Pile cap action in auger cast piles 1

[BAGW]

Hi,

I have a situation where a column is supported on a pile cap bridging between 2 piles. Attached is the image below. Column reaction is in the range of 300kips.

Pile cap is 4'-0" deep and auger-cast piles are 18" in dia. Auger cast piles are deep enough to support the 150 kips of reaction through skin friction.

When I run the analysis for the pile cap design, I get more like a simple supported behavior for a pile cap rather than a fixed behavior (which I thought would be). Is the simply supported behavior because of the cap being so much more rigid than the piles? Has anyone run into such a situation?

Thanks

 

[r13]

Yes. It is similar to a frame with the beam is much stiff than the column (pile). If modeled correctly, the pile should have a negligible amount of the moment.

 

[BAGW]

yup. The column has negligible moment.

Ok so its the stiffness btw the two that's causing the simple behavior rather than fixed. Thanks

 

[r13]

You are welcome.

 

[slickdeals]

I would recommend doing a Strut and tie analysis. It's likely that your piles are 4.5' apart at 18".

 

[ATSE]

The bending moment at the top of pile is partly dependent on the vertical doweling between the pile and cap.
If you have detailed, say, (8) #6 bars as vertical dowels, the top of pile will attract a bit of bending moment demand with just a small amount of rotation.
So what is your doweling?
And what is the clear span?
And when you say "run the analysis", that depends on what you told your software / spreadsheet to do. If there is fixity at the connection, of course you will attract moment. If you include a pin, then of course you will attract zero moment.

BTW, is there any relevance to ACIP vs CIDH or any other pile type?

 

[BAGW]

I am detailing the piles as fixed. There are dowels at the top of pile and also there are horizontal rebar at the top of the pile cap. Even if I detail it as fixed, I am getting a small negative bending at the pile as the piles are very flexible compared to the pile cap.

I am modeling them in Risa and I have modeled the caps and the pile as continuous frames. Also I have modeled horizontal soil springs.

I agree with r13, because the cap is very rigid compared to piles, this has more of a simply supported behavior

 

[HTURKAK]

The pile cap is 4'-0" deep and the piles are 18" in dia. and apparently the piles 4.5 -5 ft apart. That is, the shear span ( distance from face of the column to pile center ) to pile cap depth ratio is less than 1.0 and truss action will be dominant rather than bending action. I agree with slickdeals and a pink star to him for his comment (I would recommend doing a Strut and tie analysis. It's likely that your piles are 4.5' apart at 18".)

If you design the pile cap as bending element , the tension force for T=PL/(4d) ,( d= depth of pile cap ) and you would not need shear reinforcement. However, when overloaded, may exhibit brittle failure due to a small amount of longitudinal reinforcement and may not have any transverse shear reinforcement.

I will suggest also , STM and provide transverse shear reinforcement.

 

[r13]

If you design the pile cap as bending element , the tension force for T=PL/(4d) ,( d= depth of pile cap ) and you would not need shear reinforcement.

ACI no longer has deep beam provision?

 

[HTURKAK]

I have ACI 318 -14 and regarding the pile caps 13.4.2 , the code specifies the design with sectional method (designed for flexure and shear at assumed critical sections ) and allows the use of STM (13.2.6.3 Foundation design in accordance with strut-and tie modeling, Chapter 23, shall be permitted ).

Section 9.9—Deep beams ( 9.9.3.1 ) specifies min. distributed reinforcement along the side faces of deep beams.

 

[r13]

Thanks. At one point of time, I thought they want to switch to STM entirely.

 

[HTURKAK]

This is my respond to similar thread=

My interpretation of ACI 318 for the pile cap design is, satisfy the shear strength requirement for one way or two way shear ( if two way is applicable) and choose the reinforcement for flexural. That is, design for for flexure and shear at assumed critical sections .

If a pile cap is designed and satisfy two conditions, ACI will be a good shield . But , IMO,in order to sleep with comfort, a reinforcement cage should be provided for pile caps having more than 3 piles and for two piles case, the cap should be designed as beam element and minimum transversal shear reinforcement should be provided.