Information on "Tube Structure"

[vasilios13]

Hello,
I am a new structural engineer. I am looking at a example of a eight story building 60' x 60'. I am trying to understand the way the structural system works.
The exterior walls are moment frames. The columns are spaced closer together than the interior columns. The beams on the inside are connected to the exterior columns with a pinned connection. I have found some information calling this a tube structure. This book says that the moment frame resists the lateral forces and some gravity forces. The interior columns are designed only to carry vertical forces. I do not understand why this is true. I would think that the effect would be felt also on the interior. Does it have to do something with the reaction between the metal deck and the floor system?
Thanks

 

[csd72]

vasilios13,

What you describe sounds more like a sway framed building than a tubed structure, you may want to do a bit of reading in that direction.

As for what resists the lateral forces, it largely depends on how you design it. If the interior columns have pinned connections to the beams then they will take no significant part in the lateral load resisting system.

Here is a good website to look at:

http://www.kuleuven.be/bwk/materials/Teaching/master/wg14/l0700.htm

csd

 

[shin25]

The behavior of a structure depends on how they are detailed. If a joint is detailed as a pinned connection, then by theory it will be unable to carry any moment. Since it will be unable to carry moment, it will not take any lateral force effect. On the other hand, if the joints are detailed as fixed they will take the moment from the lateral force effect, like the exterior frame for your case.

Also the structural system (tube structure) that you have described, are well effective during a progressive collapse.

 

[hokie66]

The term "tube structure" is normally reserved for high rise buildings, but the 8 storey building you described works in a similar fashion. The lateral loads are distributed through the floors to the external moment frames, with the internal columns carrying gravity loads only.

 

[DaveAtkins]

A tube structure would not be used (typically) in an 8 story building. For a building that small, shear walls would be the best lateral load resisting system. Braced frames could also work. The external moment frames you describe may do the job, but they won't be very efficient. Oh, and as was said above, the interior beams and columns are not rigidly connected, so they cannot resist lateral loads.

DaveAtkins

 

[jike]

I agree with the comments regarding tube structures are normally used for high rise construction. The term "tube" refers to that fact that exterior moment frames with closely spaced columns are like perferated tubes. The closely spaced columns help reduce the shear lag affect in the columns. This means that all the columns are working effectively in resisting overturning forces rather than some just going along for the ride.

The World Trade Center Towers were examples of tube structures (one extrerior tube). The Sears Tower is a bundled tube (9 tubes acting together).

I hope this helps!

 

[shin25]

jike-

And this is why (mainly) both the structures stood without collapsing for 1 1/2 to 2 hours after impact. This saved lot of lives. 'tube' Saved the structure from immediate progressive collapse.

 

[csd72]

vasilios,

I assume you have an elevator shaft on this building, is there a reason why you are not using this to resist the lateral loads?

csd

 

[jike]

shin25:

I absolutely agree! The redundacy of this system was remarkable in saving lives!

 

[vasilios13]

csd,
Thanks for the replies, that link was great. There is a elevator shaft, but it is all the way in the front off the structure (right against the exterior wall). In the rear of the buillding it is all open.
I was thinking that using the elevator shaft to resist the lateral loads would cause some excentricity in the design/building. Is there a way that I could incorporate it?
Thank You

 

[csd72]

You could possibly use it to take out a portiion of the lateral load and a frame at the other end to take out the eccentricity. Being such a stiff member the lift shaft will take part in the lateral system regardless of how you design it, so you may as well use it.

If this is in a seismic zone then this may make picking a structural response value a bit tricky.

csd