double top plate on stud wall

huangyhg

double top plate on stud wall
i have a very long span floor (29')for a residential house.  the floor joists are supported by existing 2x4 stud wall.  the studs are spaced at 16" oc (hf #2). by using dl=15psf, ll=40psf, my analysis shows the single 2x4 top plate can not carry the load.  i do not think it is right to consider double 2x4 working as a composite beam since it is impossible to provide that many nails if you calculate the shear flow between the two plates.  but ibc does indicates 2x4 stud wall can carry one floor plus one roof ( i always feel strange it does not talk about the span limit).  would someone help me to explain why?  thank you!  
well, if the studs can take the axial load then i would just make them try to line up the joists with the studs.  then the double plates will only act to resist lateral load (chord force).  if it is not lining up, as long as it is 3" away then it should not create any moment.
never, but never question engineer's judgement
if this is an exterior wall with a rim joist, the rim joist would help spsan out the load between the studs.  
if this is an interior bearing wall, if the joists lap over the top plate, then the load is spread out further to 6" instead of 3" if the joists are spiked together.
if this is an interior bearing wall with joists only on one side, hopefully it too hsas a rim joist like the exterior wall.
mike mccann
mmc engineering
" i do not think it is right to consider double 2x4 working as a composite beam"
that's correct, but the will act as two independent beams.
it is an interior bearing wall with joists only on one side.  but it does not have rim joist.  there is blocking between the floor joist, which i do not think it would help to distribute any load.  i could prove it works for bending by using reduced live load ( normally i don't use this for residential )and just single 2x4 top plate.  but it would not work for shear even with reduced live load.  i am wondering why i can not use double 2x4 top plate for shear analysis since it is there.  i do not quite understand the 3" and 6" mike and coengineer mentioned above.
it took me a while to agree with sdz.  i used to think the plate on top will carry all the load until it fails, then the load will be carried by the one at bottom.  but the more i think of it, the more i agree with sdz.  once the top plate carry the load and deform, the bottom plate will deform the same amount, which makes the two plate each carries about half of the load.  any disagreement or comments?     
if you need a further endorsement of sdz's post, you have it.  the two top plates will work together, but not compositely.  but you need to watch where splices occur.
in actuality, depending on the nailing and any gluing, partial composite is more likely.  it's just that using two independent beams is conservative, and full composite is unconservative.  
the truth is somewhere in between.  can you handle the truth?
mike mccann
mmc engineering
thanks hokie66.  it does give me more confidence on my understanding.  btw, is 29' floor span commonly used in residential?  there are nonbearing walls below and cross the long floor joist at middle of the span.  the inteior nonbearing wall is built on existing 2x6 t&g plank floor.  will it be a concern that the nonbearing wall will support the floor joist even it is not intented to?        
once initial deflection occurs, composite action is mobilized, though weakly.  as mike noted, the two should be "spiked" together.  if the spiking occurs at a reasonable interval, i.e, nailing the top plate together at 6 inches or so, staggered, then some composite action occurs.
long term deflection will be a concern and accidental load sharing can happen, vibration may also be a factor with this span. back to the original question, can you add studs or double up to reduce the problem/issue of the double plate? agree with trying to line up joists with studs, but if that cannot be done, or is already built, then you will need to do something to beef it up so you can sleep at night.