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Notation
Ec = modulus of elasticity of concrete, psi
Es = modulus of elasticity of reinforcement, psi
fc'= specified compressive strength of concrete, psi
fy = specified yield strength for non-prestressed reinforcement, psi
εt = net tensile strain in the extreme layer of longitudinal tension reinforcement at nominal strength, excluding strains due to effective prestress, creep, shrinkage, and temperature
εty = value of net tensile strain in the extreme layer of longitudinal tension reinforcement used to define a compression-controlled section
ϕ = strength reduction factor
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Download ideCAD to define strength reduction factors according to ACI 318-19 |
Strength reduction factors ϕ used in the design are shown in ACI Table 21.2.1.
Section Design Force Classification | ϕ |
|---|---|
Moment, axial force, or combined moment and axial force | 0.65 to 0.90 |
Tension Controlled Moment and axial tension | 0.90 |
Compression Controlled Moment and axial compression | 0.65 |
Shear | 0.75 |
Shear Seismic | 0.60 |
Joint Shear | 0.85 |
Torsion | 0.75 |
Bearing | 0.65 |
Strength Reduction Factor for Combined Moment and Axial Force
Strength reduction factor ϕ shown in section (a) of Table 21.2.1, for a moment, axial force, or combined moment and axial force, are given in ACI Table 21.2.2.
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An upper ϕ-factor is used for tension-controlled sections than compression-controlled sections because tension-controlled sections have more ductility. Columns with spiral reinforcement are assigned a higher ϕ-factor than columns with other types of transverse reinforcement because spiral columns have greater ductility.
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Strength Reduction Factor for Earthquake-Resistant Structures
The value of ϕ for shear is taken as described in ACI 21.2.4 for earthquake-resistant structures with elements in (a), (b), or (c):
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Download ideCADto define strength reduction factors according tofor ACI 318-19? |
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