Subject: Re: For Urgent Review and Comment - Blue Book Commentary
Date: Sat, 22 May 1999 23:41:43 EDT
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)aol.com
<< 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.
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
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.
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
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
improvements. Thank you all for your input.