Beam Analysis: Beam cross section view different nature of stress at different beam length

[thakurai]

Hi,
I am doing a problem of I bean subjected to uniform transverse load actiong downward on beams. Since the load is acting same (downward) through out the beam, I understand that the nature of stress should be same through out the beam i.ie. compressive at top fibers and tensile at bottom fibers. The contour plots I get from NX 11.0, shows that the stress is negative near the centre of beam length and positive near the two corners. Also when I see the beam cross section view, at element near beam centre it shows, top fibers in compression and bottom fibers in tension while when I choose an element near beam ends, it shows top fibers in tension and bottom fibers in compression.
Can anyone explain why am I getting theses kind of results or is my understanding wrong and I am supposed to get theses kind of results only.
I cross checked using few other problems also and getting the same type of results. .
I have attached the problem and NX results

 

[cowski]

Since the load is acting same (downward) through out the beam, I understand that the nature of stress should be same through out the beam i.ie. compressive at top fibers and tensile at bottom fibers.

Wrong. If the beam had roller supports at both ends, your assumption would be true; however, the fixed end supports introduce more complex behavior. Take a look at the moment diagram of the theoretical case for confirmation.

www.nxjournaling.com

 

[thakurai]

Thanks, yeah moment diagram is helpful.

Also just wanted to ask like few of my friends did the same problem but the stress results they got has stress value negative near beam ends and positive near center, and if you look at cross section view at center top fibers are in tension (+ve stress ) and bottom fibers are in compression (-ve stress) as shown in attached document. Is it possible to happen in nx, is there some explanation to why they got this results or did they do something wrong. As my results are complete opposite.