This document discusses hail impact testing conducted at the IBHS Research Center. It notes that over 75% of US cities experience hailstorms annually, which can cause billions in damages. The goals of the research center are to reduce hailstorm impacts through testing building materials and developing methods to accurately simulate natural hailstones, including varying sizes and densities. Initial testing will focus on artificial hailstones from 1-3 inches in diameter with different compositions and freezing conditions to replicate natural hailstone properties. Conglomerate hailstones will also be tested to match real-world hailstone formations.

1. Hail Impact Testing at the
IBHS Research Center
Tanya M. Brown, Ph.D.
Research Engineer, IBHS Research Center
tbrown@ibhs.org

2. Background & Motivation
• More than 75% of cities in the continental U.S.
experience at least one hailstorm each year [1]
• $1.2 billion in damages to structures & crops in
the 1990’s [2]
• A single hailstorm in DFW metroplex caused $1.1
billion in 1995 [2]
• A long-lived thunderstorm with a path length of
over 360 miles caused over $1.5 billion in
damages in Kansas, Missouri, and Illinois in
2001 [3]

3. Background & Motivation
• In April, 2009 NWS changed the criteria for
severe hail from ¾ inch to 1 inch in
diameter [4]
• Hail damage:
– 1 inch in diameter for shingles and other
roofing materials [5]
– ¾ to 1 inch in diameter for aircraft
– 1.6 inch in diameter for crops [6]

5. Code Development: Steel Ball
Testing
• Density of approximately 0.9 g/cm3 (same as pure ice)
• Balls dropped from a height necessary to duplicate the kinetic
energy of hailstones of identical diameter
• Assumptions [7]:
– Each hailstone is spherical
– Hailstones do not deform on impact
– Some recovery of impact material is allowed
• Problems:
– One study found only 58% of hailstones were spherical [8], while another
found only 75% [7], remaining stones were conical or irregularly shaped
– Largest hailstones are often conglomerates of smaller stones
• UL 2218 [9] & ASTM 3746 (modified) [10]

6. Code Development: Ice Ball
Testing
• Density of approximately 0.9 g/cm3 (same as pure ice)
• 1950’s: Ice balls launched at roofing materials
• 1960’s: Ice ball testing expanded to include testing of wall
materials
• 1980’s: Haag Engineering developed & published procedures
for evaluating hail damage & determining repair difficulty [11]
• 1990’s: Experiments with more kinds of roofing materials, and
varying the angle of impact to account for wind-blown hail
• Problems:
– Ice balls are harder and denser than natural hailstones
– Ice balls do not have the air bubbles & layer structure seen in natural
hailstones
• FM 4473 [12]

7. Goals & Objectives for Hail
Testing at the IBHS Research
• Reduce the impact ofCenter structures by
hailstorms to
increasing the resilience of building products & materials,
particularly roof & siding materials
– Methodologies to accurately create artificial hailstones
– Methodologies to create conglomerate hailstones
– Methodologies to test both vertically-falling and wind-blown hail
impacts
– Laboratory testing & analysis of materials impacted by artificial
hailstones
– Development of a rubric outlining various damage states &
failures with respect to replacement/insurance claims
– Development/refinement of laboratory testing methods
– Post-disaster field studies
– Understand spatial effects of hailfalls

9. Artificial & Conglomerate
Hailstones
• Initial testing for sizes of 1” - 3” in diameter
• Density experiments
– Chemical composition
• Tap water
• Distilled water
• Soda water
– Freezing conditions
• Changing freezing temperature
• Freezing in layers
– Using compacted crushed/shaved ice
• Conglomerate stones
– Fusing small artificial hailstones together
– Fusing broken pieces of large artificial hailstones together

10. Artificial & Conglomerate
Hailstones

11. References
1. Changnon, S.A. (1996). Climatology of Hail Risk in the United States, Publication CRR-40, Changnon
Climatologist, Mahonet, IL.
2. Changon, S.A. (1999). “Data and Approaches for Determining Hail Risk in the Contiguous United States,”
Journal of Applied Meteorology, 38, 1730-1739.
3. Changon, S.A., and Burroughs, J. (2003). “The Tristate Hailstorm: The Most Costly on Record,” Monthly
Weather Review, 131, 1734-1739.
4. National Weather Service Quad Cities, IA/IL, (April 2009). “What is a ‘Severe’ Thunderstorm?” <http://
www.crh.noaa.gov/dvn/?n=oneinchhail>
5. Marshall, T.P., Herzog, R.F., Morrison, S.J., and Smith, S.R. (2002). “Hail Damage Threshold Sizes for
Common Roofing Materials,” 21st Conference on Severe Local Storms, San Antonio, TX.
6. Gringorten, I.I. (1971). Hailstone Extremes for Design, AFCRL-72-0081, Air Force Cambridge Research
Laboratories, Bedford, MA.
7. Schleusener, R., and Jennings, P.C. (1960). “An Energy Method for Relative Estimates of Hail Intensity,”
Bulletin of the American Meteorological Society, 41(7), 372-376.
8. Weickmann, H. (1953). “Observation Data on the Formation of Precipitation in Cumulonimbus Clouds,”
Thunderstorm Electricity, University of Chicago, 66-138.
9. Underwriters Laboratories Inc. (2002). Impact Resistance of Prepared Roof Covering Materials, UL 2218,
Northbrook, IL.
10. ASTM International, (2002). Standard Test Method for Impact Resistance of Bituminous Roofing
Systems, ASTM 3746, West Conshohocken, PA.
11. Marshall, T.P., and Herzog, R.F. (1999). “Protocol for Assessment of Hail-Damaged Roofing,” Proceedings of
the North American Conference on Roofing Technology, Toronto, Canada, 40-46.
12. FM Approvals, (2005). Specification Test Standard for Impact Resistance Testing of Rigid Roofing
Materials by Impacting with Freezer Ice Balls, FM 4473, Johnston, RI.