Roof analysis - does this sound acceptable 1

[OHEcoEng]

Hello, everyone.

First of all, no I am not a student, and I am not a structural engineer. I am a PE with two degrees in Civil Engineering and 15 years experience in the environmental/wastewater field. I have posted a couple times in the past couple months with specific questions regarding wind loading. I never really asked about the actual methodologies I have been using for an overall roof analysis.

A little background (again): I've recently jumped ship and gone to work for a solar design/installation company designing and certifying electrical plans for solar PV systems. Part of my job has been to review roof structures for capacity to support additional dead and wind loading exerted by these PV systems.

I have become somewhat proficient in using ASCE 7-05 for my additional loading and factored combinations, and I have consulted with a structural engineering professor at the local university as well as a local structural engineer. Both have said that I am doing a "fine job", and agree with my methods. Of course, I am also relying on my memories of undergraduate structures classes...and I have to say that I was not impressed with the structural engineer I spoke with. Especially when he looked at me with a puzzled expression when I started talking about ASCE 7-05.

Anyhow, I wanted to get additional opinions here. You all seem like a somewhat talented bunch of folks.

I am not designing new structures. I wouldn't go there without some additional experience and/or classes; only looking at existing structures. Typical buildings are flat (or very low pitch) roofs with membrane. The solar industry typically does not like to penetrate these roofs, so we go with a "ballasted" system, using concrete cap blocks to keep the system in place. At their worst, I have found some of these systems to exert upwards of 8-10 psf in additional dead load. These warrant careful consideration.

Methods: Typically, I am looking at OWSJ or wide-flange beams supporting insulation, decking, etc. I have decided that these new loads can be evlauated as uniformly distributed loads, though they do not typically affect the entire span. What I am doing is back calculating maximum shear and moment based on the SJI loading tables for the member in question. Then, I calculate actual shear/moment from the existing and new loads. The two are then compared. I perform the same analysis for the decking (I have already held up one proposed major installation because these methods indicated that the existing structure wasn't suitable).

The structural engineering professor said that this is the method he would also use; he reviewed my work for one project and said that he also "looked at a few other things" but didn't elaborate.

This is the extent of what I need to do.

Are there any other considerations that anyone can think of? Is there a better, more accurate/effective method for this type of analysis?

Thanks!

 

[hokie66]

OHEcoEng,
I think your approach is sound in regard to the gravity load cases. As to uplift, I am not familiar with the US codes, but would have expected that in some cases you would get more than 8-10 psf, particularly if near roof edges. Other issues to consider are whether the ballast is adversely affecting drainage and whether ponding and/or drifting are worsened.

 

[OHEcoEng]

Thanks, hokie. Yes, I am aware of the need to evaluate deflection, too, and the possibility of ponding due to blockages/deflection. As for snow, after careful consideration, I made a judgement call that drifting is not a controlling factor, as the modules sit only 14" from the roof surface at their highest point. By the time drifting becomes an issue, the modules will be completely buried and will not influence snow depth.

As for uplift, since these are not attached to the roof, the joists/beams and decking will not experience any uplift; only the modules and racking. They are adequately ballasted to resist the 90 mph wind speed for this area (ASCE 7-05 calculations). I calculate my zone 2 and 3 areas for each project and instruct designers to keep the system out of that area. And, yes, I have calculated 25+ psf for some roofs where modules are mechanically attached near roof edges and corners.

Thanks again for your feedback!

 

[SteelPE]

I would be concerned with wind loads as well. Make sure you are calculating them properly.

Back figuring moment and shears are pretty much the way that I do them however, you do need to be aware of shear deficiencies that may exist in you joist. To do this you would draw the shear diagram for the joist and the shear diagram for the loading and then overlay the two. Any place were the loading exceeds the allowable shear you may have a problem. You may want to pick up the book designing with steel joists, joist girders and steel deck written by Fisher. I believe he quickly goes through that analysis.

In snow country, I would be really surprised if you could put any additional loads on a joist roof. I have seen some designs where a roof would place 349 plf on a joist and have the joist selected be capable of resisting 350 plf. Your only hope would be if the company used a large dead load on the roof. I even remember seeing a roof were there were 150 joist loading diagrams on the structural drawings. That means not one extra pound could be added to the roof.

In other parts of the country where roof LL governs you may have a chance as I believe some jurisdictions have ruled that you can not get roof LL where you have solar panels.

 

[JedClampett]

You seem to have a sound approach to these tasks. SteelPE is right as far as joists being designed to the gnat's butt. As a matter of fact, there's a way of specifying joists were you give them the DL/TL and they'll tell you that's all it's good for. That's why I've given up letting the joist manufacturer design the joists and started calling them out myself.
And also don't belittle working on existing structures. In many cases working on an existing structure is much more challenging than designing a new one. You have to figure out load paths and design for someone else's work. There's a real art to that. Wait until you figure out that an existing structure is designed inadequately and you have to make the choice of breaking the news to the owner or trying to figure out how it's stood all these years.

 

[OHEcoEng]

Thank you; sounds like I am on track. I appreciate discussion about the "little" details when doing this. One recent project was a relatively new (10 year old) commercial building with EPDM, lightweight insulating concrete, decking, and a maze of different bar joists and I beams. This is one where my calculations showed this place was designed to the bare minimum.

As a side note, it's amazing to me in this new position how people either don't care or don't understand the responsibility I have in ensuring that our installations are safe and comply with our building code. I've had people basically yelling at me to put my stamp on the plans and forget about it! Kind of a shocker in going from a review role for waste water projects to actually putting my certification and livelihood on the line!!

Mike

 

[OHEcoEng]

Oh...and Jed...I hope I didn't come across as belittling the evaluation of existing structures. I have a great deal of respect for all of you "true" structural engineers. Being in a position where it is ME that has to put the stamp on the plans has shown me how exhausting even a "simple" roof structural review is. Man, I cannot imagine going through the entire design process for a complete structure.

 

[SteelPE]

OHEcoEng

A few years ago when I was a little disgruntled I was flipping through the job listings. I came across a solar company that was looking for a structural engineer. It looked like a good opportunity with a company that was growing fast. I started the application process online to see what they were looking for. I ended up on a page where they wanted me to answer a bunch of questions. The way the questions were worded it appeared as they were looking for someone to be the scapegoat. They wanted to put panels on every roof and they wanted and engineer to take responsibility. Being in the business for a while, and being somewhat conservative with my designs, I realized that I was going to end up fighting everyone I worked with so I didn't bother to complete the application.

I couldn't imagine explaining structural design to a bunch MBA's who only know how to count dollars and cents. When money gets in the way all logic goes out the window with those people (my opinion).

Good luck.......

 

[SteelPE]

Oh, don't forget to look at IBC chapter 34.... they may have a few things in there that may help you out a little. I seem to remember something about 5% overstress.

 

[hokie66]

OHEcoEng,
Welcome to the club...and to the frustrations!

 

[OHEcoEng]

Thanks for all of the tips. And, SteelPE, luckily I am working for a company where the owners respect my opinions and actually value me as an employee!!! When I say no-go, we all sit down and look for an acceptable alternative. Nice change from where I used to work!

 

[Ron]

SteelPE...way to go...too many engineering firms, not just the specialty things, are being run by MBA's and CPA's.....they have no concept of "safety, health, and welfare" of the public. They attempt to drive engineers to do things they shouldn't do, but get by with it in the interest of "risk management". Also, because engineers are what they are, some tend to avoid conflict and yield to this crap. While I feel badly for them, I'd also like to flog them...they bring our profession down. NO JOB IS MORE IMPORTANT THAN YOUR PROFESSIONAL INTEGRITY.

 

[cb1331]

Gentlemen,

A wonder thread... This has been my life for the past 5 months and have little to no support from more senior structural engineers as well.. there aren't any.

Trying to describe justifications for structural design to Owners and Contractors has been needless to say an art and/or like running gauntlet. I am beginning to feel like the only people that actually get this thing of "safety" and "engineering judgement" are engineers and engineers only. Frustrating.

 

[HSIII]

Ron - "NO JOB IS MORE IMPORTANT THAN YOUR PROFESSIONAL INTEGRITY."

I absolutely could not agree more. No employer should have the ability to force an engineer to sign off on something that he believes is not right or unsafe.

I have run up against this a few times where I would not sign off on certain documents because I did not like how they were worded (putting me in the liability seat if and when everything would go south), or did not agree with the design method (junior engineer who kisses royal @$$ with the company CEO who doesn't have an engineering degree).

In the end, I won out, but the battles shouldn't be as tough as they are to protect your integrity, as well as your stamp, within your own company.

 

[beton1]

Sorry if I am late on this thread, just back from out of town. We had a similar project in February. Similar discussions: Thread 507-265716. Yes there is a lot of 'push' from solar manufacturerers/installers and 'their' engineers, you have to hold your ground.