What Is the Relationship Between MPH and PSF?
You typically hear weather forecasts that include wind speeds in your neck of the woods. But how does a wind speed in miles per hour (mph) translate into a wind load in pounds per square foot (psf)? The general public is not aware of how strong (in the structural sense of the word) a 50-mph wind is in comparison with a 100-mph wind or even a 150-mph wind.
According to the American Society of Civil Engineers, Standard 7-16 (ASCE 7-16) “Minimum Design Loads and Associated Criteria for Buildings and Other Structures” describes the means for determining design loads including dead, live, soil, flood, tsunami, snow, rain, atmospheric ice, seismic, and wind loads and their combinations for general structural design.
Among all of the structural design loads described above, ASCE 7-16 provides the requirements to calculate the magnitude of wind loads applied to a building, for example in pounds per square foot, starting with a required minimum design wind speed in miles per hour.
At a first glance, you would think that a 100-mph wind applies a load that doubles in magnitude in comparison to a 50-mph wind...and you would be wrong. This is a typical notion when speaking with the general public. However, wind speed and wind load have an exponential relationship, not a linear one. The load exerted on a building as a result of blowing winds is a factor of the square of the wind speed. Other factors and coefficients affect the final wind load calculation, such as geographical and geometrical ones; however, the speed-to-load exponential relationship is maintained.
Wind Load = 0.00256 * (Wind Speed)2
Not including the numerous factors, let us look at the following table to illustrate how much stronger wind loads get as wind speeds increase:
Wind Speed (mph)
|Corresponding Wind Load (psf)||Increase to||Wind Speed (mph)||Corresponding Wind Load (psf)||How much faster?||How much stronger?|
|50||6.4||100||25.6||2 times faster||4 times stronger|
|50||6.4||150||57.6||3 times faster||9 times stronger|
|100||25.6||150||57.6||1.5 times faster||2.25 times stronger|
The table shown above illustrates the exponential relationship between wind speeds and wind loads and puts into perspective how much the magnitude of a wind load increases as a result of an increment in the wind speed. The load applied on a building from a 100-mph wind is 4 times stronger than the load from a 50-mph wind, and 9 times stronger when the wind reaches 150 mph!
The 2020 Florida Building Code requires typical residential structures in Southeast Florida to withstand nominal design wind speeds of approximately 136 mph. Using the formula above, a comparison of the magnitude of wind loads applied on a building between the code-required wind speeds and the measured wind speeds at a specific site is possible. You may be surprised by the difference in wind load magnitudes as a result of the two wind speeds.
About the Author
Sergio G. Arratia, P.E., M.S.C.E. is a Consulting Engineer in our South Florida Office. Mr. Arratia provides consultation related to construction evaluation of residential and commercial structures, failure analysis, evaluation of scope and cause of damage to structures, wind and hail damage assessment, water intrusion investigation, remediation analysis, structural load analysis, and structural design analysis. You may contact him for your forensic engineering needs at firstname.lastname@example.org or 954-743-4500.
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