Can you insert a smaller HSS pipe into a bigger HSS pipe to reinforce it? 4

[structure567]

I am working on an existing cantilevered column that has an increased lateral load due to the client wanting to install a bigger sign on the existing column. We want to minimize construction so instead of replacing the existing column, I was wondering if it was possible to insert a smaller HSS pipe(3.5" dia) inside a bigger HSS pipe(4"dia). Right now the column does not have the necessary moment capacity to support the new sign. If this was allowed is it ok to just add the the strength of both columns to get an increased moment capacity?

 

[JStephen]

Depending on how the tolerances work out, you may or may not be able to put the smaller inside the larger.
I would think they would share the load in proportion to their stiffness (with perhaps some allowance for the slop between them), rather than just summing the strength of both. Or if you calculated the strength of each, and found the deflection corresponding to that strength, the smaller pipe should have a larger deflection at failure- so when the larger pipe is loaded to its ultimate strength, the smaller pipe is still below its ultimate strength.

 

[jayrod12]

Without appropriate planning ahead of time, I.e. preparation of the larger hss during fabrication, my bet is no. Unless specified, the weld material on the inside of an hss isn't always removed. Therefore it's quite likely you won't be able to shove the new one inside the existing.

 

[12345abc6ttyui67]

Without anything connecting the pipes together, surely you can't just add the strengths.

At best you need to apportion the load to each based on the stiffness.

The same reasoning as why simply laying one plate on top of another doesn't result in the modulus being (t1 + t2)^2.

You need to connect them to prevent slip at the interface.

As also noted, interference issues may be present.

 

[Guest262653]

For a simply supported beam I could see this as a solution, taking into account the advice already posted, analyzing relative load distributions by considering the same deflected shape.

However, I think that for a cantilever this wouldn't probably work as the critical section at the base, where you probably can't get proper access to weld the new pipe to the base plate, would remain with the outer HSS moment capacity.

 

[SteelPE]

So, you have an existing cantilever column and you want to reinforce it by putting a new pipe inside the existing? If you can get the reinforcing to take load, how do you plan on getting this load out of the reinforcing at the base??

I have run into this issue in the past. We either reinforce the outside (welding the reinforcing to the base). This reinforcing could be covered with a metal skin if unsightly. Another option would be to extend the footing/foundation above grade encasing the column in concrete (reducing it's effective length).

 

[canwesteng]

avs hit the nail on the head. If simply supported and using plastic capacity, you could simply sum the capacities though, as there will be sufficient deflection to yield the weaker pipe.

 

[structure567]

So, you have an existing cantilever column and you want to reinforce it by putting a new pipe inside the existing? If you can get the reinforcing to take load, how do you plan on getting this load out of the reinforcing at the base??

Thank you for your comments. I shall try doing the calculations according to the stiffness and see if that is acceptable. In relations to the base, the existing column has a pile foundation that is 5' into the ground. So the plan is to insert a smaller column that accounts for the total height including the below ground. If this is the case wouldn't the base reaction be transferred into the ground directly? We were also planning on pouring concrete in the remaining hole to help with the deflection.

 

[Celt83]

We were also planning on pouring concrete in the remaining hole to help with the deflection.

If your going to fill it with concrete anyway have tried looking at it as a filled composite member?

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[structure567]

If your going to fill it with concrete anyway have tried looking at it as a filled composite member?

Since this is a compact member, I am allowed to design using the plastic stress distribution method but I havent done that approach yet. I have only designed the combined axial and flexural force using the equations with steel values only which have failed. Even if I did put concrete inside the column I would still consider to insert a steel column anyways just for additional reinforcement.

 

[dhengr]

Structure567:
Regarding placing (telescoping) a smaller round tube/pipe inside a larger round pipe, comparing the o.d. of the smaller with the i.d. of the larger, to make the telescoping fit, take a look mechanical tubing for the smaller tube. Some sizes may be hard to find, but you have much more choice in dia’s. and wall thicknesses. You have to take great care in understanding the tolerances of all sizes and dimensions and the quality of surface finishes inside and out to start to know if you can make things fit. Also, as you are trying to reinforce the outer pipe which is a ‘flagpole’ type, cantilevered sign post, be sure you understand where the max. moments and stresses are, and how the two pipes, acting in unison and properly interconnected, will work to improve this picture. Note that in a structure like this sign post, once you start forming any plastic action (even approaching a plastic hinge) you essentially have a failed structure.