Shear reinforcing pile cap 2

[slickdeals]

Folks,
I am required to design a pile cap where I am limited in depth and controlled by one way shear at "d" beyond the face of the column. I am using ACI prescribed value for Vc = 3.5 sqrt (fc').

Can anyone provide pointers to how shear reinforcing could be used to make up the difference needed to carry the shear through reinforcing?

I have looked in vain online for suggestions and I am sure someone here has done something similar.

 

[WillisV]

Add j-hooks between top and bottom reinforcing mat to use as shear reinforcing.

 

[civilperson]

Pile caps are use thickness and concrete strength to resist shear. Most steel additions for shear strength are not used. (This is a war that I have fought with DOTs for 25 years).

 

[kslee1000]

civilperson has valid point here. Do not take shear in pile cap lightly. In the case with thickness restricted by whatever reasons (really want to know why can't make it thicker), suggest to look into ACI code for two way flat slab shear reinforcement around column penetrations.

 

[slickdeals]

The reason I can't make it deeper is because my boss does not want to . We hit water at 1'-0" below grade and he wants to have as less a depth as possible, so I was exploring the option of using shear reinforcing. Any pointers for some research or design examples for shear reinforcing in pile caps?

 

[kslee1000]

How does your pile layout look like? Single row, or multiple piles arrangement?

 

[csd72]

Need more info, but what is commonly done in australia for wide 'band beams' is that they us the typical U shear stirrups and over lap them horizontally (e.g first U goes to first and fourth top bar, second U to second and fifth, or whatever works)

Also have you considered flat slab punching shear reinforcement if you can get any deep enough?

We need more info to give you better advice.

 

[slickdeals]

It is a modified 16 PC, 18" dia. 250 Ton piles. Piles are spaced at 4'6" o.c., edge distance of 2'6".

It is picking up 2 columns centered on the Pile cap.

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[kslee1000]

How about design embedded beam grids, and reinforced accordingly to take on shear as regular narrow beams, bridging the 12 center piles (in both directions). Or you may try to use embedded structural shapes, watch out for concrete-steel shape interface though.

 

[csd72]

okay, good you posted that one up.

Strength of the pile cap is not you major concern with this flexibility is!

The shallower you make it, the more load goes on the nearer piles and the less load goes on the farther piles. This always happens but it is small enough with the stiff pilecaps that it evens out with the soil. In your case you will make it much more flexible.

You have not said what the piles are bearing on, if it is rock then this is definately an issue, if soil it may be depending on the flexibility of the pile cap relative to the soil. Any deflection more than about 1/4" will have significant effect on pile loads.

 

[kslee1000]

csd72 brought up the crucial concern - pile cap rigidity. Make sure your pile cap can be assumed rigid, and linear distribution of loads apply. Otherwise you are having hands full of problems.

 

[slickdeals]

@csd72
It is on rock. I have a 7.5 ft thick pile cap which I want to try to reduce to 5.5 - 6'.

 

[kslee1000]

Better than 12" implied in the opening
Make sure the stress cone covers the N/S outer piles.

 

[vandede427]

Yes, while usually pile caps don't require stirrups, there are times where you are limited in depth or your effective depth is reduced.

For example, in pile caps for a tower crane foundation, there are embedments placed in the cap that can decrease your effective depth by 24". You may have a 60" thick cap above your piles but only a 36" thick cap at these inserts.

What I have done is similar to what the previous guy described as "band beams" (I think). Place your typicall bottom and top steel as needed for flexure distributed over the width and length of the cap, but a grid of stirrups in a cage over your piles. So in essence, the 2 top and 2 bottom bars over a pile will be encased by a stirrup and the top/bott bars between piles don't have stirrups. This should solve your punching/beam shear problems.

Stirrups are a pain in grade beams, but sometimes they're unavoidable.

 

[csd72]

Rigidity is even more important when you are on rock as the failure strain is low.

You should check the deflection of the new pile cap.

 

[kslee1000]

csd72:

Not clear what you mean by saying "Rigidity is even more important when you are on rock..". Why we need piles in such case - sound rock at footing level?

 

[csd72]

kslee, as I said above, this is because the failure strain of the rock is often very low (8th or 16th of an inch) so if the pile cap deflects more than that then some piles can fail before the others take up their load.

With soils there is a lot more give and the uneven loads tend to even out a little more because of the higher failure strains.

 

[slickdeals]

@CSD72,
That is very informative. I have a subsequent question regarding distribution of reinforcement.

The ACI code requires T&S reinforcement of 0.0018 Gross Area. What is a good practice regarding distribution of this steel?
I does not seem appropriate to throw in all that reinforcing on the bottom of the cap and have no other reinforcing.

I have looked in ACI for requirements but found none. Would some skin reinforcing and top mat be placed on a deep pile cap (say 5' and deeper)?

 

[slickdeals]

As a follow up, what is the requirement for shear reinforcing if a pile cap is designed using the strut and tie method?

 

[csd72]

If you design for strut and tie then there is usually no shear reinforcement required.

Reinforcement layout depends on the loads exerted, if there is always a downward force then only bottom reinforcement should be okay.

for a good article on strut and tie go to:

http://www.gostructural.com/pg.asp?id=27