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.

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