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Re[2]: Hardy Frame, between code changes

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The 2:1 ratio is not arbitrary - it was established from cyclic tests done at 
the University of California Irvine and the University of Idaho. Also,APA did 
some preliminary testing - enough to know that the the deflection formula in UBC
Std 23-2 is not applicable to the 3 1/2:1 wall; the deflection formula is based 
on a standard ASTM E72 monotonic racking test with a panel aspect ratio of 1:1. 
The limits of the 3 1/2:1 panel were extrapolaed far beyond the range of 
applicability of  monotonic racking data and applied to dynamic, cyclic 
behavior. In this case beeter to use the new limits as prudent until APA or 
others do extensive cyclic testing  to establish appropriate limits. 

______________________________ Reply Separator _________________________________
Subject: RE: Hardy Frame, between code changes
Author:  <seaoc(--nospam--at)seaoc.org> at net
Date:    3/27/98 11:18 AM


Design is based upon Engineering Judgement as much as it is based upon 
minimum standards established in code. The engineer should be hired for his 
experience and judgment not his fee.
I do not know of too many lawsuits that have been won one the charge of 
over-designing. However, I know of many that were won when damaged occurred 
from an insufficient design.
I am not entirely in agreement with Bruce since I believe that the public is 
being penalized for poor construction practices in the past. The majority of 
failures that I explored after Northridge were not due to improper design or 
insufficient H/b ratio of shear panels. I discovered that the panel 
connection to the studs, the uplift restraint attachment at the studs and 
anchorbolt placement were more the cause due to improper installation.
I'd like to ask a question. I am under the impression that the H/b 
requirement is required only where the design engineer does not desire to 
perform a lateral drift analysis. All minimum standards in the code, 
including unsupported plate heights, web crippling (for a few examples) are 
established unless they can be proven otherwise by analysis.
If the engineer can determine that a 3.5:1 ratio deflects less than 0.005h 
and can provide sufficient restraint for the wall, the issue of an H/b 
equivalent to 2:1 is moot.
I have not chosen to design based upon the 1997 UBC simply because it is not 
yet adopted. If what Bruce feels should be mandatory practice based upon 
good engineering judgement, the Building officials as well as the EOR would 
be liable for not adopting a more restrictive measure before it's compliance 
date.
Good engineering judgement can also disagree with he provisions of a code - 
which in this case my judgement is not yet made.
I believe that with the adoption of Structural Observation requirements 
within the City of Los Angeles (and most other places), we should see better 
performance on panels that are designed by past code slender ratio's. Just 
because the code says it should be this way is not enough. If testing proves 
(and it has) otherwise, then I'm inclined to accept empirical data over 
subjective writing. I say this because the 2:1 limit started as an emergency 
measure shortly after Northridge and has never been reduced in Los Angeles. 
I don't feel it was based on any rational test data.
The Blue Book indicates in section C802.2.4 of the commentary that the APA 
had not planned Cyclic retesting of plywood shearwalls until some time in 
late 1996. This means that the 1997 UBC requirement was passed prior to 
testing. Section C802.1.2 states "The height-to-width ratios indicated in 
Figures C802-8 and C802-9 are ANTICIPATED for the calculation of 
height-to-width ratios.
     
I believe that Bruce is jumping the gun on this one, but I would never fault 
his judgment and this is the essence of good engineering.
     
Dennis Wish PE
La Quinta, California
wish(--nospam--at)cwia.com
ICQ# 6110557
http://wwp.mirabilis.com/6110557
     
"Silence is the virtue of fools."
Francis Bacon
     
|-----Original Message-----
|From: T [mailto:vicpeng(--nospam--at)vtcg.com]
|Sent: Friday, March 27, 1998 9:02 AM 
|To: seaoc(--nospam--at)seaoc.org
|Subject: Re: Hardy Frame, between code changes 
|
|
|Subject: Re: Hardy Frame, between code changes 
|
|
|As a
|>professional licensed engineer, you have a responsibility to design to 
|>incorporate the best available thinking into your design not just blindly 
|>finally the letter of the Code book.  Particularly in
|circumstances following
|>a "learning experience" such as an earthquake, are we bound to 
|wait for code
|>changes to be written and adopted before we try to pass on our 
|experience into
|
|
|Do you not have the conundrum that existing legal proscription may 
|weigh against you if you use parameters that are not "legal" at 
|time of use?  I.e. even if new thinking is "superior" I wonder if 
|you don't have the potential for a case against you for using 
|parameters not yet "legal", especially if it results in:  a) a 
|more expensive structure or b) the new thinking fails in an event. 
|
|Thor Tandy  P.Eng,  MCSCE
|Victoria BC
|
|
|
|