Design of waffle slabs using strut-and-tie model.

摘要

Multi storey buildings with waffle slabs as concrete floor and roof systems are very common in Saudi Arabia and in many parts of the world. Because of the complex geometry of intersecting ribs, the analysis and design of waffle slab is not easy. Current design methods neglect the existence of ribs and treat the slabs as solid slabs and are approximate in nature. Finite element based methods are also used for design of waffle slabs. Strut-and-tie model is being used increasingly for analysis and design of concrete structures with pronounced D-regions. With a grid of intersecting ribs, waffle slab essentially forms a D-region. This thesis presents a new approach based on strut-and-tie method for analysis and design of waffle slabs. A three dimensional strut-and-tie model is proposed for analysis and design of simply supported waffle slabs and to predict the strength and corresponding mode of failure. The result obtained from proposed method, generated in ANSYS software and analyzed using its nonlinear analysis features, is compared with experimental results from literature. The strut-and-tie model is in good agreement with experimental results for all configurations of simply supported waffle slabs, whether the mode of failure is flexural, shear or slip bond failure, and it can be employed for the design of waffle slabs. A Visual Basic based user friendly software "STWAF" has been developed for analysis of the 3D truss model using STAAD and for the design based on ACI-318-05 recommendations for waffle slabs using the proposed strut-and-tie method. Parametric studies conducted using STWAF shows that ACI code provisions for the rib dimensions and spacing of waffle slab is done in such a way that the slab fails under flexure in case of overloading for spans up to 12m.