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Strain, εt | Section Classification | ϕ |
|---|---|---|
εt ≤ εty | Compression Controlled Moment | 0.65 |
εty < εt < (εty + 0.003) | Transition region | 0.65 + 0.25[(εt - εty)/0.003] |
εt ≥ (εty + 0.003) | Compression Controlled Moment | 0.90 |
Flexural Strength
Nominal flexural strength Mn and axial strength are calculated using Design Assumptions. While finding the flexural design strength, combined with axial force ϕMn, it should be found in which control zone the cross-section is. When the section is tension controlled, a ϕ factor for tension control is used. A ϕ factor for compression control is used when the section is compression controlled. When the section is within the transition region, ϕ is linearly interpolated between the two limit values.
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Design assumptions
The flexural and axial strength of a member calculated by the strength design method, two basic conditions should be satisfied:
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Maximum strain at the extreme concrete compression fiber is assumed to equal to 0.003.
The tensile strength of concrete is neglected.
The equivalent rectangular concrete stress distribution method represents the relationship between concrete compressive stress and strain.
Concrete stress of 0.85fc' is assumed to be uniformly distributed over. Equivalent rectangular concrete stress zone bounded by edges of the cross-section and a line parallel to the neutral axis located a distance α from the fiber of maximum compressive strain, as calculated by:
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fc' , psi | β1 |
|---|---|
2500 ≤ fc' ≤ 4000 | 0.85 |
4000 < fc' < 8000 | 0.85 - 0.05(fc' -4000)/1000 |
fc' ≥ 8000 | 0.65 |
Combined Flexural and Axial Strength
Nominal flexural strength Mn and axial strength are calculated using Design Assumptions. While finding the flexural design strength, combined with axial force ϕMn, it should be found in which control zone the cross-section is. When the section is tension controlled, a ϕ factor for tension control is used. A ϕ factor for compression control is used when the section is compression controlled. When the section is within the transition region, ϕ is linearly interpolated between the two limit values.
Nominal flexural strength Mn with zero compression is calculated as described in the title of Flexural Strength per ACI 318-19 with ideCAD. Similarly, with the same design assumptions combined nominal flexural and axial strength Mn and Pn are calculated as shown below.
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From the equation of equilibrium:
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Nominal flexural strength Mn:
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