Re: [TowerTalk] Rohn Tower Loading: Exposure B and C

[n1rr@comcast.net]

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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