Moment magnification method for prestressed piles 1

[yakpol]

Hi all,

By ACI moment magnification factor = Cm/(1-Pu/0.75Pc), where Pu - factored axial load, Pc - critical buckling load. In case of prestressed piles, shall Pu include prestressing force?

Thank you for your thoughts!

 

[KootK]

I vote no. Particularly with bonded pre-stressing, the associated axial load is internal and doesn't contribute to instability. It's kind of tricky to explain via text. One way to think of it is this: while the pre-stressing load might initiate / exacerbate lateral pile movement, that very movement would generate a restoring force in the pre-stressing strand.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[yakpol]

Thank you, I had similar thoughts but not quiet convinced yet. If pile is long enough will it buckle under prestress load only? When precast in 200+ feet beds piles are restrained until cut on shorter pieces.

 

[KootK]

Theoretically, there should be no buckling due to pre-stress at any length. I imagine that there is some tolerance to be expected in the location of the strands which would induce a moment in the columns. That moment wouldn't need to be amplified however as it would be unaffected by pile slenderness.

There's no magnification of moment once the pile is in the ground, correct? It's braced by the surrounding soil?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[canwesteng]

I don´t think pre stress can cause axial buckling if it is applied the shear centre/centroid. I can't upload a sketch, but imagine a column with negligible flexural stiffness and applied prestress. Now add some small deflection at the mid point. Cut a section here and you should see that the prestressing strands now actually apply a force that in addition to putting the section in compression will act in the opposite direction of the deflection.

 

[KootK]

Here is an excerpt from Bazant's exellent book Stability of Structures. It deals with precisely this question. If you figure out why the piles are restrained prior to cutting, please do report back.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[yakpol]

Thank you very much, it was an excellent, very to the point answer.