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Re: rigid wood diaphragms

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Well, of course there was the Northridge Meadows apt. in which both loss of life
and major structural damage occurred. And there were lots of other wood frame
buildings (condos) which sustained substantial structural damage (see Ben Schmid
case history for an example, of which there were many others). However, your
point is well taken - many of these structures had questionable design
assumptions or construction features, in my opinion, even though these were
permitted by UBC at the time of construction. I could not think of an example
where rigid or flexible diaphragm performance could be a factor in the damage
that was observed, however.
John Rose/APA, Tacoma

Seaintonln(--nospam--at)aol.com wrote:

> 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< >>
>