Infinite
Technologies, Inc. RCS is
committed to achieving the highest levels of technical excellence and quality
in all we do. This is especially true in the design and fabrication of
our composite radomes. One of the
greatest challenges faced by engineers designing radomes is accurately
predicting the wind loads these structures can expect to see, given the
uncertainty of storm patterns in any particular area. Despite this, it is possible to obtain a
rough, but conservative, estimate of the frequency with which radomes will
experience high wind loads.
As an example, ITI engineers were asked to design a radome that could
withstand multiple category IV hurricanes over a 20 year period. Several steps were taken to determine the
number of loading cycles that the radome could reasonably be expected to
encounter. These steps are listed below.
Step 1) Define the
duration of a hurricane
Hurricanes travel at about 30 mph. The slower, more dangerous hurricanes travel
as slowly as 13 mph (Hurricane Andrew, 1992).
Winds rotate counter-clockwise around the eye with the highest wind
speeds recorded in the right-hand side.
Hurricane force winds can occur as far as 300 miles from the storms
eye. The stronger winds are generally
contained to a much smaller corridor.
Hurricane Katrina, for example, was a category IV hurricane at landfall
and had category III wind speeds recorded over an area approximately 80 miles
wide. At 13 mph it would take an 80 mile
wide circle 6.15 hours to completely pass over a single point.
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| Figure 1: Wind speed map of Hurricane Katrina at landfall |
Step 2) Determine how often
wind loads a structure during a hurricane
Hurricane wind speed is measured as the average speed in a one
minute interval. Wind gusts are the
highest measurement taken during a one minute period. Hurricanes are classified based on wind
speed, not gust speed. This means that
Category III hurricanes can achieve gusting wind speeds at category IV levels
for brief periods. Assuming 4 gusts per
minute, a radome would experience 1476 gusts during a 6.15 hour storm.
Step 3) Check
Historical Hurricane Data
The National Oceanic and
Atmospheric Administration has record of every hurricane from the past 150+
years. With this data it’s possible to
determine how many category III+ hurricanes have passed through an area. Some regions in the Gulf coast experience
less than one category III+ hurricane every 20 years. The highest number of occurrences was in Florida
City, FL which experienced 15 category III+ in the last 150 years; 7 during a
20 year period from 1931 to 1951. The absolute
worst case scenario of 7 slow-moving, category III+ hurricanes hitting one spot
over a 20 year period translates to 10332 loading cycles. Realistically, since category III hurricanes
only produce category IV gusts infrequently, a more accurate assumption would
be 5608 cycles (assuming three category IV+ hurricanes plus 20% of the gusts
from category III hurricanes).
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| Figure 2: Category III+ hurricanes passing near Florida City from 1931 to 1951 |
Conclusion
Given the above assumptions and weather data, a conservative
estimate for the number of loading cycles a radome near Florida City would need
to sustain during the worst 20 year hurricane season ever recorded is 5608.
References
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