Design Wind Speeds from the IRC

Julie Ruth
May 9, 2015
COLUMN : Code Arena | Codes & Standards

The series of columns on the new provisions of the 2015 International Codes for fenestration continues this month with a discussion about design wind speeds from the 2015 International Residential Code.

The design wind speed model used in the 2015 IRC has been revised to the Strength Design (SD) model. With this change, the design wind speed model used in the 2015 IRC will be consistent with that used in the 2015 International Building Code.

Prior editions of the International Codes (2009 and earlier) were based upon the Allowable Stress Design (ASD) model. This is also the design model used in all editions of AAMA/WDMA/CSA 101/I.S.2/A440 (NAFS). This design wind speed model was based upon ASCE 7 – 00 and ASCE 7 – 05, which established the ASD model for wind load.

ASD considers loads that a building or its components are likely to experience once or more during its service life. The building or component is then designed to experience that load and remain in service (not need to be replaced).

Between the 2005 edition and the 2010 edition of ASCE 7, however, the design wind speed model was converted from ASD to SD. In the 2012 edition of the International Codes, the 2012 IBC used the SD model while the 2012 IRC used the ASD model.

SD considers higher loads that a building or its components have a lower probability of experiencing during their service life. The building or component is then designed to not fail (rupture) if it ever were subjected to that load. The building and its components, however, are not designed to be undamaged when subjected to these loads. In other words, it is anticipated that, if a building were subjected to extreme loads, it would not collapse due to them, but some of its components might need to be replaced after the loads are removed for the building to remain in (or be returned to) service.

Reducing Potential Confusion

Using two different models in the two different codes was similar to using metric units in one and IP units in another. Although the net result was very similar, confusion could result if the two sets of units were intermixed. Fortunately, the 2012 IBC contains provisions for converting from one model to the other, and in most cases it does not require retesting of fenestration products.

Basically, if a required design wind pressure is based upon the SD model used in the 2012 IBC, it is to be multiplied by 0.6 before comparing it to the Performance Grade of AAMA/WDMA/CSA 101/I.S.2/A440. If a required design wind pressure is based upon the ASD model used in the 2012 IRC, however, no conversion is necessary (the required design wind pressure is not to be multiplied by 0.6 before comparing it to your product’s PG rating). See Table 1 (above) for guidelines on when the 0.6 modifier should be applied.

Our industry was concerned that potential confusion over this might result in code officials rejecting the 0.6 conversion on some projects. For this reason AAMA, WDMA, DASMA and FMA issued technical bulletin - Relating ASCE/SEI 7‐10 Design Wind Loads to Fenestration Product Ratings, in 2011. As the Technical Bulletin explained, values based on the SD model cannot be intermixed with values based on the ASD version.

Converting the 2015 IRC to the same design wind speed model as the 2015 IBC reduces the potential for confusion between the application of the two codes. But it will still be necessary to apply the 0.6 modifier to design wind pressures from either of those codes before comparing them to your product’s PG rating that is based upon any of the editions of AAMA/WDMA/CSA 101/I.S.2/A440 (NAFS).

Code Arena is brought to you by the America Architectural Manufacturers Association. Julie Ruth may be reached through AAMA at 847/303-5664 or via e-mail at julruth@aol.com.