ABSTRACT
Reinforced concrete structures must be designed to withstand extreme-case scenarios such as fires. Structural engineers are interested to analyze the behavior of reinforced concrete structures subjected to a combination of mechanical loads and elevated temperatures. In the present study, an engineering mechanics approach is used to describe the structural behavior of a segment of a subway station subjected to regular service loads and a moderate fire. This approach combines fundamental concepts of thermo-elasto-mechanics with beam analysis software. The three-dimensional reinforced concrete structure is idealized as a frame consisting of straight beams. The rectangular columns are transformed into cylindrical ones with equivalent extensional stiffness. The obtained temperature changes of the structural elements are used to quantify thermal eigenstrains. The latter are decomposed into three parts: an eigenstretch and an eigencurvature of the axis of the structural element, as well as an eigenwarping of its cross-sections. Beam analysis software is used to study the load carrying behavior of the frame structure subjected to mechanical loads as well as to thermal eigenstretches and eigencurvatures of all structural elements. The obtained axial forces and bending moments result in axial stresses which are linear across the cross-sections. The latter remain plane even under combined mechanical and thermal loading. This activates self-equilibrated thermal eigenstresses which are spatially nonlinear across the cross-sections. Total axial stresses are obtained from adding the thermal eigenstresses to the axial stresses quantified based on the axial forces and bending moments. The total stresses agree well with the results of a three-dimensional thermo-elastic Finite Element simulation. It is concluded that the subdivision of the developed engineering mechanics approach into a sequence of several smaller problems allows for relating causes to effects in a clear and insightful fashion, such as appreciated by structural engineers.
