×
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

Gravity Foundation to Resist Lateral Forces?

Gravity Foundation to Resist Lateral Forces?

Gravity Foundation to Resist Lateral Forces?

(OP)
Hi there,

I am new to structural design and I am designing a foundation for a tower. The tower itself does not weigh much (450kg) and because I am in a hurricane prone area I want to ensure that the foundation stays in place in 150 mph winds. The lateral forces therefore govern. I used a very conservative estimate of the area of the tower that the wind hits (I assumed the area as a block instead of as a tubular area as shown in the attached sheets) and applied that force at the very top of the tower in the analysis. I am not sure if I went about determining the restoring moment in the right way but any advice on how to ensure this stays put with the applied forces shown in the sheets will be appreciated. The foundation will be cast on grade and on top of sturdy rock. Would I need to use anchors to anchor the concrete into the existing rock? Is the foundation too big for the application? Is my live load estimate reasonable? Also how can I determine how deep the bottom of the tower should be embedded into the concrete and how can one tie the tower into the rebar (when I get to that stage in the design)?

Points I should mention is that the tower is made of aluminium and is a tube with a diameter of 194mm.

Any help would be appreciated.

RE: Gravity Foundation to Resist Lateral Forces?

For a tower design you'd need to include at least a safety factor of about 1.5 as a minimum against overturning.

This shows up in the building codes as a load combination of:
0.6D + W (1/0.6 = 1.66).

This includes a bit for underestimating the dead load as well.

You included your "live load" of five people. Don't do that - just include dead load and the lateral wind per above.

Using the tower as a block and then placing all of the lateral load at the very top is pretty conservative...it's OK to be conservative but just be aware that a portion of the wind pressure on the tower doesn't have the full height moment arm.

Check out Eng-Tips Forum's Policies here:
FAQ731-376: Eng-Tips.com Forum Policies

RE: Gravity Foundation to Resist Lateral Forces?

(OP)
@JAE

Thanks for your advice. That was silly of me to include the LL. I removed that and bumped up the factor of safety to 1.5 shown in the attached sheet. Any tips or advice for the other questions posed? Also in your opinion, does the foundation seem too large? If so I should perhaps reconsider the block area that I used.

RE: Gravity Foundation to Resist Lateral Forces?

And sadly, 5 modern-day, Western society humans don't have a combined mass of 350kg smile

RE: Gravity Foundation to Resist Lateral Forces?

From simple observation, it seems to me that the aluminum tube will fail at the maximum moment section before it can transfer the calculated moment onto the base.
Some guy-wires could change that situation.

RE: Gravity Foundation to Resist Lateral Forces?

Is your splice there for erection convenience? If not, get the tubular section in a single piece to delete the splice. Since this is aluminum, you'll have a significant reduction in allowable stress because of the gusset and flange welds.

Also, because of the relatively large box area at the top, I would check for torsion. Probably not significant but could be an issue.

Agree with JAE's comments.

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