Roof load question [RE-wrenches]
Peter Parrish
peter.parrish at calsolareng.com
Wed Jul 13 10:57:01 PDT 2005
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Randy,
It depends on the roof. You didn't mention what kind of commercial building.
A warehouse, office building, retail?
[We just did a 36 kW job in the City of Los Angeles on a "tilt up" warehouse
building. There were glulam beams and purlins, all supported by 5" dia metal
posts. We did all of the calculations you did, figured out where we could
place our support brackets (over beams, not purlins) and gave the
information to a structural engineer. The engineer then checked our calc.
determined the number of brackets and where they should be located.]
First, we had to calculate "dead loads" and "wind loads".
The roof will need to handle the extra dead load (~3-4 lbs per sq. ft.?). To
do this calc accurately, you will need the help of a structural engineer or
bone up on structural calc. yourself (there are books to help you). Hiring
registered structural engineer might cost you in the neighborhood of $1,000.
I'll assume that you did the "wind load" calc. correctly. The net "uplift"
load is the difference between the "wind load" and the "dead load".
There is some fine tuning to the distribution of the ballasting, but I am
not an expert on this. First, most of the uplift occurs at the edges of the
array, if it is essentially one large monolithic area. Second, 100% of the
uplift load is only experienced by the leading edge of the modules; so if
the prevailing winds come from the NW (as they do in LA), then you should
skew your ballasting to the NW leading edges of your array. In our case,
this lead us to placing more ballasting on the West and North edges of our
array, less on the East and South edges, and the least in the center.
One last thing, you might want to re-consider your "non-penetrating"
approach. We found a very neat approach for penetrating anchors (which would
eliminate most or all of your ballast). It is not cheap ($40/anchor in
material alone), If that is an avenue you
are interested in learning more about, give me a call.
- Peter
Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave.
Los Angeles, CA 90065
Ph 323-258-8883
Mobile 323-839-6108
Fax 323-258-8885
peter.parrish at calsolareng.com
-----Original Message-----
From: Barbra Kerr [mailto:bkerr at energy-exchange.org]
Sent: Wednesday, July 13, 2005 9:29 AM
To: RE-wrenches at topica.com
Subject: Re: Roof load question [RE-wrenches]
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Allan Sindelar writes:
> Wrenches,
> We are proposing a non-penetrating roof-mount system for a commerical
> installation. We plan to use a UniRac SolarMount platform for this
system.
> Working through the calculations provided by UniRac, we determined the
> ballast required for this system is 11 lbs. per sq. ft. excluding the
> modules and rack.
>
> My simple question is: what are flat roof building structures designed to
> carry? Are there any rules of thumb for this? I will require the customer
> to get an engineers approval for this, but I would like to know up front
if
> this is ridiculous or in the ball park?
> Thanks,
> Randy
>
> Randy Sadewic
> randy at positiveenergysolar.com
> Positive Energy
> 3225A Richards Lane
> Santa Fe, NM 87507
> Phone 505 424-1112
> Fax 505 424-1113
>
> Enjoy the Sun!
>
>
Randy -
My experience with commercial flat roofs (and the structural enginnering
firms)lead me to suspect that the "total" anticipated load may exceed not
only the intended design load, but also the margin for future possible loads
that many engineers and building departments have built into both their
designs and building codes. You are wise to put the issue of engineering
approval in the hands of the Owner.
More directly to your question. Each county has their own specific building
codes. What works in San Diego may not be acceptable for Sante Fe. You can
go to the building department having jurisdiction over that building and
view the original building plans and the engineering/load calcs for the roof
and the entire building. You will be able to walk away with copies of the
engineer's original calculations, take those to the engineer reviewing the
PV design and load, and not only will you save the Owner money, but you will
keep the project moving.
Should the PV design call for "beefing up" the glu-lams/joists/etc...please
be reassurred that this additional work doesn't have to result in excesive
costs. We have had to do modify existing glu-lam beams that resulted in
only 2 extra days worth of work for the City of San Diego for another
self-ballasting non-penetrating roof mounted PV system.
Barbra K. Kerr
President
Kerr Enterprises, Inc
209-586-9011
209-586-8607 Fax
858-337-5097 Cell
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