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Re: For Urgent Review and Comment - Blue Book Commentary

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Sorry, the listservice cut off the remainder of my post indicating it was too 
long to send. My intention was to reprint the five paragraphs from the Blue 
Book Draft that Ali Sadre posted to the list. Paragraph 2 needs to be 
commented on by Monday the 24th. 
I hope this goes through this time. If not - please refer to the post "Re: 
Seismic Upgrade ..... Appeal to those who created the code" Dated May 21 from 
Ali Sadre for the same information.

In a message dated 5/21/99 11:47:45 PM Pacific Daylight Time, Sadre(--nospam--at) 

<< 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 
 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 
 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.  
 However, because of a long tradition of past desgin practice, a large 
 majority of engineers perform their lateral force analysis by considering 
 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 
 a reasonably regular configuration and where redundancy exists due to the 
 presence of the non-calculated partition walls that are common in 
 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 
 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.  
 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 
 1994 Northridge earthquake [Mendes, 1995].
 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 
 improvements. Thank you all for your input.