Wind Pressure Forces on Structures

Engineers who design or analyze structures such as wireless communication and broadcast towers must account for the external forcing on the structure which results from wind pressure, and ice loading of the structure which affects the wind pressure.

Wind speed and icing are both highly variable regionally and are also affected by local roughness of the surface, as well as topographic obstructions to flow which act to slow down or speed up the wind. The engineer needs information on what to expect for the maximum ice and wind load for the structure.

Building Codes used for the analysis of towers, such as the TIA-222 and ASCE7 in the US and S37 in Canada, provide a mapping of the Basic extreme wind and ice, and a prescription for adjusting the basic values to account for the terrain and topographic corrections of the basic winds.

The codes recognize that the mapping introduces generalizations which may not provide the most appropriate values for wind load calculations in some regions (designated as special wind regions) and recommends Site Specific assessment in those cases.

ICE Approach to Site Specific Wind Extreme Assessment

The Site Specific assessment uses the most appropriate local hourly data and statistical procedures and the latest research literature to extract the wind extremes, coupled with the terrain and topographic corrections in deriving the wind and ice values for a site.

In carrying out thousands of such site specific assessments for ASCE7, TIA and S37 we at ICE Inc. have been able to identify some shortcomings in the codes which lead to failure to provide a complete picture of hazard conditions. These include the effect of tower location on elevated terrain in freezing rain events, the potential for rime icing on such structures, as well as rime icing on very tall towers on level terrain.

We have also used experimental and theoretical results to generalize the effect of changes in surface characteristics using a continuous measure of roughness to improve on the transition between differing terrain along the fetch to the structure, and to account for topographic effects which are not dealt with in the isolated hill or ridge case.

Directional Site Specific Wind Extreme Assessment

ICE Inc now offers a directional extreme wind assessment for more complex sites which:

• Analyzes terrain and topographic influences on the observed airport winds in order to correct (standardize) the observations
• Performs statistical analysis of the hourly winds for 8 sectors of 45°
• Performs terrain and topographic corrections for each of the sectors
• Determines the maximum impact sector by comparing the profiles of winds for desired MRI over the height of the structure
• Provides documentation of the data sources and procedures, and the exposure and topographic characterization by sector, as well as full profiles of wind and icing (glaze and rime).

The Directional Extreme Report involves additional assessment and reporting which involves a higher cost. Quotes for the Standard or Directional assessment are provided free of charge on request.