Thickness of Slab : How to determine?
The thickness of slab is a vital factor in the building design and construction and it directly related the cost of the structural system.
For example, in a high-rise building, an increase in the thickness of slab by 5mm result significantly increase column axial loads. Then we have to increase the column sizes, reinforcements, foundations sizes, etc.
Finally, it affects the cost of construction.
Therefore, we should limit the thickness in any construction to the limits required by the design(serviceability and ultimate limit state).
Key factors affecting the minimum thickness of slab can be listed as follows.
- Applied loads
- Durability of Concrete
- Fire requirements
- Serviceability requirements such as deflections
- Serviceability requirements such as floor vibrations
- Construction requirements
Different standards could specify different thickness requirements. However, we can calculate the basic requirements of minimum thickness considering the above factors.
The calculation is only based on the construction and detailing requirement according to BS 8110 Part 01.
- Cover to the reinforcement = 20mm which is minimum specified in the code for mild exposure condition with one-hour fire resistance.
- Reinforcement bar diameter = 10mm; at the beam, we have four-bar with the top reinforcements.
- Minimum Clear spacing between bars based = aggregate size + 5; generally, we use 20mm aggregate for concrete works.
Therefore, the minimum concrete thickness can be calculated as follows.
Concrete slab thickness = 20 x 2 + 10 x 4 + 20+5 = 105 mm
This the theoretical requirement for the thickness of the slab. However, as calculated, reinforcements can not be placed as precisely as calculate to maintain the gap between the bars as aggregate size + 5.
Further, slab reinforcement will be obstructed by the beam reinforcement, and they won’t be able to place as considered in the calculation.
Therefore, it is very very difficult to achieve these requirements. Thus, limiting the thickness to 105mm is a practically difficult task.
In this background, the widely used concrete slab thickness is 125mm.
Standards like ACI 318 have specified the minimum thickness of the slab based on the span of the slab.
- Simply Supported Slab = Span / 20
- One End Continuous Slab = Span / 24
- Both End Continuous Slab = Span / 28
- Cantiliver = Span / 10
However, in other most of the standards, they have not clearly specified the minimum slab thickness directly.
