Analysis of Existing Structure with Added Solar Panels

[cruzinbear]

I've been assigned my first solar installation job to be mounted on an existing commercial building. I need to provide adequate framing for the solar panels however the biggest challenge I have on this job is that I do NOT have the original building plans.

What I know is that the building has been constructed in the 1960's. It's a single story industrial structure with a light corrugated steel roof supported by purlins spanning to trusses. The lateral force resisting systems include moment-frames and braced-framed. The solar panels are very light (about 2.7 psf with framing)with framing connecting to existing roof every 48" o/c.

My plan is to conduct a site visit and physically size the purlins and truss systems. Following this, perform structural analysis to determine whether or not the existing roof system can handle the concentrated wind loads (based on the support system of panels 48" o/c and proposed panel layout). Wind load case will most likely govern the design for the connections and direct elements such as the purlins and roof trusses. If additional framing or retrofitting is required I will provide this for the existing roof system.

Does anyone have recommendations regarding this? What conservative assumptions can I make given the existing old construction? Also, should I continue to investigate other structural elements (ex: checking seismic resisting system due to added seismic weight)? Regarding seismic weight, existing roof is probably not more than 10 psf (which I will confirm once at the site) which now will be 12.7 psf (in some areas, about 25% of the entire roof area). However, not having the original plans makes it impossible to know exactly what dead load the engineer of record assumed for the original design.

Any advice or recommendation will be much appreciated. Thank you.

 

[MotorCity]

Here are a few thoughts:

1. I would assume A36 steel framing, pretty typical for this time frame

2. I would be very cautious about checking the seismic aspects. Solar panels are a building component, so according to ASCE 7-05, the minimum seismic load due to the panels would be 0.3*2.7 psf = 0.81 psf. I would call it negligible. I would be very surprised if that had any effect on your building. In the 1960's, seismic design (in the Midwest) was an after thought - if it was even done at all. So if your calculations show that the building requires reinforcement to resist seismic loads (with or without the solar panels) you could be opening a can of worms.....which is ok, as long as you are prepared to explain to the owner that significant upgrades are required.

3. You don't mention if we are talking about 1 solar panel or 1000 solar panels, but if there are only a few solar panels I have seen them anchored to the roof by simply placing CMU blocks on their base to hold them down. The advantage of course is its a cheap installation and you don't have any roof penetrations (a potential maintenance issue)

 

[tstructural]

I work in high seismic regions and am guessing that you are in a low seismic region (I don't know how much our seismic design requirements differ) but a few thoughts:
1. @MotorCity's post above - Although I have used your idea in the past in wind load only designs Per ASCE 7-10 Section 13.4 - second paragraph - Component attachments shall have positive attachments - friction only is not allowed.
2. If you are only adding 5% weight or less to a structure you can avoid a full seismic upgrade of the structure which I'm guessing your situation fits as far as the entire building. But, if you really only have a 10 psf roof diaphragm and you are adding 2.7 psf over 25% of the surface area that works out to a 6.75% in the total roof weight. I would check the roof diaphragm shear capacity and roof connection to walls.

 

[msquared48]

No mention of snow or drift loads. Are you in a snow area?

Mike McCann, PE, SE (WA)

 

[MacGruber22]

My advise is to be very careful with your wind analysis and anchoring or ballast weight computations. ASCE 7-10 does not explicitly contain gust pressure coefficients(GC)for solar panels (particularly for those panels NOT flat to the roof surface). I hear all the time from other engineers, "Oh just use ASCE 7's components and cladding". That is an incomplete and misleading statement. While it is generally true that solar panel elements, and connections are governed by small effective wind areas (placing them in the realm of components and cladding), trying to apply a mismatched ASCE 7 components and cladding procedure could get you significantly underestimated wind pressures from reality.

From that, my suggestion is to read "Wind Design for Low-Profile Solar Photovoltaic Arrays on Flat Roof" by the Structural Engineers Association of California (SEAOC) 2012. Although it has been a while since I spoke with SEAOC, I *think* the they are advising ASCE 7 Committee on incorporating their findings and procedures into a future ASCE 7 release. When you read it you will understand why.

Good luck!



"It is imperative Cunth doesn't get his hands on those codes."

 

[RFreund]

Normally the solar panel company will have done some testing which usually results in lower pressures than ASCE7 (but this may not always be the case depending on the system/geometry). In any case you may want to check with them as they will often provide some uplift and downward pressure information.

EIT

http://www.HowToEngineer.com

 

[MacGruber22]

Normally the solar panel company will have done some testing which usually results in lower pressures than ASCE7

I am fairly certain that those tests are limited to wind tunnel simulation on the arrays/panels only, without consideration of the particular installation.

I have never seen a solar panel company perform a site-specific wind tunnel test, just a cut sheet for the panel/frame with allowable wind loads.

"It is imperative Cunth doesn't get his hands on those codes."

 

[cruzinbear]

Thank you everyone who responded to my thread.

MotorCity, I don't have an exact count yet, but it's going to be about 25% of the roof surface area as noted in my initial post.

tstructural, regarding your note #2, are you referencing any code when you mention the addition of 5% weight or less limit?

MacGruber, thank you for the wind loading reference. The panels for this project are going to be flat on the roof.

 

[MacGruber22]

You're welcome.

That is good news for you, as your GC coefficient will be very similar to the roof surface, and thus make your analysis much easier.



"It is imperative Cunth doesn't get his hands on those codes."