Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
(OP)
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.
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)






RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
I think you will need to use non-destructive testing to determine the location of the reinforcement.
BA
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
Are the beams big enough that you'd be comfortable that you're adding a small percentage of load? Maybe look at providing structure to bridge back to the beams and then try to prove the new load is a small amount of the beam capacity given minimum steel amounts.
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
Why not dip into the 20 psf live load capacity of the roof, I assume it flat? They won't be re-roofing with the panels in place, so capturing the live load is possible. Possibly a letter to the owner as such, informing them.
Also, with a concrete roof that is much heavier, I could easily see 5 psf added and not even worry about it..It when you add these panel to a 15 PSF dead load (panelized wood roof) where it could be a problem.
What is these panel weights per sqft? If you were in california, I would also be concerned about additional seismic mass if the area is large, but since it appears you are in midwest this isn't an issue! Will the panels change the wind load to the MWFRS? i would assume not.. but another valid concern.
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
Assuming reinforcement is useless. You are essentially making stuff up at that point, and with concrete you can talk yourself into anything.
In general I think that signing off on anything that you have only seen in pictures is risky. Regardless of the original capacity how do you know there aren't other issues - corrosion, alterations, damage etc. Whoever is sending you the photos is probably not doing an in depth evaluation and sending you photos of questionable areas.
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
"Dear Sirs:
"It appears that the added equipment load will only be a small fraction of the total live and dead loads, and in my opinion, will probably not cause structural problems for the existing roof.
"However, as no structural details of the existing roof and roof reinforcing are available, it is impossible to confirm the adequacy of the existing roof, and in fact, it is not possible to confirm that the roof is adequate prior to the work, or that it met building codes when it was built."
Now, if they say that letter just won't do, you need to reword it, etc etc., then THEY are being the persnickety ones that are holding up the job. Or if that letter is all it takes, then there you go.
That's assuming it IS your opinion, of course.
Another thought- would it tell you anything to measure deflection (if any) when the equipment was added?
And if you do calcs and the roof isn't adequate, what do you do then? And can you do that now?
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
Throw them away.
Erect a tower/truss/frame from the floor below right up through the holes to support the AC panels. Install gaskets/buckets/whatever for weatherproofing etc.
Problem solved.
Mike Halloran
Pembroke Pines, FL, USA
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
Do you know the year of construction and the most probable design code (UBC) that would have been used?
I have to do this a lot, and you can get reasonable results, if the client will pay for the testing. If not, then fail the addition. Do not try to be Mr. Nice Guy here.
Mike McCann
MMC Engineering
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
What sort of weight/footprint are we talking about? Is it super heavy, or just alot of them?
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
It's probably as close as you're going to get, short of growing up on Krypton or going to Hogwarts.
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
hokie66, I intended to right PV panels (solar). I have another job that involves a lot of modifications to window AC units, and I guess my wires got crossed. I probably need some more sleep. Why is that the week when work goes ballistic is the same week I'm packing up my hole house for a move on the weekend?
Hopefully I can get enough information to prove the new load is an insignificant increase compared to existing and if needed, I can reduce the roof live load.
Thanks
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
Either do the necessary testing, strengthen the roof or back off and let somebody else be a hero.
BA
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
My guess is the reason they want you, a long distance Engineer, is that they can't find anyone local who's willing to do what they want.
John
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
I’ll bet they never really asked you to do any serious engineering on these jobs, did they? You know..., these are just small jobs, and we can’t afford real engineering, we need your fee money to increase our own profits. All we need from you is your signed letter, saying all is well, and a few phonied-up pages of what look like calcs. and engineering ramblings, so we can get our building permit. You don’t really need to know that some of the rafters are already cracked or rotten, and they probably haven’t looked anyway. That will come to light when the roof fails and you can deal with it them, because that company will likely be out of business. Then, please keep your insurance in force, even if the fees we paid you won’t pay your premiums. We will need you to help cover, or take the fall, the bulls we shoot. That’s the primary reason you’re involved, not for proper engineering advice, but as the insurer of last resort, another deep pocket to distribute the blame and liability.
This approach to engineering and the thought that the general public thinks this is how engineering is done, is really frustrating and scary to me. That really isn’t why I got into engineering, or how I want to do it, or what I like about my work.
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
Dik
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
BA
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
Mike McCann
MMC Engineering
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
Mike McCann
MMC Engineering
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
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.
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
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.
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
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.
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
Dik
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
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.
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
"especially since the building is in Hawaii." Yea, right Hokie...
Mike McCann
MMC Engineering
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
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
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
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.
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
Why is that?
EIT
www.HowToEngineer.com
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
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.
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
RE: Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?
BA