Effect of preload on the axial capacity of concrete-filled composite columns

Concrete-filled tubes are often preferred for the construction of high-rise buildings because of their high strength and stiffness compared to conventional reinforced concrete or steel columns. However, prior to infilling of concrete, the steel tubes are subjected to preloads from upper floors arising from construction loads and permanent loads of the building. These preloads cause initial stresses and deformations in the steel tubes which would affect the load carrying capacity of the composite columns. In this paper, a design method based on a modified Eurocode 4 approach, incorporating the effect of preload, is proposed to evaluate the axial capacity of concrete filled composite columns. Eight full-scale composite column specimens were tested, Parameters studied included the strength of the concrete infill, slenderness of the columns and the amount of preload applied on the steel tubes. Results obtained from the proposed method are compared against test results and other published data. Comparison studies show that the test results are on average 3% higher than predicted results with a standard deviation of 0.089. Finite element analyses are also performed for systematic verification, and the results are 8% higher than predicted results. It is conclude that the proposed design method is accurate and mostly conservative and can be readily used in the context of Eurocode 4: Part 1.1 for designing composite columns. (C) 2009 Published by Elsevier Ltd