Importance of Wind

As a buyer of engineered products it is important to understand exactly what the suppliers are offering in response to your RFQ. The one often-misleading area of disagreement is wind specifications. Most customers specify tower wind design requirements in terms that relate to velocity. Because the structure is really being designed to resist various pressures, wind load being one of them, a conversion has to be made from velocity to pressure. It is here that various standards define and measure wind and wind velocity in different ways and therefore the formulas used to convert these velocities to pressure produce results that can vary as much as 25%. That translates into a 25% difference in the cost of tower material, a 25% difference in design loads that will produce different foundation sizes all of which means a totally different installed cost.

Wind Speed Definition

The subject of wind speed can often times be a controversial subject. A properly written specification will define a design wind speed and usually give the origin of the specification such as ANSI or British Standards, in order to know the formula for conversion to wind pressure, wind escalation with height, and safety factors. Often times specifications give only wind speeds in terms such as "Withstand" or "Survival". Reference to these terms by a user who is not familiar with wind load terms, leaves the customer open to getting designs and offers that read the same, but can fall short by as much as 20 to 30 percent in actual structural capacity. EIA recommends the use of an escalated wind for design, or "Basic wind speed", however EIA designs can also apply to a Uniform wind speed.

Basic Wind Speed

This design escalates the wind load from 10 meters above ground to the top of the structure. For example, for a 90 meter (300 foot) tower with a basic wind speed design of 115 kph (70 mph), the wind load design at the top of the tower is 160 kph (100 mph).