Table 8. Return periods for large ice loads.
0.5 in. to 1.5 in. for the two superstations in
upstate New York and northwestern Vermont
Uniform
Upstate
Coastal and
and in coastal and central Maine and New
ice
New York
Central Maine
Hampshire. The larger equivalent uniform ice
thickness
and
and
thicknesses from this storm ranged from 0.75
(in.)
NW Vermont
New Hampshire
to 1.25 in., which corresponds to return peri-
0.5
15
5
ods ranging from 35 to 100 years in the NewY-
0.75
35
20
orkVermont superstation and 20 to 85 years
1.0
65
40
in the New HampshireMaine superstation.
1.25
100
85
These return periods are consistent with the
1.5
145
160
information from Storm Data and other sourc-
es in Appendix A on severe ice storms in the
same area earlier in the century. Extensive tree damage and associated damage to the electrical
distribution system can be expected with only 0.5 in. ice loads, which have a return period of
between 5 and 15 years. Because of the sparse weather data and the rough terrain in interior New
Hampshire and most of Vermont we have not estimated the return period for a storm like this in
that region. The ice loads in the January storm varied from none to a lot over very short distanc-
es, associated with changes in elevation, exposure, and the local temperature profile.
7.4 Regional loads
The concept of regional loads, rather than point loads, applies to both wind loads and ice
loads for the design of transmission lines. The ice load map in ASCE 7 shows 50-year return-
period loads at a point. It is used in the design of communication towers and power lines. How-
ever, power line systems have large horizontal extents compared to communication towers. So
the risk of exceeding the 50-year return-period point loads anywhere in the system is higher than
the risk of exceeding that load at a particular point. For example, in Essex Junction there is a
64% probability that the ice load will exceed the 50-year return-period ice load on the ASCE 7
map at least once in any 50-year period (Table 6). However, a transmission line that extends from
Essex Junction to White River Junction encounters ice storms that occur anywhere between the
two cities. Thus, designing a transmission line, which extends tens or hundreds of miles, and a
single TV tower for the same ice load results in a greater risk of failure for the transmission line
than for the tower. The risk of exceeding the 50-year return-period point ice load increases with
the horizontal extent of the transmission line.
8. CONCLUSIONS
The January ice storm in New England and upstate New York was notable for both the large
area it covered and the amount of ice that accreted on trees and structures. Both of these factors,
along with the rural character of the region, contributed to the long power outages during and
following the storm. In some areas in the United States customers were without power for three
weeks and in Canada outages lasted even longer. This storm, however, is not without precedent.
The ice storm in December of 1929, in particular, appears to cover almost the same region with
ice loads comparable to those in this storm. This is consistent with return period estimated by our
extreme value analysis of between 35 and 85 years for storms of this magnitude in the Northeast.
Other storms have produced ice loads comparable to those in this storm, but over a smaller region.
For the most part, structures that were designed for heavy ice withstood the loads imposed by
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