Non-linear analysis of concrete-filled steel elements

Composite columns and beams are a combination of concrete and steel elements realising the advantages of both types of materials. According to codes for concrete-filled column, the plastic resistance of the cross-section is given as a sum of the components and taking into account the effect of confinement in the case of circular sections. In this study the stress state in composite columns is determined taking into account non-linear relationship of the modulus of elasticity and Poisson's ratio on the stress level in the concrete core. It is determined that the effect of confinement occurs at a high stress level when structural steel acts in tension and concrete works in lateral compression. The stress state and load bearing capacity of section in bending is determined taking into account nonlinear dependence on position of neutral axis. The ultimate limit state of material is not attained for all the parts simultaneously. In order to improve the working conditions of a composite element and to prevent the possibility of a failure because of a small thickness of structural steel and fire, the appropriate strength classes of concrete and steel should be used.