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 the IBC."

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