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Received Oct 4, 2017; Accepted Dec 5, 2017
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1. Introduction
Pretensioning techniques are widely used for beams and slabs in concrete structures due to their ability to efficiently compensate for the reduced tensile strength of concrete through precompression. The improvement in strength achieved by pretensioning enables the use of thinner sections and provides an efficient solution for controlling material splitting and avoiding excessive deflections. For materials that have a tensile strength similar to or higher than their compressive strength, such as timber or steel, pretensioning offers significantly reduced advantages. Moreover, timber has problems with its long-term behavior; its inherent creep deflection can reduce the pretensioning effects over time, decreasing its capabilities. Therefore, the pretensioning technique is rarely used for timber.
Defect-free timber has a higher tensile strength than compressive strength. However, the inherent defects (cracks, knots, grain deviations, etc.) that are present in structural timber reduce its tensile strength. Furthermore, typical bending failure is reached instantaneously after a brittle fracture is caused by tension in the fibers. This phenomenon has motivated research on different reinforcement solutions to improve the bending behavior of timber.
The reinforcement systems that have been used can be divided in two basic typologies: passive and active reinforcements. Passive reinforcement is made of metallic elements [1–3] or fiber-reinforced polymers (FRPs) [3–13] that are glued to the timber with structural adhesives. Active reinforcement can be made with unbonded tendons or with bonded tendons that are glued to the timber with adhesives. Active reinforcement has been used to both reinforce frame connections [14, 15] and improve the behavior of beams [16–25].
Some authors [20, 24] have studied the loss of prestressing force for LVL and glulam beams prestressed using unbonded tendons. They found a reduction in prestress from 1.4% to 10% for beams loaded parallel to the grain exposed to controlled and uncontrolled environmental conditions. They concluded that controlling the relative humidity would reduce the losses. The use of bonded reinforcements could contribute to reduce the creep deformations in wood members [5].
This paper focuses on using active reinforcement with unbonded tendons. Therefore, an...