Margin of safety

Margin of Safety

Margin of safety (M.S.) is a way of representing a structure's capability above certain requirements, for example its strength relative to a design ultimate load.

\begin{align} M.S. & = \frac{ Maximum Allowable Load }{ Actual Load }-1\\ M.S. & = \frac{1,600}{1,550}-1\\ M.S. & = +0.03\\ \end{align}

Program requirements typically dictate minimum required margins of safety relative to certain design parameters.  It is important to keep in mind what is being used for allowable loads and actual load used in the analysis.  Two types of loads are very commonly used, limit loads and ultimate loads.  Limit is what the "design load" is, or in other words the largest load the structure is expected to experience in use.  Ultimate load, in aerospace, is often 1.5 times limit load (LL or DLL, design limit load).  What is used is program dependant.  For example, perhaps the structure is not allowed to have permanent deformation at 1 x DLL, however may experience permanent deformation between limit load and ultimate load - while not failing at ultimate load.

You can also use this handy Margin of Safety Calculator to check your work: