Design of two steel columns with one inside another 2

[structure567]

I am in the process of designing a steel pipe column that will require inserting a new smaller column inside of an existing column to increase the allowable strength. The smaller column will have an outside diameter 0.2" smaller than the larger column's inner diameter.
If I were to treat the columns as two separate columns how should I calculate the force applied to both the columns?
The columns has an existing circular deep foundation. The columns will be embedded the whole length of the foundation.

 

[STrctPono]

Since this is clearly related to the OP's other post Link I'm guessing the owner probably doesn't want to worry about pumping concrete/grout up to the top of an 11 ft pole or doesn't want to haul the buckets of concrete/grout up a ladder to do so.

structure567, seeing that your foundation is existing, and you're dropping a new pipe down the center to increase the section modulus, you still have the issue that your highest force demand is at the base of the pole/top of foundation. Dropping a new post down the hole and grouting it up will still leave this location as your weak point in the design since it's not getting extended into the foundation. Was the existing post wet set into the concrete foundation or set and then the concrete poured around it?

 

[canwesteng]

The flexural rigidity of the column is what prevents the column from buckling in the first place, so you'll need to account for that when resisting buckling whether it's one pipe or two, and in fact that's what we do. You can't buckle both until the flexural rigidity of both has been overcome. There is however combined bending and axial load that occurs, because the slight gap between pipes will cause secondary moments.

 

[structure567]

Since this is clearly related to the OP's other post Link I'm guessing the owner probably doesn't want to worry about pumping concrete/grout up to the top of an 11 ft pole or doesn't want to haul the buckets of concrete/grout up a ladder to do so.

haha you are correct. The two issues are not exactly the same. There are multiple columns with various issues.

 

[phamENG]

Ok. Thank you for clarifying the loading scenario - everything makes more sense now (and sorry if you'd said it earlier and I missed it). I didn't put this and your other post together.

I see STrctPono revised his last post - that was going to be my question. If it was wet set, this won't work. The concrete will have filled your pipe below the surface of the foundation. If not, then there's some hope, but I doubt you'll get it all the way down.

And I disagree about the large column not being feasible. You probably don't need the full height reinforced - just some distance up from the ground - maybe 4-6 feet of it? So if you run a larger column up on the outside (cut a pipe in half, wrap it around, and weld it back together) and stop it a couple feet below the sign head, problem is solved. It doesn't interfere with your sign, and your fit up is a lot easier.

 

[structure567]

if you run a larger column up on the outside (cut a pipe in half, wrap it around, and weld it back together) and stop it a couple feet below the sign head, problem is solved.

hmm how do you go about the base connection to the existing foundation with this method?

 

[phamENG]

How much room do you have? You could do a donut base plate, also cut in half, with 2 holes on each half to accommodate wedge or adhesive anchors.

 

[STrctPono]

Geeze, I'm starting to lean toward BridgeSmith's opinion... just rip this thing out and start over.

 

[phamENG]

I don't disagree, but sometimes it's fun to explore strange possibilities to satisfy irritatingly impossible clients.

 

[structure567]

How much room do you have? You could do a donut base plate, also cut in half, with 2 holes on each half to accommodate wedge or adhesive anchors

it is a very interesting method but I'm afraid that I'll need to increase the existing foundation. I was hoping that all the lateral force will be transferred through the pile foundation by placing the column all the way down the pile. But now having some of the reaction shifted to the base of the pole will complicate things even further.

 

[STrctPono]

structure567, all the theory aside, I think you really need to figure out if this is possible. This would be my suggestion:

1. Measure the distance from the top of the existing post to the top of existing foundation.
2. Drop a probe down the center of the post and take a measurement to the top of the post.
3. If the measurement from step 2 is at least 3 ft greater than that taken in step 1, then the strengthening option is feasible.
4. If the measurement from step 2 is less than 3 ft greater than that taken in step 1, then the strengthening option is not feasible.
5. If feasible, drop the new pipe in the center of the existing post as deep as it will go. Install some centralizers to keep the new pipe centered in the hole. Grout the annulus between the two with non-shrink grout. We're talking a very small amount.... less than 1 cubic foot of material. Weld a cap plate on top to keep water out.
6. See attached sketch.

 

[structure567]

STrctPono, I agree. I'll need to check field site and hopefully all this discussion wasn't for naught.