As we pray for the people caught in the path of Hurricane Ian, it’s important to consider the strength of this storm when compared to the applicable design standards. As the storm made landfall in Southwest Florida, morning advisories on Wednesday, September 28, 2022 reported maximum sustained winds of 155 mph (https://www.nhc.noaa.gov/archive/2022/IAN.shtml). This is only 2 mph short of a Category 5 hurricane in the Saffir-Simpson scale.
The 2020 Florida Building Code (FBC) – 7th Edition prescribes Ultimate Basic Design Wind Speeds following ASCE Standard 7-16 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures). The accompanying Commentary to ASCE 7-16 provisions includes an approximate comparison of the Saffir-Simson Scale to the FBC Wind Speed Maps. Note that these two scales cannot be directly converted as one utilizes sustained wind speed with an averaging time of 1-minute over open water, and the other utilizes 3-second gust speed over ground considering open terrain.
Notably, 155 mph (3-sec gust) is the approximate Ultimate Basic Design Windspeed (Vult) specified by the 2020 FBC for a typical residential or commercial building in the areas near Ian’s landfall. Refer to 2020 FBC Figure 1609.3(1). However, ASCE Table C26.5-2 (Approximate Relationship between Wind Speeds in ASCE 7 and Safﬁr-Simpson Hurricane Wind Scale) shows that a 1-min average sustained windspeed of 156 mph on Saffir-Simpson correlates to approximately 172 mph 3-sec gust over open terrain. Extrapolating this result, and pending further confirmation of recorded windspeeds by reporting agencies, Ian may have had wind speeds over 16 mph (3-sec gust) higher than the specified design wind speed for the area.
We will be monitoring how the Authorities Having Jurisdiction review and interpret these recent storms and if this will lead to updates in the Design Wind Speed Maps.