# Two-Way Slab Design Strength

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 =***nominal shear strength provided by concrete, lb*

**V**_{c }=*nominal shear strength, lb*

**V**_{n }=*nominal shear strength provided by shear reinforcement, lb*

**V**_{s }=*factored shear force at section, lb*

**V**_{u }=**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 is 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 all sections along the span in each direction for one-way shear

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.

**Moment**

M_{n} is determined according to 22.3 as nominal flexural strength * M_{n}* is calculated as shown below. Detailed information: Flexural Strength

**Shear**

For one-way shear V_{n} is determined according to 22.5. Nominal one-way shear strength at a section, * V_{n}*, is calculated by eq. 22.5.1.1.

The shear strength is based on an average shear stress over the effective cross section, * b_{w}d*. Therefore dimensions of the cross-section should be satisfy

**ACI Eq. (22.5.1.2)**. Detailed information: One-Way Shear Strength

For two-way shear *v _{n}* is determined according to

**22.6**.

**Openings in slab systems**

**ACI 22.6.4.3** are 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 .