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 prestress loss
must be calculated separately. The effective prestress 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 prestress 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 prestress force. Even though most design
offices do not need to calculate prestress losses, it is informative and relevant to
understand how losses are calculated. Therefore, each type of prestress 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 prestress 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 prestressing 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 prestressing
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 prestressing and the moduli of elasticity of the prestressing 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 prestressing and the moduli of elasticity of the prestressing steel and the 28day
concrete strength. For posttensioned 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...
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