Obtaining the correct size for a steel beam lintel is tricky. That's why it's usually left to a Structural Engineer. But if you're comfortable working out the approximate load acting on your beam, then there are simple methods you can use in order to estimate the size you need.
Working out the required "I value"
The "I value" refers to the Second Moment of Area, which is a measure of the stiffness of a beam. Using the following simple formula you can work out the minimum I value you need to stop the beam deflecting (bending) too much in the middle. But be careful: this only works for beams supported at each end with a uniform load.
Ireq = 2.24 x total load x length x length
written as: 2.24 WL² (Length L is in metres, load W is in kN)
Now take the value you have obtained and look for a beam with a higher value in the blue book steel tables. This should give you approximately the right steel size (download the FREE tutorial below).
An even easier approach:
Beam depth = L/16
This can be used for very preliminary sizing of beams where finding an approximate depth is important (for example where ceiling space is limited).
When using any approximate method, you will now have to get the beam checked out by a Structural Engineer who can prepare detailed calculations for Building Control.
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