Table 2. Frequency of failures associat-
1164-ft KFDI tower in Kansas, only the top 79 ft of the
ed with ice type and wind speed.
tower fell when melting ice slid down a guy cable and
smashed the cable grip at the anchor. As the top fell, it
Icing source
Southern U.S.*
Northern U.S.*
became entangled in a lower guy cable and slid down
20
36
its length all the way to an outer anchor, resulting in a
In-cloud
2
36
60% collapse radius.
Mixed
8
18
Although many failures occurred without warning
and with station personnel on site (28 of 82 sites were
Estimated
wind speed
Central
All other
known to be manned at the time of failure), only two
plains†
(mph)
regions
resulted in injuries to station employees. There have
Low (< 10)
37
20
been no injuries to passersby. The worst injury occurred
Med (10 to 30)
13
12
in 1960 at a remote site in New Mexico when the 1610-
High (> 30)
15
23
* Northern and southern U.S. as divided by lati-
which four people were working, one of whom suf-
tude N37.
fered a broken knee. Three other buildings, which
† Including the states of Illinois, Minnesota, Iowa,
housed the employees' families, were damaged by fall-
Missouri, Oklahoma, Kansas, Nebraska, North
ing debris, but no one in them was injured. The second
Dakota and South Dakota.
injury occurred in the 1983 collapse of the 578-ft WCIQ
tower atop Alabama's highest point, Mt. Cheaha. The
for example, freezing rain and drizzle (Table 2). Of the
transmitter technician sustained minor cuts while climb-
30 incidents occurring in the south for which ice type
ing out of the debris after the collapse. In earlier years,
has been determined, 20 (67%) were the result of pre-
many transmitter sites were manned, but that is less
cipitational icing. Regions farther north experience low-
common with today's more automated equipment, re-
er temperatures for longer periods, so that in-cloud ic-
ducing the risk of employee injury in the future.
ing, or rime icing, is more prevalent (54 of 90 cases
[60%] involved rime or a rime-glaze mix).
2.4 Concurrent weather
Failures in the central Great Plains more frequently
I made a qualitative appraisal of the on-site weather
occurred at low wind speeds. Fifty-seven percent (37
and ice conditions prior to tower collapse, using any or
of 65) of those cases happened when the estimated
all of these four sources: 1) interviews with station per-
winds were less than 10 mph, whereas only 36% (20 of
sonnel, 2) local newspaper articles, 3) Storm Data
55 cases) in all other areas of the country were accom-
(NOAA 19591995), and 4) meteorological data from
panied by such low winds.
nearby weather stations. During my interviews with sta-
Figure 7 shows the distribution of the factors that
tion personnel, I obtained their subjective estimates of
contributed to these failures. In most cases, I assessed
the ground level wind speed, tower ice thickness and
the available wind and ice load information to deter-
ice type. Newspaper articles about a collapse often pro-
mine the cause of failure; however, in a few cases other
vided additional qualitative information on tower con-
specific factors were cited. For example: a tower fell
ditions. Storm Data provided a county-specific over-
after being hit by an adjacent tower that fell; a gin pole
view of the storm conditions, the storm's progression,
used for tower construction was in place near the top of
and its consequences. Storm Data also mentioned many
the tower (causing catastrophic imbalance when load-
tower failures that I had not previously known of, which
ed with ice or wind-on-ice); or the tower was galloping
were, in turn, researched and added to the database. I
(oscillating severely) under the combined wind and ice
also used NCDC's Local Climatological Data publica-
loads. Six failures were directly attributable to ice shed-
tions, which provided quantitative meteorological mea-
ding under warming conditions. That is, either a cylin-
surements at nearby weather stations; I interpolated or
drical piece of ice slid down a guy and destroyed the
extrapolated these to the collapse site.
cable grip at the anchor, or the sudden release of ice
My preliminary analysis suggests that most con-
induced a catastrophic load imbalance.
firmed icing-related tower failures in the southern U.S.
When possible, I categorized the failures based on
were the result of a few very large and very severe
an assessment of the ice and wind loads derived from
storms. All of the confirmed failures in the south (47 of
available information. I classified each failure as re-
140) resulted from only 12 separate storms, whereas
sulting primarily from ice load (if there was little wind
the 93 failures in the north occurred during 48 distinct
and much ice), wind load (if there was much wind but
storms.
little ice), or wind-on-ice load (if both were probably
The ice that destroyed towers in southern storms was
important). Note that this categorization does not take
more frequently the result of freezing precipitation from,
into consideration the specific loads that the towers were
7
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