Windstorm Standards Outline:
On September 28, 2022, Hurricane Ian made landfall near Cayo Costa, FL as a Category 4 hurricane with sustained wind speeds of up to 150 mph. Along with the strong winds and rain, it brought a catastrophic storm surge causing widespread destruction to the area. It ranked as the 5th strongest hurricane to make landfall in the mainland US and the 6th category 4 hurricane to make landfall in only the past 5 years. The initial loss estimates are in the $40-70 B range which would make this event amongst the most expensive catastrophes of all time.
This event is a reminder that proper design and construction in wind-prone locations remain critical to minimize potential damage and business interruption from windstorms. The pivotal event of Hurricane Andrew in 1992 sparked industry-wide changes to codes and design standards. Forensic evidence after subsequent hurricane events has since shown that facilities can in fact be better designed and constructed to be more resilient to severe windstorms.
So, what are the prevailing wind standards? In the US, American Society of Civil Engineers (ASCE) 7 is the prevailing wind standard referenced and used as the basis for the International Building Code (IBC) and Factory Mutual Global Property Loss Prevention Data Sheet (FMDS) 1-28. This standard has steadily improved, in some cases dramatically, since the first major revision of ASCE 7-95, especially in recent years. Because the IBC and FM standards lag the ASCE changes, it is worth reviewing these changes and noting when they went into effect. When reviewing the evolution of windstorm standards, of particular significance are the (often confusing) topics of wind speed, safety factors, and roof perimeter zone definitions.
Often heard from clients is “I am in Miami, my building is designed to 140 mph, and I have an FM 1-90 roof. So I’m safe, right?” This is a common misconception. The confusion begins firstly with equating an arbitrary wind speed to a wind design. Even more confusing is that hurricane wind speeds reported by the media are usually based on the “1 min. sustained” definition. The wind speeds used by ASCE, IBC, and FM are based on the “3 sec. gust” definition, which is significantly different than the “1 min. sustained” definition, as shown below.
More importantly, even if you did know the appropriate wind speed, the reality is that the wind speed alone means very little. It is only one of many factors that determines the building pressures on a structure which is really needed for the basis of the design. IE, as shown in the formula below, wind pressures are derived from not just the speed but numerous other factors such as
- Surface Roughness Exposure, B, C, D
- Roof Height, h
- Is h < or > than 60 ft?
- Enclosure Classification
- Roof Slope
- Building Dimensions
- Building Shape
- Surrounding topography, escarpment, etc.
- Risk Category or Importance Factor
- Basic Wind Speed, V
- Velocity Pressure Coefficient – Kz
- Topographic Factor – Kzt, ie, Hills and Escarpments
- Directionality Factor – Kd (0.85 for all building structures)
- Ground Elevation Factor – Ke (1.0 for up to 1,000 ft above sea level)
Proper design pressure (p) of the building components, i.e., cladding, roofing, and framing, is what will determine if the building can survive these pressures. So, for example, a FM1-90 rated roof means that it is appropriate for a roof area needing a design pressure of 45 psf with a 2.0 safety factor, but it may or may not be appropriate for a building in a 140 mph wind zone depending on all of the other factors that go into determining the pressure for that particular part of the building. Also, if the structure is in a windborne debris region and has unprotected openings, a partially enclosed condition will add additional wind pressure.
The basic pressure formula above is complex and has also changed frequently through time. It is useful to briefly review the history of wind standards to remind us just how the science has changed in determining pressures.
In the 1940’s the only standard was the American National Standards Institute’s – Minimum Design Loads for Buildings and Other Structures ANSI A58.1 which had a universal simple one-size fits all pressure table.
It was later revised and improved but mainly focused on aerodynamics of buildings. The first introduction into modern building codes was when ASCE took over the design guidance from ANSI.
Here is a brief timeline of the modern wind standards:
- ASCE takes over from ANSI
- Minor changes to wind design portion
- Changed from “fastest-mile” to “three-second gust” wind speed measurement. 3 Sec Gust is approximately ~ 20-25% higher than Fastest Mile measurement.
- The average speed of a particle traveling with the wind over a distance of one mile
- Used in ASCE until 1995
3 second gust:
- The highest average speed over a 3-second duration
- Included design factors for exposures, gust, topography
- Added more factors and multipliers
- Modified Surface Roughness Exposures
- Included windborne debris regions and need for glazing protection
- Basis for FMDS 1-28 wind pressure tables
- More revisions…
- Methods for ease of calculations
- Surface Roughness Exposure Categories – “A” removed
- Even more revisions…
- Basis for FMDS 1-28 US Wind Speed Maps
- Reinstated Exposure D in some hurricane prone regions.
- MAJOR changes to wind speed maps. Maps changed from a single 50 – 100 yr. wind map to four maps, based on Risk Category or Importance factors (I) and included Safety Factors (SF) incorporated into the maps. More specifically the calculation went from being based on Allowable Strength Design (ASD)=1.0 (or nominal design) where (I) and Safety Factors needed to be applied after determining pressures, to (I) specific maps with Load and Resistance Factor Design (LRFD)=1.0 (or ultimate design) where the Safety Factors and (I) became included in the wind maps. Effectively, the new wind maps now have the following mean recurrence intervals:
- More changes to wind speed maps, especially Hawaii
- Wind load guidance for solar panels added
- After using the same standard perimeter and corner definition for decades, major changes to the roof wind perimeter and corner zone definitions were made as shown below. The significance of this can’t be overstated. For example, for a typical 30 ft. high, 200 x 200 ft. warehouse building, the perimeter definition (a) went from a single 12 ft. wide zone to two perimeter zones (0.6h), each 18 ft. wide. Further, the zone pressures are drastically different. IE, the additional outer perimeter zone (2) has a significantly higher pressure coefficient than previously calculated (1.7 vs. 2.3). After years of practice under the previous definitions, this has led to major confusion amongst roof designers and roofers alike and is still not well understood.
- More changes to wind speed maps, decrease along the NE coast, as well as in most non-hurricane prone regions.
International Code Councils’ International Building Code (IBC)
- As noted earlier, ASCE is the design standard, but its implementation is subject to adoption.
- IBC adoption generally lags at least ~2 yr. or more after ASCE updates
- IBC does not necessarily adopt all provisions of ASCE
- State by State may not adopt all provisions of ASCE or IBC
- Time frame on adoption varies greatly, IE, statewide adoptions as of Aug 2021 is as follows:
- 2003 IBC – TX, Based on local jurisdictions
- 2009 IBC – Guam
- 2012 IBC – AR, AK, TN
- 2015 IBC – AL, LA, NC, PA, RI, VT, WV
- 2018 IBC – CA, FL, GA, HI, MS, NJ, NY, SC, VA, PR, US VI
FM Data Sheet 1-28, although similar to ASCE, has some important differences, most importantly of which is that it is based on ASD design guidelines, so Safety Factors have to be applied after determining pressures. To illustrate this, the table below helps to summarize some of the differences between ASCE 7-10 and FM 1-28 and the implied Safety Factors within.
Safety Factor (SF)
What is a safety factor and why does it matter?
A SF is a term describing the structural capacity of a system beyond the design loads. IE, a system needing a 50 psf design with a 2.0 safety factor will need to withstand 100 psf. This margin of safety is important to compensate for the variability in design, workmanship, materials, performance deterioration over time, etc. FM specifically calls for a 2.0 SF for all new roof construction. ASCE 7 and IBC use a combination of guidance, and these are enforced differently based on the authority having jurisdiction. But for roofs, new designs typically require a 2.0 SF on roofs and roof decks. When selecting the appropriate roof rating for a particular roof area, you may OR may not need to apply a SF based on the particular listing being used. IE,
|Listing Service||Wind Rating Includes SF?||Apply SF to the Design Wind Uplift Pressures?|
|UL’s Product iQ||No||Yes|
|FBC Product Approvals||Yes||No|
|Miami-Dade County Product Control Approvals||Yes||No|
|TDI Product Evaluation Index||Yes||No|
So how can you know if your building or structure is properly designed? As laid out in this article, this will depend on many factors such as the design standard used for construction, the design pressure calculation methods, the roof perimeter zone definitions used, the condition of the roof, deck, and windows, exposure to windborne debris, etc. The only way to accurately assess the windstorm risk is to complete a detailed windstorm evaluation. Risk Logic offers an advanced windstorm survey service that can accurately assess the risk and provide necessary recommendations for improvement based on current conditions and standards.