Numerical study of progressive collapse in reinforced concrete frames with FRP under column removal

The growing need for blast-resistant designs in structural engineering is driven by the rising threat of terrorism, accidental explosions, and the significant risk of progressive collapse. This study presents a numerical procedure for analyzing the progressive collapse of Reinforced Concrete (RC) framed structures due to blast loads, addressing the gap in current methodologies. Traditional approaches such as the Alternate Load Path method, which are code-based and threat-independent, mainly focus on sudden column loss scenarios but do not fully capture the dynamic nature of blast-induced, threat-dependent collapses. Hence this study addresses the need for a computationally efficient and reliable method to predict and model progressive collapse under blast loading. This study employs a comprehensive numerical investigation using Finite Element Method where a seven-storeyed RC building is assessed for progressive collapse under blast loading. Progressive collapse analysis was executed using a commercially available Finite Element Analysis software, adhering to the Linear Static Procedure specified in the General Services Administration (GSA) guidelines, while blast load scenarios were examined through a nonlinear direct integration time history analysis. Different blast parameters, including charge weight and standoff distances, are varied to evaluate their impact on the structural integrity of the building. The study differentiates between threat-independent analysis, considering four column removal locations, and threat-dependent (blast-induced) scenarios with three distinct column removal positions. The scope of this study delves only into the perimeter blast scenarios neglecting the internal explosions. Demand Capacity Ratios (DCR) of columns were calculated to determine the susceptibility of the building to progressive collapse, with a DCR greater than 1 indicating failure. The numerical model was validated against the GSA baseline model. In threat-independent analysis, 30%, 60%, 22%, and 44% of the considered columns under corner, long side, short side, and interior column removal scenarios respectively exceeded the acceptable DCR criteria. In threat-dependent analysis, 100% of the considered columns under each blast induced column removal scenario exceeded the acceptable DCR criteria. This emphasizes the need for scenario-based planning in structural design to reduce collapse risks. The identification of critical columns, for threat-independent analysis as those directly above the removed column and on the topmost floor and for threat-dependent analysis as ground floor columns adjacent to the removed column, reveals potential weak points for progressive collapse initiation. The analysis of blast-induced progressive collapse reveals significantly higher DCR values than threat-independent assessments. Even the minimum percentage increases of DCRs when transitioning from threat-independent to threat-dependent analysis reaches high values up to 995%, 325%, and 981% for corner, long side, and short side column removal scenarios respectively. This specifies the importance of integrating blast resistance into the structural design of high-risk buildings. As a result, this study contributes to the understanding of structural dynamics under blast loads and offers a framework for the analysis of progressive collapse in RC buildings.

To evaluate and ensure the safety of a prefabricated connection scheme for future engineering applications, this study investigates the life-cycle progressive collapse resistance of prefabricated reinforced concrete frame structures (PRCS) with steel tube stud connections. A simplified prefabricated column model was developed in SAP2000 using multi-segment linear plastic connection elements, and a six-story PRCS was established for analysis. Progressive collapse behavior of the PRCS and a comparable cast-in-place concrete frame structure (CPCS) was evaluated through the column removal method under four scenarios: corner column, long-side middle column, short-side middle column, and interior column removal. Time-history responses of internal forces and displacements were obtained. In addition, the effect of chloride-induced corrosion on the collapse performance of the structure was analyzed through pushdown analyses. The results indicate that the proposed connection model accurately reproduces the behavior of prefabricated column joints. Among the four scenarios, interior column removal had the most severe effect on PRCS, with displacement increasing by 72.2% compared to CPCS. Pushdown analysis revealed that PRCS exhibited 25.4% lower beam mechanism capacity when the corner column was removed, and 33.1% lower catenary mechanism capacity when the interior column was removed. Furthermore, long-term corrosion significantly reduced the progressive collapse capacity, underscoring the necessity of considering durability effects in design.

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 work evaluates the steel frame stability of the sand sleeve filter of the processing plant of the consequence class KS-3. Purpose : The aim of the work is to consider all possible scenarios of the removal of steel frame columns leading to a possible progressive collapse using the stress-strain state simulation in a three-dimensional model of the base–foundation–building system. Methodology : The stress-strain state of the steel frame is simulated in Ing+ 2021 MicroFe software package using the kinematic method of the limit equilibrium theory for building protection from progressive collapse. Research findings : It is found that at the specific load combination and the removal of any one column of the steel frame, an emergency situation does not occur and progressive building collapse does not occur also. According to calculations, at the load level exceeded by 39% at the removed corner column and by 34% at the removed serial column, the steel frame transmits to a kinematically variable mechanism, while its load-bearing capacity is exhausted.

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.

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.

Performance of RC beam-column assemblies during and after elevated temperature to resist progressive collapse

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.

CFRP (Carbon Fiber Reinforced Plastics) sheet has been used for the retrofitting of a number of existing concrete buildings and structures because of its excellent properties (high strength, lightness and high durability). To be an effective retrofitting system, however, bond effectiveness between CFRP sheet and concrete should be verified, and understanding of the peeling mechanism is especially important. This study employed both the experimental and the analytical methods to quantify the maximum bond stress τ_y and the effective bond length l_u, where the bond stress distributes. Here the interface is modeled as an interphase of 1 mm thickness, that is, a composite which consists of epoxy resin and concrete. The values of τ_y and l_u are finally proposed, and they are 4.56MPa and 45.2mm, respectively.

Experimental study on the collapse-resistant performance of unbonded prestressed PC beam-column sub-assemblages with pinned connections

This paper presents an analytical model for predicting the flexural strength of Reinforced Concrete (RC) beams strengthened with Carbon Fibre Reinforced Polymer (CFRP) sheets including the effects of freeze-thaw (F/T) exposure. The model integrates nonlinear material behaviour, and accounts for parameters influencing the flexural behaviour of CFRP-strengthened RC beams. The model combines the fibre method with established failure criteria and was validated against an experimental database of 98 RC beams strengthened with CFRP sheets. Also, the model includes F/T-concrete stress-strain curves as well as the degradation of concrete/CFRP interface and accounts for elastic-plastic properties of steel reinforcements and elastic properties of CFRP reinforcements. The validation confirms the accuracy and reliability of the developed model in predicting the behaviour of CFRP-strengthened RC beams under freeze-thaw conditions. A comparison of the proposed analytical model versus the available design provisions indicates that the analytical model has the best fitting with the experimental data

The utilization of fiber-reinforced polymer (FRP), can be an efficient, and long-lasting solution for increasing the durability of reinforced concrete (RC) structures in hostile environments. FRP can be used as a reinforcement or as a strengthening technique that may recover the load capacity of a structure. This paper aims to investigate the flexural behavior of one-way concrete slabs with openings reinforced by glass fiber-reinforced polymers (GFRP) bars and strengthening using carbon fiber-reinforced polymer (CFRP) sheets. The experimental work consists of five one-way concrete slabs that were tested up to failure. All slabs had the same dimension, 750*2650 mm, with a thickness of 150 mm, and were reinforced with the same reinforcement ratio. The experimental work included one specimen without openings and without strengthening as a solid slab. While, the remaining slabs were fabricated with two different openings, rectangular and square 250*500 mm, and 250*250 mm respectively, and strengthened by CFRP sheets. The test results show that the failure load of existing slabs with mid-span openings is decreased by around 41% and 43%, for one and two openings respectively, compared to the solid slab. The utilization of CFRP sheets enhances the flexural load-carrying capacities by about 52% and 44% and improves the stiffness at the ultimate load by about 101%, and 95%, and decreases the deflection at a service load by about 56% and 54%, respectively compared with the un-strengthened slabs. Furthermore, the experimental data were used to validate a finite element (FE) model along with an additional parametric study.

In the past decades, several guidelines have been developed for designs against progressive collapse based on a number of experimental and numerical studies that have been conducted. However, the majority of the previous experimental and numerical investigations focused on the response of buildings under the scenarios of loss of interior or exterior columns. However, the loss of a corner column in the event of a terrorist attack is more likely to trigger progressive collapse given the paucity of surrounding elements that could help to redistribute the axial force initially resisted by the lost corner column. In order to investigate the progressive collapse of RC frames under the loss of a corner column scenario, an analytical model was proposed and subsequently validated through comparison with the experimental results. The influences of the beam longitudinal reinforcement ratio, beam depth, beam width, column width, and joint transverse reinforcement ratio on the progressive collapse performance ...

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.

Factors influencing the progressive collapse resistance of RC frame structures