September 2021 marked the 20th anniversary of the terrorist attacks on the World Trade Center and the Pentagon. This special collection of papers focuses on the advances in structural engineering research and professional practice worldwide that have been motivated by these tragic events, including efforts to better understand the effects of extreme loads on structures, measure the vulnerability of structural systems to the initiation and propagation of damage within a system regardless of cause, and develop practical criteria for design to mitigate the risk of disproportionate collapse. The 19 papers in this special collection represent an international perspective from authors from the United States, Canada, the United Kingdom and Western Europe, South America, Australia, India, and China on new developments in disproportionate collapse-resistant analysis and design of buildings and bridges, highlighting the lessons learned over the past two decades and their significance to the practice of structural engineering.Overview of the Special CollectionThe special collection opens with two papers that consider the collapses of the World Trade Center Twin Towers retrospectively. Le and Bazant (2022) present a mathematical model that explains the collapse process of the towers, matches all the observations, and shows that the collapses were spontaneous. Emerging trends of probabilistic analysis and design of structural against progressive collapse are also discussed. Lalkovski and Starossek (2022) consider measures that might have prevented the initial damage to the buildings from progressing to their total collapse and suggest a new concept of tall building design that inserts strengthened floor plates at intervals of 10–20 floors to create intact building sections.Three papers focus on advances in analysis and design methods for disproportionate collapse (Beck et al. 2022; Izzuddin 2022; Stylianides and Nethercot 2021) and show that their limitations no longer are a barrier to disproportionate collapse-resistant design that they once were. Beck et al. (2022) target optimal required ratios of column-to-beam strengths in column loss situations, considering several failure modes and providing conceptual design recommendations to mitigate disproportionate collapse of frame structures. Izzuddin (2022) presents concepts and methods of analysis developed over the past two decades for the rational and practical robustness design of multistory building structures, examining the sudden column loss scenario with reference to dynamic events, presenting a simplified framework for alternative path analysis, and introducing a new tying force method that has been proposed for the next generation of Eurocodes. In recognition of the need for design methods that are more accessible to structural engineers, Stylianides and Nethercot (2021) present recently developed analytical methods that are capable of providing results in a more efficient and time-sensitive manner, enabling more extensive consideration of alternative arrangements, specific changes, and different structural solutions. The reduction of computational demands enhances the appeal of these methods to a broader audience of structural engineers.Performance of concrete buildings is addressed in four papers (Guo et al. 2022; Sadek et al. 2022; Ulaeto et al. 2022; Praxedes and Yuan 2021). Guo et al. (2022) present experimental results of scaled flat-plate substructure specimens studying both upward and downward punching shear failures. In their paper, an analytical investigation is also included, offering an in-depth understanding of the mechanical behavior resulting from upward or downward punching shear failure. Sadek et al. (2022) review recent research conducted by NIST on the robustness of reinforced concrete buildings, including experimental validation of high-fidelity or reduced-order modeling approaches under column loss scenarios. The paper also presents an approach for enhancing robustness of reinforced concrete beams through local debonding of tensile reinforcing bars at the connections, where significant cracking occurs after a column loss. Ulaeto et al. (2022) focus on concrete flat slabs and present an analytical model for horizontal progressive collapse analysis considering the dynamic asymmetric residual response of column-slab connections after failure. Their findings highlight the key role of integrity reinforcement to activate tensile membrane action around the columns and therefore preventing slab failure. Praxedes and Yan (2021) present a risk-based robustness assessment methodology for progressive collapse design of reinforced concrete frames. Through this methodology, probabilistic nonlinear pushdown analysis is shown to be a computationally affordable task, while the assessment can assist in the determination of the need of an enhanced design.Steel structures are the focus of four papers (Kong et al. 2021; Qian et al. 2021; Song 2022; Kong et al. 2022). Kong et al. (2021) report their experimental and numerical findings on the progressive collapse behavior of scaled six-column subframe composite floor systems subjected to the removal of a side column. In their second paper, Kong et al. (2022) study the phenomenon under a corner column loss, which is expected to cause severe progressive collapse due to the weaker restraints from surrounding elements. An analytical model is presented that was developed using experimentally obtained load–displacement relationships and failure modes. The connection between beams and columns is found to be the governing factor of the collapse mechanism. Qian et al. (2021) fabricated and tested six multistory steel moment-resisting subframes (three bare frames and three braced frames) aiming to shed light on the effects of the existence of steel braces in the progressive collapse of moment-resisting frames. It is found that the braces can indeed increase the load-resisting progressive collapse capacity and this can be attributed to tensile braces mostly. Song (2022) investigated the required rotation and tension capacities of steel bolted connections to permit the development of catenary action in supported beams to prevent progressive collapse under column loss scenarios using both experimental and analytical methods. Acceptance criteria for steel bolted connections that are based seismic conditions (GSA 2016; US Dept. of Defense 2016) were found to be too conservative.Multistory engineered wood construction has become more prevalent in recent years because of a desire to achieve sustainable buildings, while at the same time there is little to no guidance on how to prevent progressive collapse of multistory mass timber buildings. Mpidi Bita et al. (2022) present practical solutions applied in constructed prominent multistory mass timber buildings to prevent progressive collapse. This paper also reviews draft provisions for an upcoming standard related to the prevention of progressive collapse of multistory mass timber buildings.Two papers (Fiorillo and Ghosn 2022; Wang et al. 2022) focus on progressive collapse of bridges, which may be vulnerable to disproportionate collapse due to exposure to long-term deterioration, sudden localized failures, and the traditional member-oriented design approach, which may not lead to an accurate evaluation of bridge system capacity. Fiorillo and Ghosn (2022) quantify the effect of bridge element damage or failure, taking into account element damage size and location and how such failures impact the probability of failure of the entire system of highway bridge superstructures. A probabilistic structural redundancy framework with robustness criteria is established through numerical examples, as an intermediate step toward full-fledged risk-based evaluation of bridge structural integrity. Wang et al. (2022) examine the damage to long-span suspension bridges following sudden suspender losses. Both one-at-a-time and multiple suspender losses are considered, showing that there are critical numbers and locations of suspender losses, after which the bridge sustains large inelastic deformations or undergoes progressive collapse. The bridge becomes susceptible to a progressive collapse following the loss of eight suspenders.The relationship between robustness and cost efficiency is examined in two papers. Stewart (2021) explores this relationship by presenting two case studies. The first involves fatality risks from progressive collapse caused by a large vehicle bomb and an assessment of the costs and benefits of design to mitigate progressive collapse. The second examines the risks, costs, and benefits of fiber-reinforced polymer blast-resistant strengthening of RC columns. Vrouwenvelder (2021) sheds light on the underlying effectiveness and cost efficiency of applied safety measures to achieve a sufficient degree of robustness, focusing on the large epistemic uncertainties in the relation between robustness and additional costs of providing collapse resistance to events beyond the design basis.Finally, Dusenberry (2022) discusses disproportionate collapse codes, standards, and best practices and introduces the new ASCE standard on disproportionate collapse (ASCE 2023), which is expected to be published in 2022. This voluntary consensus standard on disproportionate collapse is performance based and is the first of its kind, including general requirements and definitions, risk assessment, performance criteria, design and analysis approaches, acceptance criteria, structural detailing, existing structures, and performance qualifications. The standard was enabled, in large measure, by the two federal agency guidelines for progressive collapse (GSA 2016; US Dept. of Defense 2016) that were developed following the Murrah Building collapse of April 1995 and by the significant research and advances in professional practice that have occurred since September 11, 2001, typified by the papers in this special collection.

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