BEHAVIOUR OF PARTIALLY PRESTRESSED BEAMS

Abstract

Partial prestressed structures are those in which cracking is permitted under normal load. Hense limit state of cracking is a criteria to, be satisfied by these structures. Methods proposed for determining the crack width, on the basis of few experimental results available, are based on stress in reinforcement and fictitious tensile stress in concrete. More research is required so that better empirical expressions can be provided. This thesis presents the results of an investigation under taken (a) to provide a study of variation of crack width with load; (b) to study effect of amount of prestresses and type of reinforcement on crack width; (c) to compare available methods with experimental values and (d) to suggest modifications in IS:1343 code provisions on crack width. Experimental work consisted of casting and testing of six beams with three variation of prestress for both plain M.S. bar and HYSD bars as untensioned reinforcement. Several methods which are proposed for calculating crack width are discussed and compared with measured widths of cracks. Increase in amount of prestress and untensioned reinforcement is found to decrease the crack width IS: 1343-1980 provisions are found to be conservative for limiting crack widths, hence higher values of hypothetical stress could be permitted. New improved values of constants are suggested both for mild steel and for steel reinforcements to be used in maximum crack width formulae.