Shear and Torsion Interaction in Prestressed Hollow Core Slabs

Licentiate thesis, 2005

There are many applications in which hollow core units are subjected to combined torsion and shear, for example in floors supported on three edges and in floors with openings. The traditional calculation method for combined shear and torsion in hollow-core units adds stresses from various sources without taking into account deformations and compatibility within the unit or restraint at the boundaries. In this work, methods using advanced non-linear finite element analyses were developed and used to investigate the response of hollow core units and floors subjected to combined shear and torsion. Apart from improving the knowledge and understanding of shear and torsion interaction, the aim is to improve the traditional design method. Three-dimensional finite element models were developed, and verified by full-scale tests for hollow core units and whole floors. The models were simplified to avoid time-consuming analyses. For individual hollow core units, solid elements were used only where the failure was expected and beam elements elsewhere. The model was loaded with selected, and easily varied, combinations of shear and torsion, and the resulting capacities are presented in shear and torsion interaction diagrams. In a global model developed for whole floors, the cross-section of each hollow core unit is represented by one beam element and the blocking effect between neighbouring slabs in torsion is included. This method for floors can be used to calculate the sectional forces in units in floors with arbitrary geometries and loadings, while the modelling of a hollow core unit gives the shear?torsion resistance. The design approach was improved in steps, by combining the proposed modelling methods with each other and with the traditional analytical method. Comparison of results from tests and finite element analyses showed a good agreement, both for floors, and for hollow core units. The finite element analyses of hollow core units revealed a non-linear interaction between shear?torsion capacities instead of the linear one given by the traditional calculation methods. Compared with the traditional design approach, the modelling method for units resulted in higher capacities for most ratios of shear and torsion, and the floor model showed reduced torsional moments. Hence, much can be gained by using these methods instead of the traditional design methods