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Re: Composite Beams vs Composite Joists

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Composite joists were used in a recent technology building at Virginia Tech (Torgeson Hall, to be exact). The floors were _very_ bouncy to begin with. I don't have the drawings, but I believe the space was two sets of  40' x 160'+ spaces (two long sets of classrooms on either side of an atrium).  The analysis looked fine from a static point of view, but the floor felt like it had a major mode at about 3.5 Hz. You could walk at one end and feel the vibration along the entire length.  As bad as it felt (made the owner pretty nervous), it felt better once the partition walls went into place and the classrooms filled up with "stuff"  - there were lots of computer labs, so lots of mass.

In the end, I believe it was a non-issue, but there were a couple of months of unhappy faces concerning the result.  We did not use a composite joist system again in that office.

Disclaimer: I was not part of the design team, and was only tertially involved in the post-construction analysis. As I no longer work for that firm, I don't have the drawings with which to check any information. I have been in the building since its opening and have not noticed any ill effects. I won't reveal the name of the joist manufacturer, but it rhymes with "Dull Craft", and they use(d) the building in advertising for their composite system.

Composite vs. Non-composite:
I have seen another building done with the Hambro composite joist system (a four story hotel).  I did not notice the vibration issue above.  The GC really needs to be on board, as this is a fairly significant departure from the typical non-composite (joist/form deck)  construction.  The contractor was happy with the system on this job. He admitted that it took 20-30% longer the get the first floor set an placed, but he saved a bit overall by the time he had the whole building completed.

Jordan



Will Haynes wrote:
I am looking for some opinions on using composite steel joists instead of composite steel beams for a typical concrete floor slab in an office building. I have almost always seen composite steel beam floor slabs and I just wonder why there are not more composite joists being used?
 
I am sure it comes down to money, but I wonder how can comp. joists really be that much more expensive than beams? You also get the advantage of running the mechanical through the joists.
 
 
 
Will Haynes


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