АНАЛИЗ ОСОБЕННОСТЕЙ СОПРОТИВЛЕНИЯ ПРОГРЕССИРУЮЩЕМУ ОБРУШЕНИЮ КОНСТРУКТИВНЫХ СИСТЕМ ЗДАНИЙ И СООРУЖЕНИЙ ПРИ ВНЕЗАПНЫХ СТРУКТУРНЫХ ПЕРЕСТРОЙКАХ: АНАЛИТИЧЕСКИЙ ОБЗОР НАУЧНЫХ ИССЛЕДОВАНИЙ

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.

The design of new buildings using Progressive Collapse criteria has become better understood since the nation's largest landlord, the General Services Administration (GSA), incorporated progressive collapse design criteria into their standard design requirements. Other institutions, such as the Department of Defense (DoD), also have incorporated progressive collapse design criteria into their design requirements for both new and existing structures. The DoD presents their progressive collapse requirements in the Unified Facilities Criteria (UFC) document UFC 4-023-03 (25 January 2005), Design of Buildings to Resist Progressive Collapse. While there are numerous similarities between the UFC progressive collapse criteria and the GSA Progressive Collapse Guidelines, there are several key differences which can increase the time and cost of the design process, as well as the final construction cost of the project.

Progressive collapse studies generally assess the performance of the structure under gravity and blast loads, while earthquakes may also lead to the progressive collapse of a damaged structure. In this study, the progressive collapse response of concentrically braced dual systems with steel moment-resisting frames was assessed under seismic loads through pushover analysis using triangular and uniform lateral load patterns. Two different bracing types (X and inverted V braces) were considered, and their performances were compared under different lateral load patterns using the nonlinear static alternate path method recommended in the Unified Facilities Criteria (UFC) guideline. Eventually, the seismic progressive collapse resistance of models was compared to their progressive collapse response under gravity loads. These studies showed that models under the seismic progressive collapse loads satisfied UFC acceptance criteria and limited rehabilitation objective. The structures had better performance under seismic progressive collapse than models under gravity loads because of more resistance, ductility, suitable load redistribution, and more structural elements that participated in load redistribution. Furthermore, despite studies on progressive collapse under gravity loads, the dual system with X braces showed better progressive collapse performance (more resistance, residual reserve strength ratio and ductility) under seismic loads than the model with inverted V braces.

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.

Importance of ‘DAF’ in evaluating structural adequacy of gravity-load designed RC buildings

Blast response and safety evaluation of a composite column for use as key element in structural systems

The paper investigates the influence of dynamic characteristics of building structures on the mechanisms of progressive collapse development. It is shown that natural frequencies, mode shapes, and damping levels significantly affect the propagation pattern of collapse following the initial failure of one or more load-bearing elements. Based on the obtained results, vulnerability assessment criteria are formulated that take into account the dynamic properties of structures. Purpose of the article. The purpose of this study is to establish the relationship between the dynamic characteristics of a structural system and the mechanisms of progressive collapse development. Special attention is given to the effect of natural frequencies, vibration modes, and damping level on the likelihood of structural excitation leading to instability or further collapse. Based on a numerical analysis of a steel-framed building, a direct relationship is demonstrated between local damage and the redistribution of stiffness in the structural system. The analysis of changes in dynamic characteristics makes it possible to identify potentially vulnerable zones with reduced resistance to progressive collapse. To assess collapse vulnerability, it is advisable to determine the first 3–5 natural frequencies and corresponding mode shapes, as well as damping properties. A significant reduction in stiffness leads to the localization of vibrations, increased amplitudes, and potential energy accumulation. If the system is unable to dissipate vibrations due to insufficient damping, cascading failure may occur. Critical warning signs include a frequency decrease of more than 30 %, an increase in deformation amplitude by over 50 %, the emergence of localized vibration modes, and a reduction in damping. Clarifications are proposed to the conditions of the energy-based approach for evaluating the propagation of progressive collapse, taking into account dynamic characteristics and the localization of deformations within vibration modes (i. e., structural vulnerability). The criteria for determining the dynamic coefficient are refined to consider the frequency spectrum and damping. Design tasks are formulated based on dynamic analysis to enhance resistance to progressive collapse. Conclusions. The use of dynamic coefficients (impact factors) recommended by building codes is justified but insufficient without considering the individual properties of the specific structure. The analysis of structural vibration modes can reveal potential weak spots where deformations and stresses are expected to be the highest. This is crucial for predicting the mechanisms of progressive collapse propagation. The results of this research can be applied in assessing the vulnerability of existing buildings and in designing new structures resistant to progressive collapse in accordance with modern safety requirements.

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

<p>Progressive collapse of structures is a cascading failure phenomenon with a failure consequence that is disproportionate to the direct damage of an initiating event. The Ronan Point Building collapse in 1968 triggered the start of research on disproportionate progressive collapse in building structural engineering. Intense research was followed after the Alfred P. Murrah Federal Building bombing in 1995 and the disastrous collapse of the World Trade Centre after aircraft strikes. </p> <p>Progressive collapse has essentially two distinct features: system- rather than component-level responses, and low probability and high consequences. The majority of existing design codes and standards consider progressive collapse implicitly, by improving the performance of a structure through the specification of minimum levels of strength, continuity, and ductility. On the other hand, existing explicit design procedures largely consist of a component-based design approach. </p> <p>This study proposes an innovative system-based and risk-informed decision making framework for progressive collapse design of reinforced concrete frames. Considering the full spectrum of risk due to initiating events, a novel risk-based robustness index is proposed as a system-level performance criterion for design. From a conventionally designed structure, an optimization pro- cess identifies optimal allocation of resources that results in a robust system. An efficient design frontier is defined based on optimal designs when varying the additional expenses provided for the improvement of robustness of a structure. The efficient frontier is then used to verify the cost effectiveness of the design alternatives in conjunction with the cumulative prospect theory. </p> <p>In order to assist in the decision-making process of progressive collapse design provisions, a risk-cost trade-off framework is proposed. The design framework establishes whether additional resources must be used to enhance the robustness of a structure, and when required, it further identifies the optimal expenses that should be used to prevent potential progressive collapse. </p>

After the series of earthquakes in August 1953, most of Kefalonia’s building stock totally collapsed. The buildings that emerged as a result of a collective effort are commonly referred to as “arogi” buildings, with the term arogi meaning help or assistance. In this way, merely referring to these structures is a direct mention to the circumstances under which they were constructed. The reconstruction of the building stock of the island was based in a series of building types proposed from the authorities, and proceeded replacing the richness of the architectural forms that stood before the earthquake, with austere but necessary settlements. Nevertheless, it is these buildings that constitute today’s image of the island. This study wishes to introduce us to the “arogi” buildings structural system, as it was applied in Kefalonia after the 1953 earthquake, with reference to recent bibliography and the recent experience of the 2014 earthquake. The purpose of setting the grounds for such research would be to highlight the effectiveness of this structural system. Moreover, to emphasize the fact that “arogi” buildings and their construction procedure incorporate Kefalonia’s recent history and have eventually produced today’s available “traditional” architecture of the island.

The World Trade Center collapse has brought attention to progressive collapse of tall buildings and the study of possible countermeasures. From the viewpoint of energy transfer, this analysis explains why the collapse could not stop by itself once began. By introducing a design parameter called collapse stability index that controls design against progressive collapse, it is found that conventional design of a tall building usually leads to an inherently unstable structure in the event of a progressive collapse. In a subsequent feasibility study in this paper, a heavy-duty metal-based honeycomb energy absorbing structure is proposed. Using a finite element analysis, it is demonstrated that the structure is capable of absorbing potential energy released in a tall building collapse. The added energy absorbing devices will only occupy a small percentage of the total floor space. By properly designing and installing such devices, a progressive collapse, should it happen in a tall building, may be arrested within a few floors, and hence, the building is inherently stable to the progressive collapse. The theory is also elaborated with the example of the World Trade Center collapse.

Damage or instantaneous loss of critical structural members due to unforeseen events like impact, blast and natural disasters often propagates progressive collapse as a result of complex redistribution of stresses within the structural system. In severe conditions, in which the structure lacks the ability to absorb the stresses in seeking a new equilibrium state, it could result in partial or total collapse of the building. Current design guidelines such as GSA 2003 recommend single removal of load bearing member for progressive collapse assessment. However, triggering events may affect one or more structural member resulting in partial or total collapse of the structure. This paper presents the various effect of sudden column loss on the redistribution of forces in structural members that determine the equilibrium state of the structure.

During the 1960s, the Bangladeshi-American structural engineer and architect Fazlur Rahman Khan proposed an influential design framework for the structural systems of tall buildings titled premium-for-height. Khan argued that the challenge of a structural engineer is to design structural systems that minimise the increase in structural material weight per gross floor area with increasing building height. However, in meeting the challenges of climate change and urbanisation, minimising the embodied environmental flows of tall buildings must also be a priority in structural design frameworks. This paper proposes to expand the premium-for-height framework for tall buildings by considering the embodied greenhouse gas emissions of structural systems using a hybrid life cycle inventory analysis method. Advanced structural analysis and a comprehensive consideration of building parameters are also proposed. To demonstrate the use and potential of the framework, embodied greenhouse gas emissions of six case study tall buildings are analysed. The results arediscussed and recommendations are made to improve the reliability of the more comprehensive framework.

The spread of an initial local failure from element to element resulting eventually in the collapse of an entire structure or a disproportionately large part of it, is called as progressive collapse. This review paper provides a comprehensive overview of the progressive collapse of multistorey Reinforced Concrete (RC) buildings, focusing on the causes, analysis methods, and mitigation strategies. The paper synthesizes current research and design guidelines from various codes, highlighting key factors influencing collapse resistance and the role of modern analytical techniques which will be helpful in the assessment of progressive collapse. It also discusses the critical aspects of load redistribution, structural redundancy, and ductility in preventing the severe events.

Progressive collapse, the extensive or complete collapse of a structure resulting from the failure of one or a small number of structural components, has become a focus of research efforts and design considerations following events occurring at the Ronan Point apartment building in London, the Murrah Federal Building in Oklahoma City, and the World Trade Center in New York City. A principle research and design area for progressive collapse investigates the behavior of structural frames when column support is removed. The mechanism that results from loss of column support in structural frames characteristically involves beams that are unable to provide sufficient flexural resistance. Cable retrofit is one method to enhance existing frames and supplement or replace the post-mechanism beam load resistance with straight-legged catenary resistance after a column removal. The cables are located linearly along the beam geometry and are affixed at beam supports. This paper investigates both static and dynamic behavior of the catenary action of retrofit cables, which include both the linear and nonlinear material behavior of the cable material. Moreover, a simplified model serves as the basis for retrofit cable design is presented. Finite element modeling and experimentation in this paper verify and validate the applicability of the model. Finally, a framework for developing a procedure for retrofit cable design is presented.