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Flexural Strength per ACI 318-19 with ideCAD

How does ideCAD calculate flexural strength according to ACI 318-19?

  • Flexural strength is calculated automatically.


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 = concrete compressive 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

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.

Design assumptions

The flexural and axial strength of a member calculated by the strength design method, two basic conditions should be satisfied:

  1. equilibrium

  2. compatibility of strains

Equilibrium means balancing of forces acting on the element cross section at nominal strength. Stress-strain relationship for the concrete and the reinforcement at nominal strength is established within the design assumptions described following:

  • Equilibrium is satisfied in each section.

  • It is assumed that strain in concrete and reinforcement is proportional to the distance from the neutral axis.

Design strength is calculated by using these assumptions together with design assumptions for concrete described following:

  • Maximum strain at the extreme concrete compression fiber is assumed equal to 0.003.

  • The tensile strength of concrete is neglected.

  • The relationship between concrete compressive stress and strain is represented by the equivalent rectangular concrete stress distribution method.

  • 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:

  • 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

fc' , psi


2500 ≤ fc' ≤ 4000


4000 < fc' < 8000

0.85 - 0.05(fc' -4000)/1000

fc' 8000


Nominal flexural strength Mn is calculated as shown below.

The total forces Cc and Cs resulting from the stresses of concrete and reinforcement are shown below.

From the equation of equilibrium:

Nominal flexural strength Mn:

According to ACI 22.1.3, the design strength of a section is taken as the nominal strength multiplied by the applicable strength reduction factor ϕ. Therefore, the design force is ϕMn.

The difference between Design Strength ϕMn and Nominal Strength Mn is indicated in the figure below.

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