As you may now by now I’m a Professional Engineer here in Florida and I am a Structural Engineer in Massachusetts where I was first licensed. I spent 2 years in St. Croix doing hurricane reconstruction work as an engineer. So, the question is: How much worse is a Cat 1 to a Cat 2, or 3 or 4 or a full blown 5?
OK, so as an engineer we design for wind loads as expressed in pounds per square foot or psf. What we do is we correlate between the wind speed and how that translated into a loading on a structure. Many items affect this equation. Things like the shape of the house, the height above the ground that the item being designed is at, how obstructed (or not) the area upwind is, etc. As you might imagine the math can get complicated when considering all these factors. We however will look at the older, simple method, of calculating wind load without any adjustments for these factors to answer this basic question. That Formula is: P = 0.00256 V^2. NOTE the V squared term.
Cat 1 is 74-95 mph, Cat 2 is 96-110 mph, Cat 3 is 111-129 mph, Cat 4 is 130-156 mph and Cat 5 is 157 mph or higher. Remember that V squared term. Thus, the load in PSF produced by the wind at various wind speeds is…
| 80 MPH | 16.4 PSF |
| 100 MPH | 25.6 PSF |
| 120 MPH | 36.9 PSF |
| 140 MPH | 50.2 PSF |
The difference in the wind load, the pressure in PSF on your roof and trees and the like, is that it triples from 80 MPH to 140 MPH and doubles from a 100 to 140 MPH wind speed. AND 50 PSF of wind uplift load is an ENORMOUS load to resist.
Which is why in a Cat 4 “Catastrophic damage will occur: Well-built framed homes can sustain severe damage with loss of most of the roof structure and/or some exterior walls. Most trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas. Power outages will last weeks to possibly months. Most of the area will be uninhabitable for weeks or months.”
Hurricane Dorrian is now over the Abacos with sustained 185 mph winds or 87 psf. This is total devastation.
