Foundation design of a sign to handle windload

[3388]

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.

 

[whymrg]

ASCE 7-02 or ASCE 7-05 has procedure to calculate wind load on open signs. From this procedure you will find horizontal load at each sign and then horizontal force and overturning moment at top of foundation. From my experience you need to use drilled pier up to 3 or 3.5 feet diameter and depending on soil determine depth of it, using procedure for rigid pile. Reinforce it as circular column.

Good luck.

 

[JedClampett]

Whenever I run into a situation like this, I like to research the state DOT that I'm doing the design in. They have all kinds of typical details for large cantilever signs and other type roadway structures. They're usually on their website. Even if you calculate something smaller, you probably don't want to to use anything smaller than the DOT design. The State of Florida even has free shareware for some designs (I'm aware of light pole design, but they might have others).

 

[azcats]

I design outdoor signs full time. We use a 30 PSF windload minimum and go up from there as requried for height or windspeed. The 30# is a number that the ourdoor advertising industry came up with many years ago and has apparently served them fairly well.

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.

 

[oldrunner]

Signs are designed per the Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals published by AASHTO. Table 1.2.5A provides the wind pressures starting at 60 mph and continues up to 110 mph.

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.

 

[eric1037]

Something that is often ignored for single-pole signs is torsional loading of the foundation. The Florida DOT has some procedures for design.

In addition, Dr. Chris Byrum has written a paper for TRB that presents an analysis method.

See the link here:

http://trb.org/AM/IP/paper_detail.asp?paperid=10060

 

[eng02002]

There's a paper "Design Criteria for Embedment of Pier" by E. Czerniak. I believe published by ASCE. The paper has a couple of examples on how to design for light poles and similar structures.

I can fax or email it to you.

 

[Lutfi]

I like to offer the following humble input regarding the design of signs and their foundations:

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

 

[Lutfi]

azcats,

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

 

[oldrunner]

Lutfi advice is excellent. I've only done a few signs, but many monopoles and many flagpoles - the tallest at this point is 426 feet. The company I consulted for basic wind speed was 130 mph, bare pole, no mater where the site was. But I'm seeing higher wind speeds for Florida (did only a few there) and I believe that they are real wind speeds.

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.

 

[azcats]

Lufti,

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

 

[Lutfi]

azcat,

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