Realistic modeling of composite and reinforced concrete floor slabs under extreme loading. I: Analytical method

This paper presents a new flat shell element for composite and reinforced concrete (RC) floor slabs subject to extreme loading conditions, accounting for the effects of geometric as well as material nonlinearities. A novel feature of the proposed element is its treatment of problems associated with the geometric orthotropy of composite floor slabs, achieved through a modification of the conventional Reissner-Mindlin hypothesis. The new element is formulated in a local corotational framework, enabling the use of linear strain-displacement relationships, with the influence of geometric nonlinearity addressed through transformations between the local and global systems. In addition, a robust nonlinear material model is proposed for concrete which captures the salient response characteristics under extreme loading conditions, including the effects of elevated temperature due to fire. The proposed element is implemented within the nonlinear structural analysis program ADAPTIC, which is used in this paper to provide several verification examples, focusing principally on the significance of the assumptions made in the element formulation. Extensive verification against experiments on composite and RC floor slabs is undertaken in the companion paper, where favorable comparisons between the predictions of the proposed method and experimental results are generally achieved.