Damage Evaluation And Progressive Collapse Research Of Steel Structures Subjected To Explosion And Post-explosion Fire

Terrorist explosions and accidental explosions not only result in serious casualties and structural failure directly,but also induce the post-explosion fire,which will further damage the structures.For the widely used steel structures,the steel members,especially steel columns are vulnerable under the combined hazard of blast and fire,and its failure will further lead to progressive collapse of the steel structure.Such collapse will result in serious casualties and property loss and has immeasurable negative social impacts.To investigate the damage and progressive collapse mechanism of steel structures subjected to explosion and post-explosion fire,three main researches are discussed,and they are(1)a method for simulation and prediction of blast loads and temperatures acting on the members;(2)structural response,failure mode and damage evaluation of steel columns under blast and fire;(3)progressive collapse analysis of the steel frame under blast and fire.The primary work and achievements are presented as the following:(1)Simulation of blast wave propogation,its interaction with structures and blast-induced fire development is studied,and the formulae for prediction of blast loads and temperatures acting on structures are proposed.According to the distribution law of blast loads and the characteristic of the pressure time history,the areas subjected to blast loads are divided,and formulae of calculating peak overpressure,shock wave duration,gas pressure and blow-down time of each region are proposed.A simplified model of internal blast load is established and the influence of explosion height and confinement geometry is analyzed.The results show that explosion height and confinement geometry have a great influence on peak overpressure,but less on impulse.Based on the different situations of fire intensity and vent,the blast-induced fire can be divided into two kinds: fuel-controlled burning and vent-controlled burning.The temperature distribution laws of these kinds of fire are studied,and the results show that the horizontal temperature of fuel-controlled burning keeps the same and the vertical temperature distribution is bilinear;the temperature distribution of vent-control burning is confused,but the distribution along the column is uniform.Based on the data of simualtions,a simplified model for calculating the air temperature is proposed.(2)Structural response and failure mode of steel columns subjected to explosion and post-explosion fire are studied.According to the simplified models for predicting the internal blast loads and temperatures,the failure mode of steel columns under blast and fire is analyzed by LS-DYNA.The results show that unlike the global failure mode in the ISO834 fire environment,the failure mode of steel columns in the real fire is the local failure near the column top,and varying the axial load ratio,section dimension and restraint condition have little effect on the failure mode.(3)An efficient method for damage evaluation of steel columns subjected to explosion and post-explosion fire is studied.The method evaluates the damages of steel columns under blast and fire in two steps.In the first step,a criterion for blast-induced damage is proposed,and the P-I diagrams and corresponding expressions for predicting the global deformation,local deformation and length of local deformation zone under blast are established.In the second step,a numerical method for introducing the blastinduced damage into fire analysis is proposed and a simplified model for fire analysis is established.The simplified model is used to predict the damages under blast and fire,and the results indicate that the proposed method can give a reliable prediction of damages under blast and fire,whereas results in more than 80% savings in computational time.(4)The method for evaluating the damage degree of restrained steel columns subjected to explosion and post-explosion fire is studied.Based on the analysis model of the restrained steel column,the P-I diagram for damage evaluation under blast is generated.A further study is conducted to investigate the effect of the shape parameter of the pressure time history,and the results show that the effect only occurs in the dynamic load region.According to the definition of failure time in fire analysis,the PI-t surface plot is established to evaluate the damage of restrained steel columns under blast and fire.The comparing results between restrained and non-restrained steel columns indicate that the widely used assumption is unsuitable for the damage evaluation of steel columns under blast and fire,and the actual restraint condition should be considered.Furthermore,the parametric studies show that like the negligible effect of restraint stiffness on the failure time,the column thickness has a slight influence on the failure time when the blast-induced damage is small,whereas the thicker column becomes weaker when the damage is large.However,the restrained steel column is more vulnerable under blast and fire when the column is higher and thinner.(5)Progressive collapse mechanism of the steel frame subjected to explosion and post-explosion fire is studied.Based on the macro-model and user defined material model of the shear tab connection,an accurate and efficient model considering axial force-moment interaction effect in connections is established,and the progressive collapse mechanism of the steel frame under blast and fire is analyzed according to the simplified model of blast loads.The analytical results indicate that the columns in the interior MRFs will become the weakest link when the interior MRFs exist,thus the MRFs are suggested to be set along the exterior frames.Inducing greater robustness in the shear tab connections can improve the progressive collapse resistance of the steel frame and prevent the local collapse prior to progressive collapse.For the steel frame,the weakest area is the peripheral compartment for the fire only cases but changes to the corner compartment for the combined hazard cases.The decrese of the collapse time is less than 40% till the charge weight is larger than 200 kg.Based on the progressive collapse mechanism and proposed formule of P-I-t surface plot,an efficient method for prediction of progressive collapse is proposed,and the accuracy of the method is verified by comparing with the results from the direct simulation.