# Calculating Wind Uplift

Discussion in 'Photovoltaics' started by ken, Aug 13, 2006.

1. ### kenGuest

Does anyone here know a formula for calculating the uplift force
on a ground-mounted array of solar panels? I assume it would
use the area of the panels, angle from horizontal, wind speed,
maybe soil bearing strength, etc. etc.

ken, Aug 13, 2006

2. ### SJCGuest

"ken" <> wrote in message news-sjc.supernews.net...
> Does anyone here know a formula for calculating the uplift force
> on a ground-mounted array of solar panels? I assume it would
> use the area of the panels, angle from horizontal, wind speed,
> maybe soil bearing strength, etc. etc.

Sort of depends on the angle and the wind direction. If they are mounted
at 40 degrees and get wind from the backside, I would suspect that you
would get plenty of force behind them in a 20 knot wind.

SJC, Aug 13, 2006

3. ### daestromGuest

"Duane C. Johnson" <> wrote in message
news:...
> Hi Ken;
>
> ken <> wrote:
>
> > Does anyone here know a formula for calculating
> > the uplift force on a ground-mounted array of
> > solar panels? I assume it would use the area of
> > the panels, angle from horizontal, wind speed,
> > maybe soil bearing strength, etc. etc.

>
> It really doesn't mater what the aerodynamic forces
> actually are. The National Building Code trumps
> this.
>
> The NBC basically specifies a wind load of 10 lbs of
> force per square foot static load on a vertical
> of 90 mph.
>

The physics of it is the worst force on a blunt object is 0.5*rho/gc*V^2
where rho is the density of air (about 0.074 lbm/ft^3), and V is the
velocity. This assumes a drag coefficient of 1.0 and gives the answer in
Force/area.

So a 1 ft^2 in a 63 mph (92.4 fps) wind has a force of just about 10 pounds.
(.5*0.074/32.2*92.4^2)

Of course, if the panels are angled (very likely), then the local wind speed
as it 'squeezes' between/ under/ around the panel could be higher, and hit
the panels at almost any angle.

I wouldn't worry so much about direct upward lift (the weight of the panel
helps keep it down), but more about the side/ forward/ backward forces
tending to push them over. The tensile strength of the supports to hold
them down is probably a lot higher than the lateral bending strength to hold
them upright.

> Make the calculation with the panels in the worst
> possible orientation. For failsafe reasons, you should
> not allowed the use active systems to to ameliorate
> the effects of the wind for this calculation.
>

Agreed.

daestrom

daestrom, Aug 15, 2006
4. ### kenGuest

Thanks everyone -- as Duane surmised, it's all been covered in
the Uniform Building Code. One of the rack mfrs (UniRac) has
made a table of "design wind pressure" as a function of the
official windspeed in your area and "exposure class", both of
which your local bldg. dept. knows by heart. Mine happen to be
70 mph and class B. Plug 'em into the table, out comes 10 pounds
per sq.ft. Assume it's all straight up, and pour an equal weight
of concrete as a foundation or piers. Keeps them happy, and keeps
yer panels from blowing over (apparently )

-K

ken, Aug 15, 2006
5. ### Guest

Duane C. Johnson <> wrote:

>ken <> wrote:
>
> > Does anyone here know a formula for calculating
> > the uplift force on a ground-mounted array of
> > solar panels? I assume it would use the area of
> > the panels, angle from horizontal, wind speed,
> > maybe soil bearing strength, etc. etc.

>
>It really doesn't mater what the aerodynamic forces
>actually are. The National Building Code trumps this.

Would you have a chapter and verse? What is this
"National Building Code" and where does it apply?

>The NBC basically specifies a wind load of 10 lbs of force
>per square foot static load on a vertical surface...
>based on a wind speed of 90 mph.

The basic velocity pressure is 0.00256V^2 psf, with V in mph,
eg 21 psf at 90 mph.

>... For failsafe reasons, you should not allowed the use
>active systems to to ameliorate the effects of the wind
>for this calculation.

You mean like, giant fans blowing in the other direction?

Nick

, Aug 16, 2006
6. ### Martine RiddleGuest

"Duane C. Johnson" <> wrote in message
news:...
> Hi Ken;
>
> ken <> wrote:
>
> > Thanks everyone -- as Duane surmised, it's all been
> > covered in the Uniform Building Code. One of the
> > rack mfrs (UniRac) has made a table of "design wind
> > pressure" as a function of the official windspeed
> > in your area and "exposure class", both of which
> > your local bldg. dept. knows by heart.

>
> http://www.unirac.com/solarmount/sm_pdf/ii214.pdf
>
> > Mine happen to be 70 mph and class B. Plug 'em into
> > the table, out comes 10 pounds per sq.ft. Assume
> > it's all straight up, and pour an equal weight of
> > concrete as a foundation or piers. Keeps them happy,
> > and keeps yer panels from blowing over
> > (apparently )

>
> > -K

>
> Duane

Thanks for the link, should come in handy.

Cheers

Martine Riddle, Aug 17, 2006
7. ### daestromGuest

<> wrote in message
news:ebuv7v\$...
> Duane C. Johnson <> wrote:
>
>>ken <> wrote:
>>
>> > Does anyone here know a formula for calculating
>> > the uplift force on a ground-mounted array of
>> > solar panels? I assume it would use the area of
>> > the panels, angle from horizontal, wind speed,
>> > maybe soil bearing strength, etc. etc.

>>
>>It really doesn't mater what the aerodynamic forces
>>actually are. The National Building Code trumps this.

>
> Would you have a chapter and verse? What is this
> "National Building Code" and where does it apply?
>

As to 'where does it apply', that is much like the National Electric Code.
It depends on the AHJ (Area Having Jurisdiction).

Many parts of the US have state/local building codes that endorse the NBC as
part of themselves. NY for example has their own building code that adds
more requirements than the NBC. Similarly, FL and CA have stricter codes
when it comes to hurricane and earthquake design requirements respectively.
YMMV...

>>The NBC basically specifies a wind load of 10 lbs of force
>>per square foot static load on a vertical surface...
>>based on a wind speed of 90 mph.

>
> The basic velocity pressure is 0.00256V^2 psf, with V in mph,
> eg 21 psf at 90 mph.
>
>>... For failsafe reasons, you should not allowed the use
>>active systems to to ameliorate the effects of the wind
>>for this calculation.

>
> You mean like, giant fans blowing in the other direction?
>

No, I think he was thinking more along the lines of a tracking system that
would tilt parallel to the wind to reduce surface area, or some
retraction/lowering mechanism to reduce exposed surface.

Giant fans? You sometimes take the cake Nick. Only you would say something
that outragous.

daestrom

daestrom, Aug 17, 2006
8. ### Martine RiddleGuest

<snip>
> You mean like, giant fans blowing in the other direction?
>

Like these? www.bigassfans.com

Martine Riddle, Aug 17, 2006
9. ### Guest

daestrom <daestrom@NO_SPAM_HEREtwcny.rr.com> wrote:

>> ... What is this "National Building Code" and where does it apply?

>

>
>Many parts of the US have state/local building codes that endorse the NBC as
>part of themselves...

I don't think we've ever used NBC in PA. We just switched from BOCA to ICC.

>>>The NBC basically specifies a wind load of 10 lbs of force
>>>per square foot static load on a vertical surface...
>>>based on a wind speed of 90 mph.

>>
>> The basic velocity pressure is 0.00256V^2 psf, with V in mph,
>> eg 21 psf at 90 mph.

It looks like the ICC specifies 85 or 90 mph where I live west of Phila
(the code official is supposed to read the 50-year wind map and fill in
a blank.) Table R301.2(2) of the 2006 ICC for 1- and 2-family dwellings
says a zone 3 roof edge (within 4' of a ridge at the end of a roof) would
have 36.8 psf of uplift and a wall would have 14.6 psf of pressure in
a 90 mph wind. These would be multiplied by 1.21 for a 15' mean roof height
in exposure class C (open terrain) vs 1.0 for class B (urban and suburban)
and 1.47 for class D (wind arriving over open water.)

Nick

, Aug 17, 2006
10. ### Guest

Martine Riddle <> wrote:
><snip>
>> You mean like, giant fans blowing in the other direction?
>>

>Like these?

http://www.bigassfans.com

Not exactly, altho they certainly are big ceiling fans, 24 feet in diameter
with a 2 HP motor turning 43 rpm and moving 338K cfm. Wow. What do they cost?
Perhaps they can turn very slowly with the VFD drive, eg 1000 cfm at 0.13 rpm,
ie 8 seconds per revolution?

Nick

, Aug 17, 2006
11. ### Guest

Duane C. Johnson <> wrote:

>... my rule of thumb was what my local building
>inspector told me to use. He said the NBC, National
>Building Code, was 10psi horizontal in 90mph.

Hmmm... 1440 psf.

>And, from the chart, on page 5, this suggests that
>I would need to use about 15psi...

Hmmm... 2160 psf.

Nick

, Aug 17, 2006