ACIP Pile reinf design

[ARLORD]

When given a design moment and shear for a circular augered cast-in-place pile from the geotech, what is the procedure to design the reinforcing cage, for the moment, shear and uplift? Do you know of any good references for pile design?

 

[lrhg]

ARLORD

Take a look at ACI 336. While I've designed caisson shafts (drilled piers) and augercast piles for moment and axial loads, I don't ever recall the geotech giving me the M and V. I usually use a minimum moment of P*e, where e = 3" or the specificied out of placement tolerance of the shaft. When using the piles to take building base shear, I request the geotech to provide soil spring stiffnesses as a function of depth which I use to model the caissons (and grade beams) and determine bending due to lateral loads. For net uplift, you'll need skin friction values from the geotech, though in my area these numbers are usually quite small.

As a minimumm, I usually use a minimum of 0.5%Ac vertical reinforcement in the top 20'. If net tension, full length rebar in the shaft. I hope this helps, as I am not sure this is your question.

 

[JKStruct]

I've never heard of a geotech providing design forces on a pile... is he designing the structure? How does he know what's acting on the foundation? Typically, the geotech will give soil properties and soil design criteria for transferring the loads from the structure to the soil.

Part of your question hinges around fundamental analysis procedures. For foundation design - Bowles is a good book.

JK

 

[ARLORD]

I provided the geotech with the reactions, base shear, max pile downward capacity and uplift. I guess the geotech is using a program called LPILE or something similar to get the moments in the pile.

 

[lkjh345]

ARLORD:

We use the procdure you describe quite regularly. We give the Geotech the design parameters, they input that information ( and some judgement calls of their own regarding soil properties) into LPILE and send us back the shear and moment diagrams on the pile.

This is typical where the piles are resisting a lateral load from the structure above.

Once I have the shear and moment diagrams, its just a matter of designing a circular concrete beam to find the required perimter cage reinforcing.

For uplift, we usually run a bar down the center of the pile. #9's and #10's are common, and mechanically splice on a 90 hook at the top of this to develop the bar.

If the uplift force is higher than a #9 or #10 can handle, we occasionally will use Dwidag bars of Grade 120 (I think) to handle a lot of uplift. These bar require special details to develop at the top.

Just how we do this.