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24.5 Permissible Stresses in Prestressed Concrete Flexural Members
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For calculation of stresses at transfer of prestress, at service loads, and at cracking loads, elastic theory shall be used with assumptions (a) and (b):
(a) Strains vary linearly with distance from neutral axis in accordance with 22.2.1.
(b) At cracked sections, concrete resists no tension.
Prestressed flexural members shall be classified as Class U, T, or C in accordance with Table 24.5.2.1, based on the extreme fiber stress in tension ft in the precompressed tension zone calculated at service loads assuming an uncracked section.
Table 24.5.2.1—Classification of prestressed flexural members based on ft
| Assumed behavior | Class | Limits of ft |
|---|---|---|
| Uncracked | U[1] | |
| Transition between uncracked and cracked | T | |
| Cracked | C |
[1]Prestressed two-way slabs shall be designed as Class U with .
For Class U and T members, stresses at service loads shall be permitted to be calculated using the uncracked section.
For Class C members, stresses at service loads shall be calculated using the cracked transformed section.
Calculated extreme concrete fiber stress in compression immediately after transfer of prestress, but before time-dependent prestress losses, shall not exceed the limits in Table 24.5.3.1.
Table 24.5.3.1—Concrete compressive stress limits immediately after transfer of prestress
| Location | Concrete compressive stress limits |
|---|---|
| End of simply-supported members | 0.70fci' |
| All other locations | 0.60fci' |
Calculated extreme concrete fiber stress in tension immediately after transfer of prestress, but before time-dependent prestress losses, shall not exceed the limits in Table 24.5.3.2, unless permitted by 24.5.3.2.1.
Table 24.5.3.2—Concrete tensile stress limits immediately after transfer of prestress, without additional bonded reinforcement in tension zone
| Location | Concrete tensile stress limits |
|---|---|
| Ends of simply-supported members | |
| All other locations |
The limits in Table 24.5.3.2 shall be permitted to be exceeded where additional bonded reinforcement in the tension zone resists the total tensile force in the concrete calculated with the assumption of an uncracked section.
For Class U and T members, the calculated extreme concrete fiber stress in compression at service loads, after allowance for all prestress losses, shall not exceed the limits in Table 24.5.4.1.
Table 24.5.4.1—Concrete compressive stress limits at service loads
| Load condition | Concrete compressive stress limits |
|---|---|
| Prestress plus sustained load | 0.45fc' |
| Prestress plus total load | 0.60fc' |
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