In order to assess the safety levels of glass structures a scattered and inhomogeneous variety of mostly complicated resistance criteria is presently available, very often requiring specially developed softwares. For this reason engineers who wants to assess with reliability the actual safety level of glass structures of relevant economical importance are still obliged to undertake expensive experimental tests. In the attempt to overcome this problem, it was formulated a new semi-probabilistic failure prediction method called “Design Crack Method” (DCM), which is a compromise between the necessity to accurately model the complex mechanical behaviour of glass at breakage and the need to reduce the analytic complexity of the calculations. On the basis of Linear Elastic Fracture Mechanics, such aim has been analytically reached in the present work by defining a new quantity called Design Crack, characterized by a mathematical expression that depends on the probability of failure and on the surface damaging level. The proposed method, which is in accordance with the basic principles of the Structural Eurocodes, allows to predict glass lifetime taking into due account the influence of parameters like the surface extension and the loading time-history of the structural element. A design safety assessing procedure of glass structures is also briefly explained and several experimental tests (four point bending test) on real glass elements are numerically simulated by the D.C.M. showing a good agreement with the measured values.