At service loads and up to cracking loads, straight-line theory may be used for computing stresses in prestressed beams with the following assumptions:
Strains vary linearly with depth through the entire load range.
At cracked sections, the concrete does not resist tension.
Areas of unbonded open ducts should not be considered in computing section properties.
The transformed area of bonded tendons and non-prestressed reinforcing steel may be included in pretensioned members and, after the tendons have been bonded by grouting, in posttensioned members.
Flexural stresses must be limited to ensure proper behavior at service loads.
Limiting these stresses, however, does not ensure adequate design strength.
In establishing permissible flexural stresses, the ACI 318 Building Code recognizes two service-load conditions, that before and that after prestress losses.
Higher stresses are permitted for the initial state (temporary stresses) than for loadings applied after the losses have occurred.
Permissible stresses in the concrete for the initial load condition are specified as a percentage of Æ’ , the compressive strength of the concrete, psi, at time of initial ci prestress. This strength is used as a base instead of the usual , 28-day strength Æ’c of concrete, because prestress is usually applied only a few days after concrete has been cast. The allowable stresses for prestressed concrete, as given in ACI 318-99, are tabulated in Table 9.28.
Bearing Stresses. Determination of bearing stresses at end regions around posttensioning anchorages is complicated, because of the elastic and inelastic behavior of the concrete and because the dimensions involved preclude simple analysis under the St. Venant theory of linear stress distribution of concentrated loads. The ACI 318 Building Code formula for bearing stresses [Eq. (9.89)] does not apply to posttensioning anchorages.
Lateral reinforcement may be required in anchorage zones to resist bursting, horizontal splitting, and spalling forces. Expanded design requirements for posttensioned tendon anchorage zones were introduced into the ACI 318-99 Building Code. The Codes design requirements are compatible with comprehensive provisions adopted previously in the AASHTO Standard Specifications for Highway Bridges, American Association of State Highway and Transportation Officials.
