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,
<< Rigid wood diaphragms
Wood construction of light framed buildings.
I have read that some engineers envelope the
possible force distributions. (rigid vs. flexible
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
David B Merrick, SE