Steel Column Embeded in Drilled Shaft
Steel Column Embeded in Drilled Shaft
(OP)
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
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






RE: Steel Column Embeded in Drilled Shaft
RE: Steel Column Embeded in Drilled Shaft
>>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
RE: Steel Column Embeded in Drilled Shaft
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?
RE: Steel Column Embeded in Drilled Shaft
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.
RE: Steel Column Embeded in Drilled Shaft
BA
RE: Steel Column Embeded in Drilled Shaft
RE: Steel Column Embeded in Drilled Shaft
BA