CASCO: a simulator of load paths in 2D frames during progressive collapse

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CASCO: a simulator of load paths in 2D frames during progressive collapse Enrico Masoero1  Received: 2 March 2020 / Accepted: 11 July 2020 © The Author(s) 2020  OPEN

Abstract Modern structural design software can simulate complex collapse dynamics, but the main physical processes driving collapse propagation are often hidden among structure-specific details. As a result, it is still unclear which structural geometries and material properties should be preferred when approaching the design of a damage-tolerant structure. This manuscript presents a new approach to explore the relationships between structural geometry, local mechanical properties, and collapse propagation. The insight comes from a unique ability to trace the evolution of load paths during collapse, achieved by combining energy conservation with local mechanisms of plastic failure and a few simplifying assumptions. The method is implemented in a new simulator of collapse of 2D frames, called CASCO and programmed in MATLAB. Simulation results for reinforced concrete frames predict collapse loads and mechanisms in agreement with fully non-linear, dynamic simulations, while also providing a graphical description of the evolving structural topology during collapse. A first application of CASCO to mechanically homogeneous and heterogeneous frames, indicates certain evolutions in number and density of load paths during collapse that may be targetted to improve collapse resistance. Keywords  Progressive collapse · Simulation · Load paths · Energy · Framed structures

1 Introduction Resistance to progressive collapse is the ability to withstand local damage without triggering a chain of failures with disproportionately severe consequences. The Alternative Load Path Method (ALPM) is an established framework to analyse progressive collapse [9, 10]. The ALPM considers a model structure initially at equilibrium under characteristic loads, and then removes of one or more elements, usually columns, to represent accidental damage. The concept behind the ALPM is that each load has its own path, relying on internal forces, to reach the external supports. Accidental damage removes some paths, forcing the loads to alternative paths. Such redistributions might amplify the internal forces and trigger new failure events, prompting

further redefinitions of paths and redistributions of loads. This process can cause avalanches of failures and compromise the stability of large portions of the structure. Going from this intuitive concept to actually tracing load paths is not straightforward. The current tools for numerical simulations can realistically describe structural collapse using dynamic and nonlinear analyses. These simulations are typcially based on Finite Element Analysis (FEA) [15, 18, 26] or on the Discrete Element Method [22], and can also include stochastic analyses [35]. Detailed simulations, however, produce complex distributions of internal forces from which it is generally difficult to trace load paths, as illustrated in Fig. 1. Detailed simulations are the s