Seismic Performance of Stairs in the Existing Reinforced Concrete Building

Abstract eng:
This paper deals with the seismic performance of existing buildings and in particular on the moment resisting frame structures that could have their critical and weak points in the stair members: columns and beams or slabs. The stair increases structural strength and stiffness of a structure but attracting seismic forces it could fail into its short columns or into the slabs due to high shear forces, into inclined beams supporting the steps a cause of high axial forces. The structural solutions and design practice of stairs in gravity load designed structures are investigated to define their real geometric definition and to understand their performance. Some numerical modal linear and non linear push-over analyses are herein presented. A typical reinforced concrete building respecting the materials and design criteria of the time is considered for the analyses. In particular two types of stairs are considered: the one with cantilever steps constrained in inclined beams, and the stair composed of simply supported slabs. The modal analysis emphasizes the different modal behavior considering the stairs. A non linear lumped plasticity models allow to perform non linear pushover analysis that allow to identify the main failure mechanisms. Some numerical simulations give some interesting results and offer some good features on the problems related to the mechanical and geometrical modeling of the structural elements of the stair, and to the principle types of failure due to flexure, or shear.

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