Steel Compresion Rings used in wood construction.

[RARWOOD]

Does anyone know of a text book or published paper which covers the design of steel compression rings and its size limitations.

In my work I deal with a lot of free standing structures with opposing arches or hips. A very common detail would be a 3/8" steel octagon with a 12' to 18" diameter. However I see a lot of plans where the compression ring is 8' to 10' out to out or larger.

When I ask the EOR about the structural stability they indicate the system is stable under balanced loads, but generally have no answer to the question of unbalanced loads.

I handle the problem by using engineering judgement, and in some cases by developing a rigid connection to the comprission ring.

 

[AlohaBob]

I redesigned one for a landmark restaurant in Hawaii after a hurricane rendered the structure inserviceable. I struggled with unbalanced loading and resolving the stresses and connections to maintain stability. In the end, I reinforced from where I began to provide for added strength and detailing the connections to resolve loads through torsion and bending. The original design didn't consider unbalanced load.

 

[RARWOOD]

Thank you for your reply. I have been very busy at work so I have not be able to get back to you sooner.

I not clear, on the torsion you are checking.

The major problem I have is determining what is a reasonable size for a compression ring. When we design a system in house we generally use a ring with a 12" to 18" diameter. In a system where I have opposing arches or hips I treat the system as a three hinged arch.

The problem I am trying to work out is how the system behaves as the ring gets larger.I deal with a lot of cases where the architect wants to have a 10' opening in the system for a skylight or clear story.

I feel as the ring gets larger at some point your system behaves as a four hinged arch and is not stable under unbalanced load.

I have not seen any books or articles that do with this problem. To me the basic problem is that if you isolate the comression ring under unbalanced load you have the ring subjected to a moment which is approximately equal to the unbalanced shear times the compression ring diameter.

For the system to be stable the moment created by the unbalanced shear must be resisted by an equal and opposite moment. With a small compression ring my theory is that as the ring tries to rotate it increases the comprsssion stress on the end of the arch or hip.

Now as the compression ring gets larger the moment will increase, depending on the loading. I think then the reasonable limit on the compression ring is the point where your increased compression stress exceeds the design allowable.