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What size floor joists do i need?


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#1 martin68

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Posted 16 May 2009 - 10:46 PM

I have a loft that is 7.5m wide from wall plate to wall plate, there are no supporting walls in the space below, so what size joists should i install to make the loft habitable as a bedroom?

the floor below uses 9x2" joists, but i'm thinking that if i used 9x2's in the loft i would lose too much head room. would a 6" x 2" be sufficient, and more to the point be approved by BRegs?

#2 temp

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Posted 17 May 2009 - 08:06 AM

Almost certainly not...

The old version of Part A had joist span tables in them but I think these only go upto about 6 meters. However there is enough info in there to suggest that even 9 x 2 isn't enough for a 7.5 meter span. It looks like a 50mm x 220mm is limited to about 5.5 max meters at best.

http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_A_1992.pdf

See page 43 onwards.

I think you need proper calculations done.

#3 davem

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Posted 17 May 2009 - 01:03 PM

It'll never work for a span like that, the deflection would be massive (25mm +) - you'd need something special like engineered I-beams and even then you might be struggling a bit and need to go quite deep.

Easiest solution is probably a steel beam running from gable to gable (or party wall etc) to cut the span down.

#4 trebor

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Posted 17 May 2009 - 01:29 PM

Hi, as part of our roofing system we supply light gauge steel joists which have been tested to a 7.3m span with max deflection of 15mm. depending on loadings etc 7.5m should be achievable using the standard C section at 600 ctrs if not we can put the joists back to back or reduce the ctrs. joists are 165x65 (6.5"x2.5" approx)and 3.8kg/lm.
They are also pre-punched for fixing to the wall plate and for services. Email me at bob.lyons@u-roof.com and we can sketch up a design / quote.

#5 davem

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Posted 17 May 2009 - 06:40 PM

Bob

Satisfy my curiosity please - have you got any more details of these? I've been doing some fag-packet style calcs and come up with a bending stress of 434 N/sq mm and a deflection of 89.5mm! Obviously had to assume some stuff to get there, and ended up with 1.1mm web and 2.3mm flanges. Have you got local authority type approval based on some form of standard test? Not knocking it but interested in the system and how it works. Not much info for designers on your website and it only seems to concentrate of roof members.

#6 trebor

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Posted 17 May 2009 - 06:48 PM

Davem,

Thanks for your comments. Please either send me a private message with your email address or email; me at bob.lyons@u-roof.com and I will send our standard calcs which the system is based on. But it is a bit of a large pdf. We have had the local authority building control officers (calderdale and Bradford) out to see the projects in their areas and down to our factory to see the full size test / display model. Both have been satisfied so far.

#7 joiner

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Posted 18 May 2009 - 07:47 AM

Bob, why not put the link to the pdf on here direct so that everyone can benefit?

#8 agb657

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Posted 18 May 2009 - 09:36 AM

indeed i am interested!

#9 trebor

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Posted 18 May 2009 - 12:09 PM

I tried to upload the calcs but the file size is way over the 800kb limit. I have no problems with emailing the calcs direct. I visited the www.abcdesign-build.com site and you don't seem to have an email on your contact page. But the images showing the roof and dormers does look similar to a project on our web site. Please email me at bob.lyons@u-roof.com and I will send them on. I promise not to send loads of irrelevant stuff, just the calcs. :-)

#10 agb657

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Posted 18 May 2009 - 12:30 PM

OK bob

will do

#11 kevins

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Posted 18 May 2009 - 06:25 PM

I'm also curious about this, particularly how you get the deflection down to 15mm. Is this figure from the test results rather than by calculation? I guess it's possible that the decking is helping to stiffen the joists up, but I'm surprised its by that much.

I've just checked a Metsec cold formed mezzanine floor 'C' section which sounds very similar to yours (165.M.15 @ 3.73kg/m). The ultimate load capacity is about right, but the deflection is way off, with an imposed load deflection of around 85mm (similar to Davem's figure).

If you don't mind you can email the calcs to:

admin at quick-calcs dot com

#12 davem

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Posted 18 May 2009 - 08:09 PM

Thanks for the calcs Bob.

I think it looks a good system, but only when used as a full truss, not just as a floor joist. I think the calcs use the upright ashlar posts as hangers to cut the span of the 'floor joists' down to about 3.8m which back to the fag-packet calcs results in 8.2mm deflection. This then transfers the load to the rafters as a point load. The high level 'tie' (member 103) is then actually acting as a compression member (seems to for all loadcases presented) which cuts down the effective span of the rafters. I've not gone through all 52 pages to check my theory completely though....

#13 blasted

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Posted 26 May 2009 - 05:14 PM

I have a similar distance that I am looking at spanning. However, the joists overlap above a stud wall, so dont span the full distance. Not wanting to tamper with the existing structure that is in place, Im looking at going 90 degrees to the joists with posi-joists that are at 400 centres and attached to the gable walls by joist hangers. Obviously i will be leaving the required space for defection. This means i only have to span 5.3 meters which I believe, only require a posi-joist depth off 225mm.