Strain Rate Effect Study Of Two CFST Members Under Impact Load

Concrete filled steel tube(CFST)structure has good mechanical properties,which is widely used in high-rise buildings,piers and other structures.In recent years,with the frequent occurrence of car and train collision accidents,it has caused serious damage to our building structure,which may lead to the collapse of the structure.Therefore,it is necessary to properly consider the impact load during the structural design.Concrete-filled steel tube will produce strain rate effect under impact load,so the effect of material strain rate should also be considered.In this paper,circular concrete-filled steel tube specimen,circle section inner and square section outer composite CFST are taken as research objects.The finite element simulation is used to analyze the changes of the impact force,deflection,strain and strain rate of the component under impact load,and provide methods and suggestions for the anti-impact design of engineering structures.The research content of this article is as follows:(1)First use the ANSYS / LS-DYNA software to simulate the drop weight impact test in the reference,use the test data to modify the material parameters,and verify the correctness of the model from multiple aspects,which can be used for the parameters of the next different working conditions analysis.(2)Establish circle section CFST model,change the concrete strength,steel yield strength,steel ratio,slenderness ratio,sectional diameter,impact velocity,impactor mass and other parameters.And the peak impact force,mid-span deflection,duration,mid span bottom strain and the peak strain rate are analyzed to find the main control variables that affect each parameter,and the calculation formulas of peak impact force,duration,and peak strain rate are fitted.(3)Combined with the simplified analysis model of "equivalent plastic hinge",on the basis of parameter analysis,the formula of dynamic bending moment increase coefficient is fitted,and a simplified calculation method for the final deflection of the mid-span is proposed and the test data in the literature is calculated,The error is within 20%.(4)The whole process of the inner circle and outer square composite CFST specimen under the drop weight impact load is analyzed,and its working mechanism,stress and strain development are analyzed,and the different inner and outer diameter to width ratio,the inner and outer steel tube yield strength,and the inner and outer steel tube steel content are analyzed.It is found that the peak value of strain and strain rate at the bottom of the specimen decreases with the increase of each parameter.Among them,increasing of the inner and outer diameter-width ratio,the strength of the outer steel tube and the steel ratio of the outer steel tube have the greatest impact on the resistance of the component,especially when strengthening the existing structure,it is a very effective measure to increase the strength and steel content of the outer steel pipe.