Roarks formula for a square plate

[HanStrulo]

I have a square plate that is simply supported on 4 edges and has a load in the middle over a small cut out.

I calculate the max stress as per table 11.4 case 1b.

All is good till now but i am not sure what to compare it with. should i compare with Fy of the plate (yield strength)? or Fu of the plate (ultimate tensile strength)? or a factor times the ultimate strength?

is there a chapter in the AISC or CISC that gives those values for plates.

Sorry if this is a dumb question.

Thanks,

 

[driftLimiter]

Roark's tables are for the elastic stress solution so only the extreme fibers of the plate in bending would have the peak stress. Its analgous to the yield moment Fy * S. I would go to AISC chapter F and use Fy as the nominal bending stress, then reduce it by LRFD or ASD factors and compare with roarks (you must also appropriately adjust the load for whatever design method you choose).

You could also investigate Roarks section 11.13 which would provide you with a solution closer to the Fy * Z.

The observation that must occur is that the plate is not going to be subject to instabilities due to bending, then the only remaining limit state is yielding. If your plate (somehow oddly proportioned) could be subject to instabilities then you would seek out a reduced stress lower than Fy, but I doubt this can apply for a normal plate.

 

[HanStrulo]

Thank you for answering.
i looked at section 11.13. for rectangular plates it has the following formula for the load to induce collapse.

I understand from the table that I could go to a load more than the elastic load by a factor of at least 5 before collapsing if my plate is ductile. is my understanding correct?

By that logic, I could use a lower thickness of the plate and then check the load I get using the formulas. If the new load is less than 5times the old load (with a higher plate thickness), then my plate would not collapse.

Do you think it's a safe assumption?

 

[driftLimiter]

I would compare stress from the table with ASD loading. If you choose to use the collapse load as the nominal strength then perhaps LRFD is more appropriate.