Boy Scout Eagle Project - Simple Sign Post - Wind Load Question 1

[sign33]

Hi Engineers,

My son is doing an Eagle Scout project for a non-profit organization that hosts university students that do testing in the Mojave Desert (deep in the middle of nowhere). His project includes mounting a small stop light, solar panel, and small utility box on a single pole outdoors. He has spec'd a 3" galvanized steel pole to be 12 feet high and plans to put 4' into the ground (16' total length) set in a 10" diameter hole with concrete. The light will be mounted at about 7' high and is 17" x 8.5", the box mounted at 10' high is 10"x18", and the solar panel mounted on top is 20" x 21" and mounted at a 45 degree angle. This is private property and the group leaders have said there are no local or state regulations that apply, but everything in the desert needs to withstand a 120mph wind load. After an exhaustive search of the internet, he has determined that the only way to figure out wind load is the "ask an engineer". The questions he wants to verify would be 1) is 4' of concrete footing enough and 2) is 3" galvanized steel pipe (schedule 40) strong enough for a 120mph wind load.

Any suggestions on who could help a scout verify his calculations?

Thanks,

Mike

 

[JAE]

Just contact a local structural engineer.

 

[bridgebuster]

I have some info to assist with his calculations. I'll post it later when I get to my computer.

 

[bridgebuster]

this is the AASHTO method for calculating wind load. It's straight forward; looks more complicated than it is.

 

[bridgebuster]

here's a table with pipe sizes and section properties. If you're using fence pipe, the yield strength is typically 50 ksi. I would design the pipe as a cantilever beam.

 

[bridgebuster]

This Excel sheet has various work sheets to calculate foundation depth. I typically use the IBC tab. On thing, it can only take one horizontal force. You'll have to calculate the resultant force location.

I assume you don't need to worry about a freak ice storm? If so, use 3 psf for ice loads and a load combination for DL+Ice+ 1/2 Wind (wind load has to be a minimum of 25 psf). Good luck. I'm sure you can find a local engineer who can spare 1/2 hour of his time for a worth cause.

 

[dik]

Pipe, as opposed to HSS, can be 35ksi, too.

Dik

 

[r13]

A few things for you to consider:

1) Fastening method and hardware strength for the attachments to endure the anticipated wind force in a finite time. I'll at least talk to the attachment suppliers, and/or contractor familiar with hanging out door equipment.

2) Footing (concrete block) must have large enough safety factor to against over turning. For short duration of display, additional weight provided by sand bags can be considered.

3) Set limit on distance a person can safely view the display.

Have a fun day out with your son Be safe.

 

[oldestguy]

Interesting thing about this problem is that likely the teacher or reviewer won;'t know anything about the situation and you could run anything by them as long as a bunch of numbers show up. Going above the100 mph on the table with squaring the velocity probably is too restrictive, but no way to check on that. If it blows down at 100,mph, so will it at higher speed.

 

[sign33]

Hi Bridgebuster,

Thank you so much for the files and the background. That was very helpful. One spreadsheet (IBC) calcs to a 10" footing just over 4 ft and the other spreadsheet shows just under 3'. His plan was to put the pole 4' into the ground, but leave at least 6-12 inches below, so either model looks like it is in the ballpark. We did find a friend that is an engineer that will check his plans.

Oldest guy, you are correct, the scouts approving the plans would not know, but the guys that run the testing center absolutely know. It is a center to test (giant) rockets, so everyone is literally a rocket scientist with a deep respect for wind, drag coefficients, center of pressure on various designs, etc. They also want to know that the plans have been checked.

Thank you!

Mike

 

[dik]

Give him a star for being helpful...

Dik

 

[bridgebuster]

@sign33 - you're welcome. When it's done, could you post a picture of the finished project?