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Re: rigid wood diaphragms

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These are valid concerns. I don't think that I would argue the concept of a 
continuous beam with a floor diaphragm when the walls are placed close enough 
to increase the rigidity of the diaphragm. I think that there is validity in 
the assumption of rigidity but I don't think that the total system will 
significantly or adversly affect connected shearwalls that are designed to 
for full tributary width distribution. My concerns are more with single story 
structures where the roof is not flat and is constructed of either trusses, 
rafter tied systems, gables, hips & Valleys and where roof diaphragms occur 
at varying elevations creating a discontinuity. By code, each of these 
smaller diaphragms will display very small deflections that will require them 
to be considered rigid. 
I don't believe that there are examples from past diseasters that can point 
to a failure of a shearwall that has been designed properly with suffient 
rigidity to control drift.
My opinion from reviewing damaged structures is that failure was caused by:
1. Improperly connected or attached shear elements,
2. Narrow shear panels designed as close the the historic 3.5:1 ratio and 
highly loaded (1000 plf or more).
3. Raised first floors with inadequate cripple wall sheathing,
4. Lack of anchorage of the mudsill to the foundation to resist base shear.
5. Interior shear walls that terminate at the ceiling of a gabled roof
6. Interior shear walls connected to a 3-1/2" slab with shot pins.
7. Buildings constructed by prescripted Type V (wood framing) sheets - I 
don't know of failures causing loss of life, but I can attribute excessive 
damage and high cost of repair to prescriptive methods.
8. Construction omissions and defects.

I welcome any examples in a seismic zone of wood framed structures that 
sustained damage which resulted in loss of life or major damage to the 
structural system of the building.

Dennis S. Wish PE

In a message dated 6/2/99 9:40:25 AM Pacific Daylight Time, 
merrickgroup(--nospam--at)compuserve.com writes:

<< Rigid wood diaphragms
 Wood construction of light framed buildings.
 
 I have read that some engineers envelope the 
 possible force distributions. (rigid vs. flexible 
 diaphragms) 
 
 Does any one consider the "continuos beam 
 over supports" load distribution? ATC3 
 suggested increase factors for interior shear 
 walls. (rigid shear and flexible flexure)
 
 Does anyone assume a flexible diaphragm 
 force distribution and then specifies that the 
 gluing of floors is not allowed? Is a glued floor 
 a rigid diaphragm? If so then shouldn't the 
 force results be enveloped for when the glue 
 may break free. 
 
 Consider a glued and unblocked diaphragm. 
 When the glue breaks free, is there enough 
 breakage to transfer shear to the more flexible 
 nailing to allow a flexible diaphragm?
 
 Does any one consider the releasing of lateral 
 forces from non-shear walls? The wall must 
 reach a capacity to crack the finishes. Does any 
 one check the bearing on the joists to restrain 
 the wall to crack the finishes? What should be 
 the release capacity of finishes on a non-shear 
 wall? 
 
 David B Merrick, SE
 >merrickgroup(--nospam--at)compuserve.com< >>