Building and Construction - Re: Steel Purlins

Author

Re: Steel Purlins

Weatherlawyer

2006-04-20, 5:21 am

dom@gglz.com wrote:

> 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.
>
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."

Dave H.

2006-04-20, 5:21 pm

"Weatherlawyer" <Weatherlawyer@hotmail.com> wrote in message
news:1145518356.511831.204770@z34g2000cwc.googlegroups.com...
>
> dom@gglz.com wrote:
>
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!
[color=darkred]
> 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.

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

> 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.

Hmm, that must be why no one makes musical intruments out of wood... ;o)
>
> That's all I know.
>
> That and "Google is your friend."
>

Thanks for the comments,

Dave H.
(The engineer formerly known as Homeless)

Dave H.

2006-04-20, 5:21 pm

<dom@gglz.com> wrote in message
news:1145520228.692675.12530@z34g2000cwc.googlegroups.com...
> 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 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?
>
> 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?).

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!
>
> 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.
>

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

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