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RE: 1997 UBC PROVISIONS FOR WALL PIERS

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At 09:53 AM 9/1/99 -0700, you wrote:
>The intent of the figure 23-II-1 is to provide some alternate means of
>defining the height to width of the shear wall panels. With the new h/w
>requirements of 2:1 for zone 3 and 4, it could be difficult to achieve this
>for the overall height (sill plate to top plate) to width (pier width) of
>the shear wall.
====================

In fact, the h/w ratio for zone 3 and lower need only be the traditional
3-1/2:1, according to footnotes in 97 UBC Table 23-II-G. (98 CBC printing)

But in IBC 2000, the zone 3 exception appears to have been lost. It is not
clear to me how a wall of a particular ratio, designed for a particular
seismic load, and subjected to that load, knows what zone it is in and
performs worse for zone 4 people (and three years from now, performs worse
for zone 3 people as well.)

It is indeed difficult to achieve 2:1 ratios in residental work, especially
in front exterior walls on the first story. Disallowance of any credit at
any shear stress level for walls narrower than 2:1 is a wholly irrational
code provision that is insulting to the ability of engineers to create sound
solutions to serviceability limitations reportedly discovered in testing.

There is widespread discord and resentment arising from this new ratio
limit, and there is disbelief that the abrupt cut-off by ratio amended into
the 97 Code is the only way or best way to reckon with the known effects of
narrow ratios.

As for the situations in part (b) of Fig 23-II-1, where pier height is less
than wall height, the code fails to account for the position of sheathing
sheet edge locations, and apparently assumes that the sheathing is always a
patchwork of convenient-sized scraps the size of the shaded-in "piers", or
smaller.

My calculations using APA Plywood Design Spec data and APA shear test
reports over the years indicate that plywood internally is substantially
weaker in allowable shear than in orthogonal tension and compression, but
nevertheless is MUCH stronger in allowable shear than is given in Code shear
wall tables for the tightest listed nail spacing with the heaviest nails.
The implications in this are that sheathing internal strength is not part of
the problem, and that this internal strength offers opportunities where a
sheet pattern is intentionally designed and detailed to not have edges at
disadvantageous locations. Sheathing thicknesses greater than typically used
for shear walls enhance these opportunities. 

For public school work, California's fussy DSA has long accepted nailed
plywood gussets as timber truss connectors, a situation that obviously
employs the plywood material in direct tension and compression as well as in
shear in an incidental way. How could they possibly have committed this sin?? 

Charles O. Greenlaw SE   Sacramento CA