Section moments in Abaqus 1

[mawb]

thread799-188299

Hi.

I am having some trouble with determining the SM1 og SM2 in Abaqus for a shell element.

From ABAQUS Analysis User's Manual:
"SM1 Bending moment force per unit width about local 2-axis.
SM2 Bending moment force per unit width about local 1-axis"

The way I read this, SM1 is the bending moment about local 2-axis. When I do some simple tests in Abaqus, I would guess from the results that SM1 was bending about local 1-axis. Am I reading the user manual wrong, or am I doing the tests wrong somehow?

Thank you for your help.

 

[Shz713]

Turn on local axes from ODB display option to know the direction; of course SM1 is about local axis 2

Shoot for the Moon, even if U miss, U still land among Stars!

 

[mawb]

Could you please take a look at the pictures attached?
It may be that my basic understanding is missing, but I find these results strange when following the definition from the manual.

Thank you for helping me out.

 

[Shz713]

In your picture, moment should be about Y axis since it is transverse to the element, your vertical force (shear force) is in Z direction and axial force in X direction.
So now if you want to perfectly bend the element, then you need moment at two ends, namely you need moment in Y direction to bend the element. Also try to plot the results along a path line to visualize the results.

Shoot for the Moon, even if U miss, U still land among Stars!

 

[mawb]

I am writing some basic theory about the local axis and its connection to SM1,SM2 and SM3.
I am agreeing with you when you describe the situation in x-y-z coordinates. But when using the results to compare with the local axis the sentence "SM1 Bending moment force per unit width about local 2-axis." is throwing me off.
As shown in the picture, the local 2-axis is the same direction as x-direction. Therefore, I would assume from the manual description that SM1 is about x-axis. This is not what the results shows.

Again, thanks for helping me out.

 

[Shz713]

I suggest you compare the theoretical results with frame elements (beam elements in Abaqus), there you get sense of local axes direction.
Also in your figure, why there is high intensity for SM2 near supports? Do the following to rectify your confusion:
redraw the model with normal axis system (default coordinates that Y is in direction of gravity)
use S4R, first get DL and natural frequencies and check with hand calcs to see if your model is sound. Then, apply a point load (something that easier to check against hand calc) and plot the output.

Shoot for the Moon, even if U miss, U still land among Stars!

 

[mawb]

For the beam elements the local axis direction makes sence. The problem occurs when I look at shell element.
I tried to redraw the model, and was not able to draw it with y-direction in gravity direction because of the drawing-grid. However, I flipped the shell so that global y-direction was in line with the gravity. This is shown in the attached picture along with results for SM1 and SM2 and a overview of the local axis system 1-2-n. The same thing happens, I get largest SM1 about local 1-axis which is not right according to the manual "SM1 Bending moment force per unit width about local 2-axis".

 

[mawb]

I have now created two different models with the exact same properties, loading,boundary conditions etc. The only thing I changed was the local axis system. See picture attached. For one model I lined local 1-axis with global x-axis, and for the other model lined local 1-axis with global y-axis. The results for SM1 was exactly the same for both models. See picture attached. I still can not understand how the manual can say that SM1 is set by the local axis system for the shell element.

Your help is appreciated.

 

[Shz713]

Mawb,
your confusing yourself, you need to know that rotating an element does not change its output; the manuals says in the direction of local axes. Have a look at below links which describe basic of outputs for stress (it is for SAP2000 but concept is same and easier description for you to follow)

http://docs.csiamerica.com/help-fil...tress_Diagrams/Shell_Forces_Stresses_Form.htm

http://docs.csiamerica.com/help-files/etabs/Output_Conventions/Shell_Element_Internal_Stresses.htm

Shoot for the Moon, even if U miss, U still land among Stars!

 

[mawb]

I will check it out.
Thank you.