Load carrying capacity of a flat roof for solar panel array

[Ricyteach]

We have a client for whom we provide stamping of drawings for ballasted solar panel mounting systems. Checking the structural capacity of the actual flat rooftop is by others, usually (strictly speaking, we're not a structural firm, though as a geotech oriented outfit we do a lot of structural design work for buried structures per AASHTO, but never IBC).

One of this client's clients is asking us to check the structural capacity of a rooftop for one of the arrays. I looked in 2009 IBC, and the minimum point load carrying capacity for design seems to be 300 lbs. The loads for the system per ASCE 7 seem to break down thusly:

D = 68 lbs ballast + gravel + decking
L = ??? (I assume this is in IBC somewhere but I'm wondering if it should be zero since nobody is going to be walking on top of the solar panels)
S = 20 psf X 17.6 SF panel = 352 lb point load
W = 16 psf X 17.6 SF (roughly - need to check this)

The worst case load combo seems to be #3:
1.5D + 1.6S + 0.5W =
1.2(68 lb) + 1.6(352 lb) + 0.5(281.6) = 806 lbf

And that doesn't even include the gravel portion of dead load.

What I'm wondering is two things:

1. Am I doing this correctly, and

2. Is there any chance that this roof is going to be able to carry this point load? Should we even go to the effort? Assuming it was designed per 2009 IBC, it was only required to carry 300 lbs. Is the point load carrying capacity usually a controlling design consideration for a steel truss roof with metal decking and gravel? Or is there usually a lot of excess point load carrying capacity?

 

[Ricyteach]

This rooftop is for an office building, by the way.

 

[hokie66]

I would just decline. Practicing outside your field of expertise is poor form.

 

[Ricyteach]

We're a full-service civil firm, just originally started out as geotech. There is expertise in the office to do this. As a first attempt I'm giving it a go on my own in order to score points. However, all my work will be reviewed by a PE (I'm only EIT).

 

[JAE]

Practicing outside of your field of expertise as an EIT without a PE overseeing, teaching, or directing you is also in poor form.

Get some direction first.
The fact that you don't have a clue what roof live load suggests you need some initial direction to get started at the very least.

Is there any chance that this roof is going to be able to carry this point load?

The application of point loads to a roof load typically designed for uniform loads depends entirely on the framing and how that point load is applied to the roof....and also where it is applied relative to the framing.

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[Canuck67]

There is substantially more information that needs to be considered, beyond what basic calcs you have provided.

 

[Ricyteach]

Gotcha, I guess that's all I was really looking for. So it sounds like that typically, roofs are designed for uniform load, and just checked for point load. Do I have that right? That would seem to mean that there may end up being quite a bit of excess point load capacity. The panel array is supported in over 150 locations; I'm obviously not going to check them all individually, so I'll probably find the worst-case applied location and check that.

In any case, you guys can rest easy: I'll discuss with the chief engineer tomorrow.

Hard to learn when nobody will talk to you. Appreciate the actual answer instead of "bad form" and "don't even try". There was a point in time I didn't know how to design a cantilevered retaining wall, either, but I do that almost every day now.

Is an 800 lb point load on a roof like this pretty high, relatively speaking? Or just moderate?

 

[BUGGAR]

There's a lot of judgement in roof top solar. Do they support snow? Some say a reduced snow load because they are black to absorb sunlight. Wind will take a team of wizards to find the right ASCE coefficients. Use sawtooth roof coefficients? How to spread the individual panel foot loads? Will they fall between rafters? Do you design for personnel loads for repairmen? Earthquake issues?
And no matter what you or your boss decide, the building department will have a whole catalog of their own for the burning of your budget.

Projects like this are why there are bars located in most cities. But you will learn a lot!

Bob

 

[Ricyteach]

Thanks Bob. In checking the design of the racking systems themselves, we've already worked through a lot of those issues, especially the wind loads. They are using the wind tunnel in procedure per ASCE 7 to determine the pressure coefficients. But we also already know how to come up with the snow loads.

Seems to me that there isn't thing to be additional live loads applied to the ballasted supports since, again, nobody is going to be walking on the panels themselves and transferring that load down to the supports. Instead I think you'd just use the standard required uniform live load over whatever minimum area the ibc requires (I think it was something like 2.5x2.5 ft square from what I read earlier today).

 

[JAE]

Instead I think you'd just use the standard required uniform live load over whatever minimum area the ibc requires (I think it was something like 2.5x2.5 ft square from what I read earlier today).

You mean to say "concentrated" load - not uniform, correct? Uniform load is everywhere. Concentrated on specific points or small areas per the code.

Also - I'm not sure from your response above regarding this statement:

So it sounds like that typically, roofs are designed for uniform load, and just checked for point load.

That isn't correct. You check ALL loads in the various required combinations. So there would always be the dead weight of the panels (concentrated loads) in conjunction with various uniform loads (roof dead, snow, wind, etc.). Some wind might still be applied uniformly to the roof itself, along with interior wind pressures, and some wind would be applied onto the panels which in turn create concentrated wind loads on the roof via panel leg supports.



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[Ricyteach]

I hadn't thought of the wind applied to the roof itself in addition to the panels. I'll keep that in mind.

I am referring to 1607.4 of the IBC:

1607.4 Concentrated loads. Floors and other similar surfaces
shall be designed to support the uniformly distributed live
loads prescribed in Section 1607.3 or the concentrated load, in
pounds (kilonewtons), given in Table 1607.1, whichever produces
the greater load effects. Unless otherwise specified, the
indicated concentration shall be assumed to be uniformly distributed
over an area 21/ 2 feet by 21/ 2 feet [61/ 4 square feet (0.58
m2)] and shall be located so as to produce the maximum load
effects in the structural members.

What I'm saying is, for the load combinations with live load, I would probably add the factored uniform live load required by 1607.3 multiplied by 2.5x2.5 sf and add that to the factored concentrated load calculated from dead load, wind, and snow.

 

[Ricyteach]

To further clarify: the reason I would only add the uniform live load is there can't be an additional concentrated live load applied in the areas where there is a ballasted support. The PV panels completely cover the roof in those regions (except the edges, but at the edges the support is only carrying half the snow load since it's supporting two panels instead of four, which is by far the controlling load being applied to the ballasted support).

I'm actually wondering why I'd need to add live load at all in these regions. The roof area isn't exposed, and as I have said, nobody is going to be driving a forklift on top of solar panels.

 

[JAE]

If you have solid solar panels - you may be correct that a "roof live load" probably can't happen assuming that no one will walk on top of the panels.

But if I'm thinking like a code-writing geek-nerd and I envision a roof system (dead load) supporting a solar panel array (more dead load) it seems like there still should be an added "catch-all" live load just because. I know that doesn't sound totally rational but I've never seen a roof designed for only its dead load (i.e. roof and panel weight) just because you typically don't walk on the panels.

If you are in snow country where snow could pile up on the panels then I could see depending on the snow load and then ignore the roof live load.

I think also there is a requirement now in the codes for rain loading - and rain would fall on the panels and drip below - so maybe that is the "catch-all" load in your case.

Lastly - I don't think you take a uniform load (dead or live) and multiply it by 2.5 x 2.5. What the code is saying is that if you have a concentrated leg load from your solar panel, you can DIVIDE that concentrated load by a (2.5x2.5) area when applying it to a roof/floor surface.

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[Ricyteach]

All understood.

Yes based in looking at the load combinations it's quite clear that snow is going to be the controlling load consideration. As you seem to agree, doesn't make sense to me to compound that with even more additional live load. Unless someone decides to get u there with a snow shovel and clean off their PV panels...? Crazier things have happened.

ASCE 7 does require additional 5 psf rain on snow for S > 0 and <= 20 psf. I didn't include that in my calculation above but it obviously makes matters a lot worse.