Analysis of Composite Structures

The main aspects that have to be considered in the analysis of composite structures are presented and discussed. Most of composite structures concern floors of buildings and bridges, so that in this lecture the attention is mainly devoted to composite beams. To correctly evaluate stresses in the concrete slab an effective width is considered to account for shear lag and moreover, to account for local buckling phenomena, the sections are divided in four classes according to the slenderness of the parts in compression. The moment of resistance of sections is calculated through plastic analysis in sections belonging to class 1 or 2, whereas the elastic analysis is used for sections in class 3 or 4. In case of plastic analysis a partial shear connection may be provided in sagging moment regions; in this case the section moment of resistance is governed by the resistance of the shear connection. In the elastic analysis of sections the effects of creep and shrinkage have to be considered because they cause significant redistribution of section stresses. At serviceability limit states, besides the check on the values of stresses, the deflection and the crack width need to be limited. The tension stiffening of concrete in tension among two consecutive cracks needs to be considered to evaluate correctly the deflection in continuous composite beams. The global analysis may be carried out using elastic analysis. At ultimate limit state some moment redistribution to account for the inelastic behavior of materials and for cracking has to be applied to optimize the sections. The limits of moment redistribution are related to the maximum rotation capacity of the sections subjected to maximum hogging moments. A broad numerical study was carried out by the author to determine the permissible moment redistribution domain that satisfies at the same time both the rotation compatibility and the crack width in service. Propped cantilevers and fixed-end beams with different types of class 1 sections were considered in the study. Moreover a low ductility reinforcing steel (elongation at maximum load 2.5%) was considered. The results of the study allowed to provide a proposal for new moment redistribution limits. A numerical procedure able to analyze the structural behavior of steel-concrete composite beams subjected to very heavy moving loads, considering the actual cyclic nonlinear relationship between the shear force and the slip of the connectors, was also developed by the author. The procedure allowed determining the oscillograms of the slip and the shear force of each connector corresponding to the transit of an assigned loading pattern. Fatigue checks, based on the strain-life approach can be carried out using such a numerical tool. The results of a numerical study of a 40 m span simply supported bridge-type beam subjected to very heavy low frequent vehicles allowed to assess the damage accumulated in the connectors due to one thousand transits of the considered load. Significant damage was found in the studs close to mid-span.