RC Compression Ring Design 2

[Trillers]

Does anyone know where I can get a spec or design guide for an RC Compression Ring?

I am designing an octagonal-shaped reinforced concrete beam-slab roof with the beams meeting at the apex of the octagonal roof. I would like the connection at the peak to consist of the beams and a compression ring.

I have looked high and low but have not been successful in finding a design procedure and reinforcement detailing standard for the compression ring.

Any suggestions on how to go about this?

 

[JAE]

I'm not sure you are going to find much for a compression ring design specifically.

The ring would simply serve as a series of short, linked columns that would fall under the provisions of your applicable concrete code.
Simply design them as short columns with the applicable axial force and moments.

 

[dhengr]

Trillers:
That’s a fairly complicated problem by the time you get done with it. But, with today’s analysis software the largest hurdle can be managed. Then ACI, CRSI are your guides and reinforcing details become the problem. You have the same general moment and reinforcing requirements in many/several directions, all happening at the same location. So which steel has/gets the favorable “d” location? Then throw in plenty of torsional reinforcing and ties. Some questions to consider...
1.) Unbalanced loading on one side of the roof induces a significant torsional moment/loading into the compression ring, tending to twist it out of its horiz. plane.
2.) Do the beams meet at the apex, so they are more or less just a kinked full span beam, with a high point at the middle and lower bearing points? They induce a thrust at their outer bearings, so you need some sort of a tension ring too, out at the bearing walls.
3.) Is there a large skylight opening at the center so the open dia. of the compression ring is large? That’s a different animal too.
4.) Might the compression ring just be an 8' dia. (8 sided) thickened slab, same depth as the beams; and the beams frame into this, and are just reinforced through it.
5.) Because of reinforcing congestion at the middle, would you make a 4' dia. (8 sided) steel ring structure, and weld rebar couplings to the outside faces of this ring. The rebar gets threaded into these couplings and the whole thing is embedded into the concrete.

Search old ACI literature. It seems to me that years ago, they may have had a few papers on this topic, but I can’t point you at a specific article. Maybe TXStructural will show up here, he is pretty knowledgeable about what ACI and CRSI have available.

 

[Trillers]

Thanks dhenqr:

I haven't decided whether to have them meet at the apex or treat them as a kinked full span beam. I've previously designed other roofs both but with only 4 beams so that was fairly straightforward in terms of torsion and unbalanced loading.
There is no skylight - and we are definitely designing a tension ring beam at the bearing walls.
JAE suggested treating the ring as a series of short linked columns which is addressed briefly by ACI, but I was concerned about the imbalance with twisting which is not directly addressed by ACI.
It seems like we're on the same page as your alternatives 4 and 5, probably leaning towards 4 so we can treat the beams as kinked and then use the thickened slab as a "ring" to aid in torsion control.
I will analyze using No 5 though as that would definitely ease in the rebar congestion at the peak/apex.

 

[JAE]

I was concerned about the imbalance with twisting which is not directly addressed by ACI.

That isn't quite correct - ACI doesn't deal with analysis methods or procedures directly but assumes that the engineer can analyze and determine the axial forces, moments, torsion and shears in members properly.
ACI 318 isn't there to tell you how to analyze something.

 

[Trillers]

You're correct JAE - I should have indicated that ACI does not provide detailing for torsion resistance specifically for a poly-axial loaded compression ring.