# Steel Design | Allowable stress design | Example

ASD (allowable strength design) is one of the oldest system that is being used for the design of steel, beams, columns, roof slab, trusses etc… In this method design remains within the allowable stress. Hence, in ASD a factor of safety applies to the strength of the material. So that structure will remain within the allowable stress level.

Allowable stress for both concrete and steel (that is used in concrete as a reinforced concrete) can be easily calculated from the equations.

#### For Concrete

$F_{a}= 0.45times f'_{c}$     Where $F_{y}$ is the compressive strength of concrete

#### For Steel

$F_{a}= 0.50times F_{y}$     Where $F_{y}$ is the yield strength of steel

## Example

### Statement

A singly reinforced concrete section has a compressive strength = 60Mpa, and yield strength of the reinforced steel = 420 MPa

### Requirement

Design a bean using the allowable strength design method

#### Solution

As we have to use the allowable strength design method. Therefore, we will first find the allowable strength of concrete and steel by using formula 1 and 2.

• For Concrete

$F_{a}= 0.45times f'_{c}$

f’c  = 60Mpa

$therefore$    Fa = 27 MPa

• For Steel

$F_{a}= 0.50times F_{y}$

Fy = 420Mpa

$therefore$  Fa = 210 MPa

### Interpretation

In allowable strength design, factor of safety applies to the strength of reinforced concrete. Therefore, from the above results, it is very clear that in ASD, structure will be designed within the reduced value of stress not in the actual strength of reinforced concrete.