Punching shear strength of unbonded prestressed flat slabs at edge column connections

This thesis presents an experimental and theoretical study of the punching shear strength of unbonded prestressed flat slabs at edge column junction. Six tests were conducted under monotonic loading and two under cyclic loading. The models were approximately 1000 x 1000 x 130 mm and prestressed in two directions by unbonded tendons. The main parameters of study were: a) Prestress level, b) Column aspect ratio, c) Wind shear moment/wind shear, (Mw/Vw). The following data were collected: i) Load-deflection relationship, ii) Strain development in reinforcing bars and at the compressive face of concrete iii) Variation of force in prestressing bars. For design of the models, the slab element was regarded as consisting of two outer layers separated by an unreinforced concrete filling. The slab was analysed as an elastic plate subjected to ultimate design loads. The moments (Mw, My, Mxy) were replaced by two equal and opposite forces acting at the mid- depth of the outer layers. These forces were combined with inplane forces (Nx, Ny, Nxy) divided equally between the two layers. Each layer was considered as a membrane element and designed using Direct Design Method using Nielsen's yield criterion. A 3-dimensional nonlinear finite element program based on 20 node isoparametric brick element was developed. The constitutive laws of concrete proposed by Kotsovos and embedded representation of unstressed steel were adopted. Shear transfer across crack was allowed. Forces due to prestressing were included using the concept of equivalent loads. Steel reinforcement including prestressing steel was modelled as elastic-perfectly plastic material, 'Smeared' cracking model was used. Finally, Comparative study was carried out between the experimental and the theoretical results for the models tested by the author and by other investigators. Comparison of the experimental and theoretical failure loads with that predicted by the BS8110 and the ACI codes were included. It was concluded that the finite element method can predict failure load and behaviour of the junctions satisfactorily.