Building Design and Construction

Design Procedure for Prestressed-Concrete Beams

Beam design involves choice of shape and dimensions of the concrete member, positioning of the tendons, and selection of amount of prestress.
After a concrete shape and dimensions have been assumed, determine the geometrical properties—cross-sectional area, center of gravity, distances of kern and extreme surface from the centroid, moment of inertia, section moduli, and dead load of the member per unit length.
Treat the prestressing force as a system of external forces acting on the concrete.
Compute bending stresses due to service dead and live loads. From these, determine the magnitude and location of the prestressing force required at sections subject to maximum moment. The prestressing force must result in sufficient compressive stress in the concrete to offset the tensile stresses caused by the bending moments due to dead and live service loads (Fig. 9.61). But at the same time, the prestress must not create allowable stresses that exceed those listed in Table 9.28. Investigation of other sections will guide selection of tendons to be used and determine their position and profile in the beam.
After establishing the tendon profile, prestressing forces, and tendon areas, check stresses at critical points along the beam immediately after transfer, but before losses. Using strength-design methods (Art. 9.108), check the percentage of steel and the strength of the member in flexure and shear.
Design anchorages, if required, and shear reinforcement.
Finally, check the deflection and camber under service loads. The modulus of elasticity of high-strength prestressing steel should not be assumed equal to 29,000,000 psi, as for non-prestressed reinforcement, but should be determined by test or obtained from the manufacturer.

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