Chord Factor For Solar ASCE 07-16, 29.4.3 & 29.4.4

[smokiibear]

When panels are not tilted, what determines chord factor and/or Lp value? I cannot find anything in the code other than Fig 29.4-7 which doesn't do a good job of explaining this?

Is it related to the direction of the array frame supports, landscape for running long direction of panels and portrait for running short direction of panels? Does direction of roof slope have bearing on this Lp factor?

Also, when Lp > 6.7ft, is the only way to compute values on the panels to do wind tunnel analysis, or forced to get smaller panels to avoid wind tunnel analysis?

Thanks,

 

[driftLimiter]

Lp is simply the length of a single row of panels its totally independent of the roof or panel slope.

The Omega angle can be 0 and you can still use the 29.4.3 or 29.4.4 sections to determine wind pressure design values.

If the array follows the roof slope, I would take a section transverse to the roof slope, measure the length of the solar module there and use that as Lp.

I asked one of the Authors of the Seaoc PV document about when Lp > 6.7ft.

His response mirrored the language in the code and the Seaoc he suggested that Lp being limited was simply due to the testing parameters used to develop the equations. And a project specific wind tunnel test could be useful for differing arrangements. But my opinion is that if Lp is reasonable close to 6.7ft then these values could still be used, but it might be hard to convince a plan reviewer of that because it lacks data.

 

[smokiibear]

Thanks so much for the response.


Why does Figure 29.4-7 show Lp going the direction of the tilt?

Per SEAOC PV2, the chord factor is based on wheather Lp is taken as Portrait or Landscape, and it doesn't appear to be the row length but a orientation of panel that is shown nowhere, lol. My best judgement was that it was based on orientation of panel to array support member....but I just can't verify it against anything in PV2 or Chp 29/30 of 7-16

 

[driftLimiter]

I would take a section transverse to the roof slope, measure the length of the solar module there and use that as Lp.

If there is a 'high' side and a 'low' side of the module, the distance between them along the chord is Lp.

Its based on the total slope of the module including roof slope. For you your module us flat so its just the roof slope that changes the heights of the modules.

 

[smokiibear]

If there is a 'high' side and a 'low' side of the module, the distance between them along the chord is Lp.

Could you kindly direct me to how you found that?

Does it come from the reference:
Kopp, G. A. (2013). “Wind loads on low profile, tilted, solar arrays placed on
large, flat, low-rise building roofs.” J. Struct. Eng., doi: 10.1061/(ASCE)
ST.1943-541X.0000821.

 

[driftLimiter]

I'm just interpreting the figure.

If you're unsure you could evaluate the pressures using either of the lengths that you are considering and take the worst case?

 

[smokiibear]

Yeah....but my panels are 89". And if i give me best "guess", my Lp ends up be that made my Lp 7.41 > 6.47.

Your comment above is what I suspected was the case...

His response mirrored the language in the code and the Seaoc he suggested that Lp being limited was simply due to the testing parameters used to develop the equations. And a project specific wind tunnel test could be useful for differing arrangements. But my opinion is that if Lp is reasonable close to 6.7ft then these values could still be used, but it might be hard to convince a plan reviewer of that because it lacks data.

It's kind of crazy. I spent the last 8 hours trying to get more familiar with "the correct way to do this." In the end, I have a 10' plate with 5' parapets on a slope of 0.5:12. I use and γE = 1.5 for the entire array, and I only have 8.9psf (ASD) in zone 3, which seems ridiculously small. I'm using RT-mini 2 feet, and their tables call for a space of about 14" for exposed. Not sure of the discrpenacy.

 

[driftLimiter]

Ya and so it goes we spend hours dialing our wind pressures in to the 10th of a psf.
Bummer to find out that Lp is > 6.7ft. When this happened to me I had the designers use a smaller module to conform with the code limits. Not my favorite idea but simpler/cheaper than wind tunnel testing.

 

[smokiibear]

So, I spoke with engineer for Roof Tech. He believes Lp is the length of the tilt, weather it's a tilted panel or parallel with slope, from bottom edge of panel to top edge of panel. Still to be confirmed, I suppose.

Thanks for the dialog!!! I appreciate it.

 

[smokiibear]

Looks like I found Lp from the Wind Design Manual Based on 2018 IBC and ASCE 7-16