A Study On The Behavior Of Concrete-Filled Steel Tubular Members Under Initial Stresses

The application of concrete-filled steel tubular structures has become widespread in recent decades. In practical construction of multi-storied buildings and skyscrapers, the hollow steel tubes are usually fixed first with steel beams, bracing structures and so on, and then the concrete is pumped into the hollow steel tubes after establishing several or even dozens of stories. Therefore, before the concrete has been solidified and could carry load together with the steel tubes, the steel tubes usually are pre-subjected to some load, including the gravity of core-concrete, working load, etc., which produce initial stresses and strains in steel tubes. There have been some reports based on tests and theoretical analyses, which address the importance of considering the influence of the initial stresses. In this thesis, a numerical investigation has been presented on the effect of initial stresses on the behaviors of concrete-filled steel tubes, adopting the general finite element nonlinear analysis package ABAQUS. It mainly analyses how the initial stresses influence the load-carrying capacity of the concrete-filled square steel tubular members, including axially loaded short columns, axially loaded long columns, beam-columns, etc. Some parameters are studied such as initial stress ratio, slenderness ratio, eccentricity ratio, etc. At last, the numerical results from finite element analysis are summarized by adopting the package MATLAB, some simplified formulae are primarily deduced, and some recommendations on how to consider the effect of initial stresses in design formula are also provided. In addition, in order to verify the reliability, results are compared with the existing experimental data, and it shows that they are consistent very well. This investigation shows that the effect of initial stresses makes the concrete contribute less to the load-carrying capacity, the deformations and deflections become larger, and the load-carrying capacities reduce. Of all the parameters, the intial stress ratio, the slenderness ratio and eccentricity ratio have more important influence on the effect of intial stresses. The larger the intial stress ratio and slenderness ratio, the larger the effect of the initial stresses; when the slenderness ratio and eccentricity ratio are not very large, the larger the eccentricity ratio, the smaller effect of the initial stresses; while the slenderness ratio or initial stress ratio go beyond some range, the larger the eccentricity ratio, maybe the larger effect of the initial stresses. All of these conclusions could be references for engineers in practical construction.