Design moment of an embedded steel post 4

[Nate1093]

I am doing a simple member design of a horizontally loaded steel post embedded in the ground. The ground conditions are assumed to be damp/sandy clay with a bearing capacity of up to 150 kPa.

This is an Australian situation so I am referring to AS/NZS 4746 - Structural design requirements for utility service poles. This standard provides the following free body diagram for a variably embedded pole, with the key assumption of the centre of rotation of the footing located at two thirds of the embedment depth below the ground surface.

Now is the design moment simply the horizontal force (Hr) multiplied by the depth to the ground surface (hf)? Or do I need to consider the distance (2D/3) to the 'Pivot depth' underground as well? Please refer to the above image.

My thoughts are that we need to consider the distance to the pivot point in our moment calculation, as the post is assumed to be rotating about the pivot point.

Sorry its a simple question but I am looking for some clarification.

Thanks,

 

[DaveAtkins]

Yes, the moment is the horizontal force times the distance between the force and the pivot point. In this case, M = (Hr)(hf + D/3).

DaveAtkins

 

[dik]

Do you have the magnitudes of the soil stresses f[sub]bv[/sub] and the bottom? I accumulate the values for comparison.


It depends on the formula... some use grade as being the moment arm distance.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[HTURKAK]

Another explanation for max .Moment is, DM/dX = 0 The place where shear is equal to zero. That is, V=0 at D/3. The max. moment will be a little less than DaveAtkins ' s formula.

If we assume R1 (upper passive resistance ) triangular distribution,

M max = Hr*(hf + D/3)- Hr*(D/3)*(1/3)

However, ( I have an old book , Foundations of Structures by Clarence DUNHAM ) Suggests the formula of Dave ATKINS.

The relevant page ,

 

[dik]

Thanks guy...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

HTURKAK any chance you can provide a clear copy of that nomograph? It gets fuzzy when I try to enlarge it so it is readibly readable.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[CarlB]

The attached appears to be the same nomograph shown by HTURRAK.

 

[dik]

Thanks Carl...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[robyengIT]

Dave : the formula you wrote - M = (Hr)(hf + D/3) - or, as you say, M = (Hr)(hf + 2D/3) ??

 

[DaveAtkins]

No, the moment arm is to the centroid of the resisting soil pressure (shown as R1 on the diagram), not where the soil pressure is zero. My original post is correct.

DaveAtkins

 

[HTURKAK]

Dear Dik, i just saw your post. Although the same doc. published by CarlB , i decided to upload togetheer with some other pages since there is worked example how to use the nomograph on page 408..Pls notice that the date is 1947.

 

[dik]

Theory changes... but, dirt doesn't... haven't you heard the expression, "I'm older than dirt." Thanks

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Ron]

You might want to consider the approach by Broms (Broms Method) as compared to Rutledge. I have a paper that compares the two methods and it is attached. Might be helpful and both were fairly well known in their day.

Interesting that the Outdoor Advertising Association pirated Rutledge's nomograph without credit!

 

[BridgeSmith]

Dave : the formula you wrote - M = (Hr)(hf + D/3) - or, as you say, M = (Hr)(hf + 2D/3) ??

No, the moment arm is to the centroid of the resisting soil pressure (shown as R1 on the diagram), not where the soil pressure is zero. My original post is correct.

The analyses I have done using P-y curves have typically resulted in a max moment that would be in between those 2 points. Using D/3 may give you a result that is unconservative, while 2D/3 should give you a conservative result. Max moment is where the shear goes to zero (around D/2 or deeper for the ones I've analyzed), which depends on the soil resistance and relative values of shear and moment at the ground surface.



Rod Smith, P.E., The artist formerly known as HotRod10