Uplift Wind Loading Values [RE-wrenches]

Jeff Oldham starpower4u at juno.com
Wed Mar 19 16:33:52 PST 2003


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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