Building Design and Construction Handbook - Merritt - Ventech!

9.146 SECTION NINE
FIGURE 9.61 Prestressed-concrete beam: (a) with straight tendons; (b)
with curved tendons; (c) midspan stresses with straight or curved tendons;
(d) stresses between midspan and supports with curved tendons.
Net stresses near the supports become tensile with straight tendons.
usually gives satisfactory results. Losses greater or smaller than these values have
little effect on the design strength but can affect service-load behavior, such as
cracking load, deflection, and camber.
Elastic Shortening of Concrete. In pretensioned members, when the tendons are
released from fixed abutments and the steel stress is transferred to the concrete by
bond, the concrete shortens under the compressive stress. The decrease in unit stress
in the tendons equals P sE s/A cE c � nƒ c, where E s is the modulus of elasticity of
the steel, psi; E c the modulus of elasticity of the concrete psi; n the modular ratio,
E s/E c;ƒ c the unit stress in the concrete, psi; P s the prestressing force applied by
the tendons; and A c the cross-sectional area of the member.
In posttensioned members, the loss due to elastic shortening can be eliminated
by using the members as a reaction in tensioning the tendons.
Frictional Losses. In posttensioned members, there may be a loss of prestress
where curved tendons rub against their enclosure. The loss may be computed in
terms of a curvature-friction coefficient �. Losses due to unintentional misalignment