Unless you have a background in solid mechanics, this is difficult to explain.<p>On the outset, there are different failures possible, tensile, warping, etc.<p>Under a load distribution, a structure would fail at some point depending on the intensity of the load. It could be tensile or due to bending moment. Call that L. Now, you would add flitches or increase say, the thickness of a plate to withstand upto 1.5L. That is a design with a 1.5 times factor of safety.