Wind damage and mechanics of flying debris Wind loading and structural response Lecture 2 Dr. J.D. Holmes Wind damage and mechanics of flying debris Damage due to windstorms is increasing accounts for 70% of insured losses (difficult to separate direct wind damage from rain, storm floods) big increases in U.S. in late 1980s and 1990s Hurricanes Hugo, Andrew, Georges $ Billions also gales in Europe : 1987($3.7bill.), 1990 ($15bill.), 1999 ($10bill.) Wind damage and mechanics of flying debris Ferrybridge cooling towers - England, 1965 interference effects Wind damage and mechanics of flying debris Brighton Chain Pier, England, 1836 also, Wheeling Bridge, Ohio, 1854 aeroelastic instability Wind damage and mechanics of flying debris Tacoma Narrows Bridge, Washington State, 1940 aeroelastic instability Wind damage and mechanics of flying debris aeroelastic instability Tacoma Narrows Bridge, Washington State, 1940 Wind damage and mechanics of flying debris Low-rise buildings - tornado and hurricane damage Wind damage and mechanics of flying debris High-rise buildings - cladding (glass) damage Hurricane Alicia, Houston, 1983 Wind damage and mechanics of flying debris Wind-generated debris generates high internal pressures allows wind and rain penetration danger to occupants Wind damage and mechanics of flying debris Types of flying debris d Rod t Sheet Compact Wind damage and mechanics of flying debris Threshold of flight Compact object: I = fixity parameter (=1, for objects resting on the ground) C F = aerodynamic force coefficient aerodynamic force just balances resistance of gravity and fixity Wind damage and mechanics of flying debris Threshold of flight The higher the value of characteristic dimension,, or density, m, the higher the wind speed for the threshold of flight, U f. Example : for C F = 1, I = 1, U F = 30 m/s (67 mph) = 110 mm (4.3 in) for wooden object = 20 mm (0.8 in) for stone object Wind damage and mechanics of flying debris Threshold of flight Sheet object: Rod object: Wind damage and mechanics of flying debris Time of flight and distance travelled assume constant wind speed Accelerating force : Acceleration : Wind damage and mechanics of flying debris Time of flight : Acceleration : Time taken to reach v m : where : dimensional Wind damage and mechanics of flying debris Distance traveled : Time to reach velocity v : Distance traveled during time taken to reach v m : Wind damage and mechanics of flying debris Time of flight and distance travelled Examples : steel ball 2mm (0.315 in.) diameter timber : 100 mm(4 in.) by 50 mm (2 in.) by 1600 mm (5.25 ft.) long Wind damage and mechanics of flying debris Sphere falls under gravity and air resistance until it impacts ground or a building Wind damage and mechanics of flying debris Time and distance to impact : Some shapes (e.g. plates, prisms) have lift can reach heights greater than release height Wind damage and mechanics of flying debris Some shapes (e.g. plates, prisms) have lift can reach heights greater than release height Wind damage and mechanics of flying debris Some shapes (e.g. plates, prisms) have lift can reach heights greater than release height Wind damage and mechanics of flying debris Some shapes (e.g. plates, prisms) have lift can reach heights greater than release height End of Lecture 2 John Holmes