Building Design and Construction Handbook - Merritt - Ventech!

9.148 SECTION NINE
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 ƒ� , 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
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 Code’s design requirements are compatible with comprehensive provi-
TABLE 9.28 Allowable Stresses for Prestressed Concrete
Concrete:
Temporary stresses after transfer of prestress but before prestress losses:
Compression 0.60ƒ�ci Tension in members without auxiliary reinforcement in tension zone,
except at ends of simply-supported members
3 �ƒ� *
ci
Tension at ends of simply-supported members 6 �ƒ� ci
Service-load stresses after prestress losses:
Compression for sustained service live load 0.45ƒ� c
Compression for transient or temporary service live load 0.60ƒ� c
Tension in precompressed tensile zone 6 �ƒ�†
c
Prestressing steel:
Due to jacking force 0.94ƒ py‡
Pretensioning tendons immediately after transfer 0.82ƒ py**
Posttensioning tendons immediately after anchoring 0.70ƒ pu
* Where the calculated tension stress exceeds this value, bonded reinforcement should be provided to
resist the total tension force on the concrete computed for assumption of an uncracked section.
† May be taken as for members, except two-way slab systems, for which computations based
12 �ƒ�c on the transformed cracked section and on bilinear moment-deflection relationships show that immediate
and long-term deflection do not exceed the limits given in Table 9.14.
‡ƒpy � specified yield strength of tendons but not greater than the lesser of 80% of the specified tensile
strength ƒpu and the maximum value recommended by the manufacturer of the tendons or anchorages.
** But not more than 0.74ƒpu. ƒ� c