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RE: Glass Storefront as Shearwalls

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Frank McClure asked me how we (storefront/curtainwall manufacturers) deal with
the story drift provisions of the '97 UBC a a while back, and it looks like now
is a good time to answer that as well give some information on deflection:

1.  Story Drift:  Our systems are tested to 0.01H story drift with an ultimate
of 0.015H with regards to meeting the intent of the code - that is, life safety.
A typical storefront system will have about 0.1875" of clearance between the
glass and the aluminum, assuming that the glass is installed just right.  With
the '97 UBC allowance of 0.025H story drift you get an allowable height of
0.1875 / 0.025 = 7.5".  So, using this limit, any lite greater than 7.5" high
will result in glass-metal contact.  Obviously, no architect is going to accept
that limitation.  As I said previously, the intent of the code is life safety.
Although the glass may break, it should not fall out.  This is accomplished by
using the proper glass, such as a laminate (the interlayers tend to hold the
glass together and keep it in the opening).  I don't engineer the glass, just
the framing, so I can't elaborate more on that.  We can then qualify to the
architect the acceptable story drift and it is up to him/her to relay this to
the EOR.  Note that the acceptable story drift is usually about half that
allowed by the '97 UBC (or twice that of the '94 UBC).  Note-to-that-note:
"acceptable" story drift is the point where the unit still passes air/water
infiltration and structural tests.

2.  Deflection:  With storefront systems we typically use a receptor channel at
the head to accomodate deflection.  The standard receptor will acommodate +/-
0.25" with deeper channels going up to +/- 0.625".  We can go deeper by using
angles to sandwich the framing, but this gets ugly.  Curtainwall systems
accomodate deflection by the use of a sleeve-type anchor at the head/sill.  The
anchor slides freely inside the mullion, allowing the structure and anchor to
deflect while the framing remains stationary.  Because of limitations with the
caulk joint, deflections are usually limited to +/- 0.5", but can go deeper with
custom designs.

Things brings up a few questions of my own for those of you that do the
structural design of buildings with storefront/curtainwall framing.  Do you
usually design the building to drift as much as the code allows or do you have
some chosen criteria that you are most comfortable with (I think Dennis Wish
said he worked with 0.005H)?  When designing lintels or surrounding structure do
you shoot for the maximum allowable deflection or do you try to keep it within
some other limits (even L/360 causes problems when you get to long runs of
ribbon windows)?

Finally, for those that design rigid panel systems, how do you deal with the
issue of story drift?