Nonlinear Analyses for Thermal Cracking in the Design of Concrete Structures

A Design by Analysis procedure has been developed to incorporate the results of thermal cracking analyses into the linear based design methods for reinforced concrete containment structures. Current practice employs linear based analyses and accounts for stress reduction due to thermal induced cracking on a section by section basis. Under thermal loading, in addition to a reduction of the section forces and moments for cracked sections, concrete cracking also reduces the structural stiffness and thus the constraint against thermal expansion or contraction for the whole structure, which in turn reduces the thermal induced stresses as compared to a linear (un-cracked) analysis. The design by analysis approach employs nonlinear thermal cracking analyses using detailed modeling of the complete structure. By comparing these stress distributions with those from a linear analysis of the same model, stress reduction factors can be developed for critical sections that also include global stress redistribution. These stress reduction factors can then be used to correct linear based thermal stresses in the design basis calculation for all load combinations that include thermal loads.