Practical_Antenna_Handbook_0071639586

658 P a r t V I I I : M e c h a n i c a l C o n s t r u c t i o n a n d I n s t a l l a t i o n T e c h n i q u e s
100
90
Impact Pressure (PSF)
80
70
60
50
40
30
20
10
0
0.0
20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0
Wind Speed (MPH)
Figure 29.2 Pressure versus wind speed.
Note In 2005, the Telecommunications Industry Association (TIA) issued Revision G of TIA-Â
222, their Structural Standards for Antenna Supporting Structures and Antennas.
Determination of the appropriate wind speed ratings to apply to a tower or antenna in
Revision G represents a substantial and significant departure from the methods of earlier
revisions. In particular, the focus has shifted from wind speed defined by a fastest mile to
wind speed defined by a peak 3-Âsecond gust. Although there is no precise 1:1 equivalence
between these numbers (that pesky old “apples and oranges” thing), tower and antenna
installations designed for approximately 70 mph using the earlier Revision F standard seem
to be comparable to designs based on a 90 mph peak 3-Âsecond gust standard. Further,
Revision G introduces and requires certain adjustments (usually additions) to peak wind
speed numbers because of terrain and/or reliability requirements for the support structure.
As before, the standard provides a baseline wind speed specification for each county in the
United States. (Most local building codes officials can provide you with the current
requirements for their own counties.) As a “glittering generality”, the baseline wind speed
rating for flat, open countryside across the vast majority of the interior 48 states is 90 mph,
but this number must be adjusted upward with icing, with increased tower height, and
whenever unusual topographic features (hills, mountains, cliffs, ocean coastlines, etc.) are
encountered. Overall, the baseline speeds vary from a low of 85 mph along the Pacific coast
states to a high of 150 mph along the coastlines of south Florida and some of the other states
bordering the Gulf of Mexico. Although not a universal practice, TIA-Â222 wind speed
ratings already are, or soon will be, made a part of local building codes in many municipalities.
Now we are in a position to calculate—in very crude fashion, at least—the requirements
our tower must meet if it is to survive a high wind. For the purposes of our ex-