) buildings. In this paper, Analytical study has been conducted to develop a nonlinear finite element FE model using ABAQUS 6.12 software to investigate the behavior of precast non-prestressed reinforced concrete beam-column connection in case of sudden column removal. The FE model consider the nonlinear behavior of concrete and steel. In addition, it takes into account the contact properties between surfaces at joints. A calibration was performed between the numerical and the experimental results, using three half-scale specimens tested under mid column lose, to verify that the FE model could be used for simulating precast beam column connections. There was a good agreement between experimental and numerical results.

Progressive collapse of post-tensioned subframes following the removal of an interior column

Experimental investigations about the post-tensioned concrete substructures are still imperative to be conducted to assess the structural performances against progressive collapse. Four half-scaled framed substructures were tested to evaluate the dynamic resistant behaviors under a scenario of sudden removal of an interior column. Then, to evaluate the performances against progressive collapse at the large deformation stage, the residual substructures were tested under the pseudo-static loading regime. Various responses at the prescribed locations were monitored with the experimental instruments. The variables in the research included diameters of the post-tensioned strands (PTS), bonded form of PTS, and unboned form of PTS. By comparison, the differences among the reinforced concrete counterpart (RC), bonded post-tensioned concrete substructures (BPC), and unbonded post-tensioned concrete substructure (UPC) were highlighted. Dynamic experimental results indicate that the PTS can have significant influences on damping ratio and enhance the vertical stiffness of RC specimens. Moreover, the ultimate bearing capacity of UPC can reach 2.09 times larger than that of the RC substructures, and the PTS with a larger diameter can enhance the ultimate bearing capacity more than that with a small diameter. Additionally, the proposed analytical model can well agree with the failure modes at the catenary action (CA) stage, and the theoretical formulas can be well utilized to evaluate the ultimate bearing capacity.

Dynamic experiments on post-tensioned beam-column substructures following the sudden removal of a single interior column

Most experimental investigations about post-tensioned (PT) substructures have been conducted statically, neglecting the dynamic characteristic of progressive collapse. Dynamic performances of PT frame structures against progressive collapse are imperative to study under the sudden column removal scenario, which can reflect the actual progressive collapse more convincingly. This paper presents experimental programs on six half-scaled beam-column substructures, which were tested to evaluate dynamic responses after the sudden removal of a single interior column. During the tests, extensive instruments were installed in prescribed locations to record various dynamic responses such as displacements, loads, strains, and crack patterns. By comparison, the influences of post-tensioned strands (PTS) on the structural mechanisms of reinforced concrete (RC) frames against collapse were demonstrated. The findings cover the span–depth ratio of beam, bonded form of bonded post-tensioned strands (BPTS), and unbonded form of unbonded post-tensioned strands (UPTS). Experimental results highlight that the initial stiffness of RC substructures can be significantly enhanced by the PTS. Resistant mechanisms are mobilized to resist progressive collapse, such as compressive arch action and catenary action. The catenary action develops to resist progressive collapse if a larger span–depth ratio is applied in the design. Moreover, both UPTS and BPTS can increase the damping ratio of the corresponding RC, and the damping ratio of the PT specimen can reach up to 217.23% of that of the corresponding RC. And also, the proposed analytical model and theoretical equations can be well verified by the experimental results.

Response spectrum of a reinforced concrete frame structure under various column removal scenarios

In a frame structure, sudden removal of a load-bearing member (e.g., a column) causes structural vibrations, often leading to a partial or total progressive collapse. Since the term ”sudden column removal” is imprecise, it is worth evaluating how the rate of column removal influences the maximum displacement and how this response refers to the one produced by the static column removal. Both linear and nonlinear, inelastic modeling of the reinforced concrete members are considered using the OpenSees finite element framework. The numerical simulations consist of a gradual increase in column removal time from near zero to asymptotically approaching the static response. Due to the asymmetry of the analyzed frame building, three column removal scenarios are investigated. It is observed that the impact of column removal rate depends on the ability of the remaining structure to withstand the lack of column and the natural period of the mode shape corresponding to the downward motion of the model without the column. Also, the column removal durations producing 95% of the maximum response are identified and their ratios to the respective natural periods are given. These investigations can be helpful when planning numerical simulations and physical experiments. Finally, a novel simplified single degree of freedom system with support removal is proposed and its response spectrum is developed. This simplified model helps to understand more complex multi-degree of freedom systems.

Progressive collapse analysis of corner-supported composite modular buildings

In this study, the alternate load path analysis is carried out to investigate the progressive collapse resistance of multi-storey modular buildings constructed using corner-supported composite modules. Three sudden column loss scenarios are considered: a corner column, an edge column at the long side, and an edge column at the short side. The non-linear dynamic analysis is first conducted to investigate the peak dynamic response of the locally damaged structures. The dynamic analysis results show that the 4-storey building prototypes remain elastic whilst partial yield in the joint regions is developed in the 12-storey counterparts; however, no collapsing tendency has been observed in cases under design loads. In the non-linear static analysis, the capacity curve and the overload factor for each case are then identified by increasing the gravity load until failure occurs. Four possible failure mechanisms are identified, varying among the different column loss scenarios and the building height. The suitable dynamic increase factors (DIFs) for the proposed composite modular buildings are identified on the basis of achieving the same maximum vertical displacement at the point of column removal when the non-linear static method is carried out to predict the peak dynamic response to sudden column removal. The DIFs of 1.90 and 1.60 are recommended for the 4- and 12-storey modular buildings, respectively.

Initial damage and residual behavior of RC beam-slab structures following sudden column removal - numerical study

High fidelity solid-element-based numerical models built in LS-DYNA are employed to study the progressive collapse resistance of reinforced concrete (RC) beam-slab substructures with an emphasis on the effect of initial damage caused by dynamic response from sudden removal of a column. In particular, the numerical models consider the bond-slip behavior between concrete and reinforcement in the joint regions, and incorporate element erosion in order to predict the failure modes such as reinforcement fracturing, concrete cracking and crushing. The finite element modeling is validated by experimental results in terms of both dynamic behavior following sudden column removal and residual behavior under quasi-static loading. After the validation, parametric studies are performed to investigate the influence of the equivalent gravity load and shock-induced initial acceleration. The study clearly shows that the dynamic response due to sudden column removal can induce great reduction in the progressive collapse resistance of substructures through the initial damages. The compressive arch action (CAA) and compressive membrane action (CMA) are more easily damaged by the dynamic response following sudden removal of a column, constituting less reliable load resisting mechanisms against the progressive collapse compared to the tensile catenary action (TCA) and tensile membrane action (TMA). Structures with larger applied load would suffer more from the dynamic response. Moreover, the initial upward acceleration in the event of blast or explosion is extremely unfavorable to the development of CAA and CMA by introducing transient upward movement before the downward deflection under gravity load. A comparison of the dynamic load amplification factors (DLAFs) is also made between the cases with and without significant initial damage due to dynamic response. It is concluded that ignoring the initial damage of structures can underestimate DLAF and further overestimate structural resistance, resulting in the structural design for mitigating progressive collapse in the events of explosion and impact on the unsafe side.

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.