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Re: GSBN:Re: holding down the roof (was: Look Ma', no hands!)



Mark, and all others


On Mar 30, 2005, at 21:57, Mark Piepkorn wrote:

Back in Lars Keller's longhair days, a bunch of Euros and a couple
Aussies
came to cowboy country. I think it had something to do with Judy and
Matts.
This wild bunch raced up and down the western U.S.A., perplexing the
'Murricans with their excellent insights and clever questions. I tagged
along indiscreetly in a red Metro.
I can vividly remember the red Metro also the Songdog ranch and the
strawbaliens. Alas since then my memory has seem to have grown less
acute. (I can vaguely remember drinking some spirits on the said eve)

<a  target="_blank" href="http://www.potkettleblack.com/natbild/strawbaliens.html";>http://www.potkettleblack.com/natbild/strawbaliens.html</a>

I seem to recall a tale told by the self-builders at Songdog Ranch -
they
built their big SB house in 1987 - about sitting inside during a big
windstorm and watching a corner of the roof lift up enough to alarm the
Harley-riding Californians, then settle back down... in mostly the
right place.

Probably there was somebody there who was a better memory than me, like
Rhine-mouth Rene, with a better recollection of the details.

See the above expose regarding my sagging memory.

In most conditions the dead loads of a roof are amply sufficient to
keep a roof down. Wind can seem powerful but in fact is quite puny
compared to most building structure weights. Meaning in most cases that
even with high winds up to even force 10 storms the dead loads will
keep the hat on. There is a catch though some roofing materials are
quite light and therefore need to be attached quite well to resist the
loads imposed. This is due to the regular buffeting caused by wind
turbulence. What happens then is that the combined effects of wind and
a resonating structure helps to destroy fastenings and give the initial
push for liftoff. What this means is that edges of roofs should be well
attached, able to resist even slight movement of roofing or other edge
structures like gutters. (Easily tested by just grasping the edge and
tugging it should not move easily any sign of resonance is not a good
sign regarding resistance to storm damage.)

force 6 = 40 ft/s = 2 lb/ft^2
force 10 = 88 ft/s = 9 lb/ft^2

This is based on a steady wind and accounting for a lift coefficient of
1 for the wind load. In gusts wind loads can go up by as much as twice
the average but this is rare 1.6 is mostly what occurs. The lift
coefficient will hardly ever be much higher then 1.2 although building
codes sometimes go up to 2 which is ridiculous meaning a building is a
far better lift generating device then the average airfoil. Further
please note that the wind load goes up by wind speed squared and not
linear.

Rene