partial vs. full composite steel construction

huangyhg

partial vs. full composite steel construction
from the salmon and johnson 4th edition steel book, it appears to me that, basically, a fully composite steel beam has a sufficient horizontal shear capacity from its shear studs between the point of zero moment and max moment to pick up the entire compressive force component of the couple @ max moment, while a partially composite steel beam has enough studs to only pick up a portion of this compressive force.
so.....my question is, why would you ever go with partially composite instead of fully composite?  to save # of studs?  it just seems inefficient, because then your beam size and slab thickness may be bigger if you go partial instead of fully composite...
thanks.

most commercial construction is designed for partial composite. we design only for the required loading and not the full section capacity. the savings will be in the number of studs required.
the slab thickness is determined by the span and gravity loading of the slab, not by the composite beam section. the beam size for partial composite is generally the same as you would use for full composite, it just has less studs.
i hope this helps!
to add to jike's reply: slab thickness may be controlled by fire rating required.
just like non-composite beams, long span composite   
whyun is 100 % correct. firerating often controls slab thickness in commercial buildings. thanks for the backup!
agree with all responses...the maximum lower limit of 25% is recommended by aisc mainly due the the fact that at this low a limit the amount that the shear studs are required to deform is quite high at ultimate load and can result in brittle type failures.  a lower limit of approximately 50% composite is more reasonable.  generally imposing this lower limit results in a fairly large reserve capacity (for the beam at least - not the connection of course) for a minimum increase in studs (which are cheap).  
another consideration is that you are typically welding the deck at each flute anyway so you might as well use the labor to shoot a stud in.  
thanks...
finally, can you tell me what page in the code that 25% minimum is written about?  thanks.
it is not explicitly written as a code provision in the 3rd ed. lrfd (it was in previous editions but has since been removed), but see p16.1-220 "equations c-i3-6 and c-i3-7 should not be used for ratios [percent composite] less than 0.25.  this restriction is to prevent excessive slip, as well as substantial loss in beam stiffness."  in other words the beam does not act compositely with less than 25% composite action.  though again i believe 50% is more reasonable.  
typically we use rules of thumb for filler beam and standard girders(3 or 4 point loads) based on span and serviceability issues so for a typical 30'x30' bay with beams at 8-10' we would use min w16x26. when you run the #'s for full composite it is usually overdesigned so we reduce the number of studs but alwalys keep it at 1 stud per foot.(for composite steel deck) a quick check of the capacity at one stud per foot and it is usually ok. obviously for conditions other than typical filler beams and girders this dosent apply.