Abstract
In this paper, experimental work was conducted to evaluate the losses in prestressing force of 13 (12 perforated and 1 solid) simply supported prestressed concrete rafters. All beams had the same dimensions and reinforcements. The tested beams were divided into four main groups and three additional subgroups were driven. These groups were classified according to size, number, and configuration of the openings, and the orientation of the posts (vertical or inclined). Regarding the prestress losses that have been affected by the cross-section properties, the provision of the codes is applicable only to prismatic solid beams, so non-prismatic or moreover perforated beams also need to be studied. This paper aims to propose a method based on the same code provisions but taking into consideration the cross-section variation along the beam length. The proposed method divides the overall length of the rafter into a number of assumed prismatic segments with heights measured at centers. Then, the overall prestress loss is found as the sum of these segments weighed by the ratios of the length of each beam segment to the overall length. The experimental results of the proposed method ranged from 84.749% to 95.607% denoting its validity.
Highlights
The stresses in the tendons of prestressed concrete members are decreasing with time, but at a decreasing rate, and asymptotically level off after a long time
This study has proposed a method to estimate prestress losses in rafter beams, which are divided into a number of segments that are assumed to be prismatic along their lengths with heights measured at centers
It was observed that the losses ranged from 17.465% to 20.309% of the initial prestress depending on size, number of openings, posts inclination, and openings configuration
Summary
The stresses in the tendons of prestressed concrete members are decreasing with time, but at a decreasing rate, and asymptotically level off after a long time. Time-dependent losses such as creep, shrinkage, and those due to temperature effects and steel relaxation, all of which are determinable at the service-load limit state of stress in the prestressed concrete element. The fact that concrete is not efficient in resisting tensile stress makes very difficult reaching a long span beam in design, so the addition of prestressing reinforcements has become necessary to reach span lengths which cannot be reached by using ordinary reinforcements [1213]. To estimate losses such as elastic shortening, creep, shrinkage, and relaxation in beams, the empirical methods recommended by the codes are only applicable to prismatic beams. This study has proposed a method to estimate prestress losses in rafter beams, which are divided into a number of segments that are assumed to be prismatic along their lengths with heights measured at centers
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