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.

 

[rb1957]

fixed end moments ... your beam is a double cantilever, with -ve moments at the ends.

another day in paradise, or is paradise one day closer ?

 

[BlasMolero]

Dear Thakurai,
When you plot results in CBEAM elements is tricky, please remember that this is a 1-D line element, and the stress is CONSTANT in the element, beams are not discrete elements. For instance, the plot we have in FEMAP is combined stress "axial + bending" at End_A or End_B of each element. The maximum value is 0.00169 MPa. You can plot also stresses at recovery points, ie, in the corner points.

In FEMAP we have a postprocessing utility named BEAM CROSS SECTION STRESS CONTROL that computes the local stress distribution in the cross section of CBEAM elements based in the resulting beam forces & moments computed at each element by ANY FEA solver (not only NX NASTRAN). This way you can plot vonMises stress, axial stress, shear stress, etc.. and see the stress distribution in the beam cross section at any or ALL elements simultaneosly, at the beginning, at the end, middle, etc.. The stress distribution according to vonMises all values are positive (square root of sum of squares).

Best regards,
Blas.

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Blas Molero Hidalgo
Ingeniero Industrial
Director

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