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Bill's Technical Corner
A
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Who is Bill Lingnell?
A Brief Review of the
Background History and Use of the ASTM Standard E 1300
for Glass Analysis
By: A. William Lingnell, P.E.
In the early part
of 2011 during an ASTM task group meeting, the chairman
of the Glass Strength committee requested that a small
contingent put together background and history along
with information on the use of the ASTM E 1300
Standard Practice for Determining Load Resistance of
Glass in Buildings. Jeff Haberer of Cardinal IG and
I offered to pursue the task. The following information
in this article is part of the effort that was put into
the writing of a paper used to explain the request. It
is hoped the explanation relating to the background and
evolution of this Standard will give the reader insight
into how the group has promulgated a standard that has
been in place now well over 22 years. The Standard
continues to be updated and added to in an effort to
assist the glass industry, architects, engineers,
contractors, designers, consultants, researchers, and
others interested in glass use for buildings. This
article will take us part way through the E 1300
background and history with additional input and
information intended to come in future articles.
Currently this
standard is used throughout the United States and parts
of Canada for determining uniform lateral load
resistance (i.e. snow and windload) of glass used in
buildings. It is referenced by the model building codes
in the US, Canada, and other countries. Origins of the
Standard will be discussed along with historical and
recent updates. The Standard offers a practical approach
to lateral loading that addresses many, if not most, of
the glazing types offered today, including: monolithic,
annealed, fully tempered (toughened), heat-strengthened,
laminated (with shear modulus equal or greater than pvb),
insulating glass (double and triple glazed), asymmetric
glass constructions, various support conditions (one,
two, and three side support), and combinations of the
above.
We will be
discussing the progress made in the Standard in
providing the user with advancements in load resistance
of glass based on the development of additional charts
for non-factored loads, glass thicknesses, glass type
factors, load sharing and increased load magnitudes that
have been important to meet glass industry needs.
Introduction
Architects, engineers, glass fabricators, glazing
contractors and others have obvious needs to know and
understand the strength of glass. Wind load, snow load
and self weight are the primary concerns when it comes
to glass for use in buildings. In these times of ever
expanding designs, there is also a need to evaluate
special applications such as point loads, line loads,
partial loading, impact loading in glass designs, among
other designs beyond the typical vertical window or
sloped glazing with four side structural support to the
glass. Throughout the long history of using glass in
buildings, users have increased the thickness of the
glass in order to meet the resistance of lateral forces
that would tend to break glass. The need has always been
for a practical and convenient way to evaluate just when
to increase the thickness for added strength to meet
design load requirements.
Background history
and an update of the ASTM E 1300 standard that is
currently in use in North America and other countries
for determining the lateral strength of glass will be
explored in a practical and understandable manner. ASTM
E 1300 is a practice which lays out a methodology for
evaluating glass load resistance and allows the user to
determine if a glass type and construction has adequate
strength to be in compliance with the architectural
design requirements or achieves code compliance.
Glass Strength
Evaluation Prior to ASTM E 1300
Prior to the introduction of the ASTM E1300 Standard
there were other methods used in the United States to
evaluate glass strength for buildings. In the 1960s and
into the 1980s most architects and engineers used one
simple chart which described the assumed strength of
each glass thickness. These charts expressed maximum
load capability vs. glass area with a line for each
thickness of glass based on a probability of breakage of
8 lites per 1000 for annealed glass. As new types of
glass saw increased use, adjustment factors were
established to address the strength for
heat-strengthened, fully tempered, laminated,
insulating, and certain pattern glasses. The straight
line chart shown in Figure 1 was adopted by many
building code agencies for evaluation of glass strength
for glass used in buildings.
Figure 1 Straight line glass strength charts from
1960-80’s
These charts were
based on empirical results from testing to failure of
thousands of various sized plates of annealed glass for
each thickness. There were at least 25 lites tested to
destruction using aspect ratios of 1:1 to 1:5 for all
the glass thicknesses tested. Testing of over 3,000
lites of monolithic factory fresh glass assisted in
developing these initial glass strength values at LOF
Research Facility in Ohio. The information was commonly
used for selecting glass subjected to uniform load
conditions for 60-second durations with a design factor
of 2.5 and firm four-edge support, and was eventually
incorporated in many model building codes.
In the 1970’s,
efforts were made by glass manufacturers to improve on
the understanding of glass strength and better predict
the statistical nature of glass. Most notably among
these was work by PPG Industries, Inc. This work
employed a maximum stress approach coupled with finite
element analysis that considered plate geometry and not
just total plate area. (PPG 1979) From this work, a
series of glass thickness charts were created. While
these were a likely improvement over the earlier
straight line charts, they did yield somewhat different
results. Because of this, the charts were deemed
controversial and did not attain uniform acceptance by
code bodies, and practicing architects/engineers.
Figure 2 PPG glass strength charts, 1979
Around the same
time W. Lynn Beason produced a study introducing a glass
failure prediction model. This became the basis for
initial discussions within ASTM on glass strength for
buildings. This work is well documented elsewhere (“A
Failure Prediction Model For Glass”, 1980; “Basis for
ASTM E1300 Annealed Glass Thickness Selection Charts”
1998). A brief account is presented here. The theory
addresses reasoning for the high coefficient of
variation in glass strength. This variation is the
result of the orientation and density of surface flaws
in the glass.
The theory employs
the work of Weibul (1939) who offered a statistical
failure analysis for predicting the strength of brittle
materials. Using this analysis, the glass failure
prediction model uses two parameters called “m” and “k”
which relate to the surface flaw conditions. These two
parameters are used in a Risk Function equation
employing plate dimensions, elasticity of the glass,
load duration, distribution of stress in the plate, and
magnitude of applied load. This very complicated
function was then synthesized into a series of charts
for each glass thickness. The charts have plate width
and height on separate axes.
Moving the
Glass Strength Controversy to ASTM
As we have discussed, ASTM E1300 was born out of
controversy in the industry when new approaches to glass
strength yielded different results from the conventional
so called “straight line” charts of the 1960’s and
1970’s. Being a consensus organization ASTM (now ASTM
International) was perhaps uniquely suited to address
the controversy. Unlike other standards organizations
ASTM is a not a national standards body. Membership and
participation is open to anyone anywhere in the world.
Membership is initiated by the participants own request
not by appointment or invitation. Committees are
balanced so that no more than 50% of the participants
are producers. The remainder must be consumers or
general interest. The latter includes consultants,
architects, and academics.
With this
organizational structure, the task was set to come to a
consensus agreement on what to use for glass strength.
Initial work took nearly 10 years of exhausting debate
and discussion until a final draft document was
produced. The Glass Failure Prediction model, previously
noted, was agreed to be the basis for the new standard.
Compromises were made on the values for the surface flaw
parameters “m” and “k”. These were chosen to accurately
reflect the strength of weathered glass removed from
buildings and resolve the controversy in the industry.
Values for the parameters are given in the standard. I
was felt by the committee that the use of weathered
glass represented a more practical effort to describe
the basis for the evaluation of load resistance for
glass used in buildings.
The ASTM E 1300
Standard and some of the early revisions and updates
The first version of the Standard (ASTM E 1300-89) was
released 1989. This version only covered annealed
monolithic glass, of rectangular shape with support on
all four edges and with lateral load duration of 60
seconds. Twelve US glass thicknesses designations were
covered from 2.5mm to 22.0mm. A chart for each thickness
was provided. Optional procedures for center deflection
and estimating probability of failure were included in
an appendix. An example of the non-factored load chart
for 6mm (1/4”) glass is shown in Figure 3. This is
similar to what was presented in the original 1989 ASTM
E 1300 Standard.
Figure 3 ASTM E 1300 Non-factored Load Chart for
6mm (1/4”) Glass with Four Sides Simply Supported
It was clear that
more than monolithic annealed glass was used by the
industry. After a period of general acceptance to the
approach within the E 1300 document, strategies for
handling different glass types were addressed. In 1994,
a second version was issued that offered type factors
for annealed, heat-strengthened, fully tempered,
laminated, and two pane insulating glass containing
lites of the same type. In addition to factors for the
glass types with 60 second load duration, the revision
also included type factors 30 day load duration. This
was for evaluation of a typical snow load.
For the 1997
version of the standard, the ASTM task group began to
address other needs. These included evaluation of
insulating glass units that are asymmetric. This refers
to having mixed glass types and thicknesses on each lite
of glass in the insulating glass unit. An example would
be an exterior 6mm tempered lite, with a roomside lite
of 13mm annealed, laminated glass. This type of
construction is especially common in sloped glazing, but
there had never been any standardized way of evaluating
its strength.
This asymmetric
analysis entails far more complexity than the symmetric
case. Hence, a major restructuring of the strength
adjustment factors had to be executed. In the asymmetric
IG example cited above, the thicker lite will take more
of the load than the thinner lite. An approach to
determining each lites share of the load was needed.
Additionally, the differences in glass type (fully
tempered versus annealed laminated) entails examining
each lite individually according to its type and to the
determined share of the applied load.
To accomplish all
this, an assumption was first made that load sharing is
proportional to the stiffness of the lites. This is
estimated by taking the ratio of the cube of the
thicknesses. A table of load share factors (LS) was
created that laid out all possible combinations of glass
thicknesses for short term (60 second) loads. A second
table of load share factors was created for IG
constructions with laminated glass under long term
loading (30 days). Since laminated glass has a different
stiffness behavior under long term loading, this second
table assigned load share for each individual glass ply
in the laminated layup.
The addition of
the asymmetric IG analysis was a major step forward.
While based on assumptions that were not perfect,
results were judged to still be conservative within the
realm of the stated breakage potential of less than
8/1000. The advantage was that more common IG
constructions could be evaluated with a standardized
consensus practice.
We have just
reviewed a huge amount of information and to bring us to
a point where the next chapter will take us further into
the development of by what means the E 1300 standard got
to its present state and what is coming next. So stay
tuned as we have covered the beginning of ASTM E 1300 up
to 1998. Next we will venture into the changes and
updated additions that have occurred in the new
millennium since year 2000.
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