Extreme ice thicknesses from freezing rain
Final Report
1.0
Introduction
The Federal Emergency Management Agency (FEMA) formed in 1998 the American Lifelines
Alliance (ALA) as a public-private partnership. In 2002, FEMA contracted with NIBS through
its Multihazard Mitigation Council (MMC) to, among other things, assist FEMA in continuing
ALA earlier guideline development efforts. In 2003, ALA requested Kathleen F. Jones of the
U.S. Army Corps of Engineers (USACE) Engineer Research and Development Center (ERDC)
Cold Regions Research and Engineering Laboratory (CRREL) Snow and Ice Branch to provide
maps of extreme equivalent radial ice thicknesses from freezing rain for mean recurrence
intervals of 50, 100, 200 and 400 years for both English and metric units. This effort
complements the map of 50-yr ice thicknesses in English units that is published in the ASCE 7
Standard Minimum Design Loads for Buildings and Other Structures (ASCE 2003). The latest
revision is ASCE 7-2005 (ASCE in draft), which is expected to be published early in 2005. The
50-yr maps are also in the current draft for ASCE Manual 74 Guidelines for Electrical
Transmission Line Structural Loading (the current revision is ASCE 1991) and are proposed to
be included in the 2007 revision of the National Electrical Safety Code (the current revision is
NESC 1997).
1.1
Project Objective
The purpose of this guide is to
explain the concept of equivalent radial ice thicknesses
provide references to background information
describe how ice thicknesses are estimated for long mean recurrence intervals
provide maps in English units for mean recurrence intervals of 100, 200 and 400 years to
extend the 50-yr map in ASCE Standard 7
provide maps in metric units for mean recurrence intervals from 50 to 400 years, for use
in hard metric standards, codes, and guidelines
1.2
Project Scope
This guide addresses extremes of equivalent radial ice thicknesses from freezing rain with
concurrent gust speeds.
This guide does not address accreted ice loads from in cloud icing or from sticky snow (see the
various Proceedings of the International Workshop on Atmospheric Icing of Structures). These
winter weather phenomena occur with different spatial distributions from freezing rain. It also
does not address dynamic loads associated with icing, including galloping of conductors (e.g.
Rawlins 1979), ground wires, and tower guys, which occurs with relatively small ice thicknesses
in moderate to high winds and can cause significant damage, either in a single episode or over
many winters through accumulated fatigue damage. Spatial factors, which quantify the increase
in risk environmental loads to distributed structures, such as transmission lines and distribution
line systems, are not discussed. The spatial effect for a variety of environmental loads is
September 2004
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