ABOUT THE METHOD OF SIMULATING THE PROGRESSIVE COLLAPSE OF A REINFORCED CONCRETE FRAME UNDER THE FIRE IMPACT

Abstract

In his paper a method of mathematical modeling of progressive destruction based on the example of an existing 17-story building using an explicit method of integrating the dynamics equations was proposed. A set of provisions and assumptions was substantiated for mathematical modeling of the progressive destruction of the 17-story monolithic reinforced concrete frame. The numerical analysis was performed using the finite element method in the LS DYNA software complex. A fire under the standard mode (150 min – I degree of fire resistance) in accordance with Ukrainian standards was considered as an emergency load, which involves taking into account nonlinear processes of changes in physical and mechanical characteristics of materials and changes in temperature. Five scenarios of fire development in the premises of the building are considered, based on which the algorithm for determining the reduced mechanical characteristics of the composite layers of planar finite elements approximating the calculated area of the structural system of the building is substantiated. It is shown that in all cases of the five fire scenarios, progressive destruction covers parts and elements of the structural system of the building that go beyond the premises where the fire develops, as well as elements whose mechanical characteristics are not subject to reduction due to fire. The results demonstrating the destruction of the elements in a cascade manner which starts with the most weakened and spills over to the elements that perceive a greater load were obtained. It is noted that the elements that do not have reduced mechanical characteristics are destroyed due to the increase in bending moments from deformed overlaps with support columns and diaphragms, which have given rise to failure. As a sign of the onset of progressive collapse, a rapid increase in displacements is adopted. They spread both in plan and along the height of the building, and these displacements prevail by several orders over displacements obtained in normal (non-emergency) conditions. At the same time, the appearance of signs of progressive collapse became possible only when the load was increased by 3.72 times.