Design a One-Way Concrete Slab to AS 3600

We currently only offer one-way concrete slab design in Calcs.com and have plans to expand to two-way design in the near future. As the one-way slab design is very similar to our concrete beam module, we recommend using our help documentation and videos for concrete beams. We’re keenly interested in hearing what you would like to see. If you’d like us to add slab design calculations, please send us an email to hello@calcs.com with:

• What type of slab calculations you’d like us to offer first
• How you’re currently doing these slab calculations
• What you’d like to see us improve

We build new features and modules based on what we hear, so your feedback is vital!

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Method & scope

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Calculation method

The One-Way Concrete Flat Slab calculator analyses and designs reinforced concrete slabs spanning in one direction to AS 3600:2018 (Amdt 2). A live FEA engine solves the structural model for each load combination, producing moment, shear, and reaction diagrams that update as inputs change.

Structural model and load cases

The slab is modeled as a continuous beam of unit width (1 m) spanning across the entered supports. Supports can be set to pinned, fixed, or continuous (internal). Load cases are generated per AS/NZS 1170.0 strength and serviceability combinations from the entered dead (G), superimposed dead (SDL), live (Q), and long-term live fraction. Self-weight is computed automatically from the slab depth and concrete density.

Positive and negative moment capacity (AS 3600:2018, Cl. 8.1)

Moment demand at each critical section is taken from the FEA results under the governing strength load combination. Positive moment capacity (bottom steel governs) and negative moment capacity (top steel governs) are calculated from the reinforcement area, effective depth, and concrete compressive strength: utilization = M_u / (phi × M_n) ≤ 1.0 where phi = 0.85 for bending. Minimum reinforcement ratios per AS 3600:2018 Cl. 8.1.6 are checked for both top and bottom faces.

Shear capacity (AS 3600:2018, Cl. 8.2)

One-way shear demand V_u at each support is checked against the concrete shear capacity (no stirrups assumed for a flat slab): utilization = V_u / (phi × V_n) ≤ 1.0 where phi = 0.75 and V_n is calculated per the simplified method accounting for longitudinal reinforcement ratio and effective depth.

Punching shear (AS 3600:2018, Cl. 9.2)

Punching shear is checked at the critical perimeter around column or drop-panel support areas. The demand V_u* is compared to the punching shear capacity f_cv accounting for the critical perimeter length and effective depth. A warning is raised if the demand-to-capacity ratio approaches 1.0.

Deflection checks (AS 3600:2018, Cl. 8.5)

Three deflection limits are checked:

• Short-term deflection under 1.0G + 1.0Q (elastic FEA result, including cracked section factor k_cs)
• Long-term (total) deflection including creep and shrinkage using the multiplier approach per Cl. 8.5.3
• Imposed load deflection (live load component only, for partition damage limit)

Each deflection is compared to the span/limit ratio (L/250 or L/500 as applicable). The governing span and load case are reported.

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How to use it

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AU One Way Slab: Overview

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AU One Way Slab: Example

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Common questions

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Next steps