tance that debris landed from the tower base. From these
legs at one or several elevations. The guys radiate down-
lists, I extracted only those in the U.S that were icing
ward to sets of three anchors in the ground. Depending
related and began researching each one in more detail.
on the tower's height and design loads, single, double,
Much of my information I obtained from interviews with
triple, or quadruple sets of three anchors provide ground
station owners, transmitter engineers, tower designers,
attachment, each set being buried a greater radial dis-
fabricators, and builders who had some personal knowl-
tance from the tower's base. Although most towers have
edge of the events (telephone interviews have been com-
a constant horizontal cross section over their entire
pleted for approximately 60% of the cases). These con-
height, many towers are designed with either a continu-
tacts led, in turn, to information on a great many more
ous taper or with intermittent tapered sections from bot-
failures that were not widely known about. The survey
tom to top. Freestanding towers (without guy cables and
form that I completed during each telephone interview
anchors) are nearly always tapered, of heavier construc-
appears in the Appendix.
tion, more expensive to build, and therefore not as
I supplemented this first- and second-hand knowledge
numerous as guyed towers. Freestanding towers require
with storm records from the National Climatic Data Cen-
less land area so they may be used at sites where land
ter (NCDC 19601994, NOAA 19591995) (for approxi-
costs are high or space is limited.
mately 60% of the cases) and newspaper articles from
Only one of the failures is known to have involved a
state and local libraries (for approximately 70% of
freestanding tower, a 310-ft, two-way-radio tower that
cases). When other sources were lacking, I obtained the
was approximately 17 years old. The average tower age,
names of towers, their coordinates, heights, or ground
for the 77 cases in which that information is available,
elevations from the U.S. Geological Survey's digital data-
is 11.5 years, and the standard deviation is 10 years. Of
base of place names appearing on their 71/2-min quad-
those cases in which the structure cross section is known,
rangle maps (USGS 1993) or from the National Oceanic
most had a constant cross section, but a few tapered.
and Atmospheric Administration's Digital Obstructions
Communication towers usually serve many functions.
File (O'Brien 1994). NOAA's DOF lists all types of
Many stations broadcast both FM and AM frequencies
obstructions to aviation. Besides tall buildings, smoke-
from the same tower, and sometimes a television signal.
stacks, catenaries, grain elevators, and so forth, it lists
Often a television station leases tower space to a sepa-
43,467 communication towers (or clusters of towers) in
rate radio station and any number of two-way user
the 50 states.
groups. For this report, I have classified each tower
according to its primary use. Figure 1 shows the distri-
bution of the tower types in this database. The largest
3. Trends Derived from the
number of failures involved television and FM broad-
Major Topics of the Database
To date, I have confirmed approximately 140 tower
casters, and two-way transmitters; their total numbers
collapses in the United States, dating back to 1959, that
are almost the same. Failures of AM broadcast and
occurred with a buildup of atmospheric ice on the struc-
cable television receiver towers are noticeably fewer.
ture. These are listed as Table 1. Approximately 15 more
The numbers somewhat reflect the much greater inci-
reports are, as yet, unconfirmed. The towers include tele-
dence of certain tower types, with the exception of two-
way towers. Towers dedicated to AM or cable televi-
receivers and transmitters, ranging in height from 40 to
sion are considerably less numerous than the other three
2000 ft agl. I have obtained varying amounts data for
types. Although two-way towers (including paging and
each failure in the form of 1) structural characteristics
mobile telephone towers) vastly outnumber television
of the tower, 2) its geographic location, 3) a description
and FM towers, their collapse affects fewer people so
of the collapse, 4) concurrent weather, and 5) resulting
perhaps there is less attention paid to their demise. We
damage. The picture is constantly changing as new fail-
are less likely to find old newspaper articles and people
ures occur and as past failures are added to the database;
do not recall as readily when a private company loses
however, I will describe certain trends in the data from
its two-way tower during a storm. For these reasons,
each of these main topics and summarize the current in-
I believe that failures of two-way towers are vastly under-
formation.
reported and therefore not well represented by these data.
I have several reports of such failures that I have not
3.1 Structural Characteristics
been able to confirm, and therefore have not included in
Communication towers are usually triangular in cross
this summary.
section (though some are rectangular), with legs and cross
A histogram showing the heights of 121 of the
bracing constructed of solid rod, tubular, or angular gal-
towers appears as Figure 2. Nearly a third, or 39 of the
vanized steel. They are usually supported against lateral
towers, were 300 ft tall or less. A similar number (43 of
loads by a network of guy cables attached to each of the
121) were between 300 and 601 ft tall. One-fifth (24 of
2
Previous Page
