Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?

[IsaacStructural]

I've got what looks to be a series of little projects involving putting some AC panels on existing roofs. When previously discussed, the example projects had various steel bar joist roofs, wood roofs etc. Basically I'm supposed to do some quick calcs to verify the roof capacity and provide a simple letter, with an attachment detail or two, as required.

Now when we go live with a real project, the first project comes in and it has a concrete roof structure with no drawings and only some assorted close up photographs of concrete beams. I informed them I couldn't do a whole lot without some decent field information and I don't have X-Ray vision to see what the reinforcing is. Now the electrical engineer/PM on the project is giving me a hard time and saying local guys don't have a problem with this. I'm not sure if this is because they are able to go on site and field verify, or if they are just cavalier about writing letters certifying roofs they can't actually verify.

the only options I can think are:

1. to analyze the slab and beams as if they had minimum steel reinforcing and see if they would work for the loading (doubtful)
2. calculate the loads carefully and try to show that the panels represent only a small increase in total load (<5%) which might work

Anyone got any other suggestions?

Am I crazy? It is a small fee project, so any kind of destructive testing or X-Ray is probably not going to work.

Kind of want to send the electrical engineer a zoomed in photograph of some random piece of wire and ask him to send me a stamped and signed letter certifying I won't get killed if I pull on it.

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[ztengguy]

Write the letter saying based on your knowledge, and calcs, the rook isnt ok for the additional load, and the project shouldnt move forward. I be somehow they find the drawings, or they agree to pay for investigative work. Your not lying, or gambling, and you did your job.

 

[BAretired]

To say the roof is not adequate to carry the additional load is just as bad as saying it is. The fact is that you do not know. It is best to tell the client what needs to be done to make the roof acceptable.

BA

 

[msquared48]

And if you do not know exactly where the building is in Hawaii, don't assume that there is no snow load. There is seasonal snow on Maui...

Mike McCann
MMC Engineering

 

[msquared48]

Do not assume that there is no snow in Hawaii as it does depend exactly where the building is. Next to the coast - no snow, but on the mountain tops, particularly at the observatories, there is - I believe that is on Maui at Mt. Haleakala? Do not quote me on the spelling...

Mike McCann
MMC Engineering

 

[IsaacStructural]

That is fair point Mike. I do have a location on the building and it is not in the mountains. I'd like to just walk away from the work (or not taken it in the first place), but the contact is from within the company, so I have to walk it back more slowly. I'm thinking we'll find a way to pass it to a local, who should have done it in the first place.



Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[dhengr]

Isaac:
It should scare you a bit, that your company or someone within the company, thinks so lightly about the significance of real engineering, and what we do, and what we can and can’t do, that they would push you to stick your neck out this way. If you can’t see it, inspect it, test it, or be given original design docs. and a reliable condition report, you don’t have a thing to hang your hat on. If your company is willing to put you in that kind of position, just to sell product, whatever the contract, or whoever the contact, and their ignorance of what real engineering is, you better watch out. They’ll as likely hang you out to dry, if anything goes wrong, you know... he should have told us he couldn’t do that, etc. And, at that point all you’ve got is your knowledge, experience and P.E. licence, and only you can take good care of those. Your company should just not be putting you in this kind of position, and you should have a talk with your boss about this.

 

[JLNJ]

It's not your fault that the Owner doesn't have records for his building. Don't take on responsibilty - leave it in the hands of the Owner. You might feel bad for the Owner and want to help him out, but it's his building, not yours.

A load test is probably the only way (unless you are under the 5% increase), but that will be time-consuming, intrusive, and expensive.

My experience with ground-penetrating radar has not been good when you need to do real analysis. To find out where the rebar is when drilling a hole? Yes. To determine exact size and location? No.

 

[hokie66]

A contrary view, if I may. I don't think these solar panels are very heavy. It would be a different manner if a lot of air handling units or similar were being added. Any concrete roof that I have ever designed would have no problem supporting some glass and aluminium frames. I would be more concerned about the integrity of the roofing. But saying that, I would still require a site visit to confirm the conditions for myself, especially since the building is in Hawaii.

 

[wannabeSE]

I tend to agree with Hokie66 provided they are not ballasted panels. Ballasted panels use pavers or bricks for anchorage that keep them from blowing off the roof. They don't require any penetrations in the roofing. I have seen typical panels with frame weigh-in around 3psf, while ballasted panel weights run all over the place - 6 to 20 psf. (Disclaimer, I don't have a lot of faith in ballasted panels, but the PV company has somebody else stamp those plans and provide calculations that often times include wind tunnel testing of typical arrays on typical buildings)

Also, it is very unlikely that the roof will ever see the design live load concurrent with the solar panels. I have seen some engineers justify the additional loads by stating the roof live load will not occur at the panel.

 

[dik]

Ballasted panels just increase the load on an already questionable structure.

Dik

 

[haynewp]

I'm not sure about trading the roof live capacity for the panels. If maintenance is carried out on the roof at some point, the panels may be stored vertically in an area on the roof while maintenance operations (replace roofing membrane for example) is being carried out. Potentially exceeding the live load capacity of an affected area. If the roof was new, would you design it for a case considering only the roof live, only the panels, or both at the same time? My point is that things get stacked in areas on roofs during re-roofing and this is now something else up there that would have to be accounted and that is not clearly addressed how to deal with by Code (the combination is a function of D+RLL, many would consider the panels to be a dead load).

The panels are picking up new wind, the amount depends on the angle of the panel to the roof of course. I don't know the building height but if single story it may be a decent new percentage. There are requirements in IBC Ch 34 for existing elements picking up more than 5% new load.

I understand that more expensive pachometers can tell the bar diameter as well. That plus concrete testing is what I would look at.

 

[ztengguy]

Might only be 3-4 psf more, but how do we know it hasnt been used already ....perhaps there is a array of pipes hanging from the ceiling, last engineer said, eh, its only 3-4 psf. Yada yada yada....I would find a way out.

 

[msquared48]

"especially since the building is in Hawaii." Yea, right Hokie...





Mike McCann
MMC Engineering

 

[ztengguy]

Whats the outcome of all this? Any update?

 

[citystructural1]

First Measure the depth of the slab and calculate the Dead Load (assume the density)
if the additional load for the AC panels is within 20% of your dead load then most likely possible

Anyway Since its AC panels most probably the load is negligible if compared to the carrying capacity of the slab.

good luck

 

[IsaacStructural]

It isn't all resolved yet, but I managed to get a higher up structural engineer to go out and visit the projects. He has the authority to handle the situation and won't allow it to come to us signing for something we aren't comfortable with, so I the ship has been leveled and we are back on course. Most likely he'll be explaining to the Electrical Engineer that we can't do the work for the proposed fee range and the company will reconsider if these projects are really something we should be pursuing.

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[SteelPE]

I don't know if this is relevant to you or not.

We do similar projects like this one every once in a while. They are mostly residential structures and consist of someone going onsite and taking measurements of the existing roof. We then usually take a quick look at the roof for the revised loading. In my area we are subjected to snow loads and I always receive the "but the snow is going to slip off the roof" argument which I follow up with "but the code doesn't say that" followed by a days worth of head butting. We usually supply an attachment diagram so they know to "uniformly distribute" the load on the roof. In a few instances we have had them take off the solar panels an attach the racking in accordance with our diagram. I would say about 30-40% of the time we have a contractor reinforce the roof which I imagine will disappear once we are allowed to use ASCE7-10.

I have often seen advertisements for structural engineering jobs at solar panel companies. I imagine a bulk of the work is fighting management to OK roofs for solar panels. Not something I would wish on my worst enemy.

 

[RFreund]

...we have a contractor reinforce the roof which I imagine will disappear once we are allowed to use ASCE7-10

Why is that?

EIT

http://www.HowToEngineer.com

 

[racookpe1978]

16 Jul 13 14:46
I don't know if this is relevant to you or not.

We do similar projects like this one every once in a while. They are mostly residential structures and consist of someone going onsite and taking measurements of the existing roof. We then usually take a quick look at the roof for the revised loading. In my area we are subjected to snow loads and I always receive the "but the snow is going to slip off the roof" argument which I follow up with "but the code doesn't say that" followed by a days worth of head butting. We usually supply an attachment diagram so they know to "uniformly distribute" the load on the roof.

But, the "new" solar panels and panel frames WILL "trap" the uphill snow loads and prevent those weights from sliding off the roof. So, the total load (ice + snow + panels) will be on the roof for longer periods of time. The melted snow will also get trapped and re-freeze into heavier point loads rather than wide area general loads.

I question the "pounds per square foot" assumption too: A solar panel will "concentrate" its loads into a very small area where the lift (airfoil) loads and pressure (gravity and airfoil vibration) loads are attached to the roof itself. So, the total loads won't be spread out like an average snow load, but pinpointed into bolt hole sized areas.

 

[SteelPE]

Question away. When we are allowed to use ASCE7-10 using Pg to design the rafters change to Ps for a roof slope greater than or equal to 7 on 12 (I'm sure the jurisdictions will increase the snow loads accordingly). And I know that a beam with a uniform load placed on it has a higher moment than one with equally spaced point loads. Ice buildup may be a problem but until it is addressed in ASCE 7 I have no idea how to account for it.