ABSTRACT
Post-tensioning is a form of prestressing of concrete members to increase its tensile load capacity. The process includes (i) installing tendons inside plastic or steel ducts embedded in concrete, (ii) pulling tendons from one or both ends up to a predefined force after concrete attains a specified strength for the operation, and (c) anchoring the ends to maintain the required force in the tendons. Ducts are typically grouted to enhance durability and establish strain compatibility between tendons and the surrounding concrete. Due to wobbling effect and friction, the force along the tendon decreases during the jacking operation. Further, the force in the tendon is slightly reduced during anchoring. Different modeling techniques are used to represent the stress state in post-tensioned concrete members. One such technique is to embed the tendons in 2D or 3D structural elements representing the concrete member by excluding the influence of the ducts. This modeling techniques utilizes the gross cross-section area of the concrete member to calculate stresses. This can be a concern when empty ducts are present during multi-stage post-tensioning. A common technique implemented in commercial software is to model the concrete member using 1D line elements with the post-tensioning force applied as member loads. As a more precise technique, a refined 3D finite element model can be used to represent the geometry with empty ducts and the sequence of post-tensioning and grouting. However, this technique also presents several challenges including the simulation of tendon force variation along its length. Strain measurements recorded during post-tensioning of an instrumented tie girder of a tied network arch bridge are used to evaluate the impact of different modeling techniques on girder stresses. The paper describes different post-tension modeling techniques and their capabilities and limitations to represent the stress state of concrete members.
