Steel Column Embeded in Drilled Shaft

[ash060]

I have a large steel column resisting lateral loads as part of a fixed base moment frame. Due to site conditions I have to use one large diameter drilled shaft as the foundation. I gave the geotechnical engineer my loads at the base of the column to be imposed on the drilled shaft, and received the L-Pile output with the moments and shears to use for design.

I had planned to insert the column into the drilled shaft and count on it to do all the work, and leave the drilled shaft unreinforced, or only lightly reinforced for a short length at the top. The main reason being is that I do not have room overhead to put a standard cage in place. I am thinking that if the steel unloads through bearing than that will develop a good amount of tension in the shaft above the bearing point. So that I am going to need steel to resist that tension in the concrete.

Has anyone run into this issue? I guess I could have them lap the bars every 10' or so and use smalled diameter bars, or have them use mechanical splices but I think that timing would be an issue for installing the couplers. I was wondering if anyone has a good design methodology for this type of structure.

Thanks

 

[abusementpark]

How do you not have room overhead for a standard cage, but can fit the column in the shaft without issue?

 

[amec2004]

My reply as the followings

>>I gave the geotechnical engineer my loads at the base of the column to be imposed on the
>> drilled shaft, and received the L-Pile output with the moments and shears to use for design.

1. Please note for LPile output of moment and shear normally it’s presented as unfactored pile shaft moment and shear as the Geotechnical engineer runs it to get a plot vs deflection. For serviceability it’s always unfactored load, so you need to factor the moment and shear for pile shaft section check

2. You also may consider the concrete cracking section for the pile lateral capacity check. Most of the time the pile shaft moment exceeds cracking moment Mcr and the concrete crack. It’s not uniformly cracking through the shaft as the pile shaft moment varies. It shall be something between 0.75EI and 0.25EI depending on how big is your lateral load.

>>I had planned to insert the column into the drilled shaft and count on it to do all the work, and >> leave the drilled shaft unreinforced, or only lightly reinforced for a short length at the top.

1. You shall get the shaft reinforced especially with close spacing, say 3” spacing, circular ties on the top. It’s important to confine the pile head to prevent the concrete from bursting or breaking out as the embedded steel section impose huge pressure through bearing.

2. You shall embed a short length of steel section and splice it with full MC connection as the piling and steel contractor are different contractor

>> The main reason being is that I do not have room overhead to put a standard cage in place.

You should have room for the cage plus circular ties

>> So that I am going to need steel to resist that tension in the concrete.

It’s mainly through bearing.

anchor bolt design per ACI 318-11 crane beam design

http://www.civilbay.com

 

[azcats]

Say you have something like a 36" pipe column being inserted into a 48" diameter hole. For the Lpile analysis you could ignore the 4' and only use 3' for the design pile diameter and use the EI for just the steel column. Generally - it only adds a bit of extra deflection into the foundation analysis when comparing the 3' to 4' on the design.

However, in your case, we don't know the column/foundation size ratio.

I'm also curious what the geotech used for EI in his model. Was he aware of your intent?

 

[ash060]

Main reason the column will fit and not the reinforcing is that I have a square hole that will barely fit the column thru. I have room to put small pieces of cage, but nothing for the full depth of the shaft without alot of splicing.

I am not to worried about the deflection caused by cracking as he did not use the EI of the steel column, just the gross EI of the concrete shaft. Even with the reduced EI the deflection would still be tolerable. The geotech only ran the worst case (and I was very conservative with the loading supplied).

I am thinking now of actually using anchor bolts and welding reinforcing to them and using a standard baseplate. If they have to do a bunch of splicing than that is what it will be.

 

[BAretired]

Inserting the column into the shaft is a difficult operation. It would be much better to establish the top of pile low enough to accommodate a pile cap, extending the pile reinforcement into the pile cap, then later, providing anchor bolts into the pile cap as required to anchor the column base plate. That way, the piling contractor can complete his work without having to worry about the precise placement of the column.

BA

 

[ash060]

A one pile, pile cap?

 

[BAretired]

Yes, a one pile pile cap. It is used frequently in my area. The upper two feet of shaft is usually cast in a cylindrical form (sonotube)placed after the piling contractor has left the site. Its main purpose is to provide the general contractor an opportunity to accurately place his anchor bolts and establish an accurate elevation for the top of concrete.

BA