Composite Joists 2

[haynewp]

Does anyone have experience with steel joists composite with concrete slab? To my knowledge and experience, composite beams are much more popular. Any opinions from someone that has used composite joists instead? (Structural or cost related)

 

[WillisV]

I have used composite joists before with moderate success. They are very useful for moderate to long span floor/roof conditions that will have significant post-composite live or superimposed dead load. Typically the governing limit state for their design tends to be the pre-composite deadload of the wet concrete etc. The composite action then kicks in to give you a much larger post-composite capacity than a typical joist. If the floor or roof you are placing is not expected to require a lot of post-composite capacity, then you are better off just using regular joists spaced closer together IMHO.

Composite joists are not just picked out of a catalog like regular joists, loads (precomposite and post composite dead and live) must be provided to the joist manufacturer on the design drawings (see p23 of

http://www.vulcraft.com/downlds/catalogs/composite/comnoncomcat.pdf).

Just like regular joists, it is much more difficult to field modify composite joists as opposed to composite hot-rolled beams. I have run into many situations where I had wished I had used beams just because if any major change in conditions occurs it must be run back through the joist manufacturer as opposed to you fixing it yourself.

 

[dik]

I tried locating one of the papers I have on Composite action with OWSJ and have been unable to find it. I'm not sure who published it, the CISC, the NRC of Canada, or the AISC. The paper is approximately 25 years old and will continue to rummage through my pile of stuff. As I recall (dangerous <G>), the analysis was similar to regular composite design and that the effects of composite action provided by nominal attachment of the steel deck was sufficient up to approximately half the design load... at which point the actual shear connectors continued to provide composite action to failure. The nominal connection also provided an increased stiffness (reduced deflection) up to half design load (well within the range of most applications).

Dik