How to calculate unfactored dead concrete slab load

Slab panel

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.

Structural plan

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

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