Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

perforated shearwalls

[Subject Prev][Subject Next][Thread Prev][Thread Next]
    No structural model is very trustworthy unless it has been validated
by full scale testing.  The UBC four term expression has been validated
with phoney tie downs and at only one h/d ratio (1:1), so I wouldn't get
too enamored with it just yet.
    A much more rigorous model is the one used by Richardo Foschi (at
BC) and Dan Dolan (at VT).  It has been tested over a wide range of h/d
but it is very unfriendly to use.  It models the deflection of each
nail, the paneling, and the
tie down.  However, it is a good way to envision shearwall deflection.
If  the sheathing type, tie down, and height are constant in a wall line
then deflection (and capacity) is proportional to the number of nails.
If you want to
give the framers an option (and I don't really think it is a good idea
BTW) you can call for two, heavily nailed panels with four tie downs or
a more lightly nailed wall
with two tie downs and get the same stiffness.  When you consider the
waterproofing, the extra analysis and detailing, the likelihood
ofconstruction screw ups, the extra construction supervision and
inspection, and the splitting
problems from close nailing, I think choosing the fully sheathed wall
with modest nailing and tie downs is a no-brainer (despite what your
architects/owners might
think.  You're the engineer, start educating them.)
    IMHO perforated walls without tiedowns don't contribute much
stiffness. Without a tie down, the end studs pull the sill nails at the
bottom corners of  the wall at pretty low load.  Ed Diekmann says about
600#(70 to 80 plf).  You could include them in your analysis, but with
reductions for door and window perforations,  the wall typically isn't
worth including.
    As for enveloping, it comes down to engineering judgment but
unfortunately our judgment typically has minimal basis in fact (test
results).  What we do know
from testing is that the typical floor diaphragm is pretty stiff
compared to a typical shearwall.  I run QLAT30 and a rigid diaphragm
analysis and compare the numbers.  The rigid diaphragm numbers have
looked the most reasonable so
far.  Large tributary loading onto skinny walls can produce some pretty
big numbers and I usually use the lower values that result from
stiffness considerations.  You may
not be comfortable doing that, but it is not because you "know" how the
structure actually behaves.  Your knowledge has come from crunching
numbers and
seeing your designs built, not from observing lab tests or from a design
level seismic event.  That's not a personal criticism, it just our
unhappy circumstances at present.
    Some of the "ivory tower" guys have a much better feel for the
structures' behavior than the practicing engineers do-- because they've
watched these diaphragms tested.  Unfortunately, they don't have the
benefit of analyzing
actual structures or seeing the typical construction quality problems.
The result is that they tend to get overly elaborate with their
analytical tools.  If you really want to help, perhaps there's a way to
make some meaningful
input into CUREe.  It's goal is a practical piece of software (validated
by adequate testing) that we can use for design work.
Chuck Utzman, P.E.