] [Thread] [Date] [Author]

[Seaintonln(--nospam--at)aol.com]

Okay, I've asked this question numerous times and am getting the strong 
impression that the majority of you are in the same boat as I am - simply 
don't have a clue.

Assumptions:
1. Plywood diaphragms lie someplace between flexible and rigid
2. Can disregard gables, vaulted ceilings, hips, valley's, scissors etc. 
Treat all as rigid (or semi-rigid).

Questionable Assumptions:
1. Since a plywood diaphragm is not entirely rigid, is it appropriate to 
isolate sections (for example each leg of an "L", "U" or "V" shaped 
structure) and analyze it separately? See next statement for justification.
2. I would assume that you can not make a rigid connection out of wood 
members without full embeddment and, therefore, each leg of a non-rectangular 
structure will exhibit some non-calcable deflection that makes it act like a 
hinge. If this is true, than only continuous diaphragms where the direction 
of framing, continuity of overlapping plywood (staggered load case I) should 
be considered rigid - all others considered the joint between continuous 
beams.

Let's make some definate decisions here!!!!!!!!

Suggestion 1. Buildings with diaprhagms at different elevations that transfer 
shear through cripple or pony walls can be broken into blocks and modeled 
separately.
Suggestons 2. Non-rectangular wood buildings with plywood diaphragms should 
be considered "hinged" at the junction of each "leg" and allowed to be broken 
down into blocks (or provide a means to calculate the stiffness of the 
intersection of the legs to resist rotation and calculate the deflection of 
each leg).

I, like most of those in practice, are trying to resolve these issues as we 
work through existing projects that need to be completed in a reasonable 
time. There is simply not enough published information to justify the intent 
of those who created this section of the code as to how it applies to wood 
framed structures. The methods are fairly consistent with masonry and 
concrete structures and does not represent such as major change. However, 
wood structures have never been modeled in this manner and offer too many 
creative assemblies to apply such a generic and limited code. 
Inasmuch has been adopted, we need to resolve these common yet creative 
problems in the most expedient manner. It is not reasonable for SEAOC or ICBO 
to make the community wait while Seismology committee or the wood committee's 
try and work out a solution that should have been considered a long time ago 
and ready by July 1, 1999. This is simply unacceptable to our clients, many 
of whom are paying interest on borrowed money while we sit back and debate 
issues that should have been resolved.

I urge SEAOC and ICBO to either Rescind the compliance of this code and roll 
back to the standard of practice prior to this code (and don't debate the 
wording of the '88 and '94 code which was never used for wood construction 
even though it did exist in wording). Otherwise, I urge you to address the 
problems faced by the majority of us in real world design problems.

Dennis S. Wish PE