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As = area of nonprestressed longitudinal tension reinforcement, in2
α = depth of equivalent rectangular stress block, in.
bw = web width or diameter of circular section, in.
c = distance from extreme compression fiber to neutral axis, in.
Cc = concretecompressive force, lb
Cs = reinforcement tension force, lb
d = distance from extreme compression fiber to centroid of longitudinal tension reinforcement, in.
fc'= specified compressive strength of concrete, psi
fy = specified yield strength for nonprestressed reinforcement, psi
Mn = nominal flexural strength at section, in.-lb
ϕ = strength reduction factor
εt = net tensile strain in extreme layer of longitudinal tension reinforcement at nominal strength, excluding strains due to effective prestress, creep, shrinkage, and temperature
β1 = factor relating depth of equivalent rectangular compressive stress block to depth of neutral axis
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Flexural Strength
Nominal flexural strength Mn is calculated based on the following Design Assumptions. According to ACI 22.1.3, the design strength of a section is ϕMn.
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The distance between the fiber of maximum compressive stress and the neutral axis, c, is perpendicular to the neutral axis.
The value of β1 is determined using ACI Table 22.2.2.4.3.
fc' , psi | β1 |
|---|---|
2500 ≤ fc' ≤ 4000 | 0.85 |
4000 < fc' < 8000 | 0.85 - 0.05(fc' -4000)/1000 |
fc' ≥ 8000 | 0.65 |
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Nominal flexural strength Mn is calculated as shown below.
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The difference between Design Strength ϕMn and Nominal Strength Mn is indicated in the figure below.
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