# 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

**8.5.1.1**.

Dimensions of the cross-section should be satisfy

**ACI Eq. (22.5.1.2).**It’s automatically controlled and users are warned if the rule is not satisfied.

**Symbols**

* M_{u }= *factored flexural moment at section, lb

*nominal flexural strength at section, in.*

**M**_{n }=*factored slab moment that is resisted by the column at a joint, in.-lb*

**M**_{sc }=**f'**specified comprehensive strength of concrete, psi

_{c}=*area of concrete used to determine shear stress, in*

**A**_{c-sh }=^{2}

*gross area of concrete section, in*

**A**_{g }=^{2}

*web width or diameter of circular section, in.*

**b**_{w }=*distance from extreme compression fiber to centroid of longitudinal tension reinforcement, in.*

**d =***overall thickness, height, or depth of member, in.*

**h =***center-to-center spacing transverse reinforcement, in*

**s =****γ**

*factor used to determine the fraction of M*

_{f }=_{sc}transferred by slab flexure at slab-column connections

To design two-way slab under flexural moment, two-way slab design strength is provided ΦS_{n }≥ U. Both flexural moment and shear strength are controlled according to **8.5.1.1**.

ΦM_{n} ≥ M_{u } at all sections along the span in each direction

ΦM_{n} ≥ γ_{f}M_{sc } within *b _{slab}*

**8.4.2.2.3**(flat slab)

Φv_{n} ≥ v_{u } at the critical sections defined **8.4.4.1** for two-way shear (flat slab)

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

**Moment**

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

**Openings in slab systems**

**ACI 22.6.4.3** 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 .