Analysis On Progressive Collapse Mechanism Of Light Steel Frame Structure Under Combined Effects Of Fire And Blast

In recent ten years, accidents induced by fire and explosion occur frequently. On the one hand, flammable and explosive materials which led to the potential fire and explosion accidents often occur with the rapid increase of industrial development. On the other hand, terrorist attacks occur frequently all over the world, resulting in large public buildings collapsed under explosion and fire, causing heavy casualties and huge property losses. It can be seen that fire and explosion often occur in combination. Therefore, it is of significance to research the progressive collapse of building structures under the combined effects of fire and explosion. In recent years, light steel structures are widely used in construction of public building, residential buildings and industrial warehouse. However, it is still very little at present to research the collapse mechanism of light steel frame structures and provide the protective measures under the combined effects of fire and explosion. In this dissertation, through the establishment of the new steel constitutive relationship considering the high temperature softening effect and high strain rate effect, a method for evaluating the damage degree of light steel columns under the combined effects of fire and explosion is introduced. The systematic study of the process and mechanism of the light steel frame structure collapsed under the combined effects of fire and explosion is carried on, and then some preliminary protective measures are made. More detailed introduction and some conclusions are presented as the following.(1) Establishing a new steel constitutive model considering fire (high temperature softening effect) and explosion (strain rate effect). On the basis of the traditional Johnson-Cook model and the Perzyna model, the introduction of the relationship between strain and scaled distance as well as the EOUROCODE3standard heating curve taking into account the temperature effect parameters, a modified Johnson-Cook constitutive model is established. In this paper, based on the proposed model of steel material, a numerical simulation of typical steel columns under the combined effects of fire and explosion using finite element method (FEM) is performed, considering the high temperature softening effect and high strain rate effect, column height, the scaled distance of the explosion, axial compression ratio, bearing constraints, and focusing on influencing parameters and failure modes. The numerical simulation results show that the steel column with relatively small slenderness ratio is beneficial when the scaled distance and bearing constraints are in certain of range; it has a powerful protection for the column with two fixed ends or a pin end and a fixed one; it can effectively improve the resistance capacity of light steel columns to fire and explosion by increasing the scaled distance; the failure modes under the combined effects of fire and explosion are divided into the shear failure mode, the combination mode of the shear and flexure failures, and the flexural failure mode, and the failure modes are mainly influenced by the bearing constraints.(2) Dividing, the failure process for the light steel column under the combination effects of fire and explosion into three cases of the blast loading, combination of fire and explosion, and the blast loading first with a continue heating up. During the simulation, the light steel column with a fixed end and a pin one is used as the researched object. Some conclusions can be made that the failure modes of the light steel column in each case are mostly the bending-shear failure under the combining effect of fire and blast; the deflection at the mid-span of the column goes large as temperature increasing; according to the relationship between the rotation at the end of the column and temperature, the loading cases can be divided into three:the relatively safe extent, the affected extent, the heavily affected extent; the damage index of D is proposed in the paper for the evaluation of the damage level of the light-weight steel column under the combination effect of fire and explosion; according to the basic principle of pressure-impulse curve, a method for evaluating the damage degree of the light steel columns under the combined effects under fire and explosion is introduced; based on the proposed damage index, four damage levels of minor injury, moderate injury, severe injury, collapse under the combined effects of light steel columns of fire and explosion are determined.(3) Performing numerical simulation analysis for the collapse process and mechanism of the light steel frame structure under the combined effects of fire and explosion. In this paper, the scaled distance is introduced in order to more effectively describe the destruction and collapse phenomena of light steel frame structures. Some conclusions are obtained based on the numerical analysis that the destruction will be serious and even progressively collapse with decreasing of the temperature of the light steel column for the same scaled distance under the combined effects of fire and explosion; the damage will be serious with decreasing of the scaled distance of the light steel column under the same temperature under the combined effects of fire an explosion; in the case of the combined effects of fire and explosion happening in the side-spans, the partial progressive collapse occurs as the scaled distance is greater than or equal to1.28; in the case of the combined effects of fire and explosion happening in the mid-span, the collapse of the overall continuity ocurrs as the scaled distance is less than0.8; six kinds of damages which are no damage, minor damage, moderate damage, severe damage, critical collapse, and progressive collapse.(4) Introducing the effective protective measures under the combined effects of fire and explosion. Some measures for preventing the light steel frame structure from progressively collapse under combined effects of fire and explosion are developed, which includes the following aspects:the light beam is used as a connection cable, and the strength grades of concrete are improved in the area of the floors; the combined steel and concrete columns or the steel reinforced concrete column in the surface wrapped carbon fiber and "steel jacket" are applied at the bottom floor in stead of light steel columns; some other methods for reducing the risk of fire and explosion events are slao suggested.