structural engineer question

climber101 · Posted: Thu Jun 04, 2009 9:03 pm Post Subject: structural engineer question

From structural engineer’s BM calcs he derives that I required for roof rafters = 12.65x10^6 mm^4.

He then says I of a 50x150 rafter = 14.06x10^6 mm^4 and to use that. I can agree with that for a 50x150 cross section but the normal dims of a 50x150 rafter is around 44x145 which gives an I of 11.18x10^6 mm^4- less than the 12.65 desired

(No longer in touch with the engineer)

Shytalkz · Posted: Thu Jun 04, 2009 9:49 pm Post Subject:

It will translate into about 1mm or so greater deflection. Nothing to worry about. Regularised is 47x147 btw

climber101 · Posted: Thu Jun 04, 2009 10:05 pm Post Subject:

thanks Shytalkz - cool

i'm guessing structural engineers might often work to the non regularised sizes

Shytalkz · Posted: Fri Jun 05, 2009 4:12 am Post Subject:

Correct.

You do get the odd uber-zealous checking engineer who seems to think that it makes an appreciable difference. Considering that the overall factors of safety on timber are somewhere approaching 10, it's a bit of a misnomer and I find a robust argument along the lines of suggesting taking their head out of their a**e normally puts it to bed.

Have you fallen out with your ginger beer?

^woody^ · Posted: Fri Jun 05, 2009 9:24 am Post Subject:

You don't mention rafter spacings nor the timber grade, which will make a difference to the roof as a whole.

Also, IIRC, you are having roof lights with double rafters, which means fewer actual single rafers, and the roof is relatively small in any case

Shytalkz · Posted: Fri Jun 05, 2009 11:09 am Post Subject:

And so.....?

^woody^ · Posted: Fri Jun 05, 2009 3:43 pm Post Subject:

... on, and so on

Shytalkz · Posted: Fri Jun 05, 2009 3:58 pm Post Subject:

You've lost me now - and I was wondering where you were going with and to whom your previous post was directed...

^woody^ · Posted: Fri Jun 05, 2009 4:20 pm Post Subject:

It was to the OP. Do keep up ... probably and age thing

Surely the engineer did not just work out timber section size without this in context of the roof as a whole, the grade or particular circumstances of the roof?

Well, probably

Shytalkz · Posted: Fri Jun 05, 2009 4:30 pm Post Subject:

Well, Woodster, my ol' chuck-butty, seeing as I is a function of E which is a function of the timber stress grade, and that I is also a function of the span and load which is a function of the covering amongst other things, then I reckon that the SE might just have taken all those things into account...