Flying debris behavior

Beside the building damage produced by direct wind force-either by overload caused by overstressing under peak load or fatigue damage under fluctuating loads of a lower level, a major cause of damage in severe windstorms is due to windborne debris. Penetration of the building envelope by windborne debris will lead to a change in the internal pressure which double the forces on the roof leeward and sidewalls. It also leads to a loss of building function by exposing the contents to subsequent weather damage.

Wills et al. (2000) have develop a simple engineering model to describe the behavior of flying debris, which relates the flying behavior of debris to their size, shape, and density. The model classifies the debris into three categories: cube (3D), sheet (2D) and rod (1D). Two sets of tests following the philosophy of that engineering model were conducted in TTU wind tunnel. One set of experiments examined flight initiation wind speeds for sheet debris. Another set examined sheet debris flight behavior after takeoff. Both sets were tested for varying restraining forces. An electromagnet was used to control the restraining force.

For the flight initiation tests, analysis of the results gave an average force coefficient Cp of 0.15. The restraining forces calculated from the engineering model (Wills et al., 2000) with CF of 0.15 agree well with the actual restraining force.

For the flight behavior tests, a much more detailed observation was made than previous available (Wills et al., 2000). For each flight, relative flight speed Vd/Vw was plotted against flight time. An equation Vd/Vw = a * (1-e (1-1a/T' ) of the form was used to fit the relative flight speed variation with the flight time.

For free-mounted sheets, a dimensionless figure, Vd/vwV,,vs. Pmt/(PaVw^2), was plotted. The figure shows that the relative maximum flight speed of sheet decreases with the increase of pmt/(paVw^2), and approaches a value of 0.5. Implying that the maximum speed an object can reach is approximately half of the prevailing mean wind speed. The result of sustained sheet flight tests shows that varying the restraining force does not change the maximum relative velocity for the same sheet..