Element design
How to design a concrete slab
Concrete slab design explained: one-way, two-way and flat slabs, Wood–Armer moments, bar meshes, punching shear and deflection, with a worked Eurocode 2 example.
A slab is a beam that is wide in two directions. That one difference changes the design: moments flow in two directions at once, they twist, and around columns the slab can punch through like a nail through cardboard. This tutorial explains each slab type, how a plate analysis turns into bars, and the checks that usually govern.
One-way, two-way and flat slabs
- One-way slabs are designed as 1 m wide beams, with distribution bars across.
- Two-way slabs share load between both directions; codes give moment coefficients for regular panels.
- Flat slabs need bands of top steel over the columns and a punching shear check; deflection often sets the thickness too.
From plate analysis to design moments
A finite element plate analysis gives, per unit width, two bending moments mx, my and a twisting moment mxy. Bars only run in two directions, so the twist must be carried by them too. The Wood–Armer rules do this:
Example: an element with mx = 30, my = 12 and mxy = 8 kN·m/m needs bottom bars for 38 kN·m/m in x and 20 kN·m/m in y. The top values (22 and 4) are still sagging, so no top steel is needed there.
Then each design moment goes to a 1 m wide section. With h = 250 mm, d = 205 mm, C30/37 and B500:
Ø12 at 250 mm gives 452 mm²/m. Read more on plates in Slabs and walls as plates.
Bar meshes plus local extra bars
Designing every element's bars separately would give an unbuildable patchwork. Practical slabs use a uniform mesh on each face and direction, sized for most of the slab, plus local extra bars where the demand is higher — over columns, along supports, at re-entrant corners. The mesh must respect the code minimum and maximum spacing.
Worked example: punching shear of a flat slab (Eurocode 2)
A flat slab on a 7.2 × 7.2 m column grid, 400 × 400 mm interior columns, C30/37. Superimposed dead load 1.5 kPa, office imposed load 3.0 kPa.
Try h = 250 mm (d ≈ 205 mm)
gk = 0.25 × 25 + 1.5 = 7.75 kPa; wEd = 1.35 × 7.75 + 1.5 × 3.0 = 14.96 kPa. Column load VEd ≈ 14.96 × 7.2² = 776 kN.
The basic control perimeter is at 2d from the column face (EN 1992-1-1 6.4.2):
Even with 2 % top steel over the column, vRd,c only reaches 0.93 MPa. The slab is too thin.
Try h = 300 mm (d ≈ 255 mm)
wEd = 1.35 × 9.0 + 4.5 = 16.65 kPa → VEd = 863 kN; u1 = 1600 + 4π × 255 = 4804 mm.
With k = 1.89 and ρl = 1.6 % in the column strip: vRd,c = 0.12 × 1.89 × 481/3 = 0.82 MPa → utilization 0.99 ✓. Extra top bars over the column make the 300 mm slab work without shear reinforcement.
Punching depends on d squared (in the perimeter and the depth), so a little more thickness helps a lot. Edge and corner columns are worse: the perimeter is cut short and the moment transfer (β) is bigger.
ACI 318: the critical perimeter is at d/2 from the face, and the two-way shear stress vc = 0.33 λs λ √f′c (the least of three expressions), with φ = 0.75 and moment transfer by the γv fraction (8.4.4).
Deflection
Slabs are thin, so deflection often controls. Codes give span/depth limits (EC2 7.4.2, ACI Table 8.3.1.1 for flat slabs: ℓn/30 to ℓn/33), or a calculated deflection with cracked, long-term stiffness against span/250.
Design it in Struxt3D
Struxt3D analyses slabs as plate elements, then designs the bars face by face and direction by direction with Wood–Armer, picks uniform meshes and local extra bars, checks one-way shear, punching at the columns and deflection.
- Open the Flat slab building, and set the bays and storeys in Size.
- Set the slab thickness. In Supports & loads › Slabs and walls, set the thickness of the slabs analysed as plates (where the template sets a thickness of its own, change it in Size).
- Turn slab design on. In Design settings, Slabs and plates is on by default; Thicken slabs that fail (with auto-sizing) thickens failing slabs up to twice their thickness.
- Run the design (F6). All panels of one level are designed as one slab, so punching perimeters run across bays.
- Read the results. Show › Slab moment gives the moments, Slab reinforcement the required steel in mm²/m. The Slabs & plates table lists the mesh (B bottom, T top, x / y), extras, punching and deflection ratios.
- See the bars with Show › Reinforcement bars, and open the slab's Calculation report.
Struxt3D does not add punching shear reinforcement. Where punching fails, it raises the top steel over the column (up to 2 %) for the codes that count it, and otherwise the slab must thicken — or add a drop panel or column head by hand.
Common mistakes
- Ignoring mxy and designing for mx and my alone — under-reinforces corners.
- Peak moments at column nodes. Plate models show spikes over point supports; design to the moment across the band, not the single peak.
- Forgetting edge columns in the punching check.
- Thin slabs that pass strength but crack and sag over time.