buckling stress concentration

huangyhg

buckling stress concentration
we are working on a steel girder where we want to cut a void in the web, and replace the portion with a new plate.  the hole is next to the bearing stiffener.  when running the model in staad, we get a high buckling stress concentration, right at the top corner of the void.  effectively, in two 1" elements, the stress dissipates to tolerable levels.  the only way to reduce this concentration is to put a horizontal stiffener plate at the top of the cut.  is there any way to ignore the high stress concentration in the analysis, and state that the system is stable?  if not, is there any other remedy that could reduce the stress right at this point?
thanks for any information.
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put another vertical stiffener exactly at the edge of the hole
a possible solution is to create a dense mesh of elements in the vicinity of the stress concentration region. in finite element models, loads flow from point to point through element nodes. so, a more refined mesh in that location enables the load to flow to a wider area. you can simply select the elements in the region, right click your mouse and choose generate mesh. a dialog box will follow with some options on how to refine it.
can you make the void circular or with rounded edges to avoid stress concentrations at a 90 corner?
i'm in agreement with broekie.  recall that the liberty ships were a failure from a stress concentration factor in using square portholes.  this of course, before we really understood fracture mechanics.  however, those who ignore history are doomed to repeat it!
regards,
qshake
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a buckling stress concentration implies that the stress is compressive. as a concentration then the calculated stress would be assessed against fatigue limits rather than some design stress limit as a proportion of yield. as the stress is compressive then fatigue damage would not occur unless it was near a weld where residual stresses would occur. check the stress against fatigue limits, if they apply, and run a buckling assessment if you're worried about localised buckling.
corus
corus,
i agree with your comments on the compressive nature.  and as the situation would have it with a hole near a bearing stiffener most certainly meets the criteria for near a weld as the stiffener is welded to the web of the plate girder.  since this is one of the main support areas for the girder it is not a trivial matter.
regards,
qshake
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