Pier Stem With High Moments

[ToadJones]

Say I have a spread footing with a relatively tall pier stem or pedestal with very little axial load and very high biaxial bending moments.
Would it be reasonable to simply design this pier stem as a cantilever beam with biaxial moments?
Finding literature on this has been difficult. The minimal axial load and high moments all but eliminate using interaction diagrams....this simply doesn't fit the bill.

 

[JAE]

Theoretically they should approach the same results (beam design vs. interaction diagram). On column interaction diagrams your [φ} approaches 0.9 like a beam.

 

[Lion06]

I agree, if the axial load is very small, you only get a benefit by using it. Therefore you are being conservative by neglecting it.

That being said, PCA Column handles biaxial moments and axial load pretty well.

 

[ToadJones]

I guess I'd have to make my own interaction diagram. My "e" value is inordinately large due the fact that I have little axial load and large moments.
I guess I am not really following you on the "Phi" part of your reply.
Care to inform me

 

[slickdeals]

You may want to search online for free interaction diagram spreadsheets such as this one.

http://www.yakpol.net/ShortCol.html

 

[ToadJones]

I guess I was curious what others would do; outside of spreadsheet or computer help?

 

[ToadJones]

The Textbook I have says the following:
When a beam is subjected to biaxial moments the following approximate interaction equation may be used for design purposes:

Mx/Mux + My/Muy <1.0

Which we have all seen before...

The text goes on to say that the this may be applied to axially loaded members provided that the axial load is no more than 15% of the column capacity of the member.

Would you feel comfortable with this?

 

[ToadJones]

slick-
have you used this spreadsheet?

 

[hokie66]

15% axial capacity is a long established cutoff where most of us old engineers just considered vertical elements as beams. I see nothing wrong with that approach.

 

[slickdeals]

I think ACI defines it to be a beam if axial load is less than 0.1 fc' Ag, doesn't it?

 

[JAE]

ToadJones - sorry - my bad typing ...should have been [&phi;] which is the capacity resistance factor on the strength.

For columns, [&phi;] = 0.65 while for beams it is 0.90. As your axial approaches zero, ACI allow the "column" [&phi;] to change over to a "beam" [&phi;].

 

[asixth]

a relatively tall pier stem or pedestal with very little axial load and very high biaxial bending moments

I would design it as a beam. If the pedastal is circular then there would be no need to Mx/?Mux + My/?Muy <1.0. You could resolve the bi-axial moment and solve for M/?M <1.0.

 

[ToadJones]

Okay, I'm glad to see that my logic isn't totally flawed.
There is so little axial load (relatively)that I just couldn't see applying any interaction to the design at all.
The pedestal is square and I have designed it as a biaxial beam. I wanted to make it round just for the reason that you state Asixth, but owner says no.
Thanks for the feedback.