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Re: Seismic Upgrade prob. W/97 UBC

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In a message dated 5/1/99 12:24:24 PM Pacific Daylight Time, 
lhoward(--nospam--at)silcom.com writes:

<< 
 Fellow Engineers:  I need some level headed help on this one-
 
 I find the new 1997 UBC has created a major "unforeseen" problem for us
 while doing the design for a seismic upgrade of an existing building.
 
 The building is a two story wood framed commercial building.  The
 building has been recently purchased and they want to do an addition, as
 well as change of use.  The new use is classified as a higher intensity
 of use than the existing use.  Because of the addition, remodel of
 existing structure, and the intensification of use, the building
 department is REQUIRING that the building be brought up to current code
 for seismic design, which is this case is the 1997 UBC.
 
 This building was built in the early 1970's.  It has plywood sheathing
 on the roof, floors, and walls.  There are shear walls with holdowns and
 was designed fairly conservatively for seismic loads.  
 
 The BIG problem is that the 1997 UBC says the horizontal diaphragms are
 rigid, and forces are to be distributed to the shear walls based on
 relative rigidity.  If I do the rigid diaphragm analysis, some of the
 walls fail miserably, and the Owner is going to have to shell out some
 major bucks to retrofit this building.  If I analyze the building with
 flexible diaphragms, the upgrade costs are minor.  
 
 I cannot in good conscious require the Owner to spend the money to
 upgrade the building to make the shear walls figure for a rigid
 diaphragm. Yet the building department is saying that it must conform to
 current 1997 Code.  
 
 I intend to go to the Building Official and explain to them that this
 rigid diaphragm stuff for low rise wood frame buildings is a crock of
 ****, and try and get them to allow an exception to this part of the
 Code.  I suspect that many of you will face a similar dilemma as you get
 into this Code.  
 
 I am asking what you my colleges would do in a situation like this? 
 Would you just go along and make the Owner do the upgrade, challenge the
 Building Official, or what?
 
 Any comments are appreciated.
 
 Thanks
 
 Lynn >>

I haven't run into this problem with a building official yet, however I 
believe if the wood diaphragm displacement is greater than or equal to two 
times the shearwall drift then you can consider the wood diaphragm as 
flexible.  Unfortunately, I don't have my 1997 code with me at the moment but 
i do recall reading this.

Good luck!

Michelle