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RE: Glass Storefront as Shearwalls

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Michael
Soft-story or open-front structures (as they are also called) are common in
seismic country as well. This is one of the arguments that supports the idea
(unsuccessfully I believe) that the deprogram is rigid, because the control
of the open front is suppose to be resolved by torsion into the other three
sides.
I think that since before the Northridge earthquake, the City of Los Angeles
recognized that soft stories collapse or are heavily damaged.
Since many of our open-front structures are wood framed, I'll demand a
sufficient pilaster to allow movement of an embedded (cantilevered) steel
column to control deflection equal to 0.005H (0.0025H if masonry is used).
The idea is to allow the structure to deflect within the pilaster and
sufficiently, but not in excess to stress the glass.
So far I've had no glass failures, but then again, possibly the conditions
have not been right as yet.
The other thing I do is to create sufficient allowance for movement in the
slab to allow translation of the column without spaulling or breaking out
the concrete. The column passes through a grade beam coplanar to the
storefront and the columns rest upon an erection pad below the grade beam.
The advantage here is that I don't have to frame the entire front in steel
and then work out fancy details to build out the steel in order to support a
wood truss or open joist roof. Instead I only need to drag the shear from
the roof diaphragm down into the headers and through the headers into the
columns. A welded column cap and strapping between headers is sufficient to
accomplish this.

Therefore what happens is that the framing around the glass mullions move
independent of the glass - hopefully.

You bring up an interesting problem because I design mall store soffit in
light gauge steel. One thing that I am very concerned with is deflection of
the soffit and generally am criticized for using deep structural steel
sections rather than light gauge built-up beams to control deflection. I
just can't get comfortable dealing with bending in beams by empirical values
and the idea of calculating a built-up light gauge beam in bending scares
the hell out of me.
Therefore, I make sure that the soffit is in place before the glass is
installed to be sure that the contractor adjusts the soffit (curtain wall
hung from steel beams) sufficiently to compensate for natural deflection due
the weight of the system.

How do others deal with deflection of storefront (light gauge steel ) that
only supports the soffit and signage?

Dennis S. Wish, PE

-----Original Message-----
From: Michael Zaitz [mailto:zmanpe(--nospam--at)bellsouth.net]
Sent: Monday, February 07, 2000 7:13 PM
To: seaint(--nospam--at)seaint.org
Subject: Glass Storefront as Shearwalls


Hello,

Is there any information on the use of glass storefront as shearwalls?  The
reason I ask is I see many buildings with these storefronts.  If a moment
frame is used around the storefront then supposedly the storefront takes no
loads.  However, the steel frame is going to drift and put load on the
storefront.  What happens then?  I have a guardhouse that is tube columns at
the corners and 9' tall by 12' wide storefronts.  I could cantilever the
columns or do a moment frame but if I use a drift of h/60 we are looking at
~2" of drift.  This will definitely load the structure.  But this is
commonly done here in GA.  Any comments?

Thanks,

Mike