Shear Key in Pile Caps 2

[IHcal]

Hi

I have a situation where I got huge shear, but not enough space to put lot of vertical piles . It is a industrial revamp job and the shear is due to combination of operational and wind load on the equipment. Can anyone guide me about designing a shear key in the pile cap.?
1. Are there special construction considerations for such a key.

2. What type of reinforcement is required in the key?

If there is any reference material available that will definitely help.

Thanks

 

[shin25]

What type of piles are you using, steel or concrete?

 

[IHcal]

Piles are 18'' concrete auger cast piles.

 

[msquared48]

I assume that you mean between the top of the pilecap and the grade beam(s)?

Vertical steel reinforcing will not suffice?

Mike McCann
McCann Engineering

 

[WillisV]

Use raked piles.

 

[vincentpa]

I don't really understand the question. Can't you just batter the piles. We usually batter the piles to resist horizontal machinery loads or lateral loads from wind or seismic. I don't really understand what the key will be resisting. If you are counting on passive resistence from the soil for horizontal loads, you should reconsider. For machinery loading, I would be much more comfortable battering the piles. Have you discussed this with the geotech? If you want a reference for key designs to used to resist lateral loads, look in CRSI Design Handbook in the Retaining Wall section. For retaining walls, we use keys sometimes to resist the lateral forces from earth loads.

 

[IHcal]

Thanks for all the response.

Let me give you some background about the area. It is revamp job and there are existing large spread footings around the construction area supporting heavy equipment, built 20-30 years ago. There is no space to install batter piles. Even the vertical piles are a challenge with low head room rig, which the drilling contractor is willing to do.
Shear key is to add passive resistance along with the lateral capacity of the free head piles.

Thanks

 

[vincentpa]

The CRSI Handbook as I wrote in my last post has a design example for a shear key. I'm sure someone in your office has at least an old version. The latest is the 2002 version and I used to use a version from the 80's. The procedure hasn't changed since then. Pick up a copy. Your problem is solved. If you office doesn't have a copy of CRSI, it should. So it is an easy sell to your boss. It is extremely useful for many design problems including pile cap design. It is the only book available that has a design procedure for pile caps and drilled piers not to mention, beams, columns, slabs, retaining walls, splices and development lengths, etc... Even ACI 318 refers to it. Reinforcement for a key has flexural reinforcement and temperature and shrinkage reinforcement. The loading is the triangular passive pressure obtained from the geotech. Design it as a cantilever one-way slab. Check shear also but stirrups are not required. Details are in CRSI.

 

[kslee1000]

For limited head room in a congest area, try look into the mini-piles.

 

[vincentpa]

what about helical piers?

 

[kslee1000]

vincentpa:

I am not familar with helical piers. As long as head room and space permit, both could be good solution. However, the small diameter of the mini-piles and less restrictive in pile length are advantageous in many applications.

 

[IHcal]

This is a part of big project. Only 18'' auger cast piles have been qualified and selected for this project. Any other type will include qualifying contractor etc and there is strict schedule for this project.

 

[DRC1]

I would not use auger cast in this situation. I would use conc filled pipe as it is more dependable. I would not try to augment the capacity of the piles with a shear key. The piles develop lateral resistance depending on the modulus of a deep soil profile. The shear key will utilize passive resitance over a short depth. Lateral movements to achive capacity will be different. Thus they will not gain capacity at a uniform rate and it will be hard to know the load distribution. This is complicated problem which requires careful study by a geotech consultant.