Supporting Stud Wall in line but between joists?

Hi,
Just had interesting discussion with BCO (see my Part P mail today) and I am erecting a stud wall in a bedroom in line with joists but about 3 inches into the 14 inch void. The stud is attached to another wall at one end and it will be about 12 foot long (sorry for old units but its a 50's house). The BCO is not happy with me putting 4x2 studding between the joists which are 7x2 inches. He advised my to get a structural engineer to check what extra support was required and implied noggins every 400mm. He also said I might be able to get "Super Beam" software on Trial to calculate loadings.
I had already decided to put in some noggins but would appreciate any advice as to what size and spacings would generally be suitable.
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I'd guess the BCO is looking for structural calculations for the existing floor, to prove it's good for the normal imposed load as well as the new static load of the wall.
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Peter Hemmings wrote:

The BCO is taking the p!$$.
What weight does he think is in a 12ft stud wall? 6 X 8/4 p-boards @ 4kg ea 2 bags plaster @ 25kg ea 10 x 8ft 3/2 timbers @ 2kg ea 2 X 12ft top and bottom timbers @ 3kg ea
Grand total = 100kg - and this is over 12ft X 4inch, making it 4 sq feet of area...plus, some of this weight will also be alleviated by your fixings into the wall!! - I know the above is only a rough estimate but it's unlikely to be far off. It's far less than having a cooker standing on the same amount of floor area and there must be hundreds of thousands of these all over the country, strangely, I've never heard of one falling through the floor.
Use noggins if you want to but it's unlikely he'll re-check
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Phil L wrote:

Not so sure that he in this case, although it does depend a bit on the length of the wall, the size of the existing joists.

Those sound much easier to lift than the ones I am used to. ;-)
A standard 2400x1200x12.5mm PB is about 23kg IIRC so we have 138kg of plasterboard (assuming the OPs walls are about 8' tall)
yup 50kg of that
More like 5.4kg each I make that (at an approx density of 600kg/m^3 for structural pine softwood), so 54kg of those
8.1kg / each, so 16kg for them

Pushing 260kg by my calcs!
So 2600N over 3.6m span is an extra uniform spread dead load of over 0.7 kN/m. If you also have to allow for the normal dead load on the joist imposed by the floor and anything placed on the floor against the wall (typically 0.8 kN/m) then there is as much again to add. So if the stud wall were directly over the joist, it would in effect be doubling the design load on it.

True, you were within one order of magnitude ;-)

The reality is that it is unlikely anything will break, but you will fail to meet the structural bending and deflection limits specified by the building regs. Needless to say a BCO is unlikely to sign this off.
The OP stated an original joist size of 175x50 ish, which is only just in spec[1] for the floor at a 3.6m span.
[1] Superbeam calculats the mid span deflection at just under 10mm, with a maximum allowed of just under 11mm at that span. This assumes C16 timber in a load sharing arragement.

In this situation even with the wall load shared between two joists you are going to be out of spec, since you are quite close to the limit as it is, without the wall.
The normal solution here would be to stick another joist beside it and bolt them together with timber connectors and plate washers.
--
Cheers,

John.

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John Rumm wrote:

My present situation is that the BCO implied he would accept noggins every 400mm but it looks that the "norm" should be another joist! I have just measured the total width of the room and it is 3.9 metres wide (stud will be 2.9m). My big problem is fitting a new joist. I have several ring main cables running through the joist, and 2 heating pipes on the top of it!
If I did bolt another joist on to it would I have to support it on hangers at each end? The second joist would still not quite be under the stud wall so would I also need noggins between the joists?
Is there anywhere which gives weight of a square metre of various stud constructions as different peoples estimates seem very different!?
I think I will ask the BCO (in writing) if he can confirm he would accept 400mm noggins or if I must get the calculations done. He did imply he would accept noggins!!
Thanks
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Peter Hemmings wrote:

With the 2.9m wall at one end of the 3.9m joist I take it?

The cables could be either be cut and reconnected after the joist is in, or you may find a socket close enough to disconnect them at temporarily.
You may find The joist can be positioned while laying on its side and then rotated into position - which may help solve the pipe problem.

Probably - they don't like to see joists sat in cut out sections of wall these days. How is the current one supported?

Ideally, unless is it at leat partially over the joist.

Well weights for plasterboard can be got from the manufacturers sites:
http://www.british-gypsum.bpb.com/products/plasterboard___accessories/gyproc_standard/gyproc_wallboard.aspx http://tinyurl.com/quzve
Bags of plaster are easy to calculate, so that just leaves the wood, which you could estimate from density as I did, or stick a bit on the scales and see! ;-)

I think I would also be inclined to think through how the loading will be imposed in reality. If the wall will form (for example) the side of a shower room, and there is unlikely to be furniture placed against it on either side, then the assumptions made for its designed floor load will tend to be overly pessimistic. Hence using much of the design capacity to carry a wall instead of floor loading is unlikely to result in any problems. If however you were going to stick a run of heavy wardrobes against the wall, then I would probably go with a new joist and noggings, or two new joists directly under the wall.
--
Cheers,

John.

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John Rumm wrote:

Yep
Yep but I like to try and not put in any junction boxes!

50's semi, real solid walls with joists into wall!

OK
http://www.british-gypsum.bpb.com/products/plasterboard___accessories/gyproc_standard/gyproc_wallboard.aspx

OK
When you say design floor load, is this all calculated (set) in the "super Beam" S/W, surely the loadings would be different for each type of room? I was going to download a trial version and see what it produced but #I thought I would get some facts and advice first!
I my youth (a very very long time ago) I did study mechanics and moments on beams! I thought that the as there were several joists crossed by T&G boards the loading would be spread across the floor, I assume the S/W will take this into consideration!?
Well, I will only have an ordinary bed against it and nothing at all on the stud (sink and radiator will be on the existing brick wall).
Thanks for the information
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Peter Hemmings wrote:

The actual loadings are not set in SB, you need to specify those... You can also specify if a load is a uniform one acting over all or part of a beam, or a point load, or even a varing load. There are a number of "standard" floor loadings that are typically used. For a domestic property; 0.8kN/m uniformly spread on a joist is a common loading to use.

Yes you can specify the number of joists acting together, including so called "load sharing".

Hence why the BCO is letting you "get away" with noggings between two joists! (if you have a play with SB you will see that your existing joists are pretty close to the design limit just carrying the floor).
--
Cheers,

John.

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John Rumm wrote:

I have asked the BCO to confirm in writing that noggins would be acceptable and went to a local "Engineer" to try for some more free advice. I found that he also advises the local BO and knows the BCO - my big foot in it I think! He states that the joists would be insufficient for the existing span (just over 4m) and I would probably need another joist bolted on plus noggins!
And, for this to be calculated and give recommendations he would need to visit and his cost would be 200 approx! I can seen my budget rapidly rising ATM.
Because of "Er in Doors" refusing to have the floor cut up and having to re-do the plumbing and electrics this project may well be floundering!
I did manage to talk through possible solutions with the Engineer before he showed me the door! He thought the preferred method would be extra bolt on joist with noggins (but needed to check existing wood quality and shear stresses!).
He ruled out extra plates each side of the joist because of the extra loading, but said a split steel under the stud should also be OK. I understand one of the "standard" steel sizes is within 1mm of my 7"x2" joists.
If I have to pick an option I will probably pick the steel as the floor span already flexes slightly when bouncing up and down on the bed (as can be seen in the wardrobe mirror)!!!! (please excuse any wrong terminology).
Just in case I do pursue this: how would I run 22mm copper pipe through a steel? how do I secure a floor board to a steel ie I want a removable board for access inside the en-suite which finishes at the stud which would be over the steel, I assume a piece of wood is fitted at the side of the "H" section?
Any other comments?
Thanks
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Peter Hemmings wrote:

When I stuck your existing figures into SB then it looked to be right on the limit - but that was using the 3.6m span you mentioned in an earlier post. If it is 3.9m then it is already a bit too bendy for a current design.

Seeing as he will probably look, then go model it in SB and print the results out for you, it would be cheaper to buy the full version of SB and DIY!

Doubt she will be doing the plumbing though ;-)

In reality that ought to be more than good enough. Especially as you know the real floor load will be significantly less than a typical design load.
However the slight difficulty is that your current joists don't quite meet regulations for their span. Since the wall is imposing a similar load to the floor[2] (for the purposes of the calcaulations) that is also just out of spec with the extra joist. You could however argue that the extra timber in the sole and top plates of the wall will more than compensate.
[2] What NT's post says about the extra stiffness from the sole and top plates of the wall is true, but a 3x2 "side on" over that span does not add much. You will have little if any fixing to the ceiling either I would have thought, and only one end is fixed to a wall.
Another solution is to build the wall just a little lighter - say using 9mm PB intead of 12.5mm. Perhaps even just taping and filling rather than skimming would be enough reduction.

Ooo Err! ;-)
Another solution for eliminating the bounce is to add herringbone bracing across the centre span of the joists - that makes quite a difference to the stiffness of a floor. You can either do it with traditional wodden braces, or you can get steel straps ready made for the job you just nail into place.

With difficulty!
If you need to use steel, the first option to consider would be a flitch beam - i.e. another wood joist beside the current one with a steel plate sandwiched between them. This way you could use a flitch plate that is only 75% of the height of the joist, leaving enough space to get a pipe past (although in the non preferred notching method!). In fact a quick play with SD suggests that a 6mm thick half height flitch plate might be good enough.
Another option would be some form of composite wood beam of the type with top and bottom rails and a ply web in the centre - these are easy to take services through. Not sure they come out much stronger though. Some of the makers web sites will do the calcs for you.
You would also perhaps cope if you specified an extra joist - but make it a 7x3" and specify C24 timber.

Spose - not had to use steel like this before. You would probably need to bolt or nail[1] a wood plate to the side of the steel to pick up the floor.
[1] apparently common practice with a suitable explosive cartridge nailer!
--
Cheers,

John.

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I you subtract the load supported at each end of the wall by the current wall, is there enough leeway to put a beam/large purlin above the wall, and have a hanging wall? Only reasonable if it is easy to put a beam in the loft.
John Rumm wrote:

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John Rumm wrote:

Sure, but the stud wall contributes twice that, as you've got sole _and_ header plate connected to each other by lots of uprights. On borderline floor loads this might be enough to tip the scales.
Next possible move is to make your floor joist, floor and soleplate one item structurally by gluing and screwing every 6", so youve effectively got significantly deeper woodwork there.
Then if the top is close enough to a ceiling joist to get a purchase up there, youve got more woodwork you can get into the equation.

If it lines up with a joist, all is well. If not but its close one can sometimes use small thick metal L brackets receessed a few mm into the ceiling plaster to reach a joist. Noggins are also an option, but make more mess above.
I also like Jacob's / Norman's idea of supporting the stud wall off the walls by using triangular woodwork.
Using all these tricks should get you a fair way, and paperwork is easier than joist replacement.
NT
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snipped-for-privacy@care2.com wrote:

And you could use bigger wood of course.
NT
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Sorry, can't help but count yourself lucky. My BCO insisted on double width joists under stud walling in a loft conversion. Existing joists were 8" x 2". One of several runs went in as 8" x 4" instead of two clamped 8" x 2"s. In retrospect I wish I had done everything I could in 8" x 4" but my brother-in-law initially balked at manhandling the weight.
--
Roger Chapman

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Roger wrote:

Yup, that is what the architect speced on my one as well... ended up needing to use flitch plates on some of them as well when there were other stringers that added extra point loads.

Oddly enough you can assume a slightly greater design strength for two side by side beams, than you can for a single beam of the same total cross section.
--
Cheers,

John.

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Peter Hemmings wrote:

The posts so far appear to imply that the stud wall will be an even loading on the floor structure below it, but it wont. A stud wall is a relatively rigid wooden structure fixed top, bottom and both ends, and as such a fair bit of the the weight will be borne by the end fixings, since the stud structure will reisst bending (sag). And some of the weight will be carried by the top fixings too. And finally, the wood at the bottom supoprting the remaining forces is not just the flooring joist, but flooring joist + floor + wall sole plate.
This is why in practice stud walls sitting on floorboards on 6" flooring joists work fine.
NT
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