Nonlinear analysis of composite sections under the effect of high temperatures

The arm of this research is to investigate the behaviour of compositeframes under the effect of high temperatures. A nonlinear finite elementcomputer program for the thermal analysis of composite frames underdifferent fire conditions is developed. The structural steel cross sections,columns reinforcing steel, slab reinforcing steel, and stirrups wererepresented by triangular elements to increase the number of the elements insmall areas, while the concrete areas were represented by rectangularelements.The .analysis procedure goes through three main phases. In the firstphase, the temperature distribution on the cross sections and then along theframe is obtained. Two dimensional heat flow analysis within a compositesection is carried out. The section is composed of steel reinforcing bars andstructural steel profiles embedded in concrete, and subjected to differentthermal boundary conditions.In the second phase, the minimum potential energy method and thelayered method are used for converting the obtained temperature distributioninto an equivalent thermal concentrated nodal forces. These thermal nodalforces are added to the normal case of loading in order to obtain the globalload vector.