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Anyone have a formula for calculating wind load? We are doing a 6'x8' ground sign that when applying for permits, the city has asked for the wind load. Any help is always appreciated!
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Posts: 101 | From: Douglasville, GA | Registered: Nov 1998
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An engineer usually gets top dollar for this work in my area. As I found articles in the sign trade magazines, I pulled out the pages and made a folder for all the info I found on this subject. The articles I have are: SignCraft May/June 1991 and Sign Builder Illustrated March/April 1993. Perhaps it is time for some Reprints ?? If you can't find these issues, contact me and I will give you what I have for your project. John
------------------ John Smith Kings Bay Signs Kingsland, GA
Purveyor of fine signage and Innovator of good things yet to be
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Posts: 817 | From: Central Florida - The Sunshine State | Registered: Jan 2000
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ST Publications, parent company of SIGNS OF THE TIMES magazine, publishes a book titled "Engineering Sign Structures: An Introduction to Analysis and Design." Authored by Civil Engineer Benjamin Jones, PE, the book should prove helpful to you.
The hardcover, 274-page book is the most comprehensive resource on sign engineering anywhere, as it covers:
1) Engineering perspective, analysis, practices and history 2) Load calculations for signs exposed to all types of environmental conditions 3) Freestanding sign structures, load effects, resultants and columns 4) Connections, structural bolts and welds, column splices, base plates and anchor bolts 5) Footings for sign columns, soils, structural concrete, pier and spread footings 6) Internal sign cabinet frames, cantilever arms and torsion beams 7) Building signs, wall sign anchorage, wooden post signs 8) Computed aided engineering, spreadsheets and finite element analysis
What you are asking is kind of hard to answer in a post. It almost takes a whole book. There are quite a few variables when it comes to engineering a sign structure and footing. First and foremost, you will need to know how much wind your local building department requires you to engineer for. You will also need to know how much support you can expect from the soil, and if you are required to have soil tests or if your local building department will allow you to "guesstimate" the lateral soil support.
At any rate, a quick rundown is as follows: Most areas require a structure to withstand a 70 mph wind (some require considerably more). The wind load at 70 mph will vary some with the height of the sign but as a quick rule of thumb, 70 mph wind will exert 30 lbs. per sq. ft of pressure. So, as an example, your 6' x 8' sign has 48 sq ft of area which will develop a total of 1,440 lbs of wind load. Now you need to multiply the wind load by the heigth of the sign measured at the center of the face. Lets say as an example that it is 24'. 1,440 lbs x 24' is 34,560 lbs bending moment, which is the force the support must bear at grade level. (The base of the post).
From here you can figure the size of the post and the footing. The engineering factor for new steel pipe is .42, so you would multiply the 34,560 lb bending moment (rounded off to 34.5k by .42 = a support post requiring a section modulus of at least 14.49 sq. in. Now you need to go to an engineering chart for steel posts. The chart indicates that an 8" schedule 30 pipe has a section modulus of 14.69 sq. inches which would be adequate for this project. An 8" x 8" .250 wall square tube would have a secion modulus of 18.35 sq. in. which would be even better.
Figuring the foundation is much more complicated, but from a quick chart, this structure would require a 24" concrete footing 7.5' deep, for average 300psf soil. That would be slightly less than 1 cu. yd. of concrete. If you were installing this sign in sandy Florida, you would need a much larger foundation.
In our area, anything over 35' tall has to be designed by a licensed engineer. Even if you have done the engineering yourself, it's probably a good idea to have a licensed engineer check your figures. Around here, I can get them to do that for about a hundred bucks.
OK - now that I have everybody thoroughly confused, maybe it's time I wrote a new engineering article for one of the magazines.
Jerry Mathel Jerry Mathel Signs Grants Pass, Oregon signs@grantspass.com
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Posts: 916 | From: Grants Pass, OR USA | Registered: Dec 1998
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I've found that no matter how carefully I plot the wind load calcs, the city wands an engineers stamp on it. They don't get impressed with my math, they want to be released from liability.It also releases me. For the $150 it's not really expensive insurance and you learn to factor it into the price of the job.
------------------ The SignShop Mendocino, California "Where the Redwoods meet the Surf"
Posts: 6718 | From: Mendocino, CA. USA | Registered: Nov 1998
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We are in the same situation. Doing a 10'x8' sandblasted HDU sign. I am in the process of building a massive steel frame out of 3/16" 4"x6" tubing. Im not worried about the frame or sign at all, but the footing and ground attachment is where i have questions. We are considering building 2 plates 1/2"x12"x12" with a rebar cage welded to it, and poured in the ground. Then 2 matching plates welded to bottom of 4"x6" posts will bolt up to the ones inground. But how deep to go is our concern. Any engineers out there? Or experience on a sign this size? Thanks Brad
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There are engineers that specialize in sign specs. For about a hundred bucks you get a drawing w/ specs and foundation requirements. In my area (s.w. Florida) ANY sign over 32 sq. ft. or 9' tall requires an engineers stamp and the holes have to be inspected prior to setting the post. The last 6' x 8' signs I did had 4-6"x6"x14' post, in a 12" dia.x48" deep hole w/ concrete. And the water table here is 3', No easy task! Also signs here have to withstand hurricane force winds, 125mph. If you just need to find out wind load, that info should be available. Good Luck.
------------------ Robert Thomas Creative Signs In Beautiful Naples, Fl.
Posts: 965 | From: Bonita Springs, Florida USA | Registered: Feb 2000
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