Foundation design of a sign to handle windload
Foundation design of a sign to handle windload
(OP)
Dear fellow Engineers,
I need to design a foundation for a sign to handle a 90 mph windload. Please advise me on what factors to be considered and the equations and formulas to solve the problem. Actually, there are 2 signs - 1 sign with its pole stacked on top of the other sign and its pole; like 2 "T"s on top of each other.
Thank you very much.
I need to design a foundation for a sign to handle a 90 mph windload. Please advise me on what factors to be considered and the equations and formulas to solve the problem. Actually, there are 2 signs - 1 sign with its pole stacked on top of the other sign and its pole; like 2 "T"s on top of each other.
Thank you very much.






RE: Foundation design of a sign to handle windload
Good luck.
RE: Foundation design of a sign to handle windload
RE: Foundation design of a sign to handle windload
We also rarely reinforce the concrete that is put in the hole. If you run the main column down to the bottom, I don't really see a need for it.
RE: Foundation design of a sign to handle windload
The method of analysis is in Section 3. A 30 psf load at ground level is between 90 and 100 mph.
Ground set poles using typical charts are pretty conservative, but I've never liked using them. We require a soil report and then design the foundation per their recommendations. You might end up with a spread footing or ground water problems.
We have one pole foundation that's about 56 foot square and 7 feet thick.
RE: Foundation design of a sign to handle windload
In addition, Dr. Chris Byrum has written a paper for TRB that presents an analysis method.
See the link here:
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RE: Foundation design of a sign to handle windload
I can fax or email it to you.
RE: Foundation design of a sign to handle windload
1. Use proper wind speed for the sign locale.
2. Use proper code to compute the wind pressures with all applicable coefficients. I get concerned when I read, as started above; to use a blanket wind pressure of 30 PSF. Wind pressure by itself is meaningless! I like to run the wind loads for specific location wind speed, sign height, etc. I can not see how a 30 PSF can apply to any sing anywhere. I practice in “Florida and we have various wind speed zones that vary by county! Often times we have various wind speeds within the county.
3. You must determine the soil profile for the locale. The depth of embedment will depend on soil characteristics.
4. You need to determine bending moment at the base of the sign.
5. You need to determine if the sing is braced at ground level or if it is not. The presence of concrete slab will have an impact on the depth of embedment.
6. You need to account for torsional effects on the pole and the foundation. This is required per ASCE 7. Loads cannot be assumed to act symmetrically.
7. You need to determine the bending moment in the pole foundation and if reinforcement is required or not. You may not need to reinforce the foundation all the bottom.
8. I like to recommend the following short list of related references:
a. 1997 Uniform Building Code (UBC), Section 1806.8, page 2-45
b. Design of Concrete Foundation Piers - by Frank Randall, Portland Cement Association (PCA) - Skokie, IL, May 1968
c. Resistance to Overturning of Single, Short Piles - by Eli Czerniak, ASCE Journal of the Structural Division, Vol. 83, No. ST2, Paper 1188, March 1957
d. Outdoor Advertising Association of America (OAAA) - New York, NY
e. Tapered Steel Poles - Caisson Foundation Design; Prepared for United States Steel Corporation by Teng and Associates, July 1969
9. I have designed many signs and foundations. Some where over 90 feet in height. That can be tricky and one must do a diligent job in his design.
10. One last advice, make sure that your plans reflect details and practices that normal to the industry. What I am trying to say is avoid making non-standard details that may be too costly.
11. Last but not least, seek the advice and assistance of experience engineer. This will be very helpful and could be a great way to learn.
Regards,
Lutfi
RE: Foundation design of a sign to handle windload
It may be conservative to use a minimum of 30 PSF on signs. However, I think as engineers, we owe it to our clients to give them a safe, economical and code compliant desings. If wind load calculations yield less than 30 PSF pressure, why not use the lower pressure?
That is the rational for my statement number 2 above.
Regards,
Lutfi
RE: Foundation design of a sign to handle windload
One thing that I didn't mention before is to require shop drawings and depending how you are putting the sign together, whether you will require special inspection.
One pole we did, we used squirter washers(DIT's) because I could not rely on the inspection (really none)for the A325 bolts (1-1/2" max). The next pole we used bolt tensioning machines (3-1/2"-A354 bolts) and I could more or less trust the erector.
RE: Foundation design of a sign to handle windload
I understand your point about providing economical designs. I work directly for a company that designs, details, fabricates and sells billboard structures to owners. Long before I started in the business, someone (I've been told the Outdoor Advertising Association) determined that 30 PSF was the minimum wind load to be used for design. Many municipalities throughout the country have zoning ordinances that require a minimum load of 30 PSF for outdoor advertising structures. I'm guessing that these all came from the same place many years ago.
I would hate to provide a structure designed for ASCE 7 loads only to find out after erection that there is some law in rural Arkansas that requires a minimum of 30 PSF.
Obviously, the wind pressure can change incredibly for different parts of the country.
I believe that we're providing the owners with what the industry wants. There are many shops similar to our that provide similar products. If we were providing over-engineered (and in turn over-priced) structures, the market would go elsewhere.
Regards,
John
RE: Foundation design of a sign to handle windload
I agree with your explanation.
Codes are intended to provide minimums. Many municipalities do require over and above the code minimums. If that is the case, by all means, engineers must follow the ordinances.
No one should design to ASCE without first checking for local requirements. I still run ASCE, or applicable standard, and compare results because they may come in higher than the 30 PSF.
My point is as I started above, 30 PSF by itself, as a blanket requirement is not meaningful, at least in my opinion. I face this with Cooling Tower suppliers. They tell me that their towers will support 30 PSF! In some areas in Florida that would not corresponds to the code prescribed wind speed with all applicable factors.
I am glad to see a healthy exchange of ideas and opinions. This is what this forum is all about.
Regards,
Lutfi