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Old 21 April 2009, 08:43   #1
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Calculating RSJ's

Afternoon all.

Anyone out there know the calculation for working out loadings on an RSJ?

I need to cut a door opening in wall and the biuldings inspector needs the calcs before I can.

Phil
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Old 21 April 2009, 08:55   #2
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Maybe a bit over the top but came across this software on a computer I was working on the other day.

http://www.superbeam.co.uk/sbwdemo.htm

The demo seems to do everything bar print.
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Old 21 April 2009, 08:57   #3
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Hi Phil

If you send me your email address I have loads of the professional calks produced for the last place I did up. It makes no sense to me or the building inspector by the look on his face when I submitted them but we got the all important pass. It might be worth while having a look though.
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Old 21 April 2009, 11:09   #4
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To calculate the required size of a beam you need to know how much load is going to bear on the beam. This has to take in to account the size of the rooms above, the layout of floor joists, walls, design of the roof etc etc which is why you are supposed to use a structural engineer to calculate beams. I'm suprised any building inspector would accept calcs from anyone other than a chartered engineer.
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Old 21 April 2009, 12:55   #5
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Many thanks to you all for your help.

Ribshop - just sent you an e-mail with my address.

Martini
I've e-mailed the inspector to see if this is the case and if it is I will post the information. The wall concerned is made of blockwork but only to the ceiling of the ground floor and is three skins thick. Not sure why yet!!
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Old 21 April 2009, 13:42   #6
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I ring my architect pal, he peels off all the right answers for me to repeat to any inspectors or regulatory bodies.
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Old 21 April 2009, 13:53   #7
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Cheers Mollers

Much appreciated
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Old 21 April 2009, 15:19   #8
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This is a fairly simple calc but you have to know the following things:

Load on the beam and the way in which it acts (assuming this is a beam on the top of a door frame supporting the wall/roof above, the load will be an assumed uniformly distributed load across the length of the beam).

The cross section dimensions of the beam

The material properties of the beam - tensile modulus mainly

Then you assume its clamped at each end with the distributed load running between the clamps. Calculate your second moment of area based on the equation (bd^3)/12 (thats assuming its an I-beam) and calculate your moment using an equation based on the above assumptions. Can't remember that one and my data book is at work

Then you use the equation: Stress = My/I, where M=moment, I=second moment of area and y=distance from the neutral axis.

You want to know the stress on the outer fibre of the beam as this is where it is greatest...so y=distance from the centre-line to the outside (assuming cross section is symmetrical).

A beam like this will always fail in tension so once you have found your stress on the lower edge (i.e. the edge in tension) you compare that to the tensile modulus of the material. If your calculated stress is greater then boom your house falls down. I'm not a civil engineer but I assume they will throw a safety factor in there too in order to pass inevitable regs.

The tricky bit is estimating the load and this would need to be done by an experienced architect or civil engineer. As has been said, I'm sure you have to get someone qualified to do this calc for you.

Easy eh?!
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