This situation about Rigid and Flexible diaphragms is similar to the concrete
tilt-up wall controversy about 10 or 15 years ago. I worked for a structural
engineering firm then that designed tilt-up walls with h/t ratio of 52. At
that time, I can not find any provision in the code that allows this high
h/t. The code prescribed a limit of h/t= 25( I think). I was just starting
out as an engineer and I did not have a lot of practical design and code
application/interpretation experience. I just accepted the office policy.
They were using empirical formulas found in textbooks and ACI design manuals.
I never knew how they came up with the h/t limit of 52 as the office
standard. I also remember hearing something like "as long as we use a
rational analysis that is in accordance with well-established principles of
mechanics"(UBC94 sec 1603.3.1), and the building official will accept it, we
did not necessarily have to comply with UBC code limitations. We got a lot of
project approved and built this way.
Anyway, there was some controversy about code limitations on concrete walls
h/t ratio since some building officials enforce it to the letter with no
exceptions. I don't know how it all started but there was some full scale
testing done in the field by a group of practicing engineers, contractors,
building material suppliers, etc., simulating actual slender wall conditions
and performance to test the empirical tilt up wall formula. Test results were
analyzed and presented to the engineering community, formulas were developed,
etc.The end result of all these was that a slender wall provision was
eventually included in the UBC which was pretty much accepted by the
I hope this discusion in our list about rigid plywood diaphragms, shear walls
deflection(hold-down contribution included), etc., will spur a similar
action/reaction from the engineering community. I think we should include
code writers, interpreters and enforcers in this effort to ensure a more
timely and efficient incorporation in the next code. If this method worked
for slender walls, it will work for diaphragms, or any other controversial
code provision(or non-provision). I will just take time.
While they're at it, they might as well include the testing of plywood shear
wall with openings. This is the next controversial topic that will emerge
once a provision for it is included in the code.