Help - RSJ calculations!

[Eric The Viking]

Cor, you lot aren't half a distraction (see other topic...)!

Wot I really wanted to ask about was this:

Like a twit I put too much stuff in the attic recently and nearly brought the ceiling(s) down. I've got a cunning plan, as ever, to strengthen them, but it involves hanging the ceiling joists from two L-shaped steel RSJs, like this:

I've only shown one ceiling joist - the pair of steels (pinky-red) runs across the middle of the ceiling, picking up all the joists.

Trouble is- that I can only guess at what size of steel I should specify. I've found some calculators on the net, but they assume either "I" or "U" section RSJs, which won't work here (well, "U" might but it's would probably be heavy and hard to manoeuver). I can't lift tiles, etc. to get an RSJ in - it's too high up and too awkward to reach, so everything has to get through a small loft access.

I also have to do it this way (or something like this) because it eliminates most of the need for accurate drilling of steel when it's up in the loft. It sort-of aligns itself, or so I hope.

So can anyone either suggest a calculator, or give me an idea of where to go for formulae?

Many thanks,

E.

 

[clk230]

wow what are you storing in your loft ?
you may need advice from a structual engineer .
how long are the steels as i would imagine there going to be a pain to get up there .
there may even be certain building regs as your altering the building spec.

i must add these are only my thoughts i've only got very basic experience of building works.

 

[Eric The Viking]

I'm not storing much, but I hadn't realised years ago that beams that look structural actually aren't, also that these are the only joists in the house that are 4x2 rather than bigger. They're also on a stupidly long span, going to what is effectively a purlin on one end. It's only so that we get some storage space up there (less than half the area), with most of the weight taken by a wall in the centre capable of load bearing. It's a complicated, messy arrangement as it is, and this will help the roof stresses quite considerably.

The steels will be a bit less than 4m, with one end on a pad on top of a load bearing wall, and the other let into the party wall with next door (I have permission to do this), and the bearing area increased with an overgrown version of a joist hanger.

I'm not altering the building spec. as such - It's not a loft conversion or anything like that.

Cheers, E.

 

[Dibs-h]

Why not use joist hanger and fit larger joists?

 

[Eric The Viking]

Why not use joist hanger and fit larger joists?

It's not possible. The joists in question bear on a wall in the middle of the house, and a 9x4 beam/purlin where they intersect the roof line. you couldn't increase them easily unless you removed the joists entirely and started again, probably with at least one steel involved on each side to replace the purlins. That means a huge amount of disruption and mess, and huge expense too. The sag is at point A below:

The purlin/beam takes the weight of one end of the ceiling and the back of the dormer roof, and, as it's about 1/3 up the main roof pitch, it does the usual strengthening job on the roof pitch itself.

It's daft: the ceiling joists run parallel to a load bearing wall (LBW: you're looking through it in the diagram) carrying almost nothing at all at that height in the structure (one end of the beam/purlin at one end of the wall). All the floor joists lower down run crosswise (on the z-axis of the diagram). The sagging joists/ceilings are tacked to the purlins, but only resting on top of the central wall with a 2ft overhang each side.

All I'm doing is providing a bit of support in the centre of the span, to take the spring out of the ceiling and _slightly_ improve the load-bearing. There's no intention to do anything more than use the centre section for 'box room' type storage (the actual box room is a workshop!).

It also means I can finish the insulation properly, which is rather necessary. I was trying to do that in the summer - cross-battening with 2x1s and laying more fibreglass between, with 16mm chipboard on top. It didn't like the added weight of the chipboard, so that's got to come off again and I need to use thin ply instead. I've removed most of the stuff we had stored up there (currently tripping over oscilloscopes and sledges, etc. in the house!), so there's little risk of further problems immediately, but I've got to do the insulation still.

The biggest difficulty is the access (isn't it always?): the widest object I can get in through the loft hatch is about 450-500mm, and structurally I can't improve it. Length is not really a problem, but sheet material has to be cut to width elsewhere. I think four L-shaped RSJs are the easiest option, as I reckon they'll be the easiest to move for a given strength. The current plan involves the minimum of steel drilling in situ and gives the most room for error in fitting the support brackets to the existing joists.

I'm back to my original question though - what size web of steel do I go for? I'm thinking asymetric (taller rather than wider), and probably 4"x 3" (equiv roughly to 7x5), but that's a guess. Any bigger may not be lift-able into place. They'll be 3.5m long roughly, and tied (through-bolted with ply packers) in a couple of places to keep them together.

[later] I've just found a useful calculator site. I'll see where I can get with it, assuming American steel is in the same sizes as here.

 

[wobblycogs]

Don't know about the steel but what about tying the ceiling joists to the upper braced purlins? Depending on exactly how it's arranged in the loft it might just be a matter of cutting chunks of wood to the correct length and nailing them in place. I'd be inclined to use metal strapping as well to make sure they never move. Got to be simpler than trying to get big hunks of metal up into the loft.

 

[Dibs-h]

Or just hoard less.

(I'll get my hat & coat)

Dibs

 

[Eric The Viking]

Don't know about the steel but what about tying the ceiling joists to the upper braced purlins? Depending on exactly how it's arranged in the loft it might just be a matter of cutting chunks of wood to the correct length and nailing them in place. I'd be inclined to use metal strapping as well to make sure they never move. Got to be simpler than trying to get big hunks of metal up into the loft.

Thanks for the thought.

I see what you're suggesting but I think the roof is already under-braced for the weight it takes. At some point in the building's history, probably the 1960s, it was cheaply re-roofed, with Marley concrete tiles rather than the terracotta double Romans it probably had. There was no attempt to strengthen the frame I can see.

I think that added a lot of weight, as there is evidence of additional bracing having been hurriedly added in a couple of places in the corners (to support joints in the hip timbers). The purlins themselves are only braced in the centre of a twelve-foot run (approx).

I can't afford to redo it, so I'm actually trying to get more load onto the suitable walls and remove some from the lower purlins if I can. The steel idea helps this a bit:

The blue line is the dormer. Everything's nailed together at point "X" (lower big purlin) but not at "Y" presently. If I bolt the pairs of joists where they cross on the centre wall ("Y") and take the weight off the centre sags (without lifting too much, if at all), then I'm actually applying a gentle brace to the big purlins too, which ought to help a bit. Happily there's no discernable rot nor woodworm activity!

@ Dibs: no chance on that one!

 

[wobblycogs]

Some one in the past has done the same to our roof, replacing fairly light slate with concrete. Some of the rafters have bowed a little but the main timbers in the roof seem to be fine (which is lucky). I think you've probably come up with just about the only workable solution unless you tear the house apart. If it was my place I'd do the calculations and then stick the biggest bit of steel up there that I could (assuming I could get a sufficiently large piece in there). You don't want to be going back to the job in the future after all.

Ok, alternative idea number two has just come to me...

One possible alternative solution would be to cut strips 18 or 25mm ply the same length as the rafter and then thoroughly nail them to either side of the rafter. If they rafters are securely held at both ends (the LBW end certainly is) then you could effectively make a much taller rafter which would be stronger. Much faster and easier to fit than the steel I'd guess.

 

[Dibs-h]

Assuming your loft joists are @16" centres - adding an extra one between the existing ones - I would expect it to notionally halve your ceiling deflection. If the ceiling below it is not lath and plaster - should be doable.

Dibs

 

[Eric The Viking]

The ceiling is lath and plaster (and pigeon poo). the house is 1905 (approx.). The biggest problem is there is nothing structural to hang one end of the joists onto safely. I've already done what you suggest at one end of one of the lower gables, to take the weight of the hot water tank. They're nominally about 1" above the top of the ceiling (they're sagging a bit now but they've been in there for a decade or so). It worked well, but there were load-bearing walls about 12ft apart to take advantage of.

 

[Dibs-h]

The ceiling is lath and plaster (and pigeon poo). the house is 1905 (approx.). The biggest problem is there is nothing structural to hang one end of the joists onto safely. I've already done what you suggest at one end of one of the lower gables, to take the weight of the hot water tank. They're nominally about 1" above the top of the ceiling (they're sagging a bit now but they've been in there for a decade or so). It worked well, but there were load-bearing walls about 12ft apart to take advantage of.

Sorry to sound dense - when you say nothing at one end? Which end is that - is that X or Y in the diagram below?

Dibs

 

[Eric The Viking]

Point X: that purlin/beam is only supported at each end - one in the party wall, and one onto the main internal load-bearing wall of the building.

The weakest section of it is the middle 7ft or so of the dormer - the rafters above, the ceiling joists and the roof joists of the dormer all bear on it, with no other support. It's slightly braced by the sagging ceiling joists, but only because they are cross-nailed to the descending rafters on top of it, and as far as I can see, they're only notched. The dormer joists just sit on top, possibly with 6" nails through from above - it's impossible to tell. I ought really to run another steel in, alongside or above it, but that's for another century, I think!

 

[Dibs-h]

Could you not double up that purlin? Put another one alongside it? Done right - you'd get minimal disruption.

That would then present you with additional options - i.e. doubling up the joists in the loft with somewhere to place the other end.

I'd be tempted to explore this option - it would be a better long term solution and possibly help address other options.

Dibs

 

[Eric The Viking]

Some one in the past has done the same to our roof, replacing fairly light slate with concrete. Some of the rafters have bowed a little but the main timbers in the roof seem to be fine (which is lucky). I think you've probably come up with just about the only workable solution unless you tear the house apart. If it was my place I'd do the calculations and then stick the biggest bit of steel up there that I could (assuming I could get a sufficiently large piece in there). You don't want to be going back to the job in the future after all.

Pretty much my thoughts on it. I was originally going to use something like a 9x4 structural-grade timber, but it would be very heavy and difficult to get into place, and worse, I'd have to be very accurate with alignment drilling the bolt holes in situ (they couldn't be wide otherwise they'd weaken the whole thing. Two parallel L-steels, back-to-back neatly gets round the alignment issue--I can fit the yellow brackets easily, I think), and I'm hoping they'd be quite a bit easier to get up into place.

Ok, alternative idea number two has just come to me...

One possible alternative solution would be to cut strips 18 or 25mm ply the same length as the rafter and then thoroughly nail them to either side of the rafter. If they rafters are securely held at both ends (the LBW end certainly is) then you could effectively make a much taller rafter which would be stronger. Much faster and easier to fit than the steel I'd guess.

That's not a bad idea. I'm not sure how much faster though, as I have twenty-two of them to do altogether, considering the two rooms. They're on a roughly 11" spacing. I think my difficulty would be getting it secure enough at the outer ends - there's hardly any room to work under the eaves, and there's already 75mm Kingspan sticking out from the dormer in the way (yup, I should've used Vermiculite!).

I'm going to have a go at the calculations tonight and see what I end up with. Once I know the webs of the steels, I can have a stab at the weights. Happily I've got some strong-armed help available, so if it's liftable, it's probably do-able.

Thanks to all,

S.

 

[Eric The Viking]

Could you not double up that purlin? Put another one alongside it? Done right - you'd get minimal disruption.

The purlin in question is mostly exposed in the room below, so strapping to it or fixing brackets is pretty much a non-starter. If funds were available, the simplest solution would be to do that, and lower the ceiling by 9" -- nobody would notice!. It's way beyond our resources though these days (we bought 15years ago, when we had two good, professional incomes. Not so now.).

That would then present you with additional options - i.e. doubling up the joists in the loft with somewhere to place the other end.

You're quite right, of course. If we could do that, the obvious fix would be proper floor joists, on joist hangers on the LBW, and probably bolted onto a strengthened/doubled purlin on the outer end, and dispense with the original ceiling joists altogether.

We could also fix the loft access issue too and get some proper bracing in on the upper purlins at the same time. Funds just don't permit, unfortunately though, and, given that it's a lath ceiling, and pretty filthy up there, it would be very messy in the house.

She really, really wouldn't like it*... :-(

Cheers though,

E.

(*some things are just waaay too risky!)

 

[wobblycogs]

I was thinking circular saw to cut ply into 150mm strips and then big nail gun firing ring shanks to attach them to the rafters. I reckon if there's two of you (one cutting, one fixing) you could get the job done in a day. Moving the insulation out the way would probably be the slowest bit of the job but your going to have to do that to fit the hangers for the steel by the look of it.

Either way I'd be interested to see a follow up on this project if you get the time.

 

[Dibs-h]

The purlin in question is mostly exposed in the room below, so strapping to it or fixing brackets is pretty much a non-starter. If funds were available, the simplest solution would be to do that, and lower the ceiling by 9" -- nobody would notice!. It's way beyond our resources though these days (we bought 15years ago, when we had two good, professional incomes. Not so now.).

Assuming the purlin isn't really ancient - reclaimed timber would mean that you could end up with something that is visually acceptable. This may still be cheaper than the steel and all the fittings for Plan A.

Dibs

 

[bosshogg]

Actually your assumption of braced purling between ceiling joists and apex is most likely wrong. The purlins, which presumably are nailed to the rafters, in which case the struts/braces would have been designed to hold the collar span up.
If I'm right the rafters will be of a fare size and dimension and the room(s) created in the loft. That would make more sense as the assumption that any rafters require bracing through a central load bearing wall(s) is quite [*]confounding.

That being the case, instead of angle iron etc. a simple queen post/strut should suffice

You would want to put the props and spreaders in first, slowly, to revert the ceiling back to normal position, once the new struts are nailed in place you remove the props etc. and all should be fine with extra load bearing to boot.

Hope this helps...bosshogg

Imagination is more important than knowledge...
Albert Einstein (hammer)

 

[Eric The Viking]

I see what you mean. I did consider this, early on - hangers/struts up to the purlins. The trouble is that this keeps roughly the same load on the lower ones (in the upper rooms), as the weight is still transferred through the main pitch of the roof.

There is a collar beam though - just one, joining the upper purlins in vertical line with the braces. I'll have to go up and look carefully at the way the joints are arranged. Although the braces don't look as though they're very loaded (gaps in the joints, etc), I'm pretty certain they're supposed to be, as the bottom ends on the central wall are certainly arranged so as to take a compression load. All the rafters, everywhere, are 4x2 timber. I think the upper purlins may be 4x4, but I'm not certain.

I've just done some measurements: the RSJs would be about 15 feet long. The two rooms are each about 14x16. The span of the ceiling joists in question is about 12 feet to the lower 'purlin', but at least they're on 12" spacings. The overall impression you get up in the attic is of something not very well designed and skimpily built. That fits with the rest of the property, which is quite weird in places - walls that ought to be structural actually aren't (lath and plaster instead).

More later, when I can take a camera up there.