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Composite Floor conventions

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It's been years since I've done a composite floor by hand - back when a 3/4" stud was comfortably assumed to carry 21kips. I'm knee deep in a retrofit, and have managed to lose track of where things should go, and where all the specifications are. Too many years relying on computers, I suppose. Anyway...

On composite floor system drawings, it's common to label the beams with end reactions and number of studs, which are equally spaced along the length of the beam. This is the way RamSteel outputs the data, and it flows right onto the drawings. I cannot remember whether the call out - which is normally labeled as "number of equally spaced studs" - refers to the number of studs on each half of the beam, or on the entire beam. Is there a consensus, and what is it (yes, I have a question in to RAM as well, but that's from the software side).

It has come up because I have a retrofit job, and the beam in question would probably calc in the normal load case with the 10 studs indicated. Since its 28' long, 10 studs would meet the criteria for 36" o/c max spacing. It does not, however, meet the 25% composite "rule". Which brings me to the next question: where is the 25% limitation written in the AISC spec, or is it a guideline which is not written. I have gone through the spec (13th Ed) and commentary and can't find it. I understand the inherent problems with low-composite action beams, but I'm looking for a specific limitation if it exists.

Last question...after rereading my course notes in advanced steel design, I have several indications that the studs should "never be less than 12" o/c". And, now, almost a decade later, I can't remember if the call out is for never placing studs closer than 12" (requiring double rows), or never to have fewer studs on a beam than would be in place if put on 12" centers. I have a feeling it is the latter, but don't have a definitive answer. Does anyone else use the 12" rule?


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