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RE: Net uplift on joists and joist girders

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Once a roof starts to unzip, it is difficult to assign percentages.  But
buckled chords have been observed and indicate a problem.  If you put any
degree of uplift on a joist, and submit it to Vulcraft, they will not
hesitate in adding uplift bridging.  If they think their product needs
uplift bridging, and Factory Mutual indicates a need for uplift bridging, I
don't argue.

Generally the effective wind area or tributary wind area is large enough on
beams where you treat the beams as part of the main wind force resisting
system.  When you look at the ASCE 7-98, Figures 6-5 through 6-7A you will
see that the charts reach a plateau with 100 sf of effective wind area.  If
it is a saw tooth or a building taller than 60 ft the plateau is 500 sf.

The UBC 97 differentiates elements from main wind force resisting systems in
footnote 2 of Table 16H.

I can't understand your calculation:
	"W - 0.9DL = (31psf)(0.7)(1.23) - (0.9)(15psf) = 5psf uplift"
Using your constants I get:
	W - 0.9DL = (31psf)(0.7)(1.23) - (0.9)(15psf) = 26.7 psf - 13.5 psf
= 13.2psf uplift
You are obviously using the UBC and it is not too different than ASCE 7 -98
which takes longer to calculate but would generate about the same result for
a low rise building.

There are probably more wood roof uplift failures than metal deck and bar
joists.  I run the numbers and design for the appropriate uplift for wood
roofs.  Wind will not behave differently on wood versus a metal roof, but
the response and how you design the elements does make a difference.

The wind researchers like Texas Tech and
CPP can point you to all the uplift failures that you could possibly want.
Factory Mutual also has quite a collection of documented roof failures in
any mode you can imagine.

Harold O. Sprague

> -----Original Message-----
> From:	Michael Bryson [SMTP:MBryson(--nospam--at)]
> Sent:	Wednesday, January 16, 2002 2:52 PM
> To:	'seaint(--nospam--at)'
> Subject:	RE: Net uplift on joists and joist girders
> Do you recall when and where you saw this bottom chord failure? Was it
> 100%
> attributable to inadequate bottom chord bracing?
> Also, do you design your beams for localized cladding wind pressures? For
> 90mph wind and 15psf DL I would design the beams for:
>  W - 0.9DL = (31psf)(0.7)(1.23) - (0.9)(15psf) = 5psf uplift
> You are right about satisfying Factory Mutual requirements, but would you
> say the same thing if the roof was wood construction?
> -----Original Message-----
> From: Sprague, Harold O. [mailto:SpragueHO(--nospam--at)]
> Sent: Wednesday, January 16, 2002 11:30 AM
> To: 'seaint(--nospam--at)'
> Subject: RE: Net uplift on joists and joist girders
>  In the land of higher wind loads, I
> have seen buckled bottom chords that resulted in a general failure in wind
> uplift.  That is why Factory Mutual got so active in the roofing
> throughout
> the US.  The insurance losses were real.

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