Different Deflection when adding a node. 1

[captb]

I was trying to figure out what was wrong with my model so i tried modelling a simply supported beam and i got the same results. I modeled a simply supported I-beam with uniformly distributed load and I am getting the exact deflection as we would expect, 5wl^4/384EI. Now when I either add a 1 or more nodes, or model the same beam as multiple segments I am getting a deflection about 3-4% higher deflection. The moment and shear values are identical with all beams, only the displacement is different. This is happening with both Risa 2d and 3d. Can someone kindly explain what is going on here?
Thanks

Here are my inputs.
member: 5Wx16
w: 5kip/ft
L: 10ft
(everything else is default)

 

[JoshPlumSE]

It's almost certainly due to shear deformation. RISA (by default) includes shear deformation in all nodal deflections. But, when the program is interpolating between known values on a a beam it uses simple beam theory (which neglects shear deformation).

To prove that this is indeed the culprit, all you have to do is go to the Global parameters (solution tab) and un-check the box that tells the program to include shear deformation.

 

[captb]

Problem solved. Thanks JoshPlum!