Question

Which specific prestress loss occurs in a post-tensioned concrete member due to the instantaneous deformation of the anchorage device during tendon stressing?

Accepted Answer

The specific prestress loss that occurs in a post-tensioned concrete member due to the instantaneous deformation of the anchorage device during tendon stressing is called Anchorage Seating Loss, also commonly known as Anchorage Set Loss. This loss happens at the precise moment the prestressing force, which has been applied by a hydraulic jack to stretch the tendon, is transferred and locked into the concrete member by the anchorage device. A post-tensioned concrete member is a structural element where high-strength steel tendons (strands, wires, or bars) are placed within ducts in the concrete, and then stressed, meaning they are stretched under high tension, after the concrete has hardened. Tendon stressing is the process of applying this tensile force to the tendons. The anchorage device is the mechanical assembly, typically consisting of wedges that grip the tendon and a barrel or plate that bears against the concrete, designed to permanently hold the stretched tendon and transfer its force to the concrete. When the jack releases, the wedges undergo a small, instantaneous seating or biting action into the tendon, and the components of the anchorage device itself may experience slight elastic or plastic deformation. This combined seating and deformation causes a minute, yet measurable, retraction or pull-in of the tendon into the anchorage device. This slight shortening of the effective stretched length of the tendon reduces its total elongation. Since the prestressing force in the tendon is directly proportional to its elongation, this reduction in elongation directly results in a corresponding, instantaneous loss of prestressing force. The magnitude of anchorage seating loss is a specific value, typically a few millimeters, provided by anchorage manufacturers based on empirical data and is an essential consideration in the overall prestress design calculations.

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Which specific prestress loss occurs in a post-tensioned concrete member due to the instantaneous deformation of the anchorage device during tendon stressing?