On 2/8/13 2:58 PM, Jack Berry wrote:
It appears that wind load calculation methodologies must have changed over the
past 20 years.
Purely anecdotal but it seems the same tower models have been derated over time.
Has the method changed or have lost law suits caused a shift?
Methods have changed to reflect better knowledge and more accurate
analyses, and, in some cases, to change underlying assumptions of things
like how high above the top of the tower the antenna is mounted.
AND, towers are rated in accordance with the EIA/TIA-222 standard
which changes over the years. Sometimes it's to make the structural
assumptions and analyses match other standards, other times it's to
reflect changing usage.
For instance, some years ago there was a difference in how "wind speed"
is measured, so for the exact same physical wind conditions, the
reported number in the tables, and how that is applied in the equations
to turn it into loads, is different.
Here's a little summary of the changes from the TIA website. Note the
change at Rev D in 1987 where the assumption of speeds at varying
heights changed.
2006 – TIA-222-G With the standard's most sweeping changes,
Revision G has been adopted by some states and local jurisdictions. The
analysis of tower structures is now consistent with other structures.
Design philosophy changed to limit states design from allowable stress
design. With recognition from the International Building Code, most
states and municipalities will eventually adopt the revision.
1996 – TIA/EIA 222-F This standard was used during the country's
greatest build out of towers and is currently the most adopted standard
in the nation. Expanded the scope of the previous version to include the
effects of ice loading. Basically, it provided two methods for analysis
of ice.
1991 – TIA/EIA 222-E The first iteration of the code to be defined
by the TIA and Electronics Industries Association (EIA).
1987 – EIA 222-D Wind pressures are replaced by basic wind speeds
(mph). A new wind velocity map is introduced. The value for basic wind
speed increased as a function of tower height above 33 feet AGL.
1976 – EIA RS-222-C
1972 – EIA RS-222-B
1966 – EIA RS-222-A
1959 — EIA RS-222 (Revision of TR-116 and RS-194) - The country's
first standard for antenna supporting structures and antennas is
published. The 11-page standard delineates the nation into three wind
uniform pressure zones: A, B and C. The wind loading was considered over
the full length of the structure and was measured in pounds per square
foot (PSF). The standard explained that "Ice coatings are not
specifically stated as icing seldom occurs simultaneously with maximum
wind loading."
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