huangyhg

fyi - composite beam deflections and ram ss
this is tangential to the recent thread on composite action. the green book uses an ieff for deflection checks (which accounts for partial composite action). the new black book calls this same equation iequ and recommends multiplying this by 0.75 for deflection (based on testing). ram did not recognize this. i called them last week and they were unaware of this change, but will incorporate it into the next version. in the meantime, anyone using ram should bump up the deflection criteria for composite beams to account for this (e.g. l/360 will want to be l/480).
are the new iequ values close to the old ieff values? the l/360 has worked pretty well i thought for years based on the inertia we have been using, whatever they want to call it. is aisc now proposing that the floors have been deflecting too much and should have been designed closer to l/480 with the old ieff?
the new iequ are identical to the old ieff values. aisc says, in the commentary, that tests show that the effective moi is roughly 15% to 30% lower than the equivalent moi (the old ieff). it's quite possible that deflection wasn't an issue because there actually is some end restraint, even though we don't consider it, which helps deflection.
a couple of side notes on this subject:
1. the 0.75 reduction is not new to the 13th edition manual - it first appeared in the 3rd edition commentary (see 16.1-220). it just got screwed up and had to be fixed in the 13th because the nomenclature changed (in the 3rd edition it was written as ieff = 0.75ieff which isn't very clear so it was revised to ieff = 0.75iequiv. except the nomenclature for the equation for iequiv wasn't changed in the first printing).
2. note that commentary is just that, commentary - and therefore not required. you could just as easily be arguing that ram should be using the lower bound moment of inertia which is another approved commentary method. if you feel that the 0.75 etc. is not necessary and l/360 with ieff has been performing fine then i would continue to use that.

"if you feel that the 0.75 etc. is not necessary and l/360 with ieff has been performing fine then i would continue to use that." agree.
can you all clarify what is meant by i(equiv)?
is i(equiv) simply the calculated i(eff) prior to multiplying by 0.75?

yes.
jae

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