http://www.criticaltowers.com/ROHN%20Products%20LLC/2015%20Data/Documents/1294693956_Understanding_Rev_G_web.pdf
UNDERSTANDING TIA-222 - REVISION G 14 © 2011 ROHN PRODUCTS LLC What is Rev G?
Rev G is the latest revision of the TIA-222 Standard “Structural Standards for
Antenna Supporting Structures and Antennas”. The previous version of the
Standard was Rev F. Rev G is based on a 3-second gust wind speed and Rev F is
based on a fastestmile wind speed. The wind speeds are not directly comparable
and it is very important to dene the basis of a wind speed when specifying
wind loading requirements. For a given location, the 3-second gust wind speed
represents the peak gust wind speed whereas the fastest-mile wind speed
represents the average wind speed over the time required for one mile of wind
to pass the site. Rev G presents additional factors to be considered in the
design of new structures and for the modication of existing structures. These
factors are briey discussed below. The reliability requirements of a structure
can now be accounted for by assigning a classication to a structure (Class I,
II or III). The wind speed can also be adjusted based on the type of terrain
surrounding the site (Exposure B, C or D) and if the site is located on a hill,
ridge or escarpment (Topographic Category 1-5).
UNDERSTANDING TIA-222 - REVISION G Phone (309) 566-3000 • Fax (309) 566-3079 •
www.rohnnet.com • The Industry Standard I ROHN SOLUTIONS 15 © 2011 ROHN
PRODUCTS LLC What is Exposure? Exposure categories are used to adjust wind
loading based on the type of terrain surrounding a site. Reduced wind loads are
associated with rougher terrains that tend to slow the wind down. Three
exposure categories have been designed based on terrain roughness. Wind
loading is increased as the exposure designation changes from Exposure B
(roughest terrain) to Exposure D (smoothest terrain). Exposure B: Urban,
suburban or wooded areas. The wind load at ground level is reduced compared to
Exposure C. This reduction diminishes with height, making the overall wind
reduction less signicant for taller structures. In order to qualify for the
wind load reduction, the rough terrain must extend in all directions from the
site at least twenty times the height of the structure, but not less than
one-half mile. Exposure C: Flat, open country and grasslands. Exposure D: Flat,
unobstructed shorelines exposed to wind owing over open water, smooth mud
flats, salt flats and other similar terrain. The wind load at ground level is
increased compared to Exposure C. Topographic Categories Topographic categories
are used to determine increases in wind loading for sites located on hills and
other elevated locations (other than buildings). The shape and relative height
(topography) of an elevated site determines the increase in wind load. Although
many elevated sites have their own unique features, the intent is to idealize
these sites into one of the standard topography categories described below. The
height of an elevated site above the surrounding terrain must be specified in
order to determine the increase in wind loading. Height should not be confused
with the elevation of the site. As described below, elevations of the site and
the surrounding terrain must be used to determine the relative height of a
site. For structures supported on buildings, it is only necessary to specify
the height of the building and the surrounding exposure category. Category 1:
Flat or rolling terrain with no abrupt changes in general topography. No
increase in wind loading is required for this category. Category 2: Sites
separated from a lower elevation by a gently sloping terrain (escarpment). Wind
loads at the crest are 2.0 times the wind loads for a at site and diminish
with height depending on the height of the escarpment. Height for an escarpment
is the difference in elevation between the upper and lower levels. Increased
wind loads do not apply for structures located in the lower half of the sloping
terrain or located beyond 16 times the escarpment’s height from the crest.
Category 3: Sites located at the top or within the upper half of a hill. Wind
loads at the top of a hill are 2.3 times the wind loads for a at site and
diminish with height depending on the relative height of the hill. Height for a
hill is the difference in elevation between the top and bottom of the hill.
For sites surrounded by other hills, height is the difference in the hill
elevation at the site and the average elevation of the surrounding hills
(within a 2-mile radius). In other words, height is the projection of the hill
exposed to wind. When there are other hills surrounding the site, increased
wind loads do not apply unless the height of the hill at the tower site is at
least 2 times the average height of the surrounding hills. (Refer to sketch
above.)
----- Original Message -----
From: "Richard Thorne" <rthorne@rthorne.net>
To: "TowerTalk" <towertalk@contesting.com>
Sent: Monday, August 8, 2016 8:59:15 AM
Subject: [TowerTalk] Rohn Tower Loading: Exposure B and C
I've been searching the net looking for definitions of exposure B and C
without much luck.
I thought I read some where that exposure C is used for a tower that is
in the wide open with no structures blocking wind, exposure B has
structures blocking wind.
I'm currently using exposure C for my tower projects.
Can anyone point me to a place to read up on exposure information?
Thanks
Rich - N5ZC
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