Catenary behavior of steel girders under progressive collapse-type loads

Cotter, Thomas
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University of Delaware
Progressive collapse occurs when local failure(s) of one or a limited number of structural components propagate to cause failure of a significant portion of the structure that is disproportionate to the original damage. Interest in mitigation of progressive collapse began after the full or partial collapses of structures such as the Ronan Point Apartments and Alfred P. Murrah Building. Current design guidelines exist for improving the progressive collapse resistance of structures, but these guidelines do not consider the potential catenary action of structural members under extreme deformation, which can potentially increase the load-resisting capacity of beams and girders compared to flexural capacity alone. By evaluating the potential catenary response, the actual behavior under extreme events can be more accurately considered and designed for and improved economy may also result. In this thesis, a broad literature review was first performed to understand the prior work in this area and assess the practical limitations for developing catenary action. Then finite element analyses (FEA) were utilized, using ABAQUS, to determine and quantify the behavior of steel girders under a representative design scenario for the alternate path method of progressive collapse design, where a single column is removed from the structure. Several models were created to investigate the effects of: residual stresses; geometric imperfections; and support conditions, on the behavior of the steel girders under extreme load. Girder rotation, percentage of web depth in tension, and quantification of connection forces were all measured to analyze the girder behavior. The results of the analyses revealed that girders with fixed supports were able to resist 80% more applied load after the flexural capacity of the girder was reached, regardless of the presence or absence of geometric imperfections and residual stresses. This compared to the shear support girder resisting only 30% more applied load after flexural capacity. All fixed support girders showed a constant increase in percentage of web depth in tension after the flexural capacity was reached up to a prescribed failure rotation of 0.05 radians, confirming catenary behavior is being utilized under additional applied load. At the failure rotation of 0.05 radians (a value selected based on review of past research), axial connection forces for the fixed support girders were between 350 kips and 372 kips. These results show that steel girders are able to utilize catenary behavior to resist additional applied load past their flexural capacity, which results in very large axial connection forces. It is suggested that consideration of these forces be incorporated in future efforts aimed to reduce the probability of progressive collapse.