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Re: 1997 Building drift limits

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Ali,

In reading the correspondence on the SEAINT there was a note from you to 
Dennis regarding Volume III of the Seismic Design Manual and the possibility 
of adding clarification to the last problem.  It is important that the build 
drift for the design of cladding systems, especially glass, be definite, not 
debatable.
 
The engineering analysis I perform is mostly for the glass and glazing 
industry.  This industry has developed manufactured systems of aluminum 
extrusions that are used to frame and attach glass to the building structure. 
 One area of the analysis is to check the adequacy of the glazing system for 
building drift.  In the 1994 code there was no guessing about the max amount 
of drift the glazing system needed to accommodate.  The building official 
could evaluate the engineering for the glazing system based on the provisions 
of the 1994 UBC.

In the design of glass systems there is no yield when the glass is loaded.  
Consequently when the glass becomes tight in the aluminum frame due to 
building drift it is considered broken and to have failed.

 The 1997 UBC is not specific regarding the drift that glass needs to 
accommodate before it breaks.  It could be interpreted that the glass should 
not break with a drift of .025h.  This is 5 time more than the 1994 UBC and 
would require brand new systems for a large industry of glazing suppliers and 
contractors.  It would also mean that architecture for glass covered 
buildings would change drastically because the widths of the aluminum framing 
would need to be much wider to contain the glass lights under these large 
building drifts.

I know it is a little late for a revision in the 1997 code but I am afraid of 
a general misunderstanding when engineers submit their glazing design 
calculations to the building departments that evaluate the design based on 
the provisions of the 1997 UBC.  If there was some authoritative reference 
(such as Volume III of the Seismic Design Manual) that clarified the problem 
it would help all of the structural engineers who design building cladding 
systems and all of the building officials who check the designs. 

Any comments or suggestions on this subject would be greatly appreciated.  

Jim Sadler
Structural Engineer