Learning Forum

[2020221038]

I want to calculate the force/moment summations of all elements on a specific cross-section in the local coordinate system.I am thinking of first defining the points at which to sum the moments in the global coordinate system, then switching the resultant coordinate system to the local coordinate system, and finally calculating the resultant force. Would this approach work? csys, 0 spoint,,x,y,z rsys, local coordinates fsum, rsys

[dlooman]

FSUM will give you the sum of the element forces.  If you want the individual forces use NFORCE (after RSYS) instead.  Is it important to know the moments?  NFORCE won't provide that.  If that's important you could use FSUM with only one node selected.

[2020221038]

Thank you very much for your answer. My wording was unclear, so I’ve revised the question.I want to calculate the force/moment summations of all elements on a cross-section in a local coordinate system, rather than the internal force of a single element.

[dlooman]

Then FSUM is the correct command.  You would select the elements and their nodes.  Typically the set of elements is on one side of the cross-section.  

[2020221038]

Thank you for your reply. However, I want to calculate the force/moment summations of all elements on a specific cross-section in the local coordinate system.First, I select the elements and nodes, then switch to the global coordinate system to determine the point of application of the resultant force, and finally switch the resultant coordinate system to the local coordinate system to calculate the sum.Is the following command correct?

csys, 0
spoint,,x,y,z
rsys, loc
fsum, rsys

[dlooman]

Could you describe the model, especially the types of loads and boundary conditions?  FSUM works differently with pressure vs nodal constraints and loads.

[2020221038]

Similar to a single-box, single-cell concrete box girder, the force/moment summations of the cross-section need to be determined under a uniformly distributed load.

[dlooman]

It might be helpful to run a test on a simple model with a known solution.

[dlooman]

The time-tested method of getting the resultant forces at a section is to select the elements on one side of the cross-section and the nodes at the end of the selected elements as shown below.