On Dynamic Responses Of Axially Preloaded Concrete Filled Steel Tubular Members Under Lateral Impact By A Rigid Body

A concrete filled steel tubular member (abbreviated as a CFT member) is a column or a beam which is made of a steel tube in which concrete is fully poured. It is one kind of confined concretes as well as of heterogeneous building materials.Because of two materials'interaction caused by loading on them, CFT members may dominently improve good mechanical properties of steel tubes and concretes and weaken their shortcomings.Thus, they have such advantages as high load-bearing capacity, large deformable ability, large energy dissipated ability, excellent impact ductility and good applicability to construction. And they are widely used as axially compressive stuts or eccentric columns in high-rise buildings, large span bridges, subway stations even military structures etc. The CFT buildings at service are often under large static loadings and subjected to even larger dynamic forces. Whether or not they can work functionally as usual especially in case of transversely striking? How do people evaluate their crashworthiness? All of these questions have been put forward since CFTs are born. It is urgent for scientist to answer these highlight problems. Howerer, it is much more difficult to uncover CFT's laterally impact response and failure mechanism by now. There are many puzzles such as propagation path of stress waves, dynamic material properties, interface bonding behaviors and coupling of local damage and global deformation.In this dissertation, experimental, computational and theoretical investigations are made to reveal the truth of CFT's response and crashworthiness. The main work and some conclusions are described below.1. ExperimentBoth static tests and dynamic ones are carried out consequently. The static tests consist of the tension tests on steel tube materials, compression tests on concrete cubics and flexure tests on CFTs. The tests are aimed to get yield strength of steels and concretes and P ? W curves for CFTs. The bending tests are proceeded on CFT specimens with 3 different tube wall-thickness of 1.7mm,3.5mm and 4.5 and with 2 different end conditions of pinned-pinned support and fixed-fixed support. The test machine is YAW-5000 of 500KN, which is able to exert forces on mid-span of CFTs and record P ? W curves aucomatically. The P ? W curves indicate in most of work conditions that concentrated forces appear as a flat value and that mid-span deflections become large enough before bend failures occur at the cross-section with the largest bend moment.In the lateral impact tests, 59 axially preloaded and unpreloaded CFTs with end conditions of pinned-pinned,pinned-fixed and fixed-fixed supports are struck at their mid-span by a rigid body with initial velocities. The propose of the tests is to obtain F ? t curves and final permanent deflections at mid-span. For contrast, additional lateral impact tests on 18 reinforcement concrete members and 18 hollow steel tubes are developed. In the transversely axially preloaded tests on CFT specimens, dropped hammer impact machine of DHR9401 and axial load exerting device are deployed. The axial forces are preloaded at 3 axial compression ratio of 0, 0.3,0.6 before lateral impacts start. The phenomenon observed include global bending deformations,local warping deformations,crack of tube surfaces and fracture of CFTs. Most of bending lines are in the shape of V and local deformations taking place at impact position and fixed end have appearance of surface warp and enlarging oblateness of circular cross-section. The failure modes fall into three categories: shearing failure, bending failure and unstability. The time history curves of lateral impact forces are another kind of experimental result. The curves show that development of transverse forces of specimens without preloaded loads may be devided into 3 segments: peak value, flat value and zero value. As for that with preloaded axial forces, there are only peak value segment and zero value ones. It is also found that the axial compression ratios make greater influence on resistance ability to impact. In case of the ratios less than 0.3, CFT's resistances strengthen. In case of that more than 0.6, they weaken. As we know, critical energy is an index used mostly to evaluate crashworthiness of members or structures. So, it is of great importance that scientists give an exact definition of critical impact energy for CFTs. In this paper, we think that the critical energy are reached when steel tubes are tensioned to crack during the progress of impact. The results prove that critical energied have close relationships with end support conditions. If critical energy for fixed-fixed CFTs is assumed as 1, that for pinned-pinned ones and pinned-fixed ones are 0.4 and 0.75 respectively.In the contrasting tests, the results display that CFTs have better impact-resist properties than sum of reinforcement concretes and steel tubes.2.Computer simulationWith the aid of well-known software of LS-DYNA10.0 and based on more accurate material properties and parameters, computational models for 3 boundary conditions of lateral impact are put into practice. The simulation results of dynamic response are time-history of, distribution of impact forces,stresses, strains,displacements,velocities,accelerations etc, which agree well with results from experiment. Then more simulation analysises are done to find the influence infactors and relative curves.The factors simulated are impact energy, confine coefficient, boundary condition and axial compression ratio. Some relative curves are illustrated, among which Ec r?ξcurves are more valuable. The regression formulas are Ec r= 38.8ξ2+ 12.5ξ+ 4.6(KJ)for pinned-pinned members and Ec r= 78ξ2? 20ξ+ 11(KJ) for pinned-fixed ones respectively.3.Theoretical analysisTheoretical model for fixed-fixed CFT member is built to explore the response of transverse impact CFT without axial preloaded forces. Some hypothesis are made in the model.The CFT is assumed as a straight bar with constant cross-sectional area in geometry. It has also the same material property longitudinally. Interactions between steel tubes and inside concretes are neglected and CFT is made of single material. The rigid striker and CFT are seen as a contact-impact system during whole impact progress. One static plastic hinge forms at impact point and other 2 plastic hinges appear at position ofξ0,which is a initial distance to impact point symmetrically along the bar and move toward ends in opposite diretions, before they get in left-side and right-side ends. The profiles of lateral velocities are regarded as linearly distributions.Three formula groups are set up to determine the dynamic reponses such as the values of Y-displacements, Y-velocities, shearing forces, axial forces and bend moments. Each of formula group is composed of yield formulations including axial force, shear force and bend moment, equivalent formulations and geometrical formulations. The complete solutions to impact response at any time are obtained by means of Runge-Kutta method programmed by Fortran77. The theoretical results have good accordance with experimental data.