In reinforced concrete design , there is what is called the factor of safety. The factor of safety is multiplied by the calculated load of the structural element. This is done to reduce the possibility of the structural element failing under its actual, or expected loadings.
When the factor of safety has been applied to the calculated load of a structural element, it is called design load. Note that Factored load is also the same as design load.
Determining the Design (factored) load of a Concrete Slab.
The formula for calculating the design load of a Concrete Slab is:
1.4gk + 1.6qk.
gk= dead load
qk= live load
1.4= factor of safety applied to dead load
1.6= factor of safety applied to live load.
Dead load is the same as the weight of the concrete slab element.
Live load is the movable or variable load the slab will carry such as people, furniture,machinery and so on.
The unit of the design load will be in kN/m².
The weight of concrete used in reinforced concrete design calculation is 24kN/m³
Example 1 calculating design load of slab
Let’s say a concrete slab panel is to be designed for a private dwelling. It has a depth ( thickness) of 0.2m, a length of 5m, and a width of 3m as shown above, the design load can be calculated as follows:
The dead load of the slab panel is the weight of concrete multiplied by the slab thickness.
24kN/m³ × 0.2m = 4.8kN/m²
Therefore gk = 4.8kN/m²
Live loads for buildings are standardized. In page 2 of BS 6399 part 1, the live load for private dwellings is 1.5kN/m² (shown below)
Therefore qk= 1.5kN/m²
Design load= 1.4gk+1.6qk=( 1.4×4.8kN/m²) + (1.6×1.5kN/m²) = 6.72kN/m² + 2.4kN/m²
Example 2. calculating design load of slab
Looking at part of a floor plan shown below, we will be calculating the design load for the floor plan labelled P8 as shown.Take live load for private dwellings as 1.5kN/m².
The depth of the slab panel is 150mm or 0.15m
We know that the formula for design load of slab panel is 1.4gk + 1.6qk and weight of concrete is 24kN/m³
Dead load, gk, = weight of concrete multiplied by panel thickness= 24kN/m³ × 0.15m = 3.6kN/m²
Live load , qk=1.5kN/m². (page 2 of BS 6399 part 1)
Design load is 1.4gk + 1.6qk=
Slab Design load is used for the calculation of the amount of steel reinforcement required for the slab panel.