Minimum Longitudinal Steel Requirement

[hyphenme]

A lot of the foundations I design are drill shaft/caissons/drilled piers. Recently, something came to my attention that I should revisit what I've been using to determine minimum longitudinal reinforcement but can't seem to find any set standards other than the ones I'm going to list below. These foundations are going to be resisting high moments.

We've been using ACI 318, 15.8.2.1 which is 0.5% of gross area. However, section 1.1.6 says that "This Code does not govern design and installation of portions of cencrete piles, drilled piers, and caissons embedded in ground except for structures assigned to Seismic Design Categories D, E, and F."

So I've been looking at other codes to see what kind of standards other people have been using for drilled piers.

The U.S. Dept of Transportation Federal Highway Administration (publication no. FHWA-NHI-10-016) says min reinforcement amount is determined by AASHTO 5.7.4.2-3 equation (As*fy)/(Ag*f'c)>= 0.135, but also states that 0.5% of gross concrete area of the pile is suggested practical minimum.

ACI 336.3R-20 doesn't go into detail on minimum steel requirement. If I missed the specific section, please let me know.

I've read through old threads on this forum and there doesn't seem to be consensus. Overall I've seen a lot of people use 0.5%-1% of gross area.

I've been using PLS-CAISSON to analyze foundations and the manual says that the steel reinforcement portion of the program is based on ACI Publication SP-7 "Ultimate Strength Design of Reinforced Concrete Columns" (1964). I got a copy of the book and can't seem to figure out how they incorporate this into the program. I found that the higher the moment, the quantity the rebar that is being asked for is not reasonable. Also if anyone can try to explain how to use the tables in this book, I would be most grateful. I want to figure out how PLS-CAISSON determines their rebar quantity and if it's something we should consider using in the future or if we should stick with what we've been using (0.5% * Ag).

 

[JAE]

336.1-01: Specification for the Construction of Drilled Piers
Found here:

http://www.concrete.org/PUBS/JOURNALS/AbstractDetails.asp?srchtype=ALL&keywords=336&ID=11304

Or
336.3R-93: Design and Construction of Drilled Piers
Found here:

http://www.concrete.org/PUBS/JOURNALS/AbstractDetails.asp?srchtype=ALL&keywords=336&ID=5167

But if you have high moments in the piers, I'd lean towards treating them as columns via ACI 318 (at least in the top 10 feet or so).

 

[rowingengineer]

Agree with JAE, high moments = columns or beam design depending on the axial force.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[jdgengineer]

Also check review the IBC or your other governing code. The 2009 IBC has an extensive section on reinforcing requirements for piers depending on your seismic design category.

 

[hyphenme]

Thank you so much the replies. I will look into the texts suggested.

 

[hyphenme]

JAE, I meant to type ACI 336.3R-93 in my original post. Both those documents does not reference any minimum steel requirement.

We have been treating the drilled shafts as columns (via ACI 318) though so it is reassuring that its the same that was suggested. I will have to do more research just to see if the minimum asked for ACI 318 is enough though, I will definitely look into IBC 2009 though.

 

[JAE]

hyphenme - a lot of the concrete drilled piers I've designed over the years were in Texas where we were dealing with fair clay soils (highly expansive) over a blue shale strata 20 to 50 feet deep. Typically we'd use criteria from our geotechs for a calculated uplift on the piers from the expansive clays. They would provide us with a forumla of sorts that gave a net force. Also they'd recommend 0.75% Ag as a minimum.

We usually ended up around there if not closer to 1% anyway. With your high moments near the top, and the difficulty of changing vertical steel as you go down the shaft, perhaps dealing with the piers as 1% columns in this particular project is the thing to do.

 

[hyphenme]

Most of our work is also in Texas so I know exactly what kind of highly expansive clays you're talking about. I will have to look into asking the geotechs that does our soil boring on what their recommended steel minimum since they don't usually give us that. Thank you for your response, much appreciated.