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DESIGN
OF FRAMED-OPENING SYSTEM
Summary
The framed-opening system
consists of joist clamps, framing members (channels),
T-brackets and bolts. The joist clamps connect
the primary channels to the top chords of the
joists. The T-brackets connect the secondary
channels to the primary channels. Both the joist
clamps and the T-brackets have slots. The slots,
in combination with spaced holes in the channels'
webs, permit adjustment of the framed opening's
length and width. The position of the opening
between joists can also be adjusted by means
of the slots and holes.
Analysis of the joist clamp
indicates that it can support an allowable load
of at least 1,200 lbs. This is the load that
can be applied in addition to the framing weight.
The maximum load applied in testing was 3,940
lbs. The failure load was not reached. Analysis
and testing considered the bolts, between channel
and clamp, to be at the most severe location
in the slots.
Analysis of the T-bracket
indicates an allowable load of substantially
more than 1,200 lbs. A test load of 2,000 lbs
was applied with no failure.
The tested framing system
used hot-rolled C5 x 6.7 channels (5" deep,
weighing 6.7 pounds per foot and made with A36
steel, which has a minimum specified yield of
36 ksi). The maximum span tested was slightly
more than 10'. Using the 2005 edition of the
AISC Specification, an allowable of 1,230 lbs
was calculated for a load applied at the middle
of the test span. This assumes the channels
to be unbraced over their entire span. The value
of Cb was conservatively taken to be one. A
maximum test load of 4,660 lbs was attained.
For a better comparison
of the test load to the predicted maximum load,
a reduced value of test load was calculated.
The actual test load was multiplied by the ratio
of the minimum specified yield stress to the
estimated actual yield. This is a conservative
adjustment for the approximately 10' span because
this unbraced length is close to the transition
from inelastic to elastic buckling. To account
for the effect of moment gradient in the test,
a more accurate value of Cb , greater than one,
was used to calculate an adjusted (increased)
value of allowable load. The ratio of the reduced
test load to the adjusted allowable load was
found to exceed the desired safety factor of
2.0.
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