Direct evaluation of the modification coefficient kmod for the static-fatigue design of annealed glass panes under wind load

Most standards reduce the design strength of glass through a modification coefficient that accounts for static fatigue caused by subcritical propagation of surface cracks. For wind pressures on glass plates, the coefficient is derived assuming nominal durations of maximum wind gusts or cumulative winds, which are defined based on tradition and practice. Here we derive closed-form expressions for the modification coefficient using a fracture mechanics approach which incorporates the time histories of wind velocity or their probabilistic distributions. The coefficient depends on the duration of the action and becomes constant for typical design lives. A consistent nominal duration is then defined by referring to a wind with fixed reference velocity which produces the same damage of the actual action. The approach, applicable to the European context, has been applied to data recorded in Italy. The results are compared with prescriptions from various standards showing that they are in general non-conservative.