The effective thickness of laminated glass: Inconsistency of the formulation in a proposal of EN-standards

The project of EN-standard PrEN-13474 records a simple method to calculate the effective thickness of laminated glass, i.e., the thickness of a monolith with equivalent bending properties in terms of stress or deflection. This is supposed to depend upon the shear-stiffness family of the interlayer only. Here, paradigmatic examples show that this approach leads to inconsistent results. The formulation is too simplistic, being based upon a theory that does not consider size-effect, type of constraint and load conditions, which instead heavily affect the shear coupling of glass plies through the interlayer. Another formulation is mentioned, that gives very accurate results with no additional computational difficulty with respect to the PrEN-13474 approach.