ICC Tackles Windloads and Deflection

The International Code Council (ICC) 2016 Committee Action Hearings wrapped up this week, with some changes being made to the structural section of the International Building Code (IBC).

S69-16, which was approved as modified, addresses IBC 1604—a code regarding deflection limits of roofing. New language was added that covers framing supporting glass.

With the change, the deflection of framing members supporting glass subjected to 0.6 times the “component and cladding” windloads now shall not exceed the following:

– 1/175 of the length of span of the framing member, for framing members having a length not more than 13 feet 6 inches; or,

– 1/240 of the length of span of the framing member plus 1/4 inch, for framing members having a length greater than 13 feet 6 inches.

“This proposal establishes the appropriate deflection limit for exterior wall framing members that are supporting glass,” said Julie Ruth, representing the American Architectural Manufacturers Association (AAMA). “It is based upon criteria given in AAMA TIR-11 Maximum Allowable Deflection of Framing Systems for Building Cladding Components at Design Wind Loads. This criterion has been used successfully by the fenestration industry for decades.

“For designers and contractors who are not following good practice, there may be additional cost associated with the vertical framing members that are supporting glass,” Ruth said. “This is a relatively small portion of the overall cost of an exterior curtainwall system.”

A proposal by the Window and Door Manufacturers Association (WDMA), S150, was also approved. According to Jeff Inks of WDMA, it is intended “to clarify that the use of the 0.6 conversion multiplier is allowed with respect to the determination of design wind pressures in accordance with ASCE 7 and testing of the respective assemblies in accordance with Sections 1709.5.1 and 1709.5.2, accordingly.”

He continued, “While that is what the existing provision allows, as currently written, that is not entirely clear and has led to confusion regarding windload requirements. This proposed amendment expressly states that the use of 0.6 multiplier is allowed and will alleviate the confusion that currently exists benefiting all – code officials, manufacturers and builders.”

Also, S56-16 was approved as modified. This is a coordination proposal to bring the 2018 IBC up to date with the provision of the 2016 edition of ASCE 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE 7-16). The changes proposed in all sections are designed to harmonize terminology between the code and the loading standard.

In all instances the word “ultimate” was changed to “basic” and the subscript “ult” was removed from the variable “V”. Similarly, the word “nominal” is changed to “allowable stress” in all places to be consistent with the terminology used in the loading standard, explained Jennifer Goupil of the American Society of Civil Engineers, the proponent.

“The design wind speed maps have been updated to reflect the maps adopted into ASCE 7-16. During the development of the ASCE 7-16 standard the ASCE 7 Wind Load Subcommittee made substantial revisions to the wind speed maps contained within the standard, and the number of maps went from three to eight,” she said. “These revisions include the development of separate maps for Risk Categories III and IV structures; reconstruction of the special wind regions within the maps, correcting known deficiencies in the wind speed contours; and modification of the basic wind speed based on updated climatic and weather data.”

New hurricane contours in the northeastern states were developed based on updated hurricane models and the locations of the contours along the hurricane coast line were adjusted to reflect new research into the decay rate of hurricanes over land, Goupil said.

“New maps for the state of Hawaii were developed to eliminate it as a special wind region and to provide guidance on the wind patterns for the state that occur because of the unique topography there,” she said.

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