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ACI 318-19United States

Concrete Slab

Slab reactions link to supporting beams and columns in the same project so load changes propagate through the full system without manual re-entry. Design one-way reinforced concrete slabs to ACI 318-19 with unlimited spans; bending (positive and negative), shear (ACI 318-19 Table 22.5.5.1), and three deflection criteria all run automatically as you work.

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What it calculates

Design one-way reinforced concrete slabs to ACI 318-19 with live FEA for unlimited spans. Checks cover positive and negative moment capacity, shear, and three deflection criteria. Slab reactions link to supporting beams and columns automatically.

Code standards

  • ACI 318-19

How it calculates

The Concrete Slab (ACI 318-19) calculator designs one-way reinforced concrete flat slabs per ACI 318-19 using Strength Design (LRFD). A live FEA engine resolves moments and shears for unlimited span and support configurations. Checks cover flexural capacity, shear strength, and three deflection criteria.

Flexural capacity (ACI 318-19, Cl 22.3)

Positive and negative moment capacities use the ACI rectangular stress block at a concrete strain limit of 0.003:

Mn = A_s × f_y × (d - a/2) where a = A_s × f_y / (0.85 × f'c × b)

For a unit-width strip (b = 12 in/ft), A_s is the reinforcement area per foot of slab width. The strength reduction factor phi = 0.90 for tension-controlled sections (net strain epsilon_t ≥ 0.005).

Minimum reinforcement per ACI 318-19 Table 7.6.1.1 is enforced:

A_s,min = 0.0018 × b × h (Grade 60 deformed bars)

Shear strength (ACI 318-19, Cl 22.5, Table 22.5.5.1)

For slabs without shear reinforcement, Vc from ACI 318-19 Table 22.5.5.1 depends on the longitudinal reinforcement ratio rho_w, the ratio Mu/(Vu × d), and axial load Nu:

phi × Vn = phi × Vc = phi × [8 × lambda × (rho_w)^(1/3) × sqrt(f'c) + Nu/Ag] × b_w × d

The lightweight concrete modification factor lambda is applied per ACI 318-19 Table 19.2.4.2.

Deflection checks (ACI 318-19, Cl 24.2)

Cracked-section effective moment of inertia (Cl 24.2.3.5):

Ie = Ig × (Mcr/Ma)³ + Icr × [1 - (Mcr/Ma)³]

Three deflection checks are performed:

  • Critical short-term (live load) deflection vs. L/360 for floors supporting non-fragile elements
  • Critical long-term deflection (with creep multiplier lambda_delta per Cl 24.2.4) vs. L/240
  • Simplified DL+(LL or SL) deflection

Results are compared against user-defined L/n deflection limits.

Load combinations (ASCE 7)

The slab is analyzed for all applicable LRFD strength combinations (1.2D + 1.6L + 0.5L_r, 1.2D + 1.0W + 1.0L + 0.5L_r, etc.) and serviceability combinations. Live load pattern loading can be enabled.

Assumptions

One-way action is assumed; two-way slab effects are not considered. Self-weight is included as dead load by default. Punching shear is not applicable to one-way slabs and is not checked.

Related calculators

Rectangular Concrete Column→Concrete Beam→Wall Footing (ACI 318-19)→Cantilever Retaining Wall (IBC 2021)→

Frequently asked questions

What design method and standard does this calculator follow?
Strength Design Method (LRFD) per ACI 318-19. Factored loads from ASCE 7 LRFD combinations are compared to reduced nominal capacities using ACI 318-19 strength reduction factors (phi = 0.90 for flexure, phi = 0.75 for shear).
What are the key inputs?
Key inputs are slab thickness h (inches), concrete compressive strength f'c (psi, minimum 2500 psi), concrete weight classification (normalweight or lightweight), reinforcement yield strength f_y (typically 60 ksi), clear cover, positive and negative longitudinal reinforcement bar size and spacing, span geometry, support conditions, and distributed and point loads by load type.
What checks does the calculator perform?
The calculator checks: positive moment capacity phi × Mn+ ≥ Mu+ (ACI 318-19 Cl 22.3), negative moment capacity phi × Mn- ≥ Mu- (Cl 22.3), shear demand and shear capacity phi × Vn ≥ Vu (ACI 318-19 Cl 22.5), critical live and short-term deflection, critical long-term deflection, and critical simplified DL+(LL or SL) deflection. Minimum slab thickness limits per ACI 318-19 Table 7.3.1 are also reported.
How is shear capacity computed for slabs?
One-way shear capacity for slabs without shear reinforcement uses ACI 318-19 Cl 22.5.5.1: phi × Vn = phi × Vc where Vc depends on the longitudinal reinforcement ratio, axial load, and the Mu/Vud ratio at the critical section (ACI 318-19 Table 22.5.5.1). The lightweight concrete modification factor lambda is applied per Cl 19.2.4.2.
How is deflection calculated?
Short-term deflection uses the effective moment of inertia Ie = Ig × (Mcr/Ma)³ + Icr × [1 - (Mcr/Ma)³] per ACI 318-19 Cl 24.2.3.5. Long-term deflection adds a creep/shrinkage multiplier lambda_delta per ACI 318-19 Cl 24.2.4. Three deflection checks are performed: critical short-term, critical long-term, and simplified DL+(LL or SL).
Does this calculator support load linking?
Yes - slab reactions can be linked to supporting beam and column calculations in the same Calcs.com project. When loads change, all linked calculations update automatically. Dead and live distributed loads can also be imported from project defaults.

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