In many areas of the world, wind storms remain one of the most prevalent sources of damage to buildings. It can be tough to know just how hard the wind will blow on an object, and buildings are meant to be around for many years, so we can expect they’ll see some large storms in their lives.
Engineers have to design for the worst wind storms a building is likely to see, and most buildings are (at least assumed to be) designed for about a 50-year lifespan. For many years, the design wind loads for buildings across the country were just flat, prescribed numbers, maybe varying a bit from locality to locality.
But over the last twenty years, the American Society of Civil Engineers ASCE 7 Standard “Minimum Loads for Building-type Structures” has taken on near-universal adoption throughout the US, and the section dedicated to the calculation of wind loads has ballooned from a single, short chapter to six full chapters.
These wind load calculations are based primarily on the physics formula for the stagnation pressure caused by a wind load, adjusted with numerous factors to account for local wind speed, prevailing surface roughness near the building site, the building height, building risk category, local topographic influences, and site elevation.
Risk Category, Wind Speed, and Site Elevation
Wind loads are based on the Stagnation Pressure, a formula derived from Bernoulli’s Principle, which can give us the maximum pressure, p, exerted by a fluid of density ρ traveling at a velocity of V:
Air density is a function of many things, from temperature and local latitude/barometric pressure to relative humidity, but the biggest influence in the ASCE standard is altitude/elevation.
Air at high elevation is less dense than that at sea level, which is why high-altitude baking has different instructions, and why it’s easier to hit a home run in Denver than at Yankee Stadium. This reduction in air density can actually reach about 20% for cities located above 6000′ in the US, most commonly in the Rocky Mountains.
For more on this, check out my article “How Does Site Elevation Influence ASCE 7-16 Wind Loads”.
We never want a building to fall down, it also doesn’t make sense to build every small self-storage building to the same standards as a nuclear power plant or level-I trauma center. To account for this, engineers classify buildings into different Risk Categories, based on how disruptive and dangerous the failure of a given structure would be to the general public.
More important buildings get higher Risk Categories assigned, and those higher categories correspond to higher loads.
Wind speed varies geographically over the entire United States, so tabulated weather data has been mapped out for a variety of “mean recurrence intervals” (MRIs), and is available both in the building code and on helpful websites. Different risk categories drive different MRIs, and therefore different wind speeds, for the same location.
Wind load, for example, is calculated from wind speed maps provided in ASCE 7-16, or viewable for any given location on the ASCE 7 Hazard Tool. The wind speed used for a particular building depends on the historical wind speed data for the building site, as well as the risk category of the building.
- Risk Category I buildings use the mapped 3-second wind gust with a MRI of 300 years, corresponding to a 15% probability of exceedance in an assumed 50-year design life of a given building.
- Risk Category II buildings use a 700-year MRI, for a 7% probability of exceedance in 50 years
- Risk Category III buildings use a 1700-year MRI, for a 3% probability of exceedance in 50 years
- Risk Category IV buildings use a 3000-year MRI, for a 1.6% probability of exceedance in 50 years
For more on Risk Categories, check out my article on How Risk Categories are Assigned.
Building Height & Surface Roughness/Exposure Category
Wind speed increases with height off the ground, up to a point. The Surface Roughness, how much stuff sticks up off the ground, provides drag to the wind, slowing it down near the ground.
Engineers classify the prevailing surroundings into Exposure Categories, which then point them to one of three curves of how the wind speed increases with height off the ground.
Note that these exposure categories are not the result of the wind being “blocked” from hitting the building being designed, but rather all about drag. This is why a skyscraper in the middle of suburbia still gets the suburban surface roughness designation.
To capture actual “blocking” effects from other buildings, engineers will engage the services of a professional wind tunnel study consultant, who builds a tiny model of the building being designed, and many of the surrounding buildings as well.
These models are placed on a large turntable, and once the main design building is instrumented, the whole thing is slowly turned through every angle in the wind tunnel, with the sensors documenting the actual expected wind pressures from various wind speeds and directions.
Wind tunnel studies are costly, but for large development projects in built-up areas, they can be well worth it in savings. Many buildings I work on in Manhattan and Philadelphia do spring for these studies.
Local Topographic Influences
Hills, mountain passes, and valleys can all magnify or decrease local wind speeds. ASCE 7 does provide a “topographic factor” to help adjust wind speeds for hills and escarpments, but in heavily-mountainous regions, designers are often forced to rely on local climactic data.
The national wind speed maps have several “special wind regions” for which data is not provided, specifically because of these local effects. Reno, NV, for example, is excluded from the national wind maps, but their local building code officials recommend wind speeds up to 140 mph, depending on the risk category, due to the funneling effect of their local topography.
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