Drilled Shaft Rebar Cage

[gnault]

My question is in regards to the amount of steel required to brace a steel reinforcing cage for a drilled shaft. I understand that stiffener bars are used to help maintain the shape of the cage during construction, but I am not sure how much steel is needed for this. Any thoughts or ideas?

The shaft that I am analyzing is 45' long and 6.5' in diameter. Also, I've seen some cases where the stiffening steel is positioned inside the shaft cage as cross braces and other cases where the steel is wrapped around the outside of the cage. Any thoughts on which way is preferred?

 

[structuresguy]

Depending on the soil conditions and local authority, you may be able to get away with less than 1% steel. However, I typically put 1% minimum steel in drilled piers, though the largest I have designed is 4' diameter. Usually this is because we are designing the piers as end bearing through really really bad soils, or have the potential for large voids along the length of the pier.

As for ties, i call out for hoop ties at regular intervals. I also call out for interior bracing ties to help maintain the cage shape during lifting. But you have to be very careful with these. THe shape and positioning may interfere with the grouting process, if they can't get their hose down through the pier. So if you are worried about it, you can call for these to be removed at the contractor's discretion.

I think both ties are really required. Well, at least I think the hoop ties would be required by ACI code, as I treat the pier like a compression member. The other ties are really to help maintain the shape of the cage during lifting.

 

[gnault]

Thanks for the reply.

Just to clarify, the shaft reinforcement (longitudinal bars & spirals) has already been designed for. I am specifically inquiring about the "interior bracing ties" that help maintain the cage shape during lifting. What size ties would you normally use for these bars and at what spacing would you provide them at along the length of the shaft?

 

[hokie66]

This is temporary bracing, and the design engineer should specify that it is the contractor's responsibility. Contractors have various ways of doing it.

 

[Qshake]

Agree with Hokie that any reinforcement beyond that required for design (vertical or horizontal) is up to the contractor.

That said, CRSI has a typical callout for spiral steel with stiffeners. The stiffeners are angles and this shape appears on most bar bend diagrams. However, I'm not a work and can't quote you a number.

A prudent thing to do if you're concerned about it is to call the local rebar fabricator and ask.

Regards,
Qshake

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[dik]

I typically use 10M ties at 3' o/c with 3 - 10M ties at the top. Also, I seldom use full length vertical bars... typically about 19' long... rebar in these locales comes in 12m and 18m lengths...

Duj