RC Tapered Beams and Compression Ring? 1

[Trillers]

Attached is a sketch of a rigid (?) concrete frame with tapered beams culminating in a compression ring near the peak. This structure will be built in Seismic Zone D.
In a previous thread I was proposing to design the beam joints at the apex by having them meet at the center which was a fairly straightforward procedure(to which some of you agreed), but the architect has changed his design to allow his accent lights to be recessed in the apex. This now requires me going back to the compression ring as shown in the sketch.
I have reviewed quite a bit of literature to see how I would compute moments and reactions for the beam/ring joints and the column beam joints, but am confused about how the ring would affect the entire beam/column system.
I also need to design the ring and I propose to design it treating each face of the ring as both a column (to support the adjacent ring segments) and a short beam (to support the tapered beam load as shown.
Anyone know of a good reference which addresses designing a similar system?
Or can anyone suggest a procedure to analyze the column/beam/ring interaction?
Also - I intend to treat this as a rigid frame - does this make sense? I only ask this because I have been told by colleagues that the compression ring renders the beam/ring connections as "hinged" connections. I don't agree, but looking for your opinions out there.
Thanks.

http://files.engineering.com/getfil...bfe-8d6b-51ab680af722&file=20131112151524.pdf

 

[Trillers]

Hi Gents:

I really appreciate these discussions as it puts me in good company as far as trying to figure out the direct approach to the analysis of this structure and its primary frames.
Now you guys all know how I feel about this - it's like that old song, "Which Way You Going Billy?".

JoshPlum, believe me, if a RISA module can handle this analysis AND give me the actual reactions at the joints I would purchase it in a heartbeat, but my staff has looked at the information on-site and have not seen where this type of problem can be addressed.

BA and JLNJ, the structure will be cast-in-place. And yes, I forgot to mention that stability at the beam-column connection will be provided by a ring beam all around at the eave line. I must admit that when BA suggested the three-hinged arc, it led me to deeper thinking, as I was not sure how the tension beam at the eave could be treated, other than as a member in tension for lateral forces, and then to make sure it is designed for torsion as well.

BA, in your opinion will the three-hinged arch analysis still work with the tension ring at the eave?

 

[BAretired]

JLNJ - Thank you.

So far, the construction method has not been discussed. I would think the tapered members and ring would be precast in a shop, then transported to the site and erected with a crane using temporary bracing under the ring to hold it in position while the tapered members are erected. Connections might be made using welded embed plates.

An eave member might be a good idea for stability during erection. It could be made into a tension ring if desired which would make the structure somewhat more indeterminate but that is not a bad feature.

A cast-in-place structure would seem impractical because of the amount of formwork required.


BA

 

[IDS]

I agree with Josh that a 3D frame analysis is the obvious way to tackle this, but I don't agree that tapered members are necessary. As long as each element is sub-divided into a reasonable number of short elements, each given the appropriate stiffness, then the results should give very similar results to using tapered beams in the analysis.

Of course, when calculating the stresses at any section the actual member cross section should be used.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BAretired]

Trillers,

It appears that I was still writing when you posted. Certainly a cast-in-place structure is a possibility even if not my first choice, and of course the choice is dependent on there being a precast plant in the region.

As stated earlier, the tension ring at the eave complicates the analysis slightly but is not a bad feature. The structure works with or without it but for unsymmetrical loads, the tension ring will deform from its original octagonal shape so it will have both tension and bending but no significant torsion. I believe that four 3 hinged arches with a holding force at eave level should be a good approximation of the behavior.

BA

 

[Trillers]

Thanks BA!! I really appreciate this entire discussion and everyone who contributed, and of course, if anyone thinks of anything else that information would be really welcome.

Trillers

 

[BAretired]

If the roof slab is to be cast-in-place concrete, a conical shell or octagonal pyramid with perimeter ring beam supported on a cylindrical or octagonal wall would eliminate the tapered frames. Alternatively the ring beam could be supported on eight equally spaced columns braced against lateral forces.



BA

 

[bookowski]

I don't see how you can go at this without building a 3d model. Maybe I'm missing something but if you have a tension ring around the eave then this beam will form a frame between columns that is likely to take the majority of lateral load - it is a much shorter span and has continuity, rather than trying to pass forces though the ring in bending -> torsion -> bending.

Very rough model screenshot attached with a 5k load applied to the up/left direction - the ring beam frames along that line take most of the moment. This assume a diaphragm... and probably lots of other issues.