Cantilever gas station sign 1

[balrog222]

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!

 

[Jerehmy]

what if the sign ends up being stronger than anticipated, surpassing overturning load, and your sign tips over and damages someone/property?

 

[TDIengineer]

I have designed a few signs for medical facilities. We always used the full projection of the 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.

 

[balrog222]

We would certainly put a safety factor on whatever the blowout load was determined to be, and perhaps that wouldn't end up changing the design that much. There's a very good chance that the conservatism also lies within the equations for calculating the footing soil pressure and the allowable soil capacity itself. Really the issue is that I feel like a schmuck for specifying 10' and deeper footings for a sign where the equations dictate the depth, but my practical side knows it is clearly overkill. There may be no way around it but thought it was worth asking the question.

 

[SteelPE]

The code is the code, if you violate it then you should be prepared to justify your violations.

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.

 

[RFreund]

This comes up "all" the time in PEMB's. "The cladding will blow off before the footings uplift, overturn, etc. We don't buy into it even if it is true.

EIT

http://www.HowToEngineer.com

 

[SAIL3]

I doubt if you will get a blow-out wind loading from any reliable source w/o testing....TD covered some pertinent/important points that should be factored in design....that is the relative cost of each component of design...from my own experience, the weight of steel, concrete is relatively cheap compared to fabrication, construction etc...unfortunately, the codes nowadays leave the opposite impression as they chase the last lb of material out of a design at the cost of increased complexity/opaqueness/effort in the design process.

 

[XR250]

@RFreund;

I actually think PEMB's have enough drift, that their wind loading is self limiting :>

 

[JedClampett]

To all of you; How many times has a contractor or owner called after the project and complimented you on providing a nice economical design? It seems whatever we design is considered excessive, no matter how much pencil sharpening we do.
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.

 

[wannabeSE]

Another thing to consider is what happens if the sign face is replaced with new and improved (stronger) material.

 

[MikeHalloran]

The traditional illuminated sign, comprising a skeletal box frame carrying an array of long fluorescent tubes, closed on its major faces with vacuum formed acrylic or polycarbonate sheets, will indeed blow out in a big wind, though it does not appear to be specifically engineered to do so.

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

 

[spats]

When designing a drilled shaft foundation like you're talking about, the limiting factor is almost always the allowable deflection at the top, which is normally 1". This is somewhat arbitrary, but is considered an accepted standard. Sometimes I'm just as concerned with the rotation, or top deflection slope, as this contributes to the overall movement at the top. However, when all is said and done, the limits of drilled shaft design is basically up to the designer. I don't think the 1" limitation is specified in any governing code. It also seems to me that you might think about gaining additional economy by using good software (if you're not already) to tweak the design, rather than trying to cheat the code. The Allpile program is excellent.