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Diaphragm Design Strength per ACI 318-19 with ideCAD

How does ideCAD calculate diaphragm 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.-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
Vc = nominal shear strength provided by concrete, lb
Vn = nominal shear strength, lb
Vs = nominal shear strength provided by shear reinforcement, lb
Vu = factored shear force at section, lb

To design diaphragms for each applicable factored load combination, it’s controlled automatically whether it’s satisfied ΦSn ≥ U. Interaction between load effects is considered. Φ is automatically designed according to Strength Reduction Factors per ACI 318-19 with ideCAD.

ΦMn ≥ Mu

ΦVn ≥ Vu

Design strengths of diaphragms idealized with finite-element model are determined in accordance with Chapter 22. Detailed information: Sectional Strength per ACI 318-19 §22. Nonuniform shear distributions are considered in the design for shear.

Moment and axial force

Diaphragms are automatically designed to resist in-plane moment and axial force according to ACI 22.3 and 22.4. Nonprestressed deformed bars, and strands can be used according to ACI 20.2.1 and 20.3.1.

The location of the nonprestressed reinforcement and mechanical connectors resisting tension due to moment is h/4 of the tension edge of the diaphragm.


It’s applied to diaphragm in-plane shear strength. Φ is obtained automatically as 0.75. For a diaphragm that is entirely cast-in-place, Vn is calculated according to Eq. (

Cross-sectional dimensions is selected to satisfy Eq. for a diaphragm that is entirely cast-in-place where the value of √f′c used to calculate Vn is not exceed 100 psi.

For any diaphragm where shear is transferred from the diaphragm to a collector or from the diaphragm or collector to a vertical element of the lateral-force-resisting system, (a) or (b) are applied:

(a) Vn is determined according to Eq. (, and cross-sectional dimensions are selected to provide Eq. ( Acv is determined using the thickness of the topping for noncomposite topping slab diaphragms and the combined thickness of cast-in-place and precast elements for topping slab diaphragms.
(b) Vn does not exceed the value determined according to the shear-friction provisions of 22.9, considering the thickness of the topping slab above joints between precast elements in noncomposite and composite topping slab diaphragms and the reinforcement crossing the joints between the precast members.

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