Wednesday, June 19, 2013

Hurricane Induced Radome Load Cycles

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.

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).

Figure 2: Category III+ hurricanes passing near Florida City from 1931 to 1951

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.