ledger construction

huangyhg

ledger construction
two items i have and would like to pose a question/comment on.  first is design/analysis of ledgers for non-snow loaded roof framing attached to taller wall studs.  ledgers are attached to studs at 16" o.c. usually.  framing that is hanging or resting on the ledger is typically at 24" o.c..  i analyze the ledger assuming that i have one joist/rafter/truss that lands right on the point of attachment for the ledger.  for example a 12' long half-truss spaced at 24" o.c. with a 20psf dl and a 20psf ll would have a load of 480lb tl at it's bearing points.  if this were to land right on the connection point of the ledger, the ledger would need to be attached to resist the 480lb.  (hf studs and ledger with 16d sinkers per nds 05 will give you 89lb x 1.25 (cd) = 111 lb/nail so 5 nails will be needed)  another engineer at work said no, just engineer for 6' tributary span of the rafters of the total roof load for a distance of 16" which is the tributary length of the ledger fasteners.  his way results in a reduction of the loads i get of 33% (40psf x 6' x 16" = 320 lb only 3 nails will be needed).  what are your thoughts/concerns/perceptions?
second item with regards to ledgers is at a curved wall.  we run into some custom homes that have curved walls with attached covered patios.  the plate of the patio is usually a few feet less than the house (house is at 12' plate height and the patio is at 9' or 10' with a flat roof).  if you want to use a ledger to hang the flat roof trusses on, it would need to be curved.  i have toyed with the idea of a built-up plywood ledger but have a difficult time trying to justify the allowable loads on that assembly (say 2 plys of 3/4" plywood, glued and nailed with 8d at 4" o.c. staggered and attached to studs at 16" o.c. with (3)-1/4" wood screws with 2" thread embedment into studs).  anyone else have an idea for this type of connection?  any better way to hang trusses off of a curved stud wall?  i have toyed with the idea of blocking between studs for attachment but it seems labor intensive...  another idea is to score a 2x ledger every few inches so that you can bend it, but again, i don't know what that will do to the integrity of the ledger (an old framer told me about this approach when i asked him the same question).
any ideas/thoughts would be appreciated.
regarding the curved wall:
run the a ledger beam  straight, just above cieling level and fix the end of the trusses into the side of the beam. then cantilever the trusses out to the wall similar to an eaves overhang. this only works if the wall curves outwards.
can't help you with the second part, but for the first question:
if your total span is 12 ft, i would use 6'; if the total span is 24' then use 12'.  i can see your concern about the spacing.  it makes sense to use the 16" like your coworker says, but i can also see your point if a truss falls right at the connection.  i think i would still design at the 16" spacing.  i would be curious to know other's thoughts on this also.
i use whatever the plf load would be and adjust for 16" spacing.  actual placement of the trusses will vary across the studs and i think it is too conservative to assume that each stud connection must match a truss load.  but that's just my opinion.
depends on the stiffness of the ledger.
if it is stiff enough, as the nearest stud starts to yield, the load will spread to the adjacent studs.
so 17% of the load spread to the stud each side does not sound unreasonable.