Initial Sizing of Steel Beam Section

The first step of preliminary / detailed design of structure is initial sizing of main structural members. This blog gives quick method of selecting steel section for Beam/Girder. The design of beam requires to satisfy the strength and serviceability provisions of codes. 

The Minimum Second Moment of Inertia of Beam Section I ( in cm^4) required is 

I = 0.5 x Ratio x K x L x M

Here,

M is Mid Span Beam Bending Moment is kNm

L is Span of Beam in m

K is constant based on loading and end supports, it is based on Bending moment diagram of span The value of K is taken from Table 1. ( Reference Table 3.1 of BS 8110-2:1985)

Ratio is Span / Allowable Deflection. It means Allowable Deflection = Span/Ratio

The value of "Ratio"  can be 250,360,400,500  or 600 based on what is going to be put on Beam/Girder

Table 1 Values of K for various Bending Moment Diagram
for allowable deflection = Span/400, The I required for Beam=0.5x400xK L M

I= 200K L M

I is in cm^4 unit.