About Slab Design Calculator
The slab design calculator sizes the reinforcement and verifies the serviceability of one-way, two-way, and flat reinforced concrete slabs per Eurocode 2, ACI 318, and AS 3600. It is used by structural engineers to find design moments, required steel per metre width, and to check deflection, span/depth ratio, and shear.
Set the slab type, support condition, spans, thickness, cover, material grades, and the ultimate and service loads. The tool returns the design moments, required and minimum reinforcement with suggested bar spacing, the deflection against its limit, and one-way and punching shear checks with an overall pass/fail verdict.
How It Works
- Pick the slab type and support condition, then compute the design moment: w*L^2 over a support coefficient for one-way slabs or coefficient tables for two-way slabs.
- Calculate the required steel per metre width using the EC2 lever-arm method or the ACI rho method, and compare it with the code minimum reinforcement.
- Check serviceability via the span/depth ratio, a simplified elastic deflection against the code limit, and one-way shear capacity.
- For flat slabs, evaluate the punching shear stress at the control perimeter against the concrete resistance, then suggest bar spacing and report an overall verdict.
Worked Example
A 4 m one-way simply supported slab, 200 mm thick with 25 mm cover, C30/37 concrete and B500B steel under an ultimate load of 10 kN/m2 gives M = 10 * 4^2 / 8 = 20 kN.m/m. With d = 169 mm, K = 0.023 and z = 161 mm, the required steel is As = 20e6 / (0.87 * 500 * 161) = 286 mm2/m (above the 260 mm2/m minimum), met by about T10 at 250 mm.
Formulas
- One-way slab moment (simply supported)
M = w * L^2 / 8- Two-way slab moment
M = alpha * w * Lx^2- Required reinforcement (EC2)
As = M / (0.87 * fy * z)- Minimum reinforcement
minAs = 0.0013 * b * h (EC2) or 0.0018 * b * h (ACI)- Punching shear stress (EC2)
vEd = V / (u * d)
Standards & References
- Eurocode 2 (EN 1992-1-1)
- ACI 318
- AS 3600
Frequently Asked Questions
What slab types can this calculator design?
It designs one-way slabs (spanning in a single direction), two-way slabs using moment coefficient tables interpolated by span ratio, and flat slabs with a punching shear check at the column.
How are two-way slab moments calculated?
The tool reads moment coefficients (alpha) from simply-supported or continuous tables keyed by the long-to-short span ratio, clamps the ratio between 1.0 and 2.0, interpolates to the nearest 0.1, and applies M = alpha * w * Lx^2.
What deflection limit does the tool apply?
It compares a simplified elastic mid-span deflection against span/250 under Eurocode 2 and AS 3600, or span/240 under ACI 318, and also reports the span/depth ratio against the code basic ratio.
How is punching shear handled for flat slabs?
For flat slabs the tool computes the punching stress at the control perimeter (2d from the column face under EC2, d/2 under ACI) and compares it to the concrete punching resistance, returning a pass or fail.