A slab in terms of its dimensions consists of a thickness(depth), length and width. Understanding this concept will correctly Enable you to calculate and apply slab loading.
In reinforced concrete design, the weight of concrete is usually taken as 24kN/m³. So let’s look at an example.
A reinforced concrete slab is to be constructed having a length of 5m, width of 3m and depth of 200mm. The slab loading can be calculated as follows.
Calculating slab load in kN/m²
Note that 200mm = 0.2m..
..and weight of concrete is 24kN/m³
In most design calculations the unit of the slab load is in kN/m². In this case the slab load is calculated by multiplying 24kN/m³ by thickness (depth) of slab which is 0.2m
So 24kN/m³ × 0.2m = 4.8kN/m²
(4.8kN/m² is the unfactored dead load of the slab)
Calculating slab load in kN
To represent the slab load in kN, just multiply the unit weight concrete which is 24kN/m³ by the volume of the slab.
Note that the volume of the slab is length x width x depth
So 24kN/m³ × 0.2m × 5m × 3m = 72kN
In practice, you will be calculating slab loading from structural plan or floor panel arrangement. A typical structural plan is shown below.
The slab panels are label P and a number. Let’s take a look at one of the slab panels, panel P8.
The panel’s thickness is 150mm or 0.15m. Also it is a 2 way slab. You can also easily calculate the length and width of the slab from the dimensions shown.
To calculate the unfactored dead load of the slab in kN/m², ( which is what we have been doing so far), just multiply 24kN/m³ by the slab thickness which is 0.15m;
24kN/m³ × 0.15m = 3.6kN/m²