Geographic location
revealed that losses related to the first tower totaled
million, while the second cost the insurer another
Most of the failures occurred in the midwestern
million.
states and the Appalachian highlands.
All except two failures occurred east of the Rocky
Mountains.
3. Possible Future Work
Two-thirds occurred north of latitude N37.
The database is as yet incomplete. Interviews have
been completed for only 60% of the failures, newspaper
The failures in the southern U.S. are generally the
articles have been obtained for only 70%, and an analy-
result of fewer, but more severe, storms than those
sis of the weather data is complete for only 60% of cas-
in the midwest.
es. Several failures are still unconfirmed. At this time,
Collapse
CRREL does not have a mandate or the funding neces-
sary to continue significant research effort on this sub-
Large storms in 1973, 1975, 1983, and 1994 caused
ject. However, further work is needed and includes:
48 of the 65 failures that occurred during those
1. Research of the total numbers of various tower
years.
types in the different states or regions of the country
The worst single year was 1983, in which 26 fail-
to gain a better understanding of rates of failure;
ures occurred.
2. Retrieval and analysis of meteorological data to
The most storms that caused failures in any one
profile better the typical storm conditions that cause
year was four, in 1989.
towers to fall;
More than 90% of the failures occurred between
3. Where higher-risk locations are found, examine
December 1 and March 31.
whether the ice and wind-on-ice design loads are
When a tower falls, the debris is usually contained
adequate;
within a radius of 50% of the tower's height.
4. More-detailed analysis of damage costs to under-
Two failures have caused minor injury. There have
stand better the relationships between damage and
been no serious injuries, and no passersby have
parameters such as tower type, height, age, base
been injured.
elevation, icing type, wind speed, and so forth.
Concurrent weather
Twice as many towers fell in four times as many
4. Summary of Findings
CRREL has an established database of icing-related
storms in the northern U.S., compared with the
communication tower collapses for the U.S. This data-
southern U.S.
base reveals where and when icing-related tower col-
Sixty-seven percent of the failures occurring in the
lapses have occurred in the United States. The record
south were the result of precipitational icing,
contains information dating back to 1959 on the failures
whereas 60% of northern failures involved in-cloud
of 140 towers, including radio, television, microwave,
icing.
and two-way towers. Information was compiled from
Fifty-seven percent of failures in the Great Plains
interviews with tower engineers, owners, station person-
occurred under low wind speed conditions, com-
nel, and others, from local newspaper articles, monthly
pared with 36% for all other areas of the country.
storm publications, and digital databases maintained by
I judged wind-on-ice loading to be instrumental in
the USGS and NOAA. For each failure, I am compiling
56% of 106 failures, ice loading alone to be instru-
information on the tower structure and its geographic
mental in 37% of cases, and wind loading alone
location, the collapse sequence, the concurrent weather,
(icing was incidental) in 7% of cases.
and the resulting damage. The information is incomplete,
Damage
although a summary is as follows:
The damage caused by tower collapse is both im-
Structural characteristics
mediate and delayed in nature. Immediate costs in-
The largest number of failures involved FM, televi-
clude the loss of the tower and equipment on it,
sion, and two-way towers.
but also may include buildings, equipment, vehi-
Of the 121 towers for which we have height data,
cles, the transmitter, power lines, and other adja-
one-third were under 300 ft tall, another third were
cent property. Delayed costs include lost advertis-
between 300 ft and 601 ft, and one-fifth were taller
ing revenue while the station is completely or par-
than 1000 ft.
tially off the air, employee layoffs, higher costs for
Only one tower was known to be freestanding.
restoration services, and overtime wages.
The mean age of 77 towers that fell was 11.5 yr.
The estimated mean time required for 57 stations
to return to normal operations after a collapse was
9
Previous Page
