Partially prestressed concrete internal square panel
This work is concerned with the general behaviour of an internal panel in a column supported multi-bay floor structure, under the influence of various prestressing tendon arrangements. The prestressing effects are studied with the help of a finite element package (PAFEC), representing the prestressing forces by equivalent vertical loads. From a variety of possible spacing arrangements, the distribution of prestressing moments is shown to be most effective in counteracting the moments due to externally imposed load when the tendons are narrowly banded in the column vicinity. The effects of tendon arrangement on slab behaviour are further investigated by means of an experimental programme comprising three series of ten tests of 1.5m span partially prestressed panel with typical span to depth ratio, levels of loading, levels of prestress and geometry of tendon profiles. The models were fixed along the edges to simulate the continuity of an internal panel in a multi-panel slab system. Post-tensioned tendons were arranged in various patterns, as currently employed in construction practice, to induce either a medium or a low level of prestress in the test panel, conforming to the design recommendations of a number of codes of practice. Test results indicated that the serviceability behaviour of the slabs with a low level of prestress was strongly influenced by the tendon distribution and the amount of non-prestressed reinforcement. Slabs with tendons banded closely in the column area exhibited a higher cracking load and were stiffer after cracking. The flexural strength of the test slabs was found to be greater than that predicted by yield line theory and the increase was attributed to membrane action. The experimental values of tendon stress at ultimate load and of punching shear were compared with those obtained by various design methods.