Uplift Wind Loading Values [RE-wrenches]

Bill Brooks billb at endecon.com
Wed Mar 19 17:31:18 PST 2003


Jeff,

Back to the same page. I am absolutely talking about UPLIFT. Live loads are
irrelevant with PV systems and many inspectors will allow you to use the
elimination of live loads as a reason to add weight to a roof. My
calculations are specifically related to withdraw strength of fasteners in
wood. The uplift on a 1kW PV system can be much more than 1200# depending on
the conditions. Dead load is only relevant for the added weight to the
structure. The dead load increase of the wind is the same with or without
the PV system.

Two main issues:

1. Can the structure handle the additional dead load of the array?

2. Can the fasteners handle the expected withdraw loads imposed by the
uplift of the wind? (total uplift divided by number of fasteners)

Point loading is another issue, but a good design has enough attachment
points to keep this from becoming a problem.

Bill.

-----Original Message-----
From: Jeff Oldham [mailto:starpower4u at juno.com]
Sent: Wednesday, March 19, 2003 4:34 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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