Is it composite?

[jamie2000]

The project consists of adding new ceiling hung medical equipment on the ground floor of a hospital. The building was apparently built in the 1920s, and of course there are no as-built drawings. The existing floor framing above consists of a concrete slab supported on steel beams. The concrete slab was constructed using hollow clay tile in an arch as the forms. The steel beams are at 8 feet spacing. The concrete slab is 4" thick at the midpoint between the beams and 7" thick at the beams, encasing the top flange with 2" +/- of concrete. I know this is not an encased beam, but could this be considered a composite beam? Any input is welcome. Thanks.

 

[Qshake]

Normally, we don't consider it composite without some type of mechanical anchorage which will provide for a positive means to transfer force. However, I have read, mostly as a manuscript reviewer, many articles on the various degrees of composite action developed by the very method you noted. It has been shown that a force transfer does exist and often results in the increased capacity of a concrete slab on I-girder system. The problem I find in all of this is that its largely dependent on the construction and certainly has no reliable design value without some specific testing for the system of concern. So unless you're prepared to performed some type of load testing/gauging for the system its not very useful. And because testing is expensive for what benefit may be achieved, the usual assumption of no composite connection is often used to provide a final arrangement that is without question.

Incidentally, there are several articles written on how to make existing systems composite with totally tearing everything up. See the AISC Journals.

 

[jamie2000]

Qshake,
Thanks for you response. I have also asked this question to AISC and they said that it is not considered composite according to latest codes. One book that I have that deals with structural renovations, suggests coring the slab above the beam and welding shear studs to the top flange to make it composite. I have a space limitation below the beam, so I this is my only practical option, unless someone has any other ideas.

 

[jheidt2543]

Would it be possible to support the medical equipment on its own frame? This could require punching through a few floors to get to the lowest level so you can install a footing under the columns. Just a thought, it would be more of a sure thing then guessing on how much composite action you might get with the studs.

I should think you would still have to run some type of a load test if you go the core slab/install stud route.

Just a thought!

 

[Qshake]

jamie2000,

The coring and welding of studs or channels to the beam is common and is treated from a theoretical standpoint in the AISC Engineering journal - not Modern Steel Construction.

Unless specifically stated in a code, I don't see why this method should require additional load testing.

Additionally, this method is employed sans coring/drilling with precast deck beams often and the holes are grouted afterward.