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Calculating glass thickness to absorb impact

Calculating glass thickness to absorb impact

(OP)
At work I'm required to calculate the require thickness of glass to absorb a certain impact from a falling mass. I've approached the problem using energy equations. In my calc's i've assumed that all potential energy is converted into elastic deformation (potential energy) in my glass plate. This gives me M*G*Y=1/2*K*X^2. I've obtained K from the ratio of force vs deflection in Roark's handbook for plate deflections assuming simple supported plate, load applied on a small area in the center. Having K, i've calculated X, from which I can calculate Stress and a factor of safety when compared to the glass' modulus of rupture. The problem is my calc's don't agree with reality. I'm off by at least a factor of 4. Does anyone have any experience with this. Is my approach reasonable? Any suggestions?

Materal: Chemically Strengthened Soda Lime Glass
Thickness: variable, need to find this
Modulus of Rupture: Minimum 24000 psi
Modulus of Elasticity: 10500000 psi
approximate size: 14"x11"

contact area: approx 1.5 sq inches
(load surface is deformable and not likely to form regions of high stress)

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