| Impact,as an accidental action,has the distinctive characteristics of low frequency,repeated occurrence,and high hazard.It has become an action that should be considered for the design of grand civil engineering structures,anti-collision enclosure structures,and national defensive and anti-terrorism related facilities,etc.With the increasing development and application of high-strength materials and high-performance components,high-strength concrete-filled steel tubes(HSCFST)have become one of the preferred members to be used as the main bearing components of large-scale buildings and structures,large-span bridges,national defense infrastructure,due to their advantages of high bearing capacity,good ductility,environmentally friendly,and cost-effective.In this context,investigating the dynamic behavior of HSCFST members under impact is essential to ensure the safety of major civil engineering structures and special structures.This dissertation focuses on two main scientific issues,i.e.,the dynamic mechanical properties of structural steels and dynamic behavior of square HSCFST members under lateral impact loading.The main scope and findings of this dissertation are as follows:(1)The high-speed tensile,and split Hopkinson pressure bar tests(36 tests categorized into 12 groups)of S690 high-strength structural steel,as well as quasi-static tensile tests,were carried out.The stress-strain curves of S690 steel at the quasi-static,medium and high strain rates were measured.Based on the dynamic yield strength,dynamic ultimate strength,and true stress-plastic true strain relationships,the strain rate effect of S690 steel was analyzed and the corresponding dynamic increase factors(DIFs)were obtained.For S690 high-strength structural steel,three rate-dependent models(which are suitable for different scenarios)were proposed,i.e.,strain-rate models based on C-S and J-C models under intermediate strain rates,strain-rate models based on C-S and J-C models under high strain rates,and an M-J-C model covering the entire range of medium and high strain rates.(2)A series of dynamic tests(81 tests categorized into 27 groups)under different strain rates for different grades of structural steels(i.e.,Q235,Q355,Q460,Q550,and S960)were conducted.The stress-strain curves and strain-rate effect indices were obtained.A database containing 215 dynamic tests on structural steels was then established by combining the data from previous studies and the data from this research.Using this database,continuous rate-dependent models were developed,covering intermediate to high strain rates and conventional strength to high strength structural steels.After that,a continuous dynamic stress-strain model of structural steel was established.This stress-strain model provides the basis for investigating the dynamic behavior of steel structures and steel-concrete composite structures subjected to impact and blast loadings.(3)Thirty impact tests of simply supported square HSCFST specimens using a drop hammer testing machine were conducted.The main parameters included the yield strength of the steel,compressive strength of the concrete,cross-sectional steel ratio,impact energy,and the length of the span.Test results including the failure mode,time history of the impact force,and time history of the mid-span displacement were obtained.Key performance indicators including the impact force plateau value,local and overall deformation,and energy absorption were systematically evaluated.The test results indicated that square HSCFST members have good impact resistance.These members experienced large plastic deformation of overall bending under the action of lateral impact.The yield strength of the steel tube and the cross-sectional ratio of the steel have a great influence on the impact resistance of HSCFST members.(4)Based on the developed dynamic stress-strain model of structural steel,finite element models of square HSCFST members under lateral impact were developed using ABAQUS.The developed models were then benchmarked using test results.The benchmarked models were then used to analyze the behavior of square HSCFST members under lateral impact loading and axial force-lateral impact loading,including the deformation history,force history,stress,strain,and energy absorption.These analyses revealed the state and bearing mechanism of HSCFST under impact loading.These analyses also indicated that,due to the large lateral deformation caused by the impact action and the existence of the axial force,the second-order moment increased significantly.This may eventually change the failure mode of the member from overall bending failure to dynamic instability failure.(5)Parametric analyses on the dynamic responses of square CFST members under lateral impact were conducted to investigate the effects of axial compressive load ratio,impact energy and momentum,yield strength of steel,compressive strength of the concrete,cross-sectional dimension,span-to-height ratio,steel ratio,boundary conditions,and impact position.Based on the results of the experiment tests and finite element analyses in this paper,the failure criterion that consists of impact mass m,impact velocity v,and axial compressive load ratio n(i.e.,critical failure surface)for CFST member under lateral impact was proposed,considering the failure characteristics when CFST member reaches allowable displacement or dynamic instability occurs.Results from the finite element analyses were also used to(i)find the influence of each parameter on the m-v-n correlation relationship and(ii)establish the m-v-n database.Based on the machine learning algorithm,the calculation method for the transverse impact resistance of square CFST members was proposed,and design recommendations were provided. |
PDF Full Text Request