×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

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!

*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

Foundation of Monopole: Shallow or Deep?

Foundation of Monopole: Shallow or Deep?

Foundation of Monopole: Shallow or Deep?

(OP)
The company I'm currently working for has been called to do the Geotechnical design of many monopoles in the city. They gave us a sample study (they have worked like that in other countries) and wanted us to do something similiar here.
The requirements were:
-Do SPT continously up to 6 meters.
-correct N to N60 and get friction angle and Su values from correlations.
-calculate bearing capacity of a shallow foundation (from 5x5 m. to 7x7 m.), Df= 2 m.
They insisted that they have used a shallow foundation 90% of the times that they have contructed monopoles. (Monopole height = 36 m.)

We did our job, SPT 6 meters. Found clay (N= 1 to 7) from 0 to 4.5 meters, then found fine sand (N= 7 to 13) from 4.5 to 6.5 m.
water table was at 2.00 meters down the surface. We got friction angle for sand and Su for clay (correlations)

Then we analyzed it under undrained conditions. we gave friction angle = 0 to the clay (undrained condition) and used Hansen theory. q adm was around 30 kPa.
Later they told us that the Moment of the monopole was 2100 kN-m and the wind speed was 140 Km/h -which I think is really really high. Vertical force was 1400 kN.

Should I analyze eccentricity? I analyzed it and it was (M/V = 1.5 m). and B/6 is 7/6 = 1.16 m. So I suggested deep foundations.
However, they insist in using a shallow one.

My questions are: are those wind values real? Is eccentricity calculated well? I know I have to calculate it for strip and spread footings, does the same rule apply to concrete slabs, mats or rafts?



RE: Foundation of Monopole: Shallow or Deep?

So they are designing to a 210 mph wind? Where is this monopole? The wind loads could be LRFD values.

Mike McCann, PE, SE (WA)


RE: Foundation of Monopole: Shallow or Deep?

(OP)
@msquared48, No, it's a 140 Km/h (87 mph) wind. It's in South America, no hurricanes or strong winds here. The maximum annual wind speed in this city is 24 Km/h. However, they said that their client says that's the regional wind speed. That wind causes a really high moment (2100 kN-m, according to our client).

RE: Foundation of Monopole: Shallow or Deep?

Sorry. Wrong direction.

24 km/her seems really low. What is your governing code there?

Mike McCann, PE, SE (WA)


RE: Foundation of Monopole: Shallow or Deep?

What does your contract say? Are you responsible for the job coming out to be safe and and still carry the loads they suggest? If not, I'd cross it off as a lost cause. For one thing why accept the N value correlations for design, if there are such things that are accurate. Going into a job and accepting these limitations I think was rather unwise. Sometimes it is more prudent to back away from imposed risks than to accept them.

RE: Foundation of Monopole: Shallow or Deep?

Agree with oldestguy....
Geotechnical consideration are site specific.....not "one size fits all". Shallow foundations are common for monopoles. I am in a relatively high wind zone in the US and we have used shallow foundations for monopoles, but you have to consider overturning and its effects (eccentricity) and the resulting reduced bearing area (thus increased bearing pressure). With good soil conditions and a large, thick foundation it can be done.

Keep in mind your client is probably looking more at risk assessment than geotechnical/structural design. Don't let your client dictate your engineering judgment.

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! Already a Member? Login



News


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close