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 c. Recommended Design Criteria. Several tests have been performed over
the years to determine the critical shatter pressures for different types
of glass and transparent materials (such as plexiglas), and the results of
these tests are published in several reports including those mentioned above
as well as Characteristics of Plexiglas Fragments from Windows Broken by
Airblast, by Fletcher, et al. For example, Figure 66 shows a plot of
critical shatter overpressure vs. pane thickness for different types of
plexiglas. The ranges of uncertainty appear as vertical lines. The
parallel solid lines (i.e., the lines cutting across the vertical lines) are
approximations to the test data. The dashed line was derived from an
empirical equation in the report by Taylor and Clark, Shock Tube Tests of
Glazing Materials for 22- by 22-inch panes of non-stretched acrylic. Design
criteria for maximum blast capacity vs. blast load duration and glass type,
and thickness have been developed based upon several tests performed at
Dugway Proving Ground in Utah. These design criteria, which are reproduced
here, are published in a report by Weissman, et al., Blast Capacity
Evaluation of Glass Windows and Aluminum Window Frames. Table 30 presents
the peak design blast pressure for various blast load durations vs. glass
type and thickness. The peak pressure is either incident or reflected
pressure, depending on the orientation of the structural element (glass)
with respect to the blast wave. The blast load duration is the duration of
an equivalent triangular blast load; procedures for calculating this
duration are presented in NAVFAC P-397. The peak pressures in Table 30 are
maximum design values for glass panes mounted in rigid window frames, where
continuous support for direct load and rebound is provided for the glass.
These criteria are applicable to glass areas of 20 square feet or less. The
glass capacity tends to be significantly lower with an increase in loaded
area (i.e., area of glass exposed to blast loads). It is recommended that
glass areas of 20 square feet or less be used in blast design. During the
series of tests described in the Weissman report, it was realized that the
capacity of the glass panel was directly related to the frame design. For
example, in the tests performed with glass mounted in aluminum frames, the
capacity of the windows was greatly reduced even where a strengthened frame
was used. Therefore, it will be necessary to evaluate the particular frame
design selected for use since there are several "off-the-shelf" frame types
and details. Depending on the design overpressure level, the frame may
require modification or it may be necessary to specify a frame design which
will provide sufficient strength and rigidity to develop the capacity of the
glass. In view of these probable changes in the design of the frames, a set
of specifications has been proposed by Keenan in Review Comments (Partial
List) on Design Drawings and Specifications for Unaccompanied Enlisted
Quarters, (P-040), Naval Submarine Support Facility, San Diego, CA, for
the design of blast resistant windows. The specifications, which are
presented below, require acceptance load tests involving application of
static pressures to window panes and entire frame assembly. The increased
cost of the windows will be negated by the reduction of the risk of injury
from glass fragments.
d.
Recommended Specifications for Blast Resistant Windows.
(1) Each type of blast resistant window to be used shall be proof
tested as a completely assembled unit that includes its glass panes, metal
frames, insulated metal panel (where applicable), hardware and anchorages
(joining the window frames to the walls). The window used in the proof
test shall be life-size and identical in type and construction to those
proposed to be furnished.
2.08-232
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