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Continuity ties

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Our firm has also wrestled with this LA City and County requirement on
new tilt-ups.  At first there was some flexibility with the plan
checkers as long as you provided reasonable engineering judgment. 
Recently, it has been "by the book" with no exceptions.

My position is that limiting the continuous crossties to 24 ft and the
subdiaphragm shear to 250 plf is entirely arbitrary and without basis. 
The 24 ft dimension was selected because many on the "code" committee
thought panelized roof systems were still using 4x purlins spanning to
glulams spaced 24 ft apart in a double carrying beam system (laugh). 
Most girder systems now conform to 50 ft spacings for the new ESFR fire
protection systems.  It has been tough to convince the City/County to
accept 25 ft instead of 24 ft!  There is nothing magical about 24 ft.

A better system we have proposed to the County (Steve Cloke) is to use
an approach that considers the subdiaphragm deformation potential (limit
the length/width to 1:1) and the diaphragm shear (limit the allowable
shear to 30% it's allowable capacity).  These suggestions would then not
be as arbitrary.  (The 30% of allowable is based on the rule of thumb
developed for dynamic analysis to allow for orthogonal effects.)

Some of our tiltups are well over 1000 ft long, and the continuous
crossties become absurd and costly when at 24 ft.  We do agree that the
girder lines must act as cross ties (typically at 50 ft) and we do
design them for their full wall tie force with the 1.7 factor ('96 UBC
Supplement) for steel elements.

John Lawson SE
Kramer & Associates Structural Engineers
jlawson(--nospam--at)kramerinc.com