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Re: Seismic Deflection Criteria for "Penthouses"

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Drift it self is not checked for elements such as curtain walls, penthouse skylights and such, those elements are usually checked for deflection only within their own system, L/175 for height of less than 13 ft, L/240 for height of over 13 ft, 2L/175 for cantilevered elements (vertical elements). The drift however must be considered in the design of the connections to the structure itself to allow for relative movement, and to determine the needed tolerance for water and air tightness. Please refer to UBC section 1633.2.4.2 for general requirements.
 
HTH
 
Ed Najjarine, SE
Irvine, CA

Bill Allen <T.W.Allen(--nospam--at)cox.net> wrote:

I am designing a roof element (?architectural feature?) using UBC 1632.2 and Table 16-O with ap=2.5 and Rp=4.0.

 

Does this element need to comply with the drift limits of 1630.9 (although it states that the drift limits are based on loads in section 1630.2.1)?

 

If not, what methodology would be appropriate?

 

I vaguely recall (from a seminar a long, long time ago) where a procedure was demonstrated on how to determine the seismic forces of an roof top ?appendage? based on the acceleration of the roof, but I don?t recall any of the details.

 

Even there is no drift limit based on wind loads in the UBC, the element complies with the old ?1/16? per foot criteria, but not for 0.7ÄM<0.025H.

 

TIA,

 

T. William (Bill) Allen, S.E.

ALLEN DESIGNS

Consulting Structural Engineers

 



Ed Najjarine, SE
NAJJARINE STRUCTURES
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