Design a flat slab building

A flat slab building has no beams: the concrete slab sits directly on the columns and spans both ways between them. It gives the thinnest floor zone, flat soffits for services and fast formwork — which is why it is the default choice for apartments, hotels and car parks. This tutorial uses the Flat slab building template to analyse the slabs as plates and design them, the columns and the footings.

#How a flat slab carries load

Without beams, the slab itself must collect the load and bring it to the columns. It bends in two directions at once. Near the columns the moments are hogging (tension on top) and concentrated in a column strip; at mid-panel they are sagging (tension at the bottom) and spread over a wider middle strip. Because the whole floor reaction funnels into a small column, the slab around each column is at risk of punching shear — a cone of concrete pushing through the slab. Punching and deflection usually decide the slab thickness, not bending.

Sideways load is resisted by slab–column frame action — the slab acts as a wide, shallow beam — which is flexible. Taller flat slab buildings normally add a shear wall core to take the wind and earthquake.

#Step 1 — Pick the template

1. In the Template gallery choose Buildings › Flat slab building.
2. The model starts with 4 × 3 bays of 7 m and 6 storeys. The material is concrete and the bases are fixed.

#Step 2 — Set the size

Parameter	Section	Default	Range
Bays in X	Plan	4	1–10
Bay width X	Plan	7 m	4–10 m
Bays in Y	Plan	3	1–8
Bay width Y	Plan	7 m	4–10 m
Storeys	Height	6	1–25
Storey height	Height	3.2 m	2.5–5 m

Square panels (equal X and Y bays) are the most efficient: the slab shares the load equally both ways. Long, narrow panels behave more like one-way slabs and need more steel in the short direction.

#Step 3 — Materials and slab thickness

1. Pick the Design code and the concrete Grade (for example C30/37 in Eurocode, f'c 28 MPa in NSCP 2015).
2. Under Sections there is only one group, Columns. Choose a starting size.
3. Under Slabs and walls set the Floor slabs and Roof slab thickness. The defaults (200 and 150 mm) are thin for a 7 m span — a better start is span/30, about 230–250 mm. The roof slab spans just as far as the floors, so give it a similar thickness.
4. Keep Size members for me on. In Design › Design settings…, Thicken slabs that fail lets the design increase failing slabs up to twice their starting thickness.

#Step 4 — Supports and loads

1. Leave Bases fixed. Switch on Pad footings and set the Allowable bearing if you want footings sized.
2. Choose the Floor use — Residential is typical for flat slabs — and the Finishes and services load (screed, ceilings, partitions).
3. Wind loads the cladding around the perimeter. For Earthquake, a flat slab frame has limited ductility; choose a conservative Structural system such as Ordinary moment frame unless your code allows more, and check what your code permits for flat slab systems.

#Step 5 — Analyse and read the slab results

Press Run analysis (F5). Then, in the Show list:

1. Displacement (service combination): the largest deflection is at the centre of the corner and edge panels, which have less continuity. This is an elastic, uncracked deflection — real long-term deflection of concrete is two to three times larger. The design accounts for this through the slab deflection check.
2. Slab moment (kN·m/m): look for the hogging peaks over the columns and the sagging zones mid-panel. Peaks right at the column are partly a modelling artefact of a point support — design values at the column face are lower.
3. Axial in the columns: the interior ground-floor columns carry the most, about the tributary area (7 × 7 = 49 m²) × floors × factored load.

#Step 6 — Design the slabs, columns and footings

1. Press Run design (F6). Slab panels at the same level are designed together as one slab per floor: bending reinforcement by the Wood–Armer method in each direction and face, one-way shear, punching at every column and deflection.
2. Show Slab reinforcement (mm²/m) to see where top and bottom steel is needed, and Reinforcement bars (3D) to see the designed bar meshes and extra bars. Tick Bars only (hide the concrete) to look inside.
3. Open the Slabs & plates table (Full interface) for each panel's mesh, extras, governing check, punching ratio and deflection. B = bottom, T = top.
4. Columns are designed with reinforcement and slenderness from the storey heights. Footings appear as blocks under the supports.
5. Open the Calculation report — the slab chapter shows the governing panel calculation step by step, including the punching perimeter.

#Typical problems and fixes

• Punching fails at interior columns. Thicken the slab, use larger columns (a longer punching perimeter) or a higher concrete grade. Edge and corner columns punch at lower loads because their perimeter is cut by the slab edge.
• Deflection governs everywhere. The slab is too thin for the span. Thickness is far more effective than reinforcement: deflection goes with the cube of the depth.
• Very large sway under wind or earthquake. Slab–column frames are flexible. Add a core (see the shear wall core template) or larger columns.
• Roof slab fails while floors pass. It was given the thinner 150 mm default — raise the Roof slab thickness.

#Rules of thumb

• Flat slab thickness ≈ span / 28 to span / 32 for continuous panels; 200 mm minimum is common for fire and punching.
• Economic spans: 6–9 m. Beyond that, add drops, post-tensioning or beams.
• Columns under flat slabs: at least 300 × 300 mm, more often 400–500 mm, to help punching.
• Typical slab steel: 10–20 kg of reinforcement per m² of floor.

Learn the slab theory in Slab design and the columns in RC column design.