Winds of Confusion
In the last few installments of Code Arena, we’ve looked at the more significant proposed changes to the 2015 ICC Group B codes and their potential impact on the fenestration industry. This month’s column provides an overview of the proposed changes for structural design of residential buildings that were approved during the 2016 ICC Group B Committee Action Hearings.
If the approval of these proposed changes is upheld through the Public Comment Hearings and Final Action, as well as online voting, they will be included in the 2018 editions of the International Building Code and International Residential Code.
Update to ASCE 7-16
The update of ASCE 7 from the 2010 edition to the 2016 edition (ADM94) was approved for both the IBC and IRC. The update necessitated changes to some of the text and figures of the IBC. Revision of the design wind speed maps in the IBC (S56) will be the most significant changes realized in the update to ASCE 7 for fenestration manufacturers. These maps are used to calculate the design wind pressures fenestration products must be tested to resist.
The new maps take into consideration the wide variation of wind speeds that occur in the central U.S. The previous maps established one design wind speed throughout the entire Midwest, central plains and mountain states. In most cases, the design wind speeds for residential construction and low-rise commercial buildings will be lower than previously specified (see Table 1).
|Table 1: The table above compares the design wind speeds given for select locations in the U.S. in the 2010 and 2016 editions of ASCE 7 for residential construction.|
Unfortunately, these changes to the design wind speed maps were not carried over to the 2018 IRC. Unless this oversight is corrected during the public comment phase of the ICC Group B Code Development Cycle, it could result in different design pressure requirements for residential buildings built under the 2018 IBC—such as apartment buildings, hotels and assisted care facilities—than for single family homes and townhouses built under the 2018 IRC.
Note that all of the design wind speeds given in Table 1 are based on Strength Design. Historically, design wind speeds were based upon Allowable Stress Design, which refers to wind speeds that a building was considered likely to experience at least once during its service life. The building components were designed to resist the loads caused by these wind speeds without failing or needing to be replaced.
In the 2010 edition of ASCE 7, the design wind speed model was changed from Allowable Stress Design to Strength Design. Strength Design loads are considered to be of a much lower probability. The building components are designed not to fail if they are subjected to such loads, but they may be permanently damaged to the point of needing repair or replacement. The design wind speeds for this model were referred to in the 2010 edition of ASCE 7, the 2015 IBC and 2015 IRC as Ultimate Design Wind Speeds.
The 2016 edition of ASCE 7 also uses the Strength Design model to determine wind speed, but the name of the design wind speeds has been changed to Basic Design Wind Speed. This change was approved throughout the 2018 IBC along with the update of the design wind speed maps. However, no similar change was proposed for the 2018 IRC.
As with the design wind speed maps, unless this oversight is corrected during Public Comment, the 2018 IRC will refer to Ultimate Design Wind Speeds and Allowable Stress Design Wind Speeds, while the 2018 IBC will call the exact same thing Basic Design Wind Speeds and Nominal Design Wind Speeds.
The potential for confusion is enormous.