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Re: L.A. City, Division 91

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Regarding tilt-ups:

Personally, I like the 24' continuous tie requirement for tilt-up buildings.
I think it adds significant strength and performance to the building.  Without
continuous ties, the subdiaphragms, which everyone in the previous posts is
counting on, have a tendency to open up at the chord of the diaphragm due to
the diaphragm deflection.  Since the edge of the diaphragm is typically a main
beam in one direction, and the purlins are typically hung, the purlins can
pull off of the beam collapsing an entire portion of the roof.  This type of
failure was seen in Northridge.

As a rule, I think it is fair to say that tilt-up buildings, due to their
nature (tall heavy walls laterally braced by a very light flexible diaphragm),
have performed poorer than the average building in previous earthquakes.  To
take structure out of the building and make the buildings weaker, based on
calculations and theories, seems to me to be ignoring the past.  And as we all
know, those that ignore the past are doomed to repeat it.

Just my opinion.

Bruce Resnick, SE
Parker Resnick Str. Eng.