Steel Post Embedded in Drilled Shat 1

[BadgerPE]

Is anyone aware of a good reference discussing the ability of a steel structural member to provide the needed reinforcing in a drilled shaft? I am working on a noise wall project and I have details from a previous project that show the only steel in the shaft is a w-section. These are cantilever type posts/piles so lateral wind loads are reacted by lateral soil forces. Any help would be great!

 

[KootK]

Link They discuss at least the existence of such a system around figure 3.1. I imagine that the rationale is to tread the steel as the structural element and the concrete as little more than localized soil improvement.

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.

 

[BadgerPE]

KootK,

My initial thought was to use the steel in the same manner that you suggested. However, the bending moment in the pile is significantly higher than the yield capacity of the steel post so composite action appears to be needed. Mechanically speaking though, I only see this possible through the use of shear studs or some other method of "bonding" the concrete to the structural steel.

 

[jdgengineer]

I would review Chapter I of AISC 360. They have guidance on embedded composite steel members.

 

[KootK]

For some applications, you can get composite action without any mechanical shear connection (studs etc). That may well have been what jd was alluding to in AISC. That said, when I've done that in the past for new construction scenarios, I've always provided a minimal rebar cage for durability at minimum. You know, just to hold stuff together if nothing else. That may be objectionable here where it sounds as though you're in pseudo competition with previously used details.

However, the bending moment in the pile is significantly higher than the yield capacity of the steel post so composite action appears to be needed.

This has me wondering if I understand your situation accurately. Does not your peak moment occur near grade, where you'd be dealing with the bare steel section moment capacity regardless?



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.

 

[BadgerPE]

Weak soils in the upper 10' increase the bending moment on the "pile" system to levels greater than the applied moment at the base.

 

[jdgengineer]

Peak moment in the piers are generally not near grade. They are at the point of zero shear in the pier which can occur quite a bit below grade depending on the passive pressures and neglected depths given.

 

[jdgengineer]

For these situations, we typically do design the bare steel section to take the moment but consider it to be braced by the pier for lateral torsional buckling. The concrete pier then serves essentially as cover for the steel section. We run across this most commonly with wood lagging / soldier pier retaining walls, but your situation sounds similar. If we don't run the steel down the full length, or the pier is much larger than the bare steel section we will put in a reinforcing cage in the pier.

 

[KootK]

Got it. I was thinking of systems with grade beams over the piles where I might consider the grade beam to be a point of lateral restraint for the pile.

With un-reinforced concrete, and giving due consideration to placement tolerances, sacrificial cover etc, it seems to me that composite behavior might not push your center of compression out all that much further out than the flange anyhow. But, then, I haven't run any numbers and I don't know your proposed sizes.

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.