Wind loading, am I doing this right?

[MauryMarkowitz]

We're using Windloadcalc and a similar spreadsheet to calculate the lift forces due to wind loading on a solar array. Both tools use ASCE 7-05 Section 6.5.13 for "open" buildings, which seems to be widely accepted as the proper methodology for rooftop tilted solar arrays. When we type in similar settings, the two tools give us very similar, or identical, values.

However, on a recent project we got load estimates back from another engineer that were almost an order of magnitude greater than the ones we got from our tools. This is very worrying.

I can't see under the covers of Windloadcalc, but the other tool is "open". So I'd like to run down the methodology of the sheet and see if it's doing something dumb. If its not, then it seems likely Windloadcalc is fine too.

The sheet starts by calculating a useful q. In our case we used:

h = 40 feet (this particular building)
Iw = 1
Kzt = 1 (no topography in this case)
Kd = 0.85 (default value, this is low, strong and basically inflexible)
V = 90 mph (default for our area)

which selected:

Kz = 0.76 (Cat B, 40 ft)

to arrive at:

qh = 13.4

Ok, then the sheet goes on to add in the gust factor. It uses G = 0.85, the most common default value. Given the fairly small size of the systems and their general rigidity, this looks OK?

Now it selects Cn from the table in ASCE Figure 6-18A on pg 66. I compared the table in the sheet to the table in the ASCE 7-05 and they are the same.

In our case we normally mount panels at 30 degrees, and the sheet selects -1.8, -1.8 for Case A, and -2.5, -0.3 for Case B for the "wind from back" (gamma=0) side, and 2.1,2.1 and 2.6,1 for Case A and B for the "wind from nose" case. These numbers also seem to be the correct ones from the table, at least for the unobstructed case.


Does anyone see any red flags so far?

Is the Kz reasonable?
Does it calculate qh correctly?
Are the Cn's being picked up correctly?

I'm looking for total honkers here, orders of magnitude.

Maury

 

[dcarr82775]

Table 6-18A is for Main Force Resisting Systems (MFRD). I would consider solar panels as components and cladding which often gives higher loads

 

[AVak]

Kz and qh are correct assuming terrain type B.

The Cn values are all correct except for Case B with gamma = 0. In this case they should be equal to -2.5 and -0.5.

Adam Vakiener, P.E.

 

[MauryMarkowitz]

Wow, thanks for the replies guys, this is almost real time!

Avak: That -0.3 was my typo *sigh*

dcarr: I'd prefer to use C&C because it's easier to work with IMHO (everything's in one table). But I was told the numbers came from MFRD, and the numbers he put in the notes show CN's that seem to come from the 6-18A (I see a 2.5 and a 1.8). In any event, 6-19A lift CN is the same as Case B in 6-18A, so ignoring the zone the numbers should be similar -- at least not the order of magnitude I'm worried about.

Ok, so let's continue with the assumption that the numbers are at least semi-correct. Here's the meat of the sheet...

The next line on the sheet takes the length of the panel and multiplies by the cosine of the angle to calculate L. Then it multiplies by G. Then qh. Then the series of CN's in different cells. That looks an awful lot like the equation in 6.5.13.2.

Taking the -1.8 CN as an example, it says the p = -20.5

So same question: does this p pass the smell test for you guys?

I don't know if I'm interpreting it correctly - this is 20.5 psf of lift directed along a line perp to the panel surface, right? If so, that _seems_ about right, it implies about 400 lbf of lift on a typical panel (5.5 by 3.4 feet).

Did I do that right, the sheet doesn't have an output like that, it goes on to a bunch of other calculations.

 

[AVak]

Maury,

Yes, 20.5 psf is correct. It is calculated simply as qh*G*Cn.

dcarr,

What table for components and cladding would you use for this type of rooftop equipment? It is neither a wall nor a roof and doesn't seem to fit the descriptions listed in Figure 6-21 for rooftop equipment.

I think that considering it as an open building is a fairly reasonable assumption.

Adam Vakiener, P.E.

 

[MauryMarkowitz]

Thanks again Adam.

Not to sound completely dense, but is my calculation of approximate loads correct? IE, its just p times "real" area, as it is in 6.5.15? Or should it be the real width times L, the projected length?

BTW, 6-21 could be used as a limit check. Panels are roughly h/D = 1, so if you take the wind-normal-to-face example you get a C of 1.3. This is the same C you get from most of the other simplified formulas for vertical bits and pieces. The component of the loads parallel to the roof surface should always be similar or less than this figure.

 

[dcarr82775]

AVak,

Using MFRS loads for something that is 18 sqft isn't correct. The tributary area of so small that you need to CC not MFRS. Just my opinion.

Since ASCE7 doesn't cover this condition so you are on your own of course. You could use Fig 6-14B and some judgment on how to handle the open bottom. You could pick any number of Figures and tweak them to satisfy yourself.

My point is that the MFRS tables are meant for the entire building, not small individual components subject to high localized pressures. That is why we have CC pressures instead of only MFRS.

Maury,

I have often calculated 20psf and higher for roof uplift. I have not looked through your variables, but if you are assuming MFRS loads then 20psf uplift isn't out of the realm of possibility

 

[southard2]

I would agree that they should be designed for the components and cladding wind loads. Use the same enclosure status as the building. However you eliminate the internal wind pressure provide there is a structural deck below them. This is because it would be shielded from the internal wind pressures.

I've seen several companies selling these solar panels in Florida convincing or trying to convince architects to add them to flat existing roofs claiming that they are self ballasting. Keep in mind the only thing holding them down is s wire. Me personally thinks that they things will be totally destroyed after the first good hurricane comes through seeing as how they only weigh like 3psf.

You can't have it both ways. Either its heavy and your roof is no good or its light and you had better tie them down properly. Has anyone ever come across this before?

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[Ron]

John...yes. I agree. Many times the salesmanship of these guys overrides common sense of those who should know better.

When the next hurricane hits, those panels on the east coast might be on the west coast....duck.

Go Gators.

Ron

 

[MauryMarkowitz]

This is a fantastic thread guys!

dcarr, one of the ways I checked the number from the tool was to compare with the structural strength of the panels. They're rated at 113 psf, which implies that wind loads on the panel would generally be less than that (well, maybe). Thus when I got back a number closer to 350 psf, I started looking for problems.

So if I wanted to use the C&C, assuming the panels are near the edge of the roofline (which you normally do to minimize shadows), would I simply move to the Zone 3 CN's, or is there something else I need to do as well? A rough look suggests the numbers come out around 3 times higher.


John, Unisolar is selling their laminates glued to a 10 degree metal sawtooth, so no open back. They say they're self-ballasting, but personally I think a brick here and there won't hurt!

That said, a friend has a conventional ballasted system here in Toronto. We just got hit by that huge wind storm, I think they said it was going to be 2nd worst in Toronto history (it never fully developed through) and we were getting bursts of 50 mph. His array is at 30 degrees and is weighed down with "a few bricks" (literally). It didn't move an inch. So my guess that even MFRS numbers may be coming out high.

Maury

 

[racookpe1978]

Check also the angle used in both analysis: some people use angle from vertical (for the 30 degrees for solar arrays) and others use angle (up) from horizontal.

 

[csd72]

I definately agree that these should be designed based on components and cladding.

Modern wind codes are designed based on statistical data from wind tunnel tests and tests on real buildings.

Real wind does not act as a regular pressure but comes in gusts which can provide high peaks of pressures over small areas. When averaged out over a large area then these are smaller but in small areas they can be very high.

This is why Cand C are greater than MWFRS.

 

[MauryMarkowitz]

Southard, can you expand a bit on the application of the internal wind pressure?

We mount the panels with at least a foot of open space below them, and they're only about 4 feet high at the top. Wind speeds will be equalized to a great degree.

The ultimate expression of this would be a horizontally mounted panel, which would eliminate all lift terms. This is dramatically different than the normal C&C, where those terms are maximized for the same condition.

 

[lsmfse]

Try this:

http://www.solarabcs.org/wind/

 

[MauryMarkowitz]

Fantastic link ismfse! Thanks!