PSI - Issue 64

Edward Steeves et al. / Procedia Structural Integrity 64 (2024) 1975–1982 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction The consequences of structural collapse, and by extension its prevention through robustness and redundancy, have been acknowledged by the structural engineering community for decades. Following the partial collapse of the Ronan Point apartment building in London in 1968, the risks associated with progressive collapse became a consideration for building codes and design guidelines (Adam et al. 2018). Moreover, research related to progressive collapse and structural robustness dramatically increased following the collapse of the two World Trade Center towers in New York City in 2001 (Adam et al. 2018). Despite a strong emphasis on building engineering in research, there have also been many bridge failures throughout history. Notable examples include the collapse of the Quebec Bridge in 1907, but more recently the collapse of the I-35W truss bridge located in Minneapolis in 2007 (López et al. 2023). When compiling a special collection of papers focused on the advances in collapse research and best practices after 9/11, Gerasimidis and Ellingwood (2023) only identified two papers related to the progressive collapse of bridges. Although several measures have been used in literature to assess robustness, there is 1) no universal definition of robustness that exists, 2) a lack of research specifically related to the quantification of structural robustness for bridges, and 3) a missing link between academia and industry where the application of measures are validated using real-life case studies. The objectives of this paper are to 1) perform a critical review of existing definitions for robustness and redundancy, supplemented by a review of different robustness measures, and 2) utilize the holistic structural robustness index and associated framework of analysis developed by Steeves and Oudah (2024) to quantify the improvement in structural robustness of a truss bridge after strategically upgrading elements to mitigate a brittle failure mode. The second objective is realized using a truss bridge based off a real-life structure located in New Brunswick (NB), Canada. 2. Definitions Progressive collapse refers to the propagation of local damage throughout a structure which results in collapse (Bhattacharya 2021). It is important when performing progressive collapse simulations to clearly define the point of collapse. For example, specifically related to a truss bridge that has lost the load carrying capacity of a given member, collapse has been defined as the point where any of its remaining, main members or connections fail (Miao and Ghosn 2016). More generally, if a structure is -degree statically indeterminate, then + 1 member failures (Bhattacharya 2021), or + 1 plastic hinges are required for collapse (McCormac and Csernak 2018). Others have suggested that collapse is defined as the formation of a collapse mechanism or the point at which the structure is subjected to severe damage (Ghosn and Moses 1998). As a unifying interpretation for the sake of this research, collapse is defined as the point in the global force-displacement curve where the stiffness of the system reduces to less than or equal to zero. This definition does not distinguish between partial or total collapse because the collapse volume is a function of the location, nature, and direction of the applied loading. Robustness has different meanings in different fields of engineering (Bhattacharya 2021). Even though there is no universal definition, structural robustness has been defined as the ability of a structure to absorb an initial damage and not collapse (Bhattacharya 2021), or the insensitivity to local failure (Starossek 2006); in this paper a “local” damage is extended to environmental deterioration that locally impacts all structural components. Others have defined structural robustness as the ability of a structure to withstand local damage (Miao and Ghosn 2016), or a measure of the capacity of a structure to withstand loss of local load carrying capacity (Khandelwal and El-Tawil 2011). Related to functionality, robustness has been defined as a measure of the ability of a structure to remain functional in the event of local failure (Buitrago et al. 2021). Robustness depends on the presence of multiple load paths, ductility, and strength to allow load to redistribute around a local damage (Ghosn et al. 2016). Robustness is considered in this paper to be an intrinsic structural property, implying that damage to the system is considered as “given” before a robustness assessment is performed (Bhattacharya 2021). As with the extent of collapse, robustness is a function of externally applied loading, specifically its position, direction, and distribution along the structure. From these definitions, structural robustness accounts for the capacity and ductility of the damaged system with respect to the intact version.