Post-Tensioned Concrete Fundamentals

Suncamcom copyright2010johnpmiller page27of49 ftk

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: ncrete strain, concrete creep, concrete shrinkage, and tendon relaxation in normal weight concrete was assumed to be 25,000 psi for posttensioned concrete. This did not include friction and tendon seating losses. This was a generalized method and it was subsequently discovered that this method could not cover all situations adequately. Since the 1983 ACI 318, each type of pre-stress loss must be calculated separately. The effective pre-stress force, that is the force in the tendon after all losses, is given on the design documents and it is customary for the tendon supplier to calculate all pre-stress losses so that the number of tendons can be www.SunCam.com Copyright 2010 John P. Miller Page 27 of 49 ft‐k Fundamentals of Post‐Tensioned Concrete Design for Buildings – Part One A SunCam online continuing education course determined to satisfy the given effective pre-stress force. Even though most design offices do not need to calculate pre-stress losses, it is informative and relevant to understand how losses are calculated. Therefore, each type of pre-stress loss is discussed in more detail below. Seating Loss When an unbonded tendon is tensioned, or stretched, to its full value, the jack releases the tendon and its force is then transferred to the anchorage hardware, and thereby into the concrete member. The anchorage hardware tends to deform slightly, which allows the tendon to relax slightly. The friction wedges deform slightly, allowing the tendon to slip slightly before the wires are firmly gripped. Minimizing the wedge seating loss is a function of the skill of the operator. The average slippage for wedge type anchors is approximately 0.1 inches. There are various types of anchorage devices and methods, so the calculation of seating losses is dependent on the particular system used. Friction and Wobble Loss of pre-stress force occurs in tendons due to friction that is present between the tendon and its surrounding sheathing material as it is tensioned, or stretched. Friction also occurs at the anchoring hardware where the tendon passes through. This is small, however, in comparison to the friction between the tendon and the sheathing (or duct) throughout its length. This friction can be thought of as two parts; the length effect and the curvature effect. The length effect is the amount of friction that would occur in a straight tendon - that is the amount of friction between the tendon and its surrounding material. In reality, a tendon cannot be perfectly straight and so there will be slight "wobbles" throughout its length. This so called wobble effect is rather small compared to the curvature effect. The amount of loss due to the wobble effect depends mainly on the coefficient of friction between the contact materials, the amount of care and accuracy used in physically laying out and securing the tendon against displacement, and the length of the tendon. The loss in the pre-stressing tendons due to the curvature effect is a result of the friction between the tendon and its surrounding material as it passes though an intentional curve, such as drape, or a change in direction, such as a harped tendon. The amount of loss due to the curvature effect depends on the coefficient of friction between the contact materials, the length of the tendon, and the pressure exerted by the tendon on its surrounding material as it passes through a change in direction. www.SunCam.com Copyright 2010 John P. Miller Page 28 of 49 Fundamentals of Post‐Tensioned Concrete Design for Buildings – Part One A SunCam online continuing education course Elastic Concrete Strain When a concrete member is subjected to a compressive force due to pre-stressing tendons, it will shorten elastically. If this compressive force were removed, the member would return to its original length. Although ACI 318 does not specifically give procedures or requirements for calculating losses due to elastic strain, there are references available that provide some guidance. In general, the elastic strain shortening is a simplified computation involving the average net compressive stress in the concrete due to pre-stressing and the moduli of elasticity of the pre-stressing steel and the concrete at the time of stressing. Concrete Creep A well known phenomenon of concrete in compression is that it creeps, or shortens, over time. The creep rate diminishes over time. Although ACI 318 does not specifically give procedures or requirements for calculating losses due to creep, there are references available that provide some guidance. In general, the creep shortening is a simplified computation involving the average net compressive stress in the concrete due to pre-stressing and the moduli of elasticity of the pre-stressing steel and the 28-day concrete strength. For post-tensioned members with unbonded tendons, for example, creep strain amounts to approximately 1.6 times the elastic strain. Concrete Shrinkage The hardening of concrete involves a chemical reaction called hydration between water and cement. The amount of water used in a batch of concrete to make it workable far exceeds the amount of water necessary for the c...
View Full Document

This document was uploaded on 01/28/2014.

Ask a homework question - tutors are online