Contact forces generated by hailstone impact

• A custom designed apparatus for measuring contact force generated by the impact of a solid impactor object is introduced.
• A two-step calibration procedure is proposed for evaluating dynamic compressive stiffness parameters (i.e. kn and p).
• An algebraic expression for predicting the value of the peak contact force is derived.
• A design chart for estimating contact force generated by hailstone is constructed for design checks and risks assessments.

Impact by hailstones can cause severe and widespread damage to roofs and many other exposed installations. This research aims at developing a model with which the value of the peak contact force generated by impact of a hailstone can be predicted with good confidence. A custom designed and built experimental setup involving the use of a gas gun for accurate measurement of contact force is first introduced. Some 45 impact experiments were conducted using the device. Meanwhile, an analytical spring connected lumped mass model featuring a non-linear visco-elastic compressive property of the frontal spring has been developed for simulating impact. To ensure that the analytical model is representative of real behaviour values of parameters used for characterising the stiffness properties of the frontal spring were obtained by calibrating with experimental measurements. Moderate changes in the calibrated values of the modelling parameters with increasing velocity of impact have been tracked. Finally, an algebraic expression along with a design chart has been developed to provide a convenient way for designers to estimate the peak contact force in deterministic terms for a range of hailstorm scenarios.