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Steel design Topics related to the design of structural steelwork to BS449 or BS5950

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Old 12th October 2010, 04:43 PM   #1
Join Date: Oct 2008
Posts: 6
Default Design of twin sections

I know this is probably an old, old story to many of you but I am having problems with the LA checker over a twin beam calculation and load sharing.

The beam in question is holding up a cavity wall.

I have refered her to the SCI advisory note 100, etc but she is insistant that each beam should be capable of carrying the maximum load placed on either of the beams. I have read SCI 100 until I am blue in the face and to me this particular point is refering to lateral loading not vertical loading. I accept that "twins" cannot share vertical load when tubes and spacers are used but I always detail diaphragms made from PFC sections as per the Advisory Note.

Prosteel doubles the Sx value to determine Mc and Mb which is as I would expect but unless I can find some way of convincing her I will have to run all twin beam calculations at least twice. Firstly as a single beam to determine the maximum inner leaf moment and then as a "twin" picking sections that have at least the Mc & Mb of the single, which makes a nonsense of trying to be economic. Anyone got any advice or can point me to some defining article about vertical load sharing in twin beams? Surely we can't have all been wrong since 1948 when this was first in BS449!
BS5950 Cl.16 is no use what so ever of course and Google didn't throw up anything useful
Any help much appreciated.

Last edited by H&H; 12th October 2010 at 04:46 PM.
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Old 13th October 2010, 05:48 AM   #2
Tony Bryer
Join Date: Apr 2008
Location: Melbourne, Australia
Posts: 904
Default Re: Design of twin sections

I'm not sure I can add anything useful. As it says in the manual, we assume that twin beams have double the capacity of single ones with no account being taken of any improved lateral stability. As you correctly say, tube spacers are not assumed to transfer any load so you do need to be sure that neither section will be overloaded if the load distribution is unequal.

The only other references I could find are,37241,en.pdf which doesn't say anything helpful and a discussion in Structural Engineer, Verulam section, 5 January 2010 which refers back to the 3 Nov 2009 issue which I don't have to hand.
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Old 8th November 2010, 09:30 AM   #3
John Cranna
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Join Date: Jul 2008
Posts: 35
Default Re: Design of twin sections

To add to this discussion. I often find that with a pair of steels to support a cavity wall and other things there is often quite a heavier load on the one steel, eg if it is supporting roof and floor load which will all come on to the inner skin. I therefore do a separate check on the single steel on the inner skin and if that means the steel size goes up then use that size for the other steel as well. For the BCO to say ALL load must be taken by the one is plain daft.

Last edited by John Cranna; 10th November 2010 at 05:40 PM.
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