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Re: H/b ratio for Low shear walls

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At 12:18 PM 8/17/99 EDT, you wrote about a residential wood shear wall ratio
problem that came up in actual design practice:

>Since there is only 1100 pounds [total shear] in the wall, the shear is
less than 200 plf 
>if I only consider [ie, employ] one of the [two] six foot piers [that are
13 feet tall] and less than 100 plf if I consider both piers.
>
>It does not seem that the code allows for an exception to the H/b allowance 
>in cases like this where the shear is very low. I'm confident that the 
>calculated deflection will be much less than the story drift - but the H/b 
>requirment seems to control.
>
>Are we allowed to exceed the H/b allowance when we can show by deflection 
>criteria that the wall can be controlled by design stiffness or must I use a 
>proprietary shear panel, braced frame or moment frame?????
>
>Dennis Wish PE
-----------------------------

It is true that excessive deflection was given as the reason for widening
the permissible ratio to 2:1 from the narrower former 3-1/2 to 1, in the 97
UBC cycle. Yet the code change proponents insisted on disallowing any usable
shear value to the ratios between 2 and 3-1/2, rather than add requirements
and provisions that would allow engineers to design out the offending
features or conditions that research showed caused the unwanted deflections.

You propose to show by engineering that a ratio of 13:6  will perform
acceptably in the deflection aspect (and in every other way) if the shear
stress is kept way below what the wall qualifies for if it had a 12:6 ratio
instead.  

There you go again, trying to be an intelligent engineer instead of a
mindless rule follower. But now the cat is out of the bag: the rulemakers
are revealed to not have done intelligent engineering in concocting some of
their rules. Some of their stuff fails to admit of a rational analysis that
comports with accepted principles of mechanics, and they were manifestly
indifferent to effects of their new rules on practitioners not represented
on their Committee. They knew that they didn't know. And the excuse claimed
for that is that they're volunteers.

As others have posted recently, wood shearwall deflection primarily comes
from nail slip in the sheathing, from uplift slippage and stretch in the
hold-down arrangements, and from "downcrunch" at the other vertical
boundary. Much of this is strongly non-linear, and it is consistent with
mechanics of materials that a substantial reduction in load will obtain an
even better reduction in deflection.

This is the principle you want to use so that at least some useful credit
may be given to your 13:6 ratio panels. 

I like the idea, but the code language authors and approvers don't, unless
they are hypocrits.

I submitted a 2000 IBC First Draft code change in the spring of 1998 that if
adopted would have allowed exactly what you now want to do. I proposed a
footnote change to the Chap 23 Maximum Shearwall Aspect Ratio Table. It
would allow ratios up to 3-1/2:1   "provided that the shear and overturning
forces used for earthquake design for ratios in excess of 2:1 are increased
by a factor of R/3 times those forces specified in Chapter 16."

The "Reason" I submitted was "to provide relief, by means of extra strength,
for continuing use of the narrower aspect ratios in two of the more severe
Seismic Design Categories. Such narrow ratios are commonplace. As a
practical matter, R/3 is a factor of 2 (sic) where R= 6.5. The resulting
extra strength and stiffness provides for adequate performance through
conservatism." 

The above was submitted privately, but with help and encouragement from my
local Code Committee leaders, who felt constrained by protocol to not openly
oppose Seismology Committee. Also, ICBO's Susan Dowty phoned and very
cordially suggested improvements that made my draft consistent with
formatting in 2000 IBC that was not then known to me, such as the
replacement of Seismic Zones with Seismic Design Categories. 

Alas, the IBC Hearings were in Virginia, which precluded my speaking up for
this change, so I discussed it with David Tyree and others at NF&PA to
encourage them to speak up for it. There is essentially no chance for
approval without a lot of politicking at the hearing combined with little
opposition, and it wasn't adopted. But a residual value of the attempt is to
show annoyed codewriters that complainers in the present indeed tried in the
past to make changes according to the established rules, and that new
complainers are in the same boat with earlier ones, as the Committee's
product reveals itself to be a flop. 

Charles O. Greenlaw  SE   Sacramento CA