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Re: FOS for von-Mises stress criteria

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Chris et al,
If you look at CMAA specification #74 fot Top Running &
Under Running Single Girder Electric Overhead Cranes
Utilizing Under-Running Trolley Hoist they give the
Hecky-Von Mises formula for the bridge beam bottom
flanges which are subject to vertical flexural tension
stresses, transverse bending stresses and both local
tension & compression stresses under the wheels.  They
give allowable stresses for different loading conditions
starting with 0.6Fy for normal loading and up for more
extreme conditions.  I have been called to look at UR
runways where the bottom flange was drooping down, curled down to use 
a less technical term.  Checking them against the CMAA
formulas showed they were inadequately sized.  Some of
the attempts at reinforcing these flanges were pathetic.
Gary


On 13 Jan 2006 at 10:23, Christopher Wright wrote:

> 
> On Jan 13, 2006, at 7:04 AM, Arun Sarkar wrote:
> 
> > can anyone suggest what factor of safety (for structural steel)
> > should be used when the checking criteria is von-Mises stress ?
> You're playing with fire. Sounds like you have some FEA results you're
> trying to apply to something like ASD rules.  Von Mises stress _If_
> you're considering working stress design and _if_ there are no
> stabiity issues, and _if_ there are no fatigue issues and _if_ you're
> talking about primary stress only and if the material is structural
> steel where the ultimate stress doesn't govern the allowables, the
> margin against first yield is about 1.6. So keep your von Mises stress
> to about 60% of the minimum specified yield value. If any of the
> foregoing ifs aren't met you're out of luck with simple-minded rules
> of thumb, particularly if you're trying to apply limit analysis
> procedures. You need to understand what the basis of your particular
> design code actually is, and match your FEA results to it.
> 
> 'Factor of safety' is a silly phrase. It's a number that you only need
> because you don't know what it is.
> 
> Christopher Wright P.E.    |"They couldn't hit an elephant at
> chrisw(--nospam--at)skypoint.com       | this distance"   (last words of Gen.
> ____________________| John Sedgwick, Spotsylvania 1864)
> http://www.skypoint.com/~chrisw
> 
> 
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