Table 21.2.1—Strength reduction factors ϕ
|Action or structural element||ϕ||Exceptions|
|(a)||Moment, axial force, or combined moment and axial force||0.65 to 0.90 in accordance with 21.2.2||Near ends of pretensioned members where strands are not fully developed, ϕ shall be in accordance with 21.2.3.|
|(b)||Shear||0.75||Additional requirements are given in 21.2.4 for structures designed to resist earthquake effects.|
|(e)||Post-tensioned anchorage zones||0.85||—|
|(f)||Brackets and corbels||0.75||—|
|(g)||Struts, ties, nodal zones, and bearing areas designed in accordance with strut-and-tie method in Chapter 23||0.75||—|
|(h)||Components of connections of precast members controlled by yielding of steel elements in tension||0.90||—|
|(i)||Plain concrete elements||0.60||—|
|(j)||Anchors in concrete elements||0.45 to 0.75 in accordance with Chapter 17||—|
Table 21.2.2—Strength reduction factor ϕ for moment, axial force, or combined moment and axial force
|Net tensile strain εt||Classification||ϕ|
|Type of transverse reinforcement|
|Spirals conforming to 25.7.3||Other|
|εt ≤ εty||Compression-controlled||0.75||(a)||0.65||(b)|
|εty < εt < 0.005||Transition1.2||(c)||(d)|
|εt ≥ 0.005||Tension-controlled3||0.90||(e)||0.90||(f)|
- For sections classified as transition, it shall be permitted to use Φ corresponding to compression-controlled sections.
- εt* is the greater of net tensile strain calculated for Pn = 0.1Agf'c and 0.005.
- For sections with factored axial compression force Pu ≥ 0.1Ag f'c, Φ shall be calculated using equation (c) or (d) for sections classified as transition, as applicable.
Table 21.2.3—Strength reduction factor ϕ for sections near the end of pretensioned members
|Condition near end of member||Stress in concrete under service load||Distance from end of member to section under consideration||ϕ|
|All strands bonded||Not applicable||≤ ℓtr||0.75||(a)|
|ℓtr to ℓd||Linear interpolation from 0.75 to 0.90||(b)|
|One or more strands debonded||No tension calculated||≤ (ℓdb+ ℓtr)||0.75||(c)|
|(ℓdb+ ℓtr) to (ℓdb+ ℓd)||Linear interpolation from 0.75 to 0.90||(d)|
|Tension calculated||≤ (ℓdb+ ℓtr)||0.75||(e)|
|(ℓdb+ ℓtr) to (ℓdb+ 2ℓd)||Linear interpolation from 0.75 to 0.90||(f)|
Stress calculated using gross cross-sectional properties in extreme concrete fiber of precompressed tension zone under service loads after allowance for all prestress losses at section under consideration.
It shall be permitted to use a strength reduction factor of 0.75.