Interaction diagrams and failure criteria for RC columns subjected to high temperature

Purpose: Previous works in constructing interaction diagrams have only focused on incorporating transient creep strain implicitly in the ultimate limit strain. The present paper aims to use different approaches to define concrete ultimate limit strain (failure strain) envelops at high temperatures for preloaded and unloaded, confined and unconfined, columns during heating are proposed. These approaches are chosen to understand the effect of using different techniques to determine transient creep strain on the resulted N_u–M_u diagrams. Design/methodology/approach: Transient creep strain is included within the concrete ultimate limit strain relationships, implicitly and explicitly, by four different ways, and accordingly, four different failure criteria are suggested. To define the concrete ultimate limit strain, studies are conducted to evaluate the compression strain corresponding to the maximal flexural capacity at elevated temperatures. In the analysis, the thermal and structural analyses are decoupled and, based on the resulted ultimate limit strain, the N_u – M_u diagrams are constructed at different fire exposures. Findings: The validity of the proposed model is established by comparing its predictions with experimental results found in the literature. Finally, comparative calculations regarding interaction diagrams obtained by the proposed model and by other methods found in the literature are performed. It was found that the proposed model predictions agree well with experimental results. It was also found that the suggested approaches, which include simplifications, reasonably predicted the exact column capacity. Originality/value: The model.