Part 1: General

Part 2: Loads & Analysis

Part 3: Members

Part 4: Joints/Connections/Anchors

Part 5: Earthquake Resistance

Part 6: Materials & Durability

Part 7: Strength & Serviceability

Part 8: Reinforcement

Part 9: Construction

Part 10: Evaluation

REFERENCES & Appendices

Heads up: There are no amended sections in this chapter.
This chapter shall apply to the selection of strength reduction factors used in design, except as permitted by Chapter 27.
Strength reduction factors ϕ shall be in accordance with Table 21.2.1, except as modified by 21.2.2, 21.2.3, and 21.2.4.
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.
(c) Torsion 0.75
(d) Bearing 0.65
(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
Strength reduction factor for moment, axial force, or combined moment and axial force shall be in accordance with Table 21.2.2.
Table 21.2.2—Strength reduction factor ϕ for moment, axial force, or combined moment and axial force
Net tensile stain 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 < εty + 0.003 Transition[1] (c) (d)
εt ≥ εty + 0.003 Tension-controlled 0.90 (e) 0.90 (f)
[1]For sections classified as transition, it shall be permitted to use ϕ corresponding to compression-controlled sections.
For deformed reinforcement, εty shall be fy/Es. For Grade 60 deformed reinforcement, it shall be permitted to take εty equal to 0.002.
For all prestressed reinforcement, εty shall be taken as 0.002.
For sections in pretensioned flexural members where all strands are not fully developed, ϕ for moment shall be calculated in accordance with Table 21.2.3, where tr is calculated using Eq. (21.2.3), ϕp is the value of ϕ determined in accordance with Table 21.2.2 at the cross section closest to the end of member where all strands are developed, and d is given in 25.4.8.1.
(21.2.3)
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[1] Distance from end of member to section under consideration ϕ
All strands bonded Not applicable tr 0.75 (a)
tr to d Linear interpolation from 0.75 to ϕp[2] (b)
One or more strands debonded No tension calculated ≤ (db + tr) 0.75 (c)
(db + tr) to (db + d) Linear interpolation from 0.75 to ϕp[2] (d)
Tension calculated ≤ (db + tr) 0.75 (e)
(db + tr) to (db + 2d) Linear interpolation from 0.75 to ϕp[2] (f)
[1]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.
[2]It shall be permitted to use a strength reduction factor of 0.75.
For structures that rely on elements in (a), (b), or (c) to resist earthquake effects E, the value of ϕ for shear shall be modified in accordance with 21.2.4.1 through 21.2.4.4:
(c) Intermediate precast structural walls in structures assigned to Seismic Design Category D, E, or F
For any member designed to resist E, ϕ for shear shall be 0.60 if the nominal shear strength of the member is less than the shear corresponding to the development of the nominal moment strength of the member. The nominal moment strength shall be the maximum value calculated considering factored axial loads from load combinations that include E.
For diaphragms, ϕ for shear shall not exceed the least value of ϕ for shear used for the vertical components of the primary seismic-force-resisting system.
For foundation elements supporting the primary seismic-force-resisting system, ϕ for shear shall not exceed the least value of ϕ for shear used for the vertical components of the primary seismic-force-resisting system.
For beam-column joints of special moment frames and diagonally reinforced coupling beams, ϕ for shear shall be 0.85.
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