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Re: FOS for von-Mises stress criteria

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On Jan 13, 2006, at 10:34 AM, Stuart, Matthew wrote:

I would disagree.  When the Code says that I have to have a safety
factor of 1.5 for overturning of a footing then I know that my
overturning resistance must be 1.5 the actual overturning forces, both
of which can be calculated for any given footing.
I'll stick to my guns, if only because 'actual' overturning forces are themselves are design approximations, as are methodology assumptions. Your design margin against the loads you're given is 1.5, depending on how closely your design methodology and load set mimic reality. But your given loads and the design methodology also contain margins appropriate to the approximations involved. Such margins add to the 'actual' safety margin (usually). That's why so few structures actually fail when the loading reaches the assumed service loading times the assumed 'safety factor.'

My experience is that even basic stuff like structural weights aren't known all that well by the time a structure gets built. Wind load design methodology is a design artifice using a simplified constant pressure which is practically certain to envelope practical situations with a certain amount of Kentucky windage thrown in to handle complicated stuff that is ignored. The elastic/perfectly plastic assumption that goes into LRFD is another approximation, that ignores load shifting from large deformation effects and the beneficial effects of strain hardening. They're just too complicated to consider in real-world design, even if the designers were conversant enough with non-linear response and plasticity to make use of them.

Engineering design is littered with methodologies which use simple approximations to cover complicated physics and adjust allowable loads or stresses accordingly. All this actually goes into the 'real' factor of safety, which we can hardly even guess at.

Christopher Wright P.E.    |"They couldn't hit an elephant at
chrisw(--nospam--at)       | this distance"   (last words of Gen.
____________________| John Sedgwick, Spotsylvania 1864)

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