wood blocking in stud walls

huangyhg

wood blocking in stud walls
for wood framed walls i have seen the following general notes:
-provide wood blocking at 4'-0" max o/c - typ. (seems like overkill to me!)
-provide wood blocking at mid-height of all load bearing stud walls (works fine for short walls, but i would be less comfortable with that for tall walls say 20 ft and above)
-provide wood blocking at 8'-0" max o/c - typ.  (seems more reasonable, since most walls 8'-0" high that are sheathed and drywalled don't need additional blocking)  is the blocking intended more for temporary conditions, where the wall may get loaded before it gets sheathed on one side?  what max. blocking spacings does everyone else typ. use in your general notes?
similar questions were raised in this thread:
some times the blocking is a fire stop.
thanks jae, i feel the same as boo1......you are my hero too!  i wish i would have discovered this site a few years ago, it would have saved me a lot of stress and worry when i didn't have much help as a junior engineer.
i was thinking a bit more about this and also reading up on the wood design manual and i think the 4' blocking dimension is suited for a wood shearwall application in which the plywood sheets are placed in a horizontal, overlapping pattern.  this effectively ties the edges of the plywood to the wall and gives you the full "blocked" shearwall design values.  if the blocking is not placed at all plywood edges, then a reduction factor is applied to the shearwall strengths to account for it being an "unblocked shearwall"
karlt - yes, i agree on the value of this site.  in fact, i once asked an "older" engineer mentor of mine about his preference for staying involved heavily with asce, nspe, etc. and he answered something to the effect that if you ever get in a bind, or have a serious issue to resolve, it is very helpful to have other engineers that you know that you can call on to get advice, provide objective observations, etc.
while local engineering societies provide that personal relationship tie, eng-tips sort of does this as well.  after spending some time on the site, you get to "know" some of the handles and get a flavor for their expertise and preferences.  
karlt,
i know this thread is old but can you site the literature which provides a reduction factor for an unblocked plywood shearwall?  what is it?
thank you.
are you even allowed by the ibc to have an unblocked wood shear wall?  in 2306.4 it specifically requires it, and also in footnote b of table 2306.4.1.  the code also allows calculations for shear walls to be based on the principles of mechanics, so maybe apa has something that allows you to come up with the capacity for an unblocked shear wall.
all edges of an apa rated sheathing panel used for a shear wall needs to be supported.  it's possible to eliminate the blocking if the sill plate to cap plate dimension is 8 ft and the sheathing is installed with the long side (8 ft) of oriented vertically, which is typical for most wood framed buildings i design.  however, i have designed a few buildings where the sill plate to cap plate dimension is 10 ft.  in those cases, i used blocking along the top of the 8' panel.  another instance where i used blocking was when the contractor requested approval to orient the long side of the 8 ft panel horizontally.  i told him to use blocking, which ticked him off but he did it anyway.
the nds chapters 3 and 4 provide guidance on unbraced lengths of compression and flexural (bending)   
per current codes, there is no such thing as an unblocked shear wall.  unblocked diaphragm, yes, but not an unblocked shear wall.  i always have this note on my drawings:  provide blocking between all studs at horizontal joints in osb.
daveatkins
if you use the unblocked diaphragms shear values, then the blocking may omited for shear walls.  
building tip: when we use the 10' high walls we use a two foot rip of pt plywood (w sst nails) at the bottom, fully blocked.
boo1,
where in the code does it say that you can use unblocked horizontal diaphragm values for a vertical shear wall?
daveatkins