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Re: Seismic Upgrade ..... Appeal to those who created the code

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Dennis, 

Here is the current proposal (for our members' input). Unfortunately, the 
time is compressed.  Please try to post your thoughts by May 24. 
All paragraphs are being numbered for easy reference. Please note that ONLY 
PARAGRAPH # 2 IS UNDER REVIEW.  All other paragraphs have already been 
reveiwed and approved by various local and state Committees.  I have included 
the complete Section so that Paragraph # 2 woud be better assessed (in the 
context in which it is being proposed).

Blue Book Commentary to wood diaphragms - Section C805.3. Latral Force 
Distribution:

Paragraph # 1):
The UBC has, since the 1988 edition, had a criterion to decide when 
diaphragms are to be considered flexible.  The deflection of the diaphragm 
under a unit load is compared with the average story drift of the vertical 
elements, per Requirements Section 105.6, and if the diaphragm deflection is 
more than twice the story drift, the diaphragm is considered flexible.  Most 
lateral force resisting systems using wood diaphragms in combination with 
wood shear walls would not meet this definition of flexible.  

PARAGRAPH # 2 (ONLY THIS PARAGRAPH IS UNDER REVIEW): 
However, because of a long tradition of past desgin practice, a large 
majority of engineers perform their lateral force analysis by considering the 
wood panel diaphragm as flexible and spanning between shear walls.  
Observations of light frame construction in past earthquakes suggests that a 
life safety performance has occured using this practice for structures having 
a reasonably regular configuration and where redundancy exists due to the 
presence of the non-calculated partition walls that are common in residential 
buildings.  

Paragraph # 3): 
Designers are encouraged to consider the relative flexibility of the 
diaphragms and supporting shear walls.  A rigid diaphragm analysis is 
recommended where the shear walls can be judged by observation to be flexible 
compared to the diaphragm, and particularly where one or more lines of shear 
wall are more flexible than the rest of the shear walls.  A rigid diaphragm 
analysis will generally result in more seismic force being resisted by the 
stiffer shear walls.  The designer can judge whether it is appropriate to 
design the lateral force system using rigid diaphragm analysis, or an 
envelope considering both the rigid and flexible diaphragm analysis.  

PARAGRAPH # 4):
It is important to consider the relative stiffness of wood shear walls and 
other veirtical elements of the lateral force resisting system.  An example 
would be steel moment frames used in combination with wood shear walls.  If 
the steel frames are quite flexible, the shear walls are likely to carry a 
large portion of the story shear than would be suggested by a tributary area 
analysis.  The relative flexibiltiy of steel moment frame may have 
contributed to damage to tuck-under parking (soft story) buildings during the 
1994 Northridge earthquake [Mendes, 1995].

PARAGRAPH # 5):
It has been pointed out that the inforamtion currently available for 
estimating diaphragm and shear wall deflections is greatly limited.   This 
hampers our understanding and our ability to accurately calculate relative 
stiffness.  It will be necessary to apply engineering judgement to the 
limited information that is available.

All comments will be taken back to the Committee to incorporate the necessary 
improvements. Thank you all for your input.


Ali