# Steel beam calculations

Discussion in 'Engineers' Talk' started by chodaboy1979, Jan 18, 2021.

1. ### SeverntrentScrewfix Select

As stated earlier he has used 7.25 and 5.31 values to design the beam. These will have been derived from his initial loading figures as detailed in his initial sheets, basically he starts with his loading figures and then determines the square meterage of the various parts of the structure ( new flat roof, existing sloping roof, ceiling walls etc etc) that actual bear on to the beam ( simply put one half of your flat roof bears on the division wall beam and the other half bears on the outer wall beam, what part of your existing structure bears on the division wall beam is all down to his interpretation of which way it is distributed to the various load bearing walls). THe final load figures seem to come off page 7 of his calcs ( although these are not the exact ones used?) but how these have been derived escape me, perhaps you should ask for the build up of these figures as without that we have no way of knowing if his assumptions/calcs are indeed wrong.

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2. ### SophiaeNew Member

I finally know the answer. He put the whole load of the 1/2 of the pitched roof on the beam, ignoring that half is carried by 3 walls, not just the wall that is to be removed. So his calculations were well over the reality. I am not sure what went through his mind as the place is more like a rectangle, no internal load bearing walls and a pitched roof comes in 2, meets in the middle, each half sits on 3 walls.

I am not an engineer and I have zero experience or understanding but I knew this didn’t make sense whatsoever.

I can’t thank you enough for being so kind and patient with my questions.

3. ### stevie22Screwfix Select

Very sorry but you don't know the answer and you're on the wrong track entirely. The roof does not sit on 3 walls as you suggest.

A modern truss roof is very simple to understand, it spans from outside wall to outside wall and you could remove the gable walls without the roof falling down (apart from the odd few tiles sat on top of the wall if you want to be pedantic). The whole length of 1 rafter would therefore be taken on any beam in the front or back wall.

A conventional cut and pitched roof is little more complicated as the forces are not so easy to analyse accurately as some load will wind up on the central wall via purlins and props. However for safety's sake this load on the central wall can only be assumed to be zero and the full load of the rafter would be on the beam. (Counterintuitively we would take roof load into consideration if we were putting a beam into the central wall and would wind up with a greater total design load than the actual load as we don't know accurately how to split it.)

Your SE's approach is therefore correct in this aspect.

He has done 2 things wrong:

1. He has altered the design without advising you properly
2. He has adopted an inappropriate section and left you with an unacceptable headroom

The second is obviously the greater sin!

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4. ### SophiaeNew Member

To be honest, I didn’t understand all the technical but but your last 2 points are more important to me than the science. I am more frustrated by not been given a choice in the matter and yes by him not following the architect’s plans and deciding for himself what works for me when I clearly was there asking for explanation.