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Margins for Buckling are shown as being calculated by:
M.S.(Buckling) = Allowable EV / Buckling EV – 1
With Allowable EV corresponding to the required Eigenvalue by a program and Buckling EV corresponding to the lowest Eigenvalue reported by Nastran. This should instead read:
M.S.(Buckling) = Buckling EV / Allowable EV – 1
Example 1. If an engineering development program required a buckling Eigenvalue against limit loads of 1.5, and the first Eigenvalue discovered by Nastran was 0.8, then the Margin of Safety is calculated:
M.S.(Buckling) = 0.8 / 1.5 – 1
M.S.(Buckling) = -0.46
Explanation: This is logical because the structure is expected to buckle at only 80% of limit load, the Margin of Safety is appropriately negative.
Example 2. If an engineering development program required a buckling Eigenvalue against limit loads of 1.5, and the first Eigenvalue discovered by Nastran was 1.6, then the Margin of Safety is calculated:
M.S.(Buckling) = 1.6 / 1.5 – 1
M.S.(Buckling) = +0.07
Explanation: Because Nastran predicted it would take a factor on limit load of 1.6X to buckle, and the requirement was only 1.5, there is a positive Margin of Safety against buckling.