How Thick Should a Concrete Slab be?

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Thickness of concrete slab depends on loads and size of the slab. In general, 6 inch (150mm) slab thickness is considered for residential and commercial buildings with reinforcement details as per design. Methods used for finding slab thickness varies for different types of slabs. For example, one-way slab thickness calculation is different and easier from that of two-way slab.

The selection and computation of slab thickness including different types of slabs is a major step in the design process. If a proper procedure for the calculation of a slab thickness is followed, the design period would be cut considerably, in addition to reaching reliable and economical slab thickness.

Thickness of One-way Slab

The thickness of one-way slab is based on Deflection, Bending, Shear, and occasionally Fire resistance requirements.

1. Deflection Requirements

Apart from slabs that are loaded heavily for instance slabs carry several meters of soil, the slab thickness is selected based on deflection requirements. The ACI Code sets limitations on slab thickness unless deflections are calculated and determined to be acceptable.

Otherwise, thickness of one-way slabs must be at least L/20 for simply supported slabs; L/24 for slabs with one end continuous; L/28 for slabs with both ends continuous; and L/10 for cantilevers; where L is the span.

These values can be used provided the slabs are not supporting or attached to partitions or other structures liable to be damaged by large deflections.

2. Bending and Shear Requirements

Determination of slab thickness based on bending and shear requirements is not frequent. However, these requirements shall be checked in the design even if the thickness is selected based on deflection requirements.

Procedure for checking slab thickness for bending requirements are as follows:

1. Calculate trial factored loads based on the slab thickness estimated based on deflection requirements.
2. Compute moments using suitable methods such as ACI Coefficient Method.
3. Since slabs rarely require reinforcement ratio greater than 0.01, check whether selected slab thickness is adequate for reinforcement ratio of 0.01. Use equation 1 to compute d:

Where:

d: effective depth of the slab needed to withstand moment

Mu: moment computed from loads

b: width of the slab, 1m (12inch) slab strip is considered

R: flexural resistance (MPa) computed using the following expression:

Where:

p:reinforcement ration taken as 0.01

fy: steel yield strength, MPa

fc': concrete compression strength, MPa

Procedure for checking slab thickness for Shear requirements are as follows:

1. Compute ultimate shear from loads, Vu
2. Calculate design shear strength of the slab, equation 3. If all spans are equal, the ultimate shear occurs at the exterior face of first interior slab, which is computed using equation 4, otherwise shears should be checked at the exterior face of first interior slab and typical interior slab, equation 5.

Where:

Vc: concrete shear strength of slab

b: width of slab, 1000mm

d: slab effective depth

Vu: ultimate shear on slab

W: ultimate distributed load equal to 1.2*dead load plus 1.6*live load

l: slab span

3. Fire Resistance Requirements

Sometimes, slab thicknesses are controlled by the danger of heat transmission during fire. For this criterion the fire rating of a floor is the number of hours necessary for the temperature of the unexposed surface to rise by a given amount, generally 121.1C (250°F).

For a 121.1C (250°F) temperature rise, a 76.2 mm (3-1/2 inch) thick slab gives a 1-hour fire rating, a 127mm (5-in) slab offers a 2-hour fire rating, and a 152.4mm (6-1/4 inch) slab provides a 3-hour fire rating. Lastly, slab thicknesses are usually rounded off to the nearest 10mm.

Two way-slab Thickness

Similar to the one-way slab, the thickness of two-way slab should satisfy deflection and shear requirements.

1. Deflection Requirements

Generally, the slab thickness is chosen to prevent excessive deflection in service. The ACI Code provides method to compute minimum two-way slab thickness which satisfies deflection.

This method is applicable for different types of two-way slabs such as flat slab, flat plate, slabs on beams, slabs without interior beams. To see the details of computing minimum slab thickness, please click here.

The slab thickness chosen must be adequate for shear at both interior and exterior columns. The ACI Code permits thinner slabs to be used if calculated deflection is within the specified deflection limitations.

Procedure to check the adequacy of slab thickness to withstand shear force is as follows:

1. Determine the factored uniform load.
2. Check the one-way shear
3. Check the two-way punching shear

If the slab's shear strength is less than ultimate shear imposed on the slab, then required strategies shall be considered to tackle the problem. These strategies include:

1. Thicken the slab over the entire panel. This may be counterproductive because the weight of the slab may increase shear force significantly.
2. Use a drop panel to thicken the slab adjacent to the column.
3. Add shear reinforcement.