Numerical simulation of progressive collapse of structures under blast loads

Progressive collapse of building structures under blast and impact loads has attracted great attention all over the world. Progressive collapse analysis is essential for an economic and safe design of building structures against progressive collapse to blast and impact loads. Because of the catastrophic nature of progressive collapse and the potentially high cost of constructing or retrofitting buildings to resist it, it is imperative that the progressive collapse analysis methods be reliable. For engineers, their methodology to carry out progressive collapse evaluation need not only be accurate and concise, but also be easily used and works fast. Thus, many researchers have been spending lots of effort in developing reliable, efficient and straightforward progressive collapse analysis methods recently. In the present paper, current progressive collapse analysis methods available in the literature are reviewed. Their suitability, applicability and reliability are discussed. Our recent proposed new method for progressive collapse analysis of reinforced concrete frames under blast loads is also introduced.

Progressive collapse is defined as the spread of an initial local failure from a key element to adjacent elements, eventually resulting in the collapse of the whole structure or a disproportionately large part of it. As an abnormal dynamic load, blast load has the characteristic of rapid propagation, high peak value and short duration. And it is also one of the main reasons that might cause the progressive collapse of building structures. Based on the existing research achievements on the progressive collapse of building structures, in this paper the classification method for structural progressive collapse analysis is put forward. Then the current methods for progressive collapse analysis are further divided into two categories, i.e., the method for evaluation of structural progressive collapse resistance and the method for whole process analysis of structural progressive collapse. Recent progress on the evaluation of structural progressive collapse resistance and the method for whole process analysis of structural progressive collapse are presented from literatures. Finally, the conceptual design method, structural detailing and the procedure for progressive collapse resistant design of building structures against blast loading are put forward.

Two-dimensional reinforced concrete frames with the storey of different heights are simulated to the column removal scenario to understand the progressive collapse behaviour by using General Services Administration (GSA 2003) guidelines. Numerical modelling of 4-, 8- and 12-storey structures with different analyses procedures were evaluated such as Linear Static Analysis (LSA), Linear Dynamic Analysis (LDA) and Nonlinear Static Analysis (NLSA) using ETABS 2015. The liner results demonstrate the corner column removal and thus make the structure more vulnerable to progressive collapse when compared to the middle column removal and nonlinear results indicate that the progressive collapse of the structures decreases with increasing the height of the storey.

In many studies, the alternative load path method (APM) has been used for progressive collapse analysis. In this method, one or several columns of the building are removed and the building response is investigated. This method ignores the initial local failure cause of building and this can affect the structural response. Investigation of ignoring the initial local failure cause of steel braced frames is the main purpose of this research. The variables include the type of progressive collapse investigation (ignoring the initial cause of failure or APM, considering blast loading and the heat caused by the fire as the initial causes of failure) and the location of the initial local failure in plan (outer and inner frame) and in floors (1st, 2nd and 3rd). 4-story braced steel buildings were simulated using ABAQUS software and the responses were compared using different methods. The most important results showed that the axial forces are very noticeable in the columns around the damaged site if the initial local failure caused by explosive loading; while these forces are ignored when APM is used. Therefore, due to this significant difference, if the design of a steel building is to be considered against progressive collapse, it is recommended to consider the initial local failure in order to make the appropriate design in accordance with it. Therefore, the initial loading type has a very significant effect on the structure response, and ignoring the initial local failure can lead to incorrect predictions of the structure response.

Determination of dynamic collapse limit states using the energy-based method for multi-story RC frames subjected to column removal scenarios

Progressive collapse is defined as the spread of an initial local failure of a critical load bearing structural element to the remaining structure, ultimately leading to the collapse of the entire building or of a large portion of it. After the partial collapse of the Ronan Point apartment tower in 1968, when a localised gas explosion ultimately led to the collapse of the whole corner of the building, progressive collapse firstly drew public attention due to its catastrophic consequences on human casualties and economic losses. Other recent collapse events, such as those of the Murrah Federal Building in 1995, Sampoong department store in 1995 and World Trade Centre towers in 2001, have further highlighted the importance of this topic and made engineering communities aware of the necessity to prevent the occurrence of progressive collapse in building structures. In the past decades, considerable research has been devoted to investigate progressive collapse of building structures. These research typically followed an alternate load path approach. This approach allows an initial failure to occur, while the remaining structure needs to seek alternate load paths to dissipate damage and prevent progressive collapse. In practice, the progressive collapse resistance of building structures is generally evaluated by notionally removing one or more load bearing structural elements, representing the initial failure. Despite all research efforts made, they have predominately focused on frame systems. Relevant investigations on reinforced concrete (RC) flat plate structures, a commonly used structural system, are still scarce. The flat plate system contains slabs, of uniform thickness, directly supported by columns without using beams, drop panels and column capitals. This provides structures with increased storey height, architectural convenience and significant savings in construction costs. Nevertheless, the slab-column connection is prone to punching shear failure, which may propagate to the vicinity of connections, potentially leading to progressive collapse. The present study aims at filling some of the current knowledge gap with comprehensive experimental, analytical and numerical investigations on RC flat plate structures. The experimental program consists of four quasi-static large-deformation tests performed on three 1/3-scale, 2×2-bay RC flat plate substructures subjected to critical column removal scenarios. The test specimens included one specimen tested under two consecutive corner column removal scenarios, one specimen under an edge column removal and one specimen under the scenario of concurrently removing both an edge and an interior column. For the loading scheme, a multi-point loading system was specially designed to simulate an increasing uniformly distributed load (UDL). The loading points of this system were equally positioned on the bays adjacent to the removed column(s) to apply the load until failure of the slab. Throughout each test, the structural behaviour of the slab was monitored by extensive apparatuses mounted to the slab both internally and externally. Failure and post-failure behaviours, failure modes, and collapse resisting mechanisms were observed and analysed. Additionally, a complement of analytical studies to the experimental tests, based on the observed yield lines, was carried out to estimate the flexural capacity of the test specimens. To numerically investigate the progressive collapse behaviour of RC flat plate structures, a set of 3D nonlinear finite element modelling approaches, using LS-DYNA software, was established. A total of five numerical models were explicitly created based on the four experimental tests in present research and one from the literature. The proposed modelling approach was basically composed of appropriate definitions of element types, bond-slip relationships, loadings and boundary conditions, and more importantly development of the material models for concrete and reinforcements and calibration of failure criteria for element erosion. A quasi-static loading scheme in displacement control in the numerical models was adopted, the same as that in the conducted tests, through explicitly modelling the multi-point loading system in order to replicate the true test conditions. The numerical models were respectively validated by comparing the load displacement responses, failure modes, crack patterns, and displacement and strain results against the experimental ones, which confirmed the appropriateness and reliability of the proposed models. The validated modelling techniques could be applied to further perform parametric studies of the progressive collapse resistance of RC flat plate substructures and prototype structures. In summary, the significant and original contribution of this research is to (1) obtain a solid understanding of the collapse behaviour of RC flat plate structures under critical column removal scenarios, (2) examine the analytical solution using the yield line theory to predict the flexural capacity of RC flat plate structures, (3) establish a set of numerical modelling techniques which can reasonably replicate the experimental process and provide valuable references of numerically analysing progressive collapse of RC flat plate structures, and (4) give design insights into Australian and international Standards in mitigating progressive collapse of RC flat plate structures.

This paper proposes a new quantitative algorithm to evaluate the structural robustness of moment-resisting steel frame structures against progressive collapse. The objective of the study is to enhance existing assessment methods by introducing two new damage criteria: the total number of remaining overstressed members and the mean change in Demand Capacity Ratio (DCR) before and after pre-selected column removal. The methodology involves using SAP2000 software for structural design and MATLAB for algorithm development. A ten-storey steel frame structure serves as the case study, analysing a total of 180 column removal scenarios. Key findings indicate that the proposed algorithm effectively identifies critical column locations that significantly affect the structure’s collapse potential. Nonlinear dynamic implicit analysis using ABAQUS further investigates these critical scenarios, examining displacement and energy changes under various column removal rates and locations. The conclusions demonstrate that this quantitative approach provides a robust framework for evaluating the progressive collapse potential of steel frame structures, offering valuable insights for future structural design and safety assessments.

Progressive collapse, where a localized member failure causes widespread structural collapse, has become a critical concern nowadays, due to its potential to cause significant financial losses and loss of human life. Triggers include natural disasters like earthquakes and floods, as well as accidents, attacks and explosions. Reinforced concrete flat slab structures, which are eminent for their architectural flexibility and have larger spans, are particularly susceptible to disproportionate collapse, due to the lack of floor beams, which can redistribute loads after a column failure, unlike moment frame buildings. This research examines how multi-story reinforced concrete flat slab buildings behave, under prescribed gravity load combinations, compared to conventional framed buildings. The effects of removing columns at specified locations from an intermediate floor of the multistorey building are also examined. However, this investigation covers both the column removal approaches to check a possibility of disproportionate collapse which are; static removal and dynamic instantaneous removal. Furthermore, the research also assesses the efficacy of perimeter beams, in minimising the risk of gradual collapse in flat slab structures, by scrutinising their ability to reduce joint displacement, chord rotation, and demand capacity ratio. ETABS v18 was used to analyze all the 18 models. The findings revealed that buildings are more prone to progressive collapse when corner columns are removed, as opposed to edge and interior columns, due to higher Demand-Capacity Ratios (DCR) and joint displacement. In comparison to dynamic analysis, the static evaluation exhibited greater DCR values and vertical joint displacement. Furthermore, since they have a more efficient load redistribution mechanism, traditional framed structures performed better than flat slab models. The simulations additionally indicated that, adding edge perimeter beams, substantially lowered the possibility of progressive collapse in flat slab structures. Moreover, the tested flat slab building models, with and without perimeter beams showed no indications of progressive collapse, when specified columns were removed from the intermediate floors, since the DCR values of the crucial columns stayed within the permissible range of 2.0. In conclusion, structures built in compliance with IS 1893:2016 code and designed to withstand seismic loads demonstrate stronger resistance against significant damage, brought about by column failures.

In this study, the progressive collapse of reinforced concrete structures due to vehicle collision to the columns of the ground floor was modeled and examined. For this purpose, a four-story reinforced concrete building with the intermediate moment frame system was designed using ETABS software followed by the simulation of impact loading by SAP2000 software. Performing non-linear time history dynamic analysis, the critical forces required to the column failure were determined via trial and error by considering different live load contribution. Then, the corresponding critical velocities for 4, 8, and 12 ton vehicles were determined. Finally, the progressive collapse of the building was examined by the sudden removal of the column. The results showed that by increasing the percentage of live load contribution, the force and critical velocity for the instability and damage of the column will decrease. Furthermore, comparing the perimeter and corner columns showed that the corner columns are the most critical columns for occurrence of the progressive collapse. In addition, during the assessment of the progressive collapse, it was found that the number of damaged springs in the corner column removal scenario is less than that of the perimeter column removal scenario.

This paper aims at assessing the impact of removing various numbers of perimeter columns on the integrity of RC flat plate substructures. Compared to interior columns, those at the corner and edge positions on the building perimeter are more likely to damage by accidental and abnormal loads, which in turn may cause progressive collapse of the entire structure. Given the uncertainty of the loading events, the likelihood of multiple perimeter columns being simultaneously destroyed is also high. However, existing research has primarily focused on the single column removal cases. To address this research limitation, a 3D high-fidelity finite element model of a flat plate substructure with a missing corner column (named Model C) was developed. The accuracy of the model was validated based on our previous experiment of the same substructure. The validated model was subsequently modified to cover two and multiple perimeter column removal scenarios. The two new models are namely, Model C_1e in which a corner column and an adjacent edge column were damaged and Model C_2e, with a corner column and two adjacent edge columns being removed. The simulation results reveal that flexural action was the primary load resisting mechanism in all three models. The load carrying capacity with three column removals was reduced by 75.6% compared to the removal of a single corner column. Full-depth cracks were found at the cut-off position of the top slab rebars for the columns adjacent to the removed one(s). In Model C and Model C_2e, cracks exhibited a diagonal axisymmetric failure pattern of the corner of the slab, with equal load transfer to two adjacent columns. In contrast, Model C_1e exhibited a diagonal non-axisymmetric failure pattern of the slab corner, with more loads being transferred to the adjacent column along the shorter bay. The energy-based dynamic response analyses were also performed to obtain the Dynamic Increase Factor (DIF) for the substructure models under different column removal scenarios. Based on the nonlinear regression analysis, the DIFs for the substructures with one and two column removals were found to be ranged between 1.22 and 1.51, and 1.28–1.68, respectively.

Progressive collapse is one of the factors which if not predicted at the time of structure plan; its occurrence will lead to catastrophic damages. Through having a glance over important structures chronicles in the world, we will notice that the reason of their collapse is a minor damage in structure caused by an accident like a terrorist attack, smashing a vehicle, fire, gas explosion, construction flaws and its expanding. Progressive collapse includes expanding rudimentary rupture from one part to another which leads to total collapse of a structure or a major part it. This study examines the progressive collapse of a 5-story concrete building with three column eliminating scenarios, including the removal of the corner, side and middle columns with the ABAQUS software. Then the beams and the bottom of the concrete slab were reinforced by (reinforcement of carbon fiber reinforced polymer) FRP and then the structure was re-analyzed. The results of the analysis show that the reinforcement of carbon fiber reinforced polymer sheets is one of the effective ways to rehabilitate and reduce the progressive collapse in concrete structures.

The progressive collapse of a multi-story building and its protective effect with aluminum foam under an explosion in its basement are numerically investigated in this paper. The three-dimensional coupling model composed of a multi-story frame structure with a basement and the surround soil is modeled using ANSYS/LS-DYNA. The progressive collapse of the entire structure under explosive impact and the protective effect of the aluminum foam are simulated using the proposed three-stage simulation method (TSSM). The result shows that, the aluminum foam as a protective layer can reduce the impact effects very well, and then can prevent progressive collapse effectively. The same model is also simulated by direct simulation method (DSM) and alternative load path method (ALPM). By comparison with the two methods, the TSSM is illustrated practical, accurate and economic for the analysis of progressive collapse.

Progressive collapse is a phenomenon in which a minor damage leads to total failure of the structure or the collapse of large parts of it. In this paper, influence of mass irregularity in height of the structures on the progressive collapse potential was investigated. To this end, four-, eight- and twelve-story steel moment resisting frame structures are designed and progressive collapse potential was evaluated using alternate load path method, recommended by the American General Service Administration 2003. Three dimensional buildings studied in the current paper were modeled using finite element method in ABAQUS software. Results revealed that in the cases with column removal in the first floor, increasing the number of floors, decreases progressive collapse potential. Maximum dynamic displacement under the removed column in regular 4-story building is about 1.41 times larger than that of regular 8-story building and about 2.16 times larger than that of regular 12-story building. Moreover, comparing performance of regular and irregular buildings showed that regardless location of column removal and story number, mass irregularity in height increases progressive collapse potential. Maximum dynamic displacement due to removed column in irregular buildings subjected to column removal in penultimate floor is 19, 20 and 22 percent larger than that of regular one for four-, eight- and twelve-story buildings respectively.

The progressive collapse analysis of reinforced concrete (RC) moment-frame buildings under extreme loads is discussed from the perspective of modeling issues. A threat-independent approach or the alternate path method forms the basis of the simulations wherein the extreme event is modeled via column removal scenarios. Using a prototype RC frame building, issues and considerations in constitutive modeling of materials, options in modeling the structural elements and specification of gravity loads are discussed with the goal of achieving consistent models that can be used in collapse scenarios involving successive loss of load-bearing columns at the lowest level of the building. The role of the floor slabs in mobilizing catenary action and influencing the progressive collapse response is also highlighted. Finally, an energy-based approach for identifying the proximity to collapse of regular multi-story buildings is proposed.

In current practice, the analysis of progressive collapse in building structures widely employs the alternate load path method. However, there are very few collapse tests that can be utilized under blast loads, especially for investigating the degradation mechanism of progressive collapse resistance in post-blast building structures. In this study, the degradation mechanisms and methods for damage assessment of reinforced concrete structures under blast loads were investigated through numerical analysis. A substructure model derived from explosion loads based on a drop-hammer testing machine was validated by comparing with test results, and the established model accurately captured three typical failure modes in the substructure columns and confirmed the axial tensile effects of the columns on the adjacent components. A substructure damage assessment method was used to assess the degree of damage in a 5-story reinforced concrete frame structure under different explosion scenarios. The results show that the degree of damage distribution of the reinforced concrete frame structure exhibited an S-shaped distribution under corner column blast scenarios while an arch-shaped distribution was observed for middle column blast scenarios. Furthermore, empirical formulas based on the explosive mass and distance ( M - R ) curves were established. These empirical formulas can help to rapidly predict the damage levels of RC frame structures for a given explosion scenario within a certain scope.