Efforts to Limit L/175 Deflection Exemption
in IBC Disapproved
January 7, 2010
During the International Code Council (ICC) hearings, which took
place in late 2009, two proposals (S140 and S141) that would have
limited the L/175 deflection exemption contained in the International
Building Code (IBC) were disapproved.
Thomas D. Culp, Ph.D., with Birch Point Consulting LLC, representing
the Aluminum Extruders Council, was the proponent for S140 and William
E. Koffel, with Koffel Associates Inc., representing the Glazing
Industry Code Committee was the proponent for S141.
"The proposals would have required a minimum L/175 deflection
limit for framing used in all buildings built to the IBC, except
for residential buildings three stories or less," explains
Culp. "[Currently] there is an exemption from that requirement
if you label to AAMA 101, and AAMA 101 allows deflections greater
than L/175 for light commercial products. These proposals would
have removed that exemption."
Culp's proposal stated that the reason for the change was due to
the fact that "Chapter 24 [of the IBC] and ASTM E 1300 require
that glazing be firmly supported to prevent breakage under the design
load by establishing maximum framing deflection limits. However,
certain products are currently and inappropriately exempted from
this requirement if they are labeled to the AAMA/WDMA/CSA 101/I.S.2/A440
standard. This proposal would remove that exemption to restore an
appropriate safety margin of less than eight in 1,000 probability
of glass breakage, consistent with ASTM E 1300."
Koffel's proposal offered similar reasoning.
"Chapter 24 of the IBC relies on glass design curves that
are contained in ASTM E 1300. This ASTM standard recognizes the
importance of limiting edge deflection of the glass and also recommends
a limitation of L/175 of the glass edge length. Prior to the IBC,
the legacy codes required deflection limitations of L/175 of the
span for glass holding members. It was not until the IBC was published
that this exemption was allowed.
AAMA/WDMA/CSA 101/I.S.2/A440 does require testing in accordance
with ASTM E 330 and measurement of deflection. However, AAMA/WDMA/CSA
101/I.S.2/A440 only places a limit on the frame and sash deflection
for heavy commercial and architectural products, and has no requirement
on deflection for residential, light commercial, and commercial
products. Excessive deflection of the frame or sash can have an
adverse effect on stress in the glass and could result in glass
breakage at or below design loads creating a safety concern. The
single ASTM E 330 load test required in AAMA/WDMA/CSA 101/I.S.2/A440
is not statistically significant in ensuring that the stress does
not increase the probability of breakage beyond the industry standard
of eight lites per thousand when the deflection limitation of L/175
is exceeded. Although the deflection exemption remains in the IRC
for residential buildings and as proposed in the IBC for low-rise
residential, it is inappropriate to have an exemption for these
products when used in more diverse and larger buildings built to
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