Wind effects on towers

Importance of Wind

As a buyer of engineered products it is important to understand exactly what the suppliers are offering in response to your Request For Quotation.

The one often-misleading area of disagreement is design wind specifications. As the primary driver of tower structural design, it is essential that the design wind specifications be precisely defined.  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". These terms are specifically excluded from modern structural design standards due to their ambiguity.  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 appear to meet the requirements of the RFQ, but can fall short by as much as 20 to 30 percent  from the desired structural capacity. EIA/TIA recommends the use of an escalated wind for design, or "Basic wind speed" (referenced from the "base" of the tower, see below), however EIA/TIA designs can also apply to a "Uniform wind speed" (see below).
 
Basic Wind Speed

This design escalates the wind speed from the "base" of the tower (defined as the portion of the tower from the ground to 10 meters elevation) to the top of the tower. For example, for a 90 meter (300 foot) tower with a basic wind speed design of 115 kph (70 mph), the wind speed design at the base of the tower is 115 kph (70 mph); starting at an elevation of 10 meters, the wind speed increases according to the formula specified in the design standard until it reaches a value of 160 kph (100 mph) at the top of the tower. 
 
Uniform Wind Speed

This design uses the same wind speed for the entire height of the structure. Since these are "design" wind loads, there is a safety factor included of minimum 1.25. Be careful when evaluating proposals. One easy way to determine if the designs are similar from one offer to another, is to compare the base reactions of the tower (the structural loads that are transmitted from the tower to the foundations). The reactions should be similar if the two structures have three legs, or slightly higher for the three leg tower when comparing a three leg tower to a four leg tower.