Finite Element Analyses of Hollow Core Units Subjected to Shear and Torsion

Report, 2002

Precast prestressed hollow core units are among the most advanced and widely spread products in the precast industry. The present calculation method for shear and torsion in hollow core slabs adds stresses from various influences without considering deformations and compatibility, the softening of cracking concrete, or restraint at the boundaries, and is therefore most likely conservative. The aim of this work is to improve the current knowledge and understanding of shear and torsion interaction in hollow core floors. This report deals with finite element analyses of individual hollow core units, subjected to different combinations of shear and torsion. Most of these analyses concern full-scale tests on hollow core units that were planned in cooperation with VTT and Strängbetong, and conducted at VTT during June and August 2002. Prestressed hollow core units of two thicknesses, 200 mm and 400 mm, were tested both with and without eccentric loading. The analyses were made with various levels of detailing, using the finite element program DIANA 7.2. The aim was to use a modelling technique that does not result in too time consuming analyses, but still gives a reasonable good agreement. In the final analyses presented here, only the part of the slab closest to the load and the active support was modelled with solid elements. The rest of the slab was modelled with beam elements. The concrete was modelled using non-linear fracture mechanics in a smeared rotating crack model. In general the finite element analyses of the tests were able to capture the overall behaviour, failure mode, crack pattern, and maximum obtained load, with a reasonably good agreement, though a very coarse mesh were used in the analyses. Especially for the centrically loaded specimens, the agreement was good. However, for the eccentrically loaded hollow core units the maximum load was overestimated. The reason for this is most likely that the torsional stiffness of the beam elements used in the model was too high.