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[AmerRescue(--nospam--at)aol.com]

In a message dated 10/25/2005 9:53:36 P.M. Atlantic Daylight Time, en(--nospam--at)najjarinestructures.com writes:

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 a_p=2.5 and R_p=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 0.7Ä_M<0.025H.

 

TIA,

 

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

ALLEN DESIGNS

Consulting Structural Engineers

 

Ed Najjarine, SE

NAJJARINE STRUCTURES
26 Lily Pool, Irvine, CA 92620
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(714) 412-4887 Cell ; (714) 464-4288 Fax
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The roof is normally the last part of a structure to fail and in 90% of building collapse the roof does not fail, at all. The higher you are in the building, the less weight above you...the less weight above you the smaller the applied force..the less the applied force the larger the voids..this is why bottom floors are squashed with 0-2% survivability and the top floors have 90% survivability.

 

This can vary if the frequency of the waves are more in tune with the top floors. This is a rare occurrence.

 

doug copp