A conceptual note on the definition of initial failure in progressive collapse scenarios

Progressive collapse can be defined as a cascading phenomenon in which an initial failure is followed by the collapse of adjoining members which, in turn, is followed by further collapse that is disproportionate to the initiating failure. While extensive experimental and numerical studies have focused on the topic, little effort has been put forward in defining and redefining the underlying theory and philosophy. These theories and philosophies are of primary importance since they can shape the entire research methodology. The current definitions and approaches have been developed based on frame structures within a threat-independent methodology, although this aspect is not explicitly emphasized. This study tries to challenge this idea. It is shown that the initial failure (i) is not necessarily a local damage, (ii) is not necessarily a member loss, and (iii) cannot always be defined as a threat-independent damage scenario. The consequences of this insight are deeply discussed regarding the structural type and acting threat. In particular, it is shown that the current code-based approaches do not always lead to the most critical scenario. Finally, a rational framework for the definition of the initial failure is provided

Progressive collapse of structures: A discussion on annotated nomenclature

The study of progressive collapse and structural robustness has advanced significantly after 9/11 event. There is a growing interest in the phenomenon, as well as in the development of numerical and experimental techniques that have led to great progress in understanding the structural robustness and integrity. However, the general ideas, concepts and definitions have been merely changed over the past twenty years. These concepts and definitions are first developed in the framework of a threat-independent methodology, implicitly focused on blast-induced progressive collapse (or other short-term extreme events) in framed structures, and then, generalized to other structural types, mechanisms and triggering events, without scrutinization. In this paper, the current definitions of the terms progressive collapse, initial (local) damage and progressive collapse analysis are challenged, their insufficiency is discussed and possible improvements are provided. The suggested definitions and discussions provide a deeper and more general nomenclature for progressive collapse and related topics

Design for Robustness: Bio-Inspired Perspectives in Structural Engineering

Bio-inspired solutions are widely adopted in different engineering disciplines. However, in structural engineering, these solutions are mainly limited to bio-inspired forms, shapes, and materials. Nature is almost completely neglected as a source of structural design philosophy. This study lists and discusses several bio-inspired solutions classified into two main classes, i.e., compartmentalization and complexity, for structural robustness design. Different examples are provided and mechanisms are categorized and discussed in detail. Some provided ideas are already used in the current structural engineering research and practice, usually without focus on their bio-analogy. These solutions are revisited and scrutinized from a bio-inspired point of view, and new aspects and possible improvements are suggested. Moreover, novel bio-inspired concepts including delayed compartmentalization, active compartmentalization, compartmentalization in intact parts, and structural complexity are also propounded for structural design under extreme loading conditions

Threat-Independent Column Removal and Fire-Induced Progressive Collapse: Numerical Study and Comparison

Progressive collapse is defined as the spread of an initial failure from element to element, eventually resulting in the collapse of an entire structure or a disproportionately large part of it. The current progressive collapse analyses and design methods in guidelines and codes focus on the alternate load path method. This method is suitable especially in the case of blast-induced progressive collapse. In this paper, fire-induced and threat-independent progressive collapse potential is numerically investigated in steel moment resisting frames. Affecting parameters such as location of initial failure and number of floors are considered in this study. Two different mechanisms were observed in threat-independent and fire-induced progressive collapse: while in threat-independent column removal alternative load paths play major role, in fire-induced progressive collapse the weight of the structure above the failure region is the most important parameter

Non-linear Dynamic Analysis of Steel Hollow I-core Sandwich Panel under Air Blast Loading

In this paper, the non-linear dynamic response of novel steel sandwich panel with hollow I-core subjected to blast loading was studied. Special emphasis is placed on the evaluation of midpoint displacements and energy dissipation of the models. Several parameters such as boundary conditions, strain rate, mesh dependency and asymmetrical loading are considered in this study. The material and geometric non-linearities are also considered in the numerical simulation. The results obtained are compared with available experimental data to verify the developed FE model. Modeling techniques are described in detail. According to the results, sandwich panels with hollow I-core allowed more plastic deformation and energy dissipation and less midpoint displacement than conventional I-core sandwich panels and also equivalent solid plate with the same weight and material

A multi-scale approach for quantifying the robustness of existing bridges

Bridges are among the most relevant structural engineering works in transport and mobility infrastructure. De-pending on a wide range of needs and constraints, various types of structures are found: simply supported beams on piers, box girders, Gerber decks, arches, balanced systems, etc. European infrastructural heritage has now more than 50 years of working life, with increasing traffic loads and continuous ageing needs maintenance. Recent cases of existing bridge failures have opened the problem of the robustness of such systems. To this aim, a multilevel framework is formulated. This approach is needed for studying the propagation of damage from the element level to the whole structure. In the proposed multilevel approach each single part is studied and its dam-age tolerance is assessed. The effects of the damage on the single part on the overall bridge structural scheme are then assessed. This multilevel analysis allows to define a member consequence factor, i.e., a measure of the overall effects of the local damage. The proposed methodology is applied to case studies

Friction-Based Energy Dissipation Efficiency of Thermal Elastomeric Bearings Effect on Seismic Vulnerability of Ordinary Bridge Substructure

Thermal elastomeric bearings, which generally accommodate thermal elongation and shrinkage rotations in bridge superstructure, can account as seismic isolators too. During moderate to strong earthquakes, the bearing possibly starts to slid on its concrete seat due to have no positive connection to the seat. As a result, a huge amount of seismic energy will be dissipated in wide force-deformation hysteretic cycles between elastomeric end layer of bearing and the concrete seat. This behavior initiates the idea of concurrent usability of thermal elastomeric bearings as seismic isolators as an economical response to AASHTO increment in design earthquake return period in 2008. The challenging problem with the approach is the large superstructure displacements and the risk of deck failure due to bearing unsetting. It this study, friction coefficient and elastomeric hardness have been considered as two affecting factors on deck and bearing displacements. The main effects of those parameters on substructure and superstructure are discussed and the probability of deck failure is presented

Strengthening and retrofitting techniques to mitigate progressive collapse: A critical review and future research agenda

Abnormal events, that are unforeseeable low-probability and high-impact events, cause local failure(s) to structures that can lead to the collapse of other members and, eventually, to a disproportionate progressive collapse. Ordinary design procedures, which are usually limited to gravity and seismic/wind loads, are inadequate for preventing the progressive collapse. Therefore, a focus on strengthening and retrofitting techniques to mitigate progressive collapse is necessary. Parameters such as topology of the structure, nature of the triggering event, size of the initial failure, typology of the collapse and seismic design requirements affect the strengthening and retrofitting strategy. A discussion on the impact of these parameters on strengthening strategy is first presented. Then, a comprehensive review on strengthening and retrofitting techniques to mitigate progressive collapse is provided. The paper concludes with an ambitious comprehensive list of issues covering different aspects of future research agenda

Blast-induced progressive collapse of steel moment-resisting frames: Numerical studies and a framework for updating the alternate load path method

The Alternative Load Path (ALP) method is widely used to assess progressive collapse resistance of steel framed structures. Code-based ALP is threat-independent methodology, implicitly focuses on a very special triggering event, i.e., a small near-field blast, that can lead to complete and sudden column loss. However, in a real blast-induced progressive collapse scenario, characteristics of the triggering event and subsequent initial damage control the structural response. To study these effects, a wide numerical investigation is carried out. First, the code-based ALP method is applied to assess the threat-independent dynamic column removal responses. The results emphasize the importance of initial damage location and buildingâs size. Then, the model structures were analyzed in different blast scenarios. A meaningful difference in the obtained results compared with code-based ALP is observed in both quantity and quality. Finally, a novel methodology (modified ALP) is suggested to update the code-based ALP method to capture the threat-dependent parameters, i.e., column removal time (CRT) and damage level. To serve this purpose, a substructure techniques, i.e., equivalent column model, is developed and validated. The results of three methods (threat-independent code-based ALP, threat-dependent blast analysis (BA) and the proposed modified ALP) are compared, and it is observed that the modified ALP method can effectively adjust the dynamic column removal response to reflect the blast effects

Numerical analysis of all‐steel sandwich panel with drilled I‐core subjected to air blast scenarios

This paper reports a numerical study carried out with the aim of quantifying nonlinear dynamic response of drilled I-core steel sandwich panel when subjected to air blast loading. Several parameters, i.e., boundary conditions, explosive charge weight and asymmetrical loading, that can affect structural response under blast loads, are considered. The material and geometric nonlinearities and strain-rate effects are also taken into account in the modeling. Obtained results are compared with available experimental data to verify the developed finite element model and good agreement is observed. According to the results, sandwich panels with drilled I-core allow more plastic deformation and energy dissipation and less midpoint displacement compared with equivalent structures, i.e., conventional I-core sandwich panels and also solid plate with same weight and material