TECHNICAL BRIEF - Wind Speed versus Wind Pressure

Using wind speed to evaluate your towers? Time to consider a more accurate metric.

When determining a tower suitable for a specific environment, one of two figures is typically presented - wind speed (velocity) or wind pressure. Although these are sometimes used interchangeably, there is a notable distinction. 

Wind speed is what we are most familiar with as we hear it in our day-to-day. It describes how fast air is moving past a certain point over a given unit of time. It can be recorded as instantaneous (3-second gust) or as an average (mean hourly). It often appears in marketing materials as it is a unit of measure that most of us have a relative gauge for. However, structural engineering analysis is not performed using wind speed. 

Wind pressure, or wind load, refers to the intensity of force that wind applies to a structure and is measured in Pascals (or Psi, pounds per square inch). It takes the wind speed and actually applies the wind as a force to the tower. The pressure takes into account other variables such as air temperature, humidity, elevation, and density. For example, a 170 km/h wind at sea level has a much different structural effect than that same wind on a mountain top. Wind pressure tells a much more complete story.

Wind speed is a convenient figure in understanding the general resilience of a structure. However, properly engineered designs should use wind pressure as the prevailing parameter. Suffice to say, wind speed looks pretty on a brochure, but building to a specified wind pressure gets Millard engineers' stamp of approval.