×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Concrete footing size on a cylindrical silo

Concrete footing size on a cylindrical silo

Concrete footing size on a cylindrical silo

(OP)
I have a 30ft x 6'-5dia Silo that weighs 10,000lbs empty (dead load) and 16,500lbs (live Load) when in use. The wind speed in the area is 120 mph. I need to get a rough estimate the concrete slab size for bidding purposes only.

I looked through different engineering handbooks and on this site, but I cant find a good formula to use for this.

i used this formula for wind force but i'm not sure what to do with it.

Force = A × P × Cd
A = projected area of the item (cylinder = 2Πrl)
P = wind pressure (lb/ft2) = .00256 x V^2 (V= wind speed in mi/hr)
Cd = Drag coefficient = 2.0 for flat plates. For a long cylinder (like most antenna tubes), Cd = 1.2. Note the relationship between them is 1.2/2 = 0.6, not quite 2/3

F= (2Π x30'x3.25')ft^2 x (.00256x120^2 mph)lb/ft^2 x (1.2) = 27,099.9 lbs

 

RE: Concrete footing size on a cylindrical silo

(OP)
that really 6'-6" dia

RE: Concrete footing size on a cylindrical silo

Are you sure the live load isn't higher?

 

RE: Concrete footing size on a cylindrical silo

(OP)
no that's it's actually a scrubber/ cooler. It will just have some water running through it. I just called it a silo to be generic.  

RE: Concrete footing size on a cylindrical silo

You can sometimes get into real trouble bidding on these things without a geotechnical report as these things put a high bearing pressure on the soil and could sometimes require piles.

Also, are you in a seismic region?

I suggest you talk to a local structural engineer to get a preliminary design based on local conditions.

RE: Concrete footing size on a cylindrical silo

(OP)
it will be located in north Carolina. I do not have soil or seismic information at this time. i'm just trying to get a general idea of size or amount of concrete.  

RE: Concrete footing size on a cylindrical silo

Why does A include 2*pi?

BA

RE: Concrete footing size on a cylindrical silo

(OP)
projected area of a cylinder = 2Π*r*l
 

RE: Concrete footing size on a cylindrical silo

I would have thought that projected area, A = d*l where d is diameter and l is length.  Why is pi included?

BA

RE: Concrete footing size on a cylindrical silo

(OP)
i just did a search for projected area of a cylinder. that what i came up with

RE: Concrete footing size on a cylindrical silo

(OP)
i thought this would be an easy question for someone who has done this before. all I'm looking for is some rule of thumb so i can get a round about number at this point. I just need to get in the right range

RE: Concrete footing size on a cylindrical silo

drujon,
In wind terms, the projected area is just the width x height.  What you used is the surface area all around the cylinder.

You should not be using this site for preparing bids based on guesswork.  That may get you into lots of trouble.

RE: Concrete footing size on a cylindrical silo

2*pi*r*l is the area if a rectangular piece of material needed to make a cylinder of length l and radius r.  By including it in your formula, you are off by a factor of pi.  Otherwise, your approach seems correct.

So A = 6.5 * 30 = 195 SF

p = 0.00256 * 120^2 = 36.9 psf

F = 195 * 36.9 * 1.2 = 8630# acting 15' above the base.

M = 8630 * 15 = 130,000'#

BA

RE: Concrete footing size on a cylindrical silo

I would usually use a Cd=0.8, but 1.2 is ok for bid purposes.
As csd72 pointed out that if one has questionable soil requiring piles then the cost could be much much higher than the following
normal setup.
Assune soil capacity to be 2000psf.
Size the slab to resist the overturning moment with a safety factor of 1.5....this gives a min. value.
Then check soil pressure at toe, if it is less than 2000psf then you are ok with min. size.
If it is greater then the soil allowable, then increase size of slab untill you get below the allowable soil press.
It all hinges on the assumption of the allowable soil press.
Perhaps you can include with your bid a statement to that effect, absence any soil info.  

RE: Concrete footing size on a cylindrical silo

you can get soil information here and this would be better than just assuming something:

http://websoilsurvey.nrcs.usda.gov/app/

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources