Frame vs fixed end beam

[JStructsteel]

So I am checking a beam part of a two story, single bay frame, and the moment i am getting from the frame analysis is almost opposite as that of a fixed beam. Fixed beam, I am using WL2/12 at ends, and WL2/24 at center. From the frame analysis I am almost opposite that. Has anyone ever encountered that before?

 

[JAE]

Fixed ends are infinitely rigid, attracting a lot more moment. The ends of the beam have zero rotation.

Ends of a beam in a frame are attached to columns that have some amount of flexibility - thus there is rotation at the ends of the beam and this dumps more moment into the center of the beam span.



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[JStructsteel]

Thanks for the quick reply. In my case I am getting almost the opposite, WL2/12 in the center, WL2/24 at the ends. Perhaps its a fluke, but want to make sure.

I would have thought it would be a good gut check to just run a beam from a frame setup as a fixed end, and see if they were close.

im gonna check it with another program to see if I get similar reuslts.

 

[RPMG]

JAE is correct, and your analysis is probably more correct than your intuition.

End Moments:

Zero rigidity (flexure) column: 0
Perfectly rigid column: WL2/12

So your end moment should fall between 0 and WL2/12, such as WL2/24, depending on the flexural rigidity of your column.

 

[JoshPlumSE]

An experiment to conduct with your analysis. Change the size of your columns. Instead of what they are now (I'll assume 18" squares), replace them with 48" square columns..... Something way, way stiffer than what you've got now. See how the results move much closer to your perfectly fixed assumption?

Then replace them with really weak columns... Like 6" square columns. See how the moment at the mid-span of your beam moves much closer to WL^2/8?

That should give you a better feel for why the results are the way they are and how sensitive they are to the column size. There is no such thing, of course, as a perfectly pinned or perfectly fixed condition. This experiment just demonstrates it in a way that may be easier to grasp.

 

[JStructsteel]

@ Joshplum, I did what you suggested, and got the results i expected. Thanks for everyones input