2018 IBC/IRC WIND DESIGN DISHARMONY ARISES AGAIN – David M. Sparks, P.E., S.E.

Many of us still remember the 2012 code cycle. Disharmony arose between the IBC and IRC when the two model codes referenced different versions of ASCE7 with respect to wind loading. The 2012 IBC references ASCE7-10 and the new LRFD wind speeds; while the 2012 IRC references ASCE7-05 and the ASD wind speeds from which the relatively new wall bracing tables had been derived. As designers, specifically in the residential realm, we then had to acknowledge the two different wind speeds that governed us from the parent IBC reference and the ASCE7-10 standard to the downstream IRC reference and the ASCE7-05 standard.

Here we are two code cycles later, and disharmony has arisen again. However, this time the differences are more subtle than before. By all appearances in reference, both the 2018 IRC and 2018 IBC list the ASCE7-16 as the standard reference for loads. However, if you look more closely, you will quickly see that the wind maps in the IRC, as well as the wind speeds as a result, still match the ASCE7-10. While there is harmony in that both the ASCE7-16 and ASCE7-10 wind speeds are ultimate wind speeds, the disharmony arises from the many changes that were made to ASCE7-16. Some of these changes include new wind speed maps, new design factors, and new coefficients on components and cladding pressures for roofs.

Designers, engineers, architects and code officials should all take time to understand the implications of the changes in ASCE7-16 as well as the impacts on plan submittals. Some things to discuss may include guidance for the design and plan submittal process (For Example: the wind speed indicated in the climatic design table in Chapter 3 of the IRC) as well as potential code amendments and/or revisions to help ensure consistency during the design, submittal, and review processes. The end goal should be to help clarify the municipality’s position on the conflict within the code, while making the design process for the designer, engineer, or architect more straightforward.

If you as engineers are using the IBC to provide an alternate design for elements in residential construction, please make this clear on the drawings and in the calculations to help minimize plan review comments from the Jurisdiction Having Authority.

Author Info

David Sparks