Moment reinforcement for drilled piles

[ROLaBere]

I am currently working on a design of a deep foundation. I have found my applied moment through structural analysis and now I am looking for a way to find my required flexural reinforcement of the circular concrete cross section of the pile. I'm not exactly sure how to go about this part of the design. I have searched online and have not found much about it. I would greatly appreciate thoughts and comments about this. Thanks!

 

[CELinOttawa]

The longitudinal bars are designed as for a column...

 

[TLHS]

Your pile diameter is generally governed by geotech or stiffness concerns. As such, watch out when you're reinforcing. The code minimums for steel may be hilariously high in comparison to the actually applied load and you'll want to look at any provisions your code has for going lower than min steel (i.e. design for three times the load or similar clauses).

 

[Lion06]

We provide applied loads at the tops of drilled piers to the geotech, who then provides shear and moment diagrams for design of the reinforcing. The diameter of the piers are usually dictated by required compressive or uplift forces as noted by TLHS.

 

[ROLaBere]

I used L Pile to determine my bending moments applied to the pile. I reviewed the soils report and used that to enter for soil conditions in L Pile. The program tells me the moment capacity for the chosen pile size and rebar. I am wanting to check it by hand to verify the results. My concern was determining the flexural rebar for the circular shaft configuration. As CELinOttawa stated, I should design as if it were a column. I didn't know if there was any formulas applying to circular concrete members or if I should use the the column load interaction tables for columns.

 

[a2mfk]

Lion- I don't disagree with your method, but wouldn't it be simpler to just get the max moment and shear and then design for that? You use the same flexural reinforcement full depth I assume, but perhaps you widen your stirrups spacing with depth?

 

[Lion06]

a2 - I do use the maximums from the diagrams, but I still want to look at the diagrams to make sure they make sense to me and that I'm not just taking a number from the geotech without looking at it with my own critical eye. I one time discovered that the geotech was assuming a fixed head condition with my grade beams when the drawings stated to assume that the grade beams do not provide fixity to the tops of the drilled piers.

 

[TLHS]

I personally reinforce full depth, but I'm aware of some people who only provide reinforcing for bending through the upper portion of the pile with the reasoning that below a certain point the moment becomes negligable. This probably makes some sense if you're working with extremely long piles.

Anyway, pile geotech design is mostly done by computer now, so the moment diagram isn't any harder to provde than a max value.

The standard column load interaction diagrams aren't going to be that useful. They'll all just tell you to use minimum steel, generally while showing that you're only using a few percent of the moment capacity. You may have to build your own interaction diagrams or otherwise design the members to avoid having a ridiculous amount of reinforceent.

Also, be aware that there may be separate code requirements for piles. For instance, in the Canadian code you have to ignore the outer inch of concrete for uncased ples and then reduce the capacity of the member by an additional factor of 0.9. There are also requirements for pile reinforcement in seismic areas to ensure ductility.