Cast in Place Pier Construction/Design 1

[pmour]

The ACI code provides for 3 inches of concrete cover when permanently exposed to earth for the construction of a drilled shaft pier. I would think it would be easier to have the rebar cage rest at the bottom of the pier for the purposes of construction. I am unaware of how the rebar cage supported otherwise during construction. I had been told once that the the purpose of the code was to prohibit corrosion of the steel, which requires moisture and air, and at the bottom of the pier there would be a minimum amount of corrosive action. I have seen piers designed in both ways(having the longitudnal steel extend to the end of the pier and detailed 3 inches short of the pier base) and I am looking for some clarification as to the preferred or proper design.

 

[SacreBleu]

The concrete may be honeycombed at the bottom of pier. I have always spec'd 6" clear. The are plastic "chairs" which are attached to the bottom of the rebar cage that give the 3" or 6" clear.

 

[BigH]

Is it really necessary to take the steel all the way to the bottom of the pier. We had a discussion of this in the past - depends on the design. But in many drilled shafts I have been involved with the steel was only in the upper part of the pile. SacreBleu is correct that there are chairs or other means of supporting your cage as you start the pour. I think that his suggestion of 150mm is good.

 

[jike]

Most piers, that I have seen just have rebar cages in the upper 10 to 12 feet. Any bending moment will usually be pretty small by that depth. The only time that I have run a cage completely to the bottom is for a pier in tension.

A crane usually is holding the cage in place until enough concrete is poured and the cage is in place securely. I recommend a minimum of 4" clearance laterally. I have seen guys trying to hold the the cage in position laterally with 2 x 4's wedged between the cage and the casing. Sometimes the cage will shift as the casing is pulled.

 

[jdonville]

pmour,

In addition to the plastic chair supports, it is also a common practice to hang the cage from the slots at the top of the temporary casing at the ground surface until the concrete has been poured. No need to monopolize the service crane for holding the cage in this case.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[pmour]

Thanks for all the info. Most of the foundations we use are not much deeper than 16 ft. In talking with the crews contructing the piers, they give the cage a lift into place during the pour, giving the concrete cover at the bottom of the pier.