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walkereng, Consultant
Category: Structural Engineering
Satisfied Customers: 2764
Experience:  Over 30 years of Structural Engineering experience.
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I have a one story commercial building with finished

Customer Question

I have a one story commercial building with finished basement. The foundation is concrete block with posts and steel beams running 25 feet on center from left to right every 16 feet. THE floor joists tying into I BEAMS Are 2" X 10 (OR 12) INCHES BY 16 FEET. I HAVE NEVER HAD ANY PROBLEMS WITH SNOW LOAD, BUT ALWAYS SHOVEL OFF ROOF WHEN IT GETS TO 3-4FEET. AM CONsidering SOLAR PANELS AND AM WONDERING ABOUT THE ADDITIONAL WEIGHT OF THE pANELS IN THE WINTER. Some ARE MOUNTED IN THE ROOF AND SOME ARE INSTALLED WITH WEIGHTED BALLASTS. IT SEEMS TO BE THAT THE LATTER IS ABOUT 4-7 LBS/S.F. Do you have any comments about which installation is better (thinking I would rather not have roof penetrations). I know there needs to be a complete structural inspection by an engineer, but this will not happen until after I decide on the company, so I am trying to get a rough idea about my situation.
JA: OK. Is there anything else the Structural Engineer should be aware of?
Customer: Don't think so.
Submitted: 10 months ago.
Category: Structural Engineering
Expert:  walkereng replied 10 months ago.

I can help

Expert:  walkereng replied 10 months ago.

Let me read your question a bit closer

Customer: replied 10 months ago.
I am waiting for you response. I do not want the extra charge of phone but can't get if off my screen
Customer: replied 10 months ago.
When will I hear from you?
Customer: replied 10 months ago.
Please refund my money if you can not give me a response
Expert:  walkereng replied 10 months ago.

I can give you an answer

Expert:  walkereng replied 10 months ago.

What city and state are you located in for snow load consideration?

Expert:  walkereng replied 10 months ago.

You say you have 16' span timber floor joists, what his your roof framing made of?

Customer: replied 10 months ago.
Cambridge MA
Customer: replied 10 months ago.
I don't know. I have a rubber roof with plywood sheathing and insulation. That is all I know
Expert:  walkereng replied 10 months ago.

Your Building Code Specified Snow Load is 45 psf.

Expert:  walkereng replied 10 months ago.

Would you be able to access the underside of the roof and snap a few pictures to share with me?

Customer: replied 10 months ago.
No, The building was built in early 1970's. Do you have an opinion on the mounting system for solar?
Customer: replied 10 months ago.
the 45 lbs/s.f. means that if built to code it should be at least 45 psf. Does the structure I gave you seem like this is correct.
Expert:  walkereng replied 10 months ago.

I agree with you that I would want to limit the penetrations into the roof for fear of future leaks.

The ballast mounting system would be my preference, but you need to verify that the roof structure can take the extra load.

Expert:  walkereng replied 10 months ago.

Since I do not know the specific member sizes, I can not make a judgement on roof capacity.

Customer: replied 10 months ago.
If you used the 10" x 2" x 16 " as the most conservative. what would that be?
Expert:  walkereng replied 10 months ago.

The roof should have been designed to support the Live Load of 45 psf, plus the Dead Load of the roof system materials. They do not usually over-design to allow you to add a great deal of weight on top of that at a later date.

Expert:  walkereng replied 10 months ago.

I would need to run a few numbers, can I get back to you in an hour or so?

Customer: replied 10 months ago.
There is HVAC systems on the roof that are very heavy. they have been there since the building was built. The solar panels would be going on other parts of the rood were the HVAC is not installed. Seems like if roof can support HVAC, plus several feet of snow 2 winters ago that panels should not be a problem.
Customer: replied 10 months ago.
Are you still there
Customer: replied 10 months ago.
you can get back to me at my email. Thanks Sharon
Expert:  walkereng replied 10 months ago.


Expert:  walkereng replied 10 months ago.

I’d like to point out that a Professional Engineer’s standard of care typically includes a site visit to assess field conditions and get an overall understanding of the structure. This can obviously not be accomplished through the internet. The information provided here is meant for planning purposes only (general sizing and budgeting) and is based on the information provided by you. All loading cases considered are for vertical loads only, no lateral analysis has been completed. The information should be verified by a professional engineer who can visit the site to ensure that potentially important information has not been overlooked or omitted.

I ran some quick numbers for 16' span 2x10's spaced at 16" o.c. using the 45 psf snow load and 15 psf dead load and they are overstressed at 131% of the allowable.

Since you said you may have 2x12's spanning the 16' and spaced at 16" o.c., I ran numbers for those and they are right at the allowable limit (98% of allowable bending stress).

So if you have 2x10's on your roof, they are overstressed under your existing load.

If you have 2x12's on your roof, they are right at the Code limit, so they work for your existing condition, but you can not add any additional Solar Panel and ballast load on the roof.

The heavy HVAC equipment may be located right over the top of one of your large steel beams, so that is why the heavier load is allowed in that spot.

Based on my rough numbers, you can not add additional load to your roof unless you strengthen it.

You will need to have a local Structural Engineer come out and inspect your roof and give you recommendations for strengthening.

If you feel you have received a satisfactory answer to your question, click the Rating button that is appropriate. Experts are credited for each adequately Rated answer they provide. If you have additional questions, please let me know. Thanks

Expert:  walkereng replied 10 months ago.

Let me know if you have any more questions.

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