SST automatically computes the effective skin on the two sides of a rib 
that contributes to the (also automatically computed) rib cross section 
properties The effective skin is stress level dependant. The higher the 
stress the less the effective skin.  The factor selected below modifies 
the SST computed effective skin.  

Factor default values are 1.67 for machined structures and 1.38 for 
chemically milled structures.