# How to Decide initial Size of Steel Beam ?

In this article, we are going to discuss the topic of Steel Structure that is how to Decide the initial size of steel Beam, What are the parameters kept in mind while deciding the trial size of steel beam.

For the Member of Steel Structures design, two parameters are the most important that is Slenderness Ratio and Flexural strength against bending.

Let’s understand each one by one

## Slenderness Ratio

Slenderness Ratio is the ratio of effective length to radius of gyration of members.

Slenderness Ratio = Effective Length / Radius of Gyration

Slenderness value directly depends on the length of the member. If the value of Slenderness Ratio is less then the member is stiff and gives good strength against buckling. But,

If the Slenderness Ratio of a member is high then it is easy to buckle and gives less stability. Slenderness is mostly critical for Steel columns as compared to steel beams.

To decide the initial size of steel beam, you need to assume Slenderness Ratio approx. 100 to 150 for steel beam.

And, Based on that find the Radius of Gyration (Ryy), based on that select the appropriate size of the member.

Also Read: 9 Best Structural Engineering Companies of India

## Flexural Strength

Flexural strength or strength against bending totally depends on the plastic section of modulus of the steel beam. Based on that you decide the initial size of steel beam for your design.

Bending strength of steel beam as per IS 800 : 2007 is as mentioned below.

Md = ( β x Zp x Fy)/( Ymo)

Where,

β = for the plastic section is 1

Zp = Plastic Section Modulus of section

Fy = Yield Strength (250 N/mm2)

Ymo = Partial Safety Factor (1.10)

1. Consider the plastic section,
2. And calculate the bending moment for a given span.
3. Based on the bending moment, β, Fy & Ymo, you got the initial Plastic Section Modulus.
4. Based on this plastic section modulus, select the appropriate section for the design.

After the selection of members, proceed the different checks as mentioned below.

• Slenderness Ratio
• Section classification
• Bending Strength
• Shear Strength
• Deflection
• Combined axial and bending strength of member