Uplift Wind Loading Values [RE-wrenches]

[keith]

Hey

I have a wind load question....is it possible to use self tapping screws
into a corrugated roof that is supported by z bar purling for pv racking
support?  Or is it advisable to drill the 90 degree angle iron that
makes the z bar purling assembly and have threaded rod through the roof?
I feel this might be better to limit the compression on the corrugation
and the roof.  What is the wind rating or perhaps a mechanical engineer
would know how many self tappers x width of screw or just go for
threaded rod and sleep better!

Thanks in advance!

Keith Cronin
Island Energy Solutions
318 A Kuulei Road
Kailua, Hawaii 96734
808-262-3268 Tel
808-263-0338 Fax
www.islandenergy.net



-----Original Message-----
From: Jeff Oldham [mailto:starpower4u at juno.com] 
Sent: Wednesday, March 19, 2003 2:34 PM
To: RE-wrenches at topica.com
Subject: Re:RE: Uplift Wind Loading Values [RE-wrenches]

Let's be sure to all be on the same page - 
The original question was "Uplift Wind Loading Values", this is
specificly what I was addressing. The link to the document at UniRac
does not seem to cover this directly and suggests that you contact a
engineering professional. This document and the numbers from Bill here
are only addressing dead and live loading from equipment and wind
combined (the 25-55#/sq.ft.), not UPLIFT. Live and dead loading clacs
are used for verifying the structures ability to support the load.
UPLIFT clacs (13#/sq.f.)are for determining the anchoring system that
holds it DOWN, not the buildings ability to support it. The document
does have pull-outs for various fasteners and this is the data you need
for calculating UPLIFT. A typical 1 kW array will have about 1,200# of
uplift to be anchored down from pull-out.

btw- I've had pretty good success arguing that my array is eliminating
the live load access to a roof area. Live load is usually addressing the
people and materials that might be up there for repair and maintenance.
My argument is that the 17 - 22#/sq.ft. typically allowed/designed for
roofs is taken up by my array as people and materials cannot be piled on
the array, this puts 17#/sq.ft. in my "bank". It can be construed that
wind turns into a live load, but in general most agencies put that into
the dead load. In most cases the array is not increasing roof area so
there in no additional "live"/wind load for the array, it is just a dead
load that is a fraction (typically 1/3)of the allowed live load, that is
no longer available. Give it a try, it works for me more often than not.
This does not mean that the Building Dept. foregoes wet stamped
engineering. Every structural engineer that I've told this to has lit up
and said "Of course! It makes sense to me, where do I sign?"


Jeff Oldham


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