Effect of crack formation process and tension stiffening on thermal stresses relaxation in reinforced concrete containments

Thermal stresses can be critical for the design of reinforced concrete thick containments; this can occur for consideration of steady state distributions of temperature across the section, in normal operating conditions, or of transient non-linear distributions in upset conditions. Serviceability criteria govern the design; nevertheless, such effects can frequently cause stresses above the permissible values established for normal structures, so that the assumption of a cracked reinforced concrete structure is taken into account; this considerably reduces the stiffness. In such an analysis, the most simple assumptions can lead to overestimate the stress relaxation. This can occur if the reduced stiffness of the cracked section is simply adopted for the stiffness of the members; such an assumption is safe for the calculation of maximum deflections under loads, but is unsafe for the calculation of the maxium stresses in the materials under thermal effects. In fact, crack formation is a gradual process, and not all theoretical possible cracks are formed under a given thermal action. (orig.)