Circular Safety Cage

[jayrod12]

Hey all,

We've been contacted by a potential client that uses circular cages with steel plate surrounding it as shoring for hydro vac services. These cages are approximately 5 feet in diameter and are constructed in 40" tall segments. These segments are then connected together to form whatever length of shoring is required. I have attached a sketch so you kind of get an idea. There are 3 rings that are made of 1.25x1.25x.125 hss and 4 sets of verticals between them (at the quarter points of the circle).

My question is not about the loading to design for as I have already obtained all of that information. My question is the analysis of a singular ring under confining stresses on all sides (this assumes the steel plate can effectively transfer the soil load to these rings). Would you design this the same as a pressure vessel only the members are in compression instead of tension? What factors come into play when looking at it that way?

Any advice or references would be helpful. And I was probably too vague on my description so please ask for any info you may need.

 

[jayrod12]

forgot to attach the sketch

 

[SlideRuleEra]

jayrod12 - What you are describing seems to be a miniature circular cofferdam. Compression of a vessel in not the same thing as tension. Compression is much more difficult and dependent on the exact pressure distribution. They can be unstable even under the best of conditions. IMHO, the best thing for your application is to make sure that all members have generous safety margins; don't try to "optimize" the design.

Google on "circular cofferdam design", you should get some relevant hits.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[JStephen]

If you have perfectly uninform loading all around, you get a uniform hoop compression and failure is by elastic buckling of the hoop in that condition. Roark or other sources should have the critical buckling stress. Generally, that analysis will give you a very small section requirement.

If you assume (or know) that the loading is not uniform, then you get bending in the hoops, and the section can become much larger. I would think that is the condition you'd want to design for.

 

[jayrod12]

I'll take a look for that kind of stuff sliderule thanks

As much as I would love to assume even pressure distribution it probably is more prudent to design for uneven pressure.


Basically we are trying to determine how deep he could use the ones he has already constructed and then tell him what they need to be upgraded to in order for him to use them up to a 20 foot depth.

Scary part is he's already used them for that kind of depth but my assumption is the time the cages were in the ground was so short they never saw the theoretical lateral soil pressure.

 

[SlideRuleEra]

jayrod12 - You may want to mention to the Client that this structure is likely a "confined space" that have safety concerns other than structural stability.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net