Element sign convention

[davidnoc]

Hi all,

I am new to FEA field.I prefer doing classical hand solution.

I extracted force and moment from Cbar/Cbeam element from post processor. But I do not know whether moment I got is creating tension or compression on the top fiber of the beam. plsease help me with this.

Thank you all

 

[gwolf2]

From the element names, sounds like you are using NASTRAN. Assuming this is so:

1) If you are extracting forces and moments in a post-processor you need to know whether these are internal element forces, external element forces, or a true free body force. Depends which post processor and which buttons you pressed.

2) Whatever the force type, you need to know the element local coordinate system. It is defined as local x is node 1 to node 2 and local y/z via the orientation vector. This is illustrated in the bulk data section of the NASTRAN user guide.

Combine 1) + 2) to get answer.

If you can indicate which pre/post processor you are using you may get some more detailed responses.

 

[davidnoc]

Hi,

Thanks for the reply.I am using Nastarn.

FEADMS(Boeings in house software) is my pre/post processor.

To answer ur question I am extracting internal loads for these 1D elements. When I click shown axis button in pre processor for these C bar elements I can see X axis from node 1 to 2 and similarty I can see Y and Z axis. So when I extract the load I got all axial, shear and moment1 and moment2 but I do not know teh oreaintation of these moments.

thanks

 

[gwolf2]

This is illustrated in the bulk data section of the NASTRAN user guide. Use the right hand rule on the axes shown to interpret the orientation of the moments.

 

[davidnoc]

Hi,
I saw the bulk data section of the NASTRAN user guid, its little bit confusing.It says Element X- axis is always from grid A to B. But when they show element forces in plane 1, Fx is action as tension on the element. So both axial and shear force and moment shown in these plane 1 and plane 2 are they in the posative X , Y amd Z direction.
becasue for X it contardicts.

thanks

 

[rb1957]

if it's a Boeing in-house code, check the Boeing manual.

generally with 1D elements either you define an element axis system by referencing a 3rd point or the element assumes global axes.

 

[gwolf2]

I don't have a NASTRAN manual to hand and I wouldn't trust my memory. You'll have to make a simple test model and figure it out.