Compression Piles 2

[WARose]

The way I read the IBC 2012, there is still a way for you to have the reinforcement in a cast-in-place, concrete pile terminate at a point (and just have a single bar going down the center) where the pile will always be in compression only and has little moment. (I've typically fought this approach.)

But in the AASHTO code, I really don't see a way to do that. It appears to me that you need the 1% minimum at just about all cross sections. For those of you familiar with this code (and I don't have the latest): is that how you see it?

Thanks.

 

[kleo]

I design cast in place piles using the IBC and always use a partial length cage and full length center bar, except in high seismic zones where liquefaction may occur at the interface of soft/hard zones (in which case I may extend a four bar deeper). Per the IBC you only need the full cage to the point where the ultimate moment is less than the cracking moment (they provide a formula for this). I typically take the cage slightly beyond this point depending on the required shear reinforcement.

 

[Teguci]

FHWA has publications on the design of piles which is fairly helpful. Looks like they don't have a download link for the pile design docs though - the drilled pier design doc is available for free.

http://www.fhwa.dot.gov/engineering/geotech/library_listing.cfm

For auger cast piles, the center bar is used to pop the bottom of the auger open to start the casting of the grout into the excavation under a minimum pressure head. Once the auger is removed, a rebar cage is pushed into the top of the fully grouted pile around the full length center bar. Anything more than a 10ft deep cage is frowned upon and it can be expected that for some of the piles, the full cage will not be all the way inserted (plan for it).

A drilled pier, which has a minimum diameter of 30" for skinny geotechs and 36" for our area, is fully excavated. The rebar cage is placed (I've always detailed a minimum of 4 full length bars for piers below the laterally loaded length and a full cage for lengths that will receive bending). and then the concrete is dropped into the hole. If a casing is needed, it is slowly removed under a concrete pressure head.

IBC makes a distinction between the pile length that needs to resist lateral and bending - must be reinforced like a column, and the rest of the foundation length. This is further extended as it relates to the building's Seismic Design Category.

For AASHTO, the FHWA docs should help clarify.

 

[WARose]

Thanks for the feedback and the link Teguci.

In Circular No.10 (at that link), section 16.3 says: "Furthermore, the minimum longitudinal reinforcement area in the portions of the shaft that behave as a column should be not less than 1% of the gross concrete area of the shaft. Below the section where the drilled shaft behaves as a column (i.e., is laterally supported) nominal longitudinal reinforcement may be provided. However, 0.5% of the gross concrete area of the pile is suggested as a practical minimum."

In section 16.6.1 (Axial Compression), it says nothing about being able to use a percentage less than 1%.

And the sample pier design (see Appendix A and Figs. 16-7 & 16-8) does not eliminate the longitudinal reinforcement at any point. This leads me to believe that a pier seeing AASHTo type loadings should have these minimum percentages at all depths

Can anyone who does AASHTO design regularly confirm this interpretation (from your experience)?