Cantilever gas station sign
Cantilever gas station sign
(OP)
I have been designing foundations and cantilever columns for gas station pricing signs for a client for some time. Many of these signs go up to 40' in the air and have a fairly large surface area and wind loading governs the design. Typical design is to provide a pole footing that is 4' in diameter with depth increasing as necessary to handle the overturning forces from the sign. Problem is some of these footing depths become fairly large and increasing the diameter doesn't change the depth requirements that much.
The truth of the matter is that the sign face (made of plastic typically) will blow out during a code level wind storm far before the footing/soil interaction would fail causing the sign to tip over. We have talked to the sign suppliers, and while they acknowledge this is true, they do not have any testing or capacity information on the sign face. Not really sure why their sign face doesn't have to stand up to code level wind loads but that's a different discussion I suppose.
In the interest of trying to save our client money and not specify obnoxiously sized footings we are trying to see if there are any alternate ways of determining wind pressure for sign face blow out in the hopes that it will reduce the design load of the column/footing. Does anyone have any experience with this issue? Thanks in advance for any help!
The truth of the matter is that the sign face (made of plastic typically) will blow out during a code level wind storm far before the footing/soil interaction would fail causing the sign to tip over. We have talked to the sign suppliers, and while they acknowledge this is true, they do not have any testing or capacity information on the sign face. Not really sure why their sign face doesn't have to stand up to code level wind loads but that's a different discussion I suppose.
In the interest of trying to save our client money and not specify obnoxiously sized footings we are trying to see if there are any alternate ways of determining wind pressure for sign face blow out in the hopes that it will reduce the design load of the column/footing. Does anyone have any experience with this issue? Thanks in advance for any help!






RE: Cantilever gas station sign
RE: Cantilever gas station sign
In terms of footing cost, I don't see the rebar itself or the concrete much of the cost here. It is all in getting the drilling rig to the site and then trucking the cage out there. If it's 10' long instead of 6' is there really that much cost?
If you were out there installing 100 signs, that may be a different story, but I don't see the footing conrete and rebar in itself being that much of your cost factor...
I definitely would not assume that their stuff fails, so you can reduce your footing. I would be conservative here.
RE: Cantilever gas station sign
RE: Cantilever gas station sign
I wouldn't think it is worth the risk. I assume that you are beginning to get some flack from the owner for the depth of these footings? Are you using the equations from IBC section 1807.3 to determine your depth? The only other thing I can think of is to provide a normal footing rather than a pole footing, but that may cause other problems with future excavations.
RE: Cantilever gas station sign
EIT
www.HowToEngineer.com
RE: Cantilever gas station sign
RE: Cantilever gas station sign
I actually think PEMB's have enough drift, that their wind loading is self limiting :>
RE: Cantilever gas station sign
So if you provide a 8 ft. deep caission instead of a 10 ft deep one and lose 5 minutes of sleep over it, is it worth the risk? Would you testify in court that I'm positive the plastic sign face would break out at a certain wind speed? When the lawyer asks you to prove it, is the owner going to be there backing you up?
Depending on the failure of a component to make you design work is a losing game. Don't do it.
RE: Cantilever gas station sign
RE: Cantilever gas station sign
However, those signs are last century's technology.
They're being replaced by huge LED arrays, basically giant televisions, comprising hundreds or thousands of connected modules, all dynamically programmable, to allow display of multiple images, rotated every few seconds, and the individual images can also be animated, or carry video.
They just _have_ to be heavier than a plastic faced box full of air, and I'd bet they won't blow out as easily as a plastic sheet. I'd also bet that a lot of them will be replacing traditional signs _without_ even considering whether the existing stick is adequate or not.
Mike Halloran
Pembroke Pines, FL, USA
RE: Cantilever gas station sign