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Re: L.A. City, Division 91

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In a message dated 98-07-02 17:01:16 EDT, you write:

<< 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.
  >>

I would agree with Bruce in general, I have seen the pictures of an interior
glu-lam beam in a panelized roof system which split at the top during the
Northridge earthquake from cross grain tension.  I think you need the
continuity ties to help assure the two opposite exterior walls will move
together during the earthquake, even if one of the opposite walls is a frame.

But with long narrow buildings (example: 75 feet by 300 feet with short
transverse interior shear walls) I don't know if you can practically provide
continuity ties at 24 feet o.c. for the full 300 feet, especially if you are
doing a clear span of 75 feet.  With so many splices in the continuity ties
between purlins, I wonder how affective they are because of all the potential
cumlative slip in the multiple connections.

In long narrow buildings, in the longitudinal direction, we have looked at
extending the continuity ties only for a diphragm depth where the diaphragm
shear is 250 plf or less, or diaphragm ratio is 1:1 as a minimum.  In some
cases, we have considered just extending the ties to the interior shear wall
if the diaphragm shear was less than 250 plf.  In long narrow buildings where
the end transverse wall is a open storefront, it seems like it might not be
necessary to extend the continuity ties for the full length.
Typically in long narrow buildings, the diaphragm shear stress is relatively
low.

I think if depends more on the height, weight, length of the transverse wall
and out-of-plane design forces you use as to whether you need to extend the
continuity ties for the full length of the longitudinal direction of the
building.  How often have you had a  wall in the transverse direction which
does not extend for the full width of the building. 

I will say though, that if you don't extend the continuity ties the full
length, I think there is a chance that you will have some diaphragm damage at
the chord where the continuity ties stop under the real earthquake loads due
to deformation compatibility between the opposite walls of the building.  I
believe the wall pulling away creates a larger tension force on the continuity
tie and diaphragm, then the compression force pushing from the opposite wall.
Especially in a long building, which have openings in the roof for skylights,
etc., and the compression force is transfered to the side longitudinal shear
walls in a shorter distance then the length of the building.  

If you stop the continuity tie short of the opposite wall,  the tension force
must be transfered by cross grain tension through the chord member to the
plywood sheathing framing from the other side.  I guess the question is then,
can you rationalize where you stop the continuity ties to minimize potential
damage or failure due to wall and diaphragm deflection.

just some more thoughts.

Michael Cochran