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Two-Way Slab Design Strength per ACI 318-19 with ideCAD

How does ideCAD calculate Two-Way slab design strength according to ACI 318-19?

  • Both flexural moment and shear strength is controlled automatically according to

  • Dimensions of the cross-section should be satisfy ACI Eq. ( It’s automatically controlled and users are warned if the rule is not satisfied.


Mu = factored flexural moment at section, lb
Mn = nominal flexural strength at section, in.
Msc = factored slab moment that is resisted by the column at a joint, in.-lb
f'c = specified comprehensive strength of concrete, psi
Ac-sh = area of concrete used to determine shear stress, in2
Ag = gross area of concrete section, in2
bw = web width or diameter of circular section, in.
d = distance from extreme compression fiber to centroid of longitudinal tension reinforcement, in.
h = overall thickness, height, or depth of member, in.
s = center-to-center spacing transverse reinforcement, in
γf = factor used to determine the fraction of Msc transferred by slab flexure at slab-column connections

To design two-way slab under flexural moment, two-way slab design strength is provided ΦSn ≥ U. Both flexural moment and shear strength are controlled according to

ΦMn ≥ Mu at all sections along the span in each direction

ΦMn ≥ γfMsc within bslab (flat slab)

Φvn ≥ vu at the critical sections defined for two-way shear (flat slab)

Φ is automatically designed according to Strength Reduction Factors per ACI 318-19 with ideCAD.


Mn is determined according to 22.3 as nominal flexural strength Mn is calculated as shown below. Detailed information: Flexural Strength per ACI 318-19 with ideCAD

Openings in slab systems

ACI is provided when an opening is placed at least 4h from the face of a column, concentrated load or reaction area. Detailed information: Two-Way Shear Strength per ACI 318-19 with ideCAD .

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