Concrete slab design
A concrete slab has to carry its loads, span between its supports and keep its deflection and cracking under control. We design ground-bearing and suspended slabs, one-way, two-way and flat, for clients across the UK.
What is the difference between a ground-bearing and a suspended slab?
A ground-bearing slab passes its load straight into the compacted ground beneath it, so its performance depends on the sub-base. A suspended slab spans between walls, beams or columns and carries load in bending like a floor. We choose between them on the ground: suspended slabs suit poor, filled or shrinkable ground that a ground-bearing slab would crack over.
Ground-bearing and suspended slabs
There are two fundamentally different slabs. A ground-bearing slab sits directly on a prepared and compacted sub-base and passes its load straight into the ground beneath, so its design is really about the support it is given: the reduced dig, the granular sub-base, the blinding and the mesh or fibres that control shrinkage cracking. A suspended slab spans between walls, beams or columns and carries its load in bending, so it has to be designed as a spanning structural element with tension reinforcement where it sags.
Choosing between them comes down to the ground. Where the sub-base can be relied on to give uniform support, a ground-bearing slab is the cheaper floor. Where the ground is poor, made up, filled or liable to settle or heave, a suspended slab that spans across the weak ground avoids the cracking and stepping that a ground-bearing slab would suffer, and on shrinkable clay a suspended slab with a void beneath it is often the only sound answer. We assess the ground and recommend the slab that suits it.
Spans, deflection and cracking
A suspended slab is sized first for bending and then, very often, for deflection. It has to be thick enough that under its permanent and imposed loads it does not sag more than the span over 250 overall or the span over 500 after finishes are applied, because it is deflection that cracks screeds and partitions, not strength. We set the depth using the span to depth rules of Eurocode 2, work out the reinforcement for the bending moments, and check that the section is not over-reinforced. A one-way slab spans in a single direction onto two supports; a two-way slab supported on all four sides carries load in both directions and can be thinner.
Cracking is controlled as well as deflection. Concrete cracks as it shrinks and as it bends, so we detail minimum reinforcement and bar spacing to keep crack widths small enough for the finish and the exposure, and in ground-bearing slabs we set the joint layout so shrinkage cracks form at the joints rather than randomly across the floor. Where a slab is exposed or has to be watertight, the crack width limits are tighter and drive the reinforcement.
Flat slabs, loads and detailing
A flat slab is a suspended slab that sits straight on columns with no downstand beams, which gives a clean soffit and shallow floor zone but concentrates a lot of load into small areas. The governing check is punching shear, the column trying to push through the slab, and it usually decides the slab thickness or calls for shear reinforcement or a thickening around each column. We assess the loads the floor really carries, residential, office, storage, plant, or point loads from partitions and equipment, because using the correct imposed load rather than a blanket figure keeps the slab economic.
Detailing turns the calculation into something that can be built. We set the cover for the exposure and any fire rating, detail the top steel over supports and bottom steel in the spans with correct laps and curtailment, and coordinate openings, service penetrations and any concentrated loads so they are reinforced rather than cut through later. The design is prepared to Eurocode 2 and Approved Document A and delivered to clients across the UK.
What we check.
The points our calculations resolve for a project like this.
- Ground-bearing versus suspended slab chosen on the ground conditions
- Sub-base, blinding and support for ground-bearing slabs
- Bending, span to depth and reinforcement for suspended slabs
- Deflection kept within limits that protect screeds and partitions
- Crack control through minimum steel, spacing and joint layout
- Punching shear at columns in flat slabs and correct imposed loads
Issued drawings, redacted.
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Engineering
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Issue
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Common questions
What is the difference between a ground-bearing and a suspended slab?
A ground-bearing slab passes its load straight into the compacted ground beneath it, so its performance depends on the sub-base. A suspended slab spans between walls, beams or columns and carries load in bending like a floor. We choose between them on the ground: suspended slabs suit poor, filled or shrinkable ground that a ground-bearing slab would crack over.
How thick does a concrete slab need to be?
It depends on the type and span. A domestic ground-bearing slab is often 100 to 150mm on a good sub-base, while a suspended slab is sized for its span and loads and can be much deeper. For suspended slabs deflection usually governs, so we set the depth from the span to depth rules of Eurocode 2 rather than a fixed figure.
Why does deflection matter more than strength?
A slab can be strong enough not to fail and still sag enough to crack the screed, tile or plaster partitions sitting on it. Those finishes fail long before the slab does, so we limit deflection to the span over 250 overall and the span over 500 after finishes, which usually sets the slab depth.
What is punching shear in a flat slab?
In a flat slab that sits directly on columns, punching shear is the column trying to push a cone straight through the slab. It usually decides the slab thickness or requires shear reinforcement or a thickening at each column. We check it at the code control perimeters around every column.
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