[TowerTalk] Rohn Tower Loading: Exposure B and C

[n1rr at 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 brie
y 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 diff
erence 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 diff
erence in elevation between the top and bottom of the hill. For sites surrounded by other hills, height is the di
fference 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 at rthorne.net> 
To: "TowerTalk" <towertalk at 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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