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Explosive testing to assess dynamic load redistribution in a reinforced concrete frame building Matthews, Timothy William

Abstract

Guidelines have been developed in the United States to assist in the design of buildings resistant to progressive collapse. These guidelines outline several methods for considering the effects of dynamic amplification on load redistribution following sudden member failure in a structure. Previous analytical studies have been performed to evaluate the consistency of these guidelines, but very little testing has been conducted to verify the guidelines and analytical work performed to date. This study investigates the dynamic load redistribution occurring in a field test on a two-storey reinforced concrete frame specimen. The test consisted of removing a column with explosives and recording the deformations of the remaining columns in the structure. The explosives removed the load-carrying capacity of the column but also exerted an upward force on the structure. Observations from the test suggest the building frame remained linear-elastic. A linear analytical model of the test specimen was developed in ETABS. This model effectively captured the response of the structure during testing. The model was used to examine the response of the test specimen due to gravity load redistribution alone, as well as the total response of the structure, including the effects of the upward force from the explosion. A dynamic amplification factor of 1.89 was observed for the columns where the majority of the gravity load redistributed during in the experimental test. This value supports the maximum dynamic amplification of 2.0 proposed for linear structures by previous research and progressive collapse assessment guidelines. The response at these columns was dominated by one mode of vibration. At columns where less steady-state load was redistributed, dynamic amplification factors much greater than 2.0 were observed, although the absolute peak axial force increases were relatively small. Higher modes influenced by cantilevers along one side of the structure contributed significantly to the peak demands at these locations. The upward explosive force magnified the peak demands on the structure from gravity load redistribution by as much as 68%. The upward explosive force was also observed to act unsymmetrically on the structure. These effects must be considered when considering column failures from explosives installed inside the member.

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