Failure assessment and virtual scenario reproduction of the progressive collapse of the FIU bridge

The collapse behavior of an engineering structure is a highly nonlinear and discontinuous dynamic process in reality. From the initial material yielding and damage to the subsequent cracking and failure of the component, until the final structure falls and collides with other parts. This process makes numerical simulation extraordinarily complicated. Moreover, the nonlinear behavior of the material also makes convergence difficult and the model difficult to refine. The complexity of the calculation increases greatly due to the damage softening effect of the material. On March 15, 2018, the Florida International University (FIU) pedestrian bridge collapsed while under construction. Engineers, as well as researchers, have since then issued differing explanations on the cause of the accident. At present, a unified perspective is yet to be formed. For instance, the National Transport and Safety Board reported that errors were made during design. On the same issue, some scholars have mentioned that the bridge failure was due to a punching shear at a node, while others still argue that the collapse was due to operational errors from construction personnel.

Therefore, to unravel this ambiguity, there is a need for corresponding experimental investigations to be implemented. Unfortunately, conducting a full-scale structural experiment is impossible even if a scale model test was to be conducted, because of size effect deficiency. In the rarest of cases, video recording of the collapse process is used as a reference case for progressive collapse behavior of large and complex structures and can be used to verify the reliability of numerical simulation results. When such scenarios emerge, videos can be used as effective means to verify the reliability of the numerical simulation of large-scale complex structures. In this regard, the research team on the failure assessment and dynamic behavior simulation of structures from Shandong Jianzhu University, China, performed a finite element (FE) numerical simulation, purposefully aiming to reproduce the virtual scenario of the bridge collapse and examine the cause of the FIU accident. Their work is currently published in the research journal, Engineering Structures.

In their approach, a refined FE model was established based on the concrete plastic damage constitutive relations. The researchers calculated the damage factor using the Najar damage theory. Additionally, the equivalent rebar smeared calculation model was adopted to simulate the FIU bridge collapse. The problem of numerical solution convergence difficulty due to material nonlinearity was solved, and a certain precision was ensured. Transferring results between the ABAQUS/Explicit and ABAQUS/Standard method was executed to avoid unrealistic structural response problems caused by the inertial effect in the dynamic analysis.

The authors reported that; first, the collapse of the FIU bridge was caused by the failure of the 11^th diagonal member, and second; the structural robustness of the FIU bridge was weak. Technically, the team acknowledged that the force transmission path was relatively simple, leading to a massive change of the internal force of the other parts of the structure once a breakdown occurs in an individual link.

In summary, the study presented a virtual scenario reproduction of the progressive collapse of the FIU bridge. Remarkably, the numerical analysis showed that the total duration of the structural collapse process, which was approximately 1.2s, was the same as the result of the video recording. Moreover, a comparison of the structural deformation and failure modes of each moment and the final collapse mode were almost identical to those on the video. In a statement to Advances in Engineering, Professor Feng Xi mentioned that the FE technique and material damage and constitutive failure model used in this study were appropriate, and the numerical results obtained were accurate. As such, these numerical results and conclusions can provide a reference for the cause of bridge collapse accidents.