why do wood stud walls have double top plates

huangyhg

why do wood stud walls have double top plates?
they seem to always have single bottom plates and double top plates...why not single top plates?  is this related to ease of construction or is it a structural concept?
i've always wondered this...
thanks.
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because according to ibc, you need the rafter or the joist to sit 5" away from stud.  if you put double plates however, you can put the rafter wherever you want.  therefore, it is easier to just put 2 plates (cheap and stronger).  the top plates essentially act like a beam supported by studs.
sills are usually bolted or nailed to the floor below, so doubles are not usually required.  
wood frame construction is redundant in nature, so the double top plate really does a number of things.  one, floor joists, rafters, and trusses don't always line up with the studs, especially when the studs are at 16 and the others are at 19.2 or 24 inches, so load has to be transferred to the stud through the dbl. plate.  also, it can be the only portion of the lateral resisting system that acts as a chord, but usally it has help.  if you have ever framed any, you will find it helps straighten out the top of you wall too.  
just a few listed, there are more reasons.
there is a "movement" to optimize wood framing, including not requiring top plates, under certain conditions.
thanks...
and what about doubled-up studs?  
are those installed just for extra strength and stiffness around doors and windows where some big localized point loads may occur?
thats just columns to support the door header. usually if its only a standard size door or window, you will only need 1 stud and 1 king on each side.  often framers just do everything so quick and always put an extra one just to be safe.
double top plate can't handle large loads, so studs directly beneath beams are required.  this is a column and needs to be so designed.
the number and size of king studs depends on the vertical point load and on wind loading (exterior walls).  it is a column (just like studs are) and needs to be designed for vertical and lateral loads.  note interior walls shouldn't require as many king studs, because no wind load.
i agree that double top plates allow the placement of standard length floor or roof joists anywhere.  along drag lines, double plate can be spliced using nails or bolts for typical cases.  single plates would always need metal straps in addition to fasteners.
in addition to making it easier to have a "continuous" chord for in-plane loads, having double top plates also helps to tie walls together out of plane.  often times a non-load-bearing partion wall that is parallel to the ceiling joists (or floor joist) will only be restrained from lateral movement at the top of the wall at each end of the wall (i.e. no blocking between joists connected to the wall).  if single plates were used, this would mean that only a couple toenails into a 1.5" thick   
double top plate is to provide continuity for frame construction.  all intersecting walls and corners should lap.
dik
in australia using  a single top plate is the norm, for single storey residential construction.