Minimum shear reinforcement in pile caps?

[abusementpark]

Do ACI's minimum shear reinforcement apply to pile caps? I know it says that the minimum reinforcement requirements don't apply to "footings." Do pile caps fall under this category?

 

[hokie66]

Pile caps are normally designed by strut and tie methods, as they are not flexural elements. A pile cap, in conjunction with its piles, is a type of footing.

 

[asixth]

As hokie said, compressive arching action is the structural behaviour. So as long as the compressive strut gets the column load to the piles within 45deg (roughly) and the compressive stresses limits aren't exceeded then the beam-shear requirements are not relevent.

When the strut fans out more than 45 deg is when I start introducing shear reinforcement.

 

[amec2004]

>>Pile caps are normally designed by strut and tie methods

That's true, but the strut-and-tie is to replace flexure check.

You still have to check single pile punch shear, one way shear, two way shear.

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http://www.civilbay.com

 

[hokie66]

amec2004,

No, the strut and tie method is a truss analogy approach, where a truss model consisting of compression and tension elements is used rather than a flexural model. Don't combine provisions for the two methods.

 

[ukengineer58]

we never put shear links in pile caps. although they are frequently designed as flexural elements especially if using fem models.

 

[Teguci]

11.1.1 provides an exemption to chapter 11 for D regions designed with Appendix A.

11.4.6.1 - exemption for footings and "solid slabs" (ie don't put any voids in you pile cap).

11.7 Deep Beam provisions (almost always apply regarding pile caps.)

11.7.6 use A.3.3 instead of providing shear reinf.

App A
A.2.5 - max angle between strut and tie = 25 deg. for a 4'-0" depth to bottom reinf, this gets us approximately 8'-0" from column to pile plan distance.

A.3.2.2 - b- allowance for no cross reinforcement

A.3.3 - Requires cross reinforcement if A.3.2.2 a was used (ie don't use a use b)

Back to the original question, yes the minimum shear requirements apply if you don't meet the exemptions (very useful to add for expensive excavations to minimize the pile cap depth). However, for standard pile cap designs, I'd recommend designing to the current CRSI manual so you don't go and reinvent the wheel.

 

[abusementpark]

As hokie said, compressive arching action is the structural behaviour. So as long as the compressive strut gets the column load to the piles within 45deg (roughly) and the compressive stresses limits aren't exceeded then the beam-shear requirements are not relevent.

When the strut fans out more than 45 deg is when I start introducing shear reinforcement.

Does the strut have to emanate from the column? Or can you break up the strut-and-tie model like a truss with multiple panel points to the meet the 45 degree requirement?

Where does the 45 degree requirement come from?

 

[abusementpark]

However, for standard pile cap designs, I'd recommend designing to the current CRSI manual so you don't go and reinvent the wheel.

Are their designs based on the strut-and-tie method? Because it does not appear that they apply the minimum shear requirement to their designs.

 

[asixth]

45 degrees is the maximum angle which I find the struts in strut-tie design to be efficient. The code I use for strut-tie design allows the struts to go to 30 degrees but their efficiency diminishes.

I read a good quote in a paper for strut-tie pile cap design.

"Unlike traditional design procedures, strut-and-tie models do not separate flexural and shear design; however it may be said that the "shear design" of deep members using strut and tie models involves limiting the concrete stresses to ensure that the tensile tie reinforcement yields prior to concrete shear failure".

The paper is called "Design of Deep Pile Caps by Strut-and-Tie models" and I found it by google search.

 

[abusementpark]

I just verified with CRSI that their designs are not based on the strut-and-tie method. It is just simple flexure, one-way shear, and two-way shear with some modifications to the ACI stregth provisions for shear based on other research.

It appears that they do not believe that the minimum shear provisions apply to pile caps.

 

[hokie66]

In that case, CRSI is stuck in the last century, or else they should define the "other research". But their tabulated designs could still be satisfactory if their assumptions are conservative.

 

[slickdeals]

@hokie,
Could you please share your resources/materials for S&T design of pile caps. I have come across a few for standard 4 PC's but nothing for multiple piles.

 

[hokie66]

On the road at the moment, but will try to help when I return in a few days.

 

[hokie66]

slickdeals,
I did look, but there doesn't appear to be much in the way of published resources for design of pile caps with a lot of piles. Since it is essentially a truss analysis, it would be similar to the design of a spaceframe. Follow the forces. Sorry not to be of more help. In most cases, my approach has to use bigger piles rather than a larger number of piles.

 

[TLHS]

I've done strut and tie on multi-pile caps with odd arrangements. It's sometimes the only option, but it gets really messy really quickly. The truss analogy is very straightforward for simple systems, but once you have things moving in several different orthangonal directions with several load cases and possible load reversals it gets ugly.

I can't say I have a real alternative, though.

 

[ron9876]

I believe that CRSI designs for shear per ACI Chapter 15 section 15.5. If pile is within d then "diagonal tension" can be neglected. It also checks for direct shear which is "real shear" for conditions where the pile is near the support.

They would then increase depth as required to avoid shear reinforcement.

 

[abusementpark]

I believe that CRSI designs for shear per ACI Chapter 15 section 15.5. If pile is within d then "diagonal tension" can be neglected. It also checks for direct shear which is "real shear" for conditions where the pile is near the support.

Which provision of 15.5 are you referring to that says that "diagonal tension" can be neglected?

 

[ron9876]

Section 15.5.4.2.

 

[hokie66]

ron,
I think you are reading into 15.5.4.2 something that it doesn't say. It just means that if a pile is wholly inside a given section through a footing, then it doesn't contribute to shear on that section. That is quite obvious, but the reason for including the statement, along with 15.5.4.1 and 15.5.4.3, is to define how much shear is to be included when piles straddle a given section.