Modeling of RC Frame Buildings for Progressive Collapse Analysis

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.

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.

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

Interior brick-infill partitions are usually considered as\nnon-structural components and only their weight is accounted for in\npractical structural design. In this study, their effect on the progressive\ncollapse resistance of an RC building subjected to sudden column loss\nis investigated. Three notional column loss conditions with four\ndifferent brick-infill locations are considered. Column-loss response\nanalyses of the RC building with and without brick infills are carried\nout. Analysis results indicate that the collapse resistance is only\nslightly influenced by the brick infills due to their brittle failure\ncharacteristic. Even so, they may help to reduce the inelastic\ndisplacement response under column loss. For practical engineering, it\nis reasonably conservative to only consider the weight of brick-infill\npartitions in the structural analysis.

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

Experimental investigation of the progressive collapse of reinforced concrete structures: An overview

The progressive collapse analysis of a reinforced concrete (RC) building occurs when an extreme event arises (vehicle collision, gas explosion, bomb blasts, etc.), which often cause damage to the RC structural frame were a sudden loss of primary structural element takes place or due to the failure of one or more critical structural element leading the structure to partial or entire collapse. The fear of progressive collapse is growing, due to the partial collapse of the 22-story Ronan point apartment in 1968, leading to a gas burst and attacks in Oklahoma City, the Alfred P. Murrah Federal Building in 1995, and the attacks on the world trade center in Lower Manhattan, New York in the year 2001. The trend is now to attain strong buildings which can be operational after such extreme events, especially when the building is occupied by a large number of persons. This paper reviews the progressive collapse study on RC frame structure based on various numerical, analytical, and experimental methods which are reported and discussed in the literature. In comparison with bare RC frame, the advantageous impact of the masonry infill wall on the RC structural frame is summed up against the progressive collapse.

The improvement of the seismic resilience of existing reinforced-concrete (RC) frame buildings, which is essential for the seismic resilience of a city, has become a critical issue. Although seismic isolation is an effective method for improving the resilient performance of such buildings, target-oriented quantitative improvements of the resilient performance of these buildings have been reported rarely. To address this gap, the seismic resilience of two existing RC frame buildings located in a high seismic intensity region of China were assessed based on the Chinese Standard for Seismic Resilience Assessment of Buildings. The critical engineering demand parameters (EDPs) affecting the seismic resilience of such buildings were identified. Subsequently, the seismic resilience of buildings retrofitted with different isolation schemes (i.e., yield ratios) were evaluated and compared, with emphasis on the relationships among yield ratios, EDPs, and levels of seismic resilience. Accordingly, to achieve the highest level of seismic resilience with respect to the Chinese standard, a yield ratio of 3% was recommended and successfully applied to the target-oriented design for the seismic-resilience improvement of an existing RC frame building. The research outcome can provide an important reference for the resilience-based retrofitting of existing RC frame buildings using seismic isolation in urban cities.

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 of beam-to-upright subassemblies of steel storage racks under a column removal scenario

Idealized structural unit method (ISUM) is now well recognized as one of the most efficient methods to analyze nonlinear behavior of large-sized structures under extreme or even accidental actions. This paper presents some techniques of ISUM modeling and surveys the literature on the ultimate limit state assessment of ship structures using ISUM. Theoretical formulations of ISUM elements for the progressive hull girder collapse analysis of ships that were implemented into ALPS/HULL computer code are outlined. Some benchmark studies are made by a comparison with physical test data and ALPS/HULL analyses on a frigate ship hull model under sagging moment tested by UK Royal Navy and on two double-skin tanker hull models under sagging or hogging moments tested by US Navy. Some ALPS/HULL application examples to progressive collapse analysis of a double-skin shuttle tanker hull under combined vertical and horizontal bending moments are then demonstrated. It is concluded that ISUM is very useful for the progressive hull girder collapse analysis of ships because it provides very accurate results while the computing time is very short.

Progressive collapse is defined as the expansion of an initial local failure of an element into another element of the structure and ultimately leading to the collapse of the whole structure or a large part of it in a disproportionate way. Three dimensional modeling, using the finite element method was developed and investigated to understand the progressive collapse of high rise buildings with composite steel frames. The nonlinear dynamic analysis examined the behavior of the building under two column removal scenarios. Two different types of lateral resistance systems were selected to be analysis and compared. The buildings included regular and irregular plans. The response of the building was studied in detail, and measures are recommended to reduce progressive collapse in future designs. The results of this study shows that side case removal in moment frame and moment with centrically braced frame systems was more critical and destructive compared with corner case removal. Comparing the models, for the two different lateral resistance systems, the dynamic response of columns were different, but were not remarkable.

Progressive collapse is characterized by disproportionate structural failure triggered by localized damage, such as column loss under extreme loading conditions. The objective of this study is to develop a simplified analytical model that is applicable in engineering practice without the need for high-fidelity nonlinear finite element analysis. Although current design guidelines (GSA and DoD) provide analytical procedures and acceptance criteria, they do not explicitly address the tensile resistance of girders after the acceptance criteria are satisfied, particularly under large deformation and connection failure. To address this limitation, this study proposes a simplified theoretical load–displacement model for a fixed-end girder subjected to three concentrated loads, considering the effects of secondary beams and focusing on the local girder response under a column-removal scenario. The proposed model incorporates moment–axial force interactions at plastic sections in the large-deformation range. Based on one-dimensional finite element analysis results, an early-developed axial force of 0.15Fₚ at the onset of the transition stage and a residual bending moment of 0.3Mₚ during the catenary action stage are explicitly introduced to better represent actual structural behavior. The girder response is idealized using five characteristic points: yielding (Y), full plasticity (P), transition initiation (T), pure catenary action initiation (C), and collapse governed by connection failure (Fconn). Stress distributions at plastic sections are analyzed using three-dimensional finite element models to establish stress-based formulations and a rational procedure for estimating axial force at collapse. The validity of the proposed model is verified through comparisons with finite element analysis results for girders with different span-to-depth ratios. The results demonstrate reasonable agreement in terms of collapse load and displacement, particularly for slender girders, confirming the applicability of the proposed model for progressive collapse assessment.

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.

Strengthening of precast RC beam-column connections for progressive collapse mitigation using bolted steel plates