Steel Purlins, piers for Irish Jays ;o)

[Dave H.]

Hi Knowledgeable Ones, Can anyone point me at building reg's type span tables or similar for steel purlins?

We're interested in a house that SWMBO wants to convert the loft in (should we get it), and wooden purlin tables only go to about 4m spans, we'd be looking at 6m (the current wooden 8x2" ones are braced to the tops of interior walls at about 2m spacings, and right where the means of escape windows would have to go, so there will need to be two purlins under each face of the roof, one above and one below the windows...) It's a trad' cut roof, so without the braces there's an usable space of about 25' (to the eaves/wall plates) by 17' between end walls, enough for our New Big Bedroom And En-suite, and a maximum floor joist span of 12'6", so within limits for the span tables I've seen... there's even room for the staircase, we reckon :o)

The house is brick + block, with a cast concrete floor, 2 storeys and has probably used its permitted development rights when the previous owners had the garage converted into a couple of habitable rooms and a further bedroom with en-suite built above - the area above this extension is another

8'6 wide (nice short joist span, phew!) for another loft room, again 25' to the wall plates but with an end wall suitable for a means of escape window, so no issues with moving purlins for roof windows like the main area.

The Kitchen's too small for her too (and me, I like cooking!) so we'd want to knock through the (load bearing) wall into the current dining room, with an Irish Jay spanning about 12'. I presume we'll have to have a pair of piers to support the ends - any idea on the sizing for these?

Thanks in advance, Dave H. (The engineer formerly known as Homeless)

[dom]

Hey the group is DIY, not qualified structural engineers! Get a copy of Architects Pocket Book (Charlotte Baden-Powell) - it has tables for safe loads on steelwork up to 8 meters (and formulae for timber beams). But given the seriousness of your plan, I think you'll be paying a structural engineer to get past BC.

[Weatherlawyer]

Too true. It wouldn't have ben so bad if he'd cross posted it to alt.building.construction or some such. Stupid omission rectified BTW.

British homes are regulated AFAIR to take the weight of a large man with a big bag of tools walking across the roof in a blizzard.

The weight or load on a pitched roof in an F12 is considerable as the slope is an aerfoil. The rafters are let into the wall plate and any purlins and fixed with whopping big nails cross stiched.

The wall plate is a 4 x 3 that spreads all this loading finally onto the masonry. Being wood its harmonics tend to deaden vibration. Long (and long unsuported spans, especially) can have amazing resonance problems and steel is a pretty good sounding mechanism.

That's all I know.

That and "Google is your friend."

[dom]

AIU this chaps problem, the purlins cross horizontally through the area where he wants to put velux windows. So he's considering replacing the entire length of the purlin with 2 new steel purlins above and below that line. However I don't understand the bit about the purlins being braced to the walls at 2m spacings. I can't picture this at all. Are there no roof trusses? Are the purlins supported at the gable ends only? How come there's internal walls every 2m under the line of the purlin?

The objective seemed to be means of escape windows (are the gable ends not suitable?). However if there's sufficient space above the purlin for a velux window, a built in step or steps up to the window may be acceptable if the bottom edge of the window would otherwise be too high for means of escape (1 meter max?).

Dave seems to be looking on basing his soultion entirely on the tables in the approved documents. Other (timber based) solutions are possible, but require engineers calculations to be submitted to BC.

[Christian McArdle]

Check with planning. The garage conversion might not have used any PD rights at all if it didn't increase built volume. Even if it involved extending the roof, there may be some left.

Christian.

[Slurp]

"Dave H." steel purlins?

Can't help you with your prob but just for your info your 'Irish Jay' is a Rolled Steel Joist or RSJ.

[Dave H.]

Now, I'd never have guessed... hence the ;o) in the header!

Dave H. (The engineer formerly known as Homeless)

[Dave H.]

Thanks for the recommendation re the book, agreed a structural engineer will have to look over it, buut having seen what a colleague at work got from his S. Eng., there's actually not a lot to the calculations needed to meet the B. Reg'S!

Thanks - I couldn't see the a.b.c group on the server I use, will seek it via google groups (aargh).

Known and understood, there are tables for the wind load expected "once in

50 years", "once in 100 years" as part of the notes to the reg's, I'm aware that the dead load has to be taken as a starting point, not a maximum, with a scale factor to calculate the actual dynamic loads/metre-squared over the roof area.

As well as the rafters being attached to the wall plate, there are structural requirements for dealing with the horizontal loads transmitted to the walls, usually dealt with by tying the walls to the floor and ceiling joists via Sturdy Brackets, as you say, often skew-nailed, although bolts with proper wood fixing washers are better than nails for the joist/purlin/rafter joints (more expensive though, hence rarely used and often misused).

Hmm, that must be why no one makes musical intruments out of wood... ;o)

Thanks for the comments,

Dave H. (The engineer formerly known as Homeless)

[Dave H.]

The purlins are supported at the gable ends, yes, as their replacements would be. There's an internal wall 6' or so from one gable (the stairwell and hallway, running from front to back of the house) and a 45-degree brace runs from a wall plate on top to each purlin; the other braces run from the purlins to an oversized ceiling joist spanning the house front-to-back and bearing on an internal wall in the middle (with 2 off 12'6" spans). There are no horizontal links crossing the loft between the two purlins, other than the brick gable ends. This is a traditional cut roof, not a modern fink-truss roof with lots of triangulation dividing it into 4'-sided triangles (which probably wouldn't have enough headroom to be worth converting in the first place!). Does that give you a clearer idea of the roof structure?

Unfortunately, that wouldn't meet the reg's - there's a very strict maximum distance of 1500mm from the edge of the roof (including guttering) to the bottom edge of the means of escape, and if the bottom of the window's (for instance) 1200mm above the wall plate (allowing 200mm for floor joists etc.), the roofing projects (typically) 300mm from the wall and the roof has enough pitch (eg 45 degrees) to make the space usable for a conversion, the bottom edge of the window is going to be around 2000mm from the edge... BCO's are *very* hot on means of escape, and so would I be!

Thanks for your interest, Dave H. (The engineer formerly known as Homeless)

[dom]

Thanks for your clarifications Dave - I see exactly what you're getting at now. I had forgotten the distance to the roof edge requirement on escape windows.Complete agreement also on some structural engineer calculations being within the scope of the non-structural engineer (having paid my bill for the engineer calculations on my chapel conversion - I'm revising it myself - and quite honestly can do rather better than that which paid =A3250 for).