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Re: Glass Hand Rail

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Joseph,

There are several different proprietary structural glass guardrail systems
out there.  In UBC jurisdictions, ref. section 2406.6 of the 1997 UBC.  In
general, keep in mind that glass is an inelastic and brittle material with
structural properties highly dependent on the nature and amount of
impurities in the silica from which it is made as well as the way it is
tempered during cooling.  Structural glass panels are often laminated and/or
heat strengthened.  Stress concentrations at notches, edges and restraints
are critical considerations.

In my experience, field load testing to "certify" completed glass guardrails
installations is commonly required, but the design team still needs to
specify something that is likely to pass the test.  This is usually done on
the basis of empirical data provided by the manufacturer/supplier of the
system (or of each of its components).  They should be able to provide
copies of test data to support the ability of the system to provide the
required factor of safety against failure under the code specified design
load.  For the glass itself, the data I have seen on past projects (usually
from PPG if I recall) has always been probabilistic rather than absolute
(i.e., for a particular product, the glass manufacturer will tell you what
the probability of failure is at a given stress), and I would be suspicious
of any data I got in an the form of an allowable stress or ultimate
strength.

I am currently working a project involving the system Nels referred you
(Julius Blum).  I think you will find that they make hardware (base fittings
and top rails) but do not sell or warrant the structural glass balustrade
itself.  They will probably give you an old (1985) Wiss Janney Elstner
report to substantiate the ability of their cast aluminum base "shoes" (and
the machine screws used to attach them to structural steel backing) to
resist the moment created by the code required rail load.  You will have to
design and detail the backing (e.g., an edge of slab embed or torsionally
restrained edge beam) to receive the screws and resist the cranked in
moment.

A word of caution:  the WJE tests were made with steel panels rather than
glass.  In evaluating the resulting data and suggested details, you should
keep in mind that the structural characteristics of the materials used to
grip the bottom of the panels of glass may be critical.  Blum supplies
plastic setting blocks and (I believe) stainless steel channel caps to fix
glass within the glazing channel at the center of their shoe, and using any
other method to secure it could create a different distribution of stress in
the glass at a critical location -- leading to premature brittle failure.  I
recommend that you make sure that the system you are using has been tested
in the exact configuration you will have and that the contractor understands
that the installation must be exactly per the manufacturer's
recommendations.

Drew Norman, S.E.
Drew A. Norman and Associates
Pasadena

----- Original Message -----
From: "Joseph Ward" <joseph_ward(--nospam--at)leavittengineers.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Wednesday, February 14, 2001 7:25 AM
Subject: Glass Hand Rail


> Has anyone out there seen a design method for glass hand rails?
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