Strength Reduction Factors (ø)
Strength Reduction factors are needed to take forces between ultimate (Mu) loading and nominal loading (Mn). They are a function of the net tensile strain.
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Note: The strength reduction factor ø, will vary in respect to εt |
ACI 318 Chapter 9[1]:
- Compression
- ø = 0.65
- ø = 0.70 (Spiral reinforcement)
- Tension
- ø = 0.9
- Shear
- ø = 0.75
ACI 318 Appendix C[1]:
- Compression
- ø = 0.70
- ø = 0.75 (Spiral reinforcement)
- Tension
- ø = 0.9
- Shear
- ø = 0.85
For transition regions (regions between tension and compression):
ACI 318 Chapter 9[1]:
- Transition Region
- ø = 0.48 + 83εt [ACI 9.3.2]
- ø = 0.57 + 67εt (Spiral reinforcement) [ACI 9.3.2]
ACI 318 Appendix C[1]:
- Transition Region
- ø = 0.57 + 67εt [ACI C.3.2]
- ø = 0.65 + εt (Spiral reinforcement) [ACI C.3.2]
Significance of the Strength Reduction Factor (ø)
As stated in the intro, the strength reduction factors are used to jump between Nominal Loads and Ultimate Loads.
øMn ≥ Mu
øVn ≥ Vu
References
- American Concrete Institute, "ACI 318-05", 2005