ANSYS not matching Hand Calculations - simple structure

[AFish66]

All,

I am having a hard time believing the ANSYS results that I am getting for a simple structure. Long story short, I am calculating around 11ksi bending stress for the attached structure, but when I run the ANSYS model with the same load, I am seeing stress levels that are almost double that. The rectangular stock is 9"x3"x1/2". Thoughts?

 

[rb1957]

the beam is cantilevered on the LH end, and has a sliding constraint on the RH, yes? so the RH end is reacting axial forces as the beam tries to bend.

a 0.5" hesh isn't that fine, only one element through the thickness.

not sure why the bending stresses are increasing towards the RH end ? unless there's a sizable moment at the constraint ...

Quando Omni Flunkus Moritati

 

[AFish66]

Sorry, I should have explained better. Only half of the actual structure is modeled - I simplified the ANSYS model for quicker results, that is the reason for the frictionless support on the ends of the rectangular tubes - from what I understand, this is how you treat something symmetrical, correct? I have refined the mesh through the wall thickness of the retangular tubing through 3 even divisions. The rest is a 0.5" mesh.

 

[inline6]

if you have symmetry did you halve the load? the way it is modelled the simply supported beam sees 2 x 38,000 lb in a 4 point bend configuration.

what are beam particulars to allow someone to verify your hand calc?

when you did the hand calc what restraint did you use at the fixed support (simple support of built in)? it we be closer to a simply supported as compared to a fixed

 

[Bobfromoh]

Maybe you considered this already but since the model is
symmetrical, ANSYS is probably doubling the load.
Also, I don't see this being a simple hand calculation?

 

[rb1957]

sure, model a symmetric boundary, for solid elements this'll be an axial force constraint only ... ok.

but symmetric model also means symmetric loads ... yes ?

Quando Omni Flunkus Moritati

 

[AFish66]

Thanks for all of the fast replies! Yes, the loads are symmetric, so I have halved them, which results in the 38kip per side. The hand calculation was a simple bending stress = Mc/I. My moment is created by the 38kip load at a 33" distance. The I is 80.8 in^4 for the 9x3x1/2" rectangular tube, and is multiplied by 2 since there are two tubes. This equates to around 11.7 ksi. I know that this is extremely simplified, but I did not expect to see stress levels almost twice that in my ANSYS model.

Also, as an update, I have run the full structure in ANSYS, and the results agree with the 1/2 structure that I have shown initially. The stress is more in the 20-22ksi range.

If the ANSYS is accurate, then I will have to improve this design drastically to meet our internal design standards, which is based off of UTS/5- Assuming A36 steel, this is 11.6ksi.

 

[btrueblood]

As somebody asked earlier, are you treating the left-hand side of the beam as a fixed support in your hand calc? Note that in the Ansys model, you have quite a significant rotation of the main beam section at that location, closer to a simply supported end. Suggest re-doing the hand calc using simple support ends, should result in a ~2x increase in bending moment.

 

[rb1957]

funny, for SS beam (i don't think your model is showing much fixity) i get 29ksi ...
M = 38000*33 ... yes?
I = 97in4 ... for a single tube
stress = 1254000*4.5/(2*97) = 29ksi

Quando Omni Flunkus Moritati

 

[AFish66]

Problem solved! I had my centroid wrong..... whoops. What a day. Thanks again for everyone's help.

 

[inline6]

i got 29.1 ksi with a ANSYS beam model

 

[inline6]

yeah simplest answer is usually correct