Solar Panels on flat roof

[beton1]

Our client owns a industrial/commercial building. Masonry walls with steel framed roof (OWSJ and beams, alternate demising walls are bearing walls). Building is divided into 2500 sq. ft units , approx 25' x 100', OWSJ @ 6.5 ft cc and span 25'. One tenant is planning to install a number of solar panels with concrete pad base (floating on roof). Owner obviously has concerns.The tenants engineer states the roof is fine as the solar panels only add about 4 psf (each unit is about 330 lb and footprint is 27 sq ft.) However, and this is where I am at odds, they say that the units 'occuppy' 81 sq. ft, and this is how they arrive at 4 psf additional. I am of the opinion that there is a potential for some joists to be overloaded (concentrated load) and don't agree with 81 sq. ft. Ideas? What about snow piling and wind? Something not right!?

 

[msquared48]

)nly 4 psf additinal with a concrete base pad? Maybe only considering the solar panels over the whole extent of the roof.

Something is not right here - how thick is the pad?

Does it cover the full extent of the solar panel array?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[beton1]

This is my point, 27 sq. ft is actual bearing surface, but they are using the average portion of roof devoted to one unit. The majority of the weight is the concrete base pad; but it cannot 'magically' become spread out over 81 sq ft. The total solar unit, frame and pad weigh 330 lb. One joist has a tributary area of 162.5 sq. ft., therefore by "their logic" two units are taken per joist.This adds at least 660 lb per joist. I think that a general configuration (panels will be diagonal relative to roof framing so as to point south) should be superimposed on the roof framing plan,and the most critical locations will be apparent. I think it will be apparent that some joists (or pairs) will easily carry in excess of two or more equivalent (parts of) units. I am not familiar with snow piling studies/diagrams for panels; it seems to be a gray area, but any added live load would definitely push this over the edge. I get the feeling that the incentives by government cover panels only and not roof retrofit/structural and someone is trying to find an out.

 

[Ron]

With OWSJ at 6.5' o.c., I would also be concerned about deflection and progressive overload from ponding on a flat roof.

Since the solar arrays will be tilted at some angle, snow drift could be an issue, as well as wind. Assuming your project is in Ontario or northern US, wind could be less of an issue than snow.

 

[efsinc]

1. The term 'occupy' is vague and must be defined. It may be related to the proposed spacing of the panels such that the final added weight of all solar panel units divided by the grand total floor space equals "4 psf". This may be fine for sales, and it is okay initially for some seismic calculations, but it is insufficient for verifying roof structure for vertical loads. If such spacing is what they are referring to, you must dictate that spacing in accordance with what your calcs show the roof structure can allow.
2. "floating"...i assume you mean ballasted and resting on the roof as opposed to connected to the roof with roof penetrating connections. As I understand it, this ballasted system is the solar panel industries answer to your wind problem. They in effect, by ballasting, assume the responsibility for the wind problem. The only real load wind load imposed upon your structure by the ballasted system is not uplift, but impact, as their undercalculated ballasted system slides across the roof and slams into your parapet.
3. 4 psf? agreeing with "square". It may be 4psf, but best ask the question 3 different ways (does this include the panel?, does this include the ballast, does this include everything?) to make sure they have given you all the information.
4. Drifting snow is an issue. The sun sloped panels will prevent snow from blowing. This may be your most significant load. I don't think the solar industry has any "snow piling studies/diagrams for panel". You will have to go to the basics and calculate these loads. They will add to the load on your roof.
5. The responsibility for roof reinforcement is not well established in the solar industry. The industry itself is young and barely understands its own issues. Whether the Owner can roll the roof reinforcement cost into the total solar package cost or not, I don't know. I know I would try. But our engineering responsibility is clear.

Best of luck!

 

[paddingtongreen]

Ask them how they propose to spread the load to each and every one of the 81 sq.ft.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[TXStructural]

No, you cannot just average over the panel; this sounds like point loading. Direct connection to the framing would be best, rather than the "weighted feet" that you describe. Knowing with certainty where the load is located would be best, and the reinforcing details for OWBJ point loads are quite simple.

Having worked an investigation on OWBJ collapse, where ponding contributed, I second the caution about that.

You mention that some framing is steel (wide flange?), can the load be distributed to demising walls and steel members using a rooftop frame?

And for snow loading, a quick look at ASCE 7 will clear that up. If the panels and supports create a place for drifts to form, you must consider that if snow load controls. Now you have extra dead load deflection, snow, maybe rain-on-snow adding live load deflection. As the snow melts, it can pond significantly, even if there is a typical 1/4" per foot pitch.

The joists where undoubtedly designed on the margin, but a visit to the SJI book, looking at joist tags or plans will define max DL/LL. Get a handle on the actual, as-built DL, rather than design DL, for your evaluation.

If the roof is ballasted, you can remove ballast where the concrete footings sit, to reduce loads, but don't just dig a hole in the gravel and leave it piled nearby.

 

[beton1]

Update: To start, the steel deck cannot take these additional dead loads in certain areas (there is a change in roof height, causing snow piling). This was not factored (ie. not in the 4psf 'reserve' used by the tenants engineer). The deck is pretty well maximum (optimized) loaded without solar panels at these areas adjacent to parapet. We superimposed their panel layout onto the existing roof framing:identified where deck is overloaded and where joists are overloaded. And that is without snow or wind, now the fun begins. They are backpedalling.... We have physically measured joist geometry and cross-sections, obviously they have not done any detailed analysis. So much for the 'occupy' theory.