Strand7 Beam-columns with applied moment and axial load 1

[ss2013]

Hi,

I am trying to model a beam-column in Strand7 that is simply supported, and has applied compressive axial load and equal and opposite end moments at each end.
After I run the linear static analysis I get the warning 6 (Node force/moment component at node xxxx is ignored) for each of the four applied loads. So the analysis is not considering any of the loads.

Does anyone know why this is happening and how to fix it?

Many thanks.

 

[Denial]

As a quick guess, I'd suspect some sort of incompatibility between your nodal restraints and your applied nodal loads (and/or nodal moments).[ ] For example.[ ] If your beam runs parallel to the X axis then you presumably have an X restraint at one (but not at both) of your end nodes.[ ] If you then apply equal and opposite X forces to those end nodes to model the axial force in the member, one of those X forces is applied directly into a restrained degree of freedom.[ ] This will trigger the warning you are reporting.[ ] But it is only a warning:[ ] the load component concerned is ignored (because it should not have been included in the first place), the analysis continues, and the results obtained will still be correct.

 

[IDS]

In addition to Denial's comments, check the axis of your applied moment, and your global restraints. If the beam is simply supported the applied moments should not be restrained.

If you would like to upload the model I will have a look at it.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[ss2013]

Thanks. I have attached the file.
I have modeled the beam using plate elements.

 

[IDS]

You have restrained all the nodes at both ends in the Z direction, so there cannot be any axial shortening, and any applied load will go straight into the restraints.

Restraining every node will also stop the end faces from rotating, so any applied moments will be fully restrained. I suggest you adjust the restraints as follows.

Restrain the bottom flange at both ends in the Y direction.
Restrain the bottom node under the web at one end in all three directions.
Restrain the bottom node under the web at the other end in the X and Y directions (leaving Z free).

That will provide simply supported conditions.

The middle web plates at each end, where you have subdivided them, are not properly connected to the rest of the model. You should replace them with a single plate at each end, connected to other plates at each node.

Finally the transverse axis is X, so you should apply the moment as MX, not MZ, but I would suggest applying the loads as plate edge pressures, to the top and bottom flange anyway.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[ss2013]

Thank you for your feedback.
How does the model look now?

 

[IDS]

It looks OK for the longitudinal loads.

For the moments, create a second load case, and apply just the moments, then have a look at the resulting stresses and deflections.

It would be a good idea to do some background reading to work out what is going on.

I'm not trying to be unhelpful here, but you will learn more if you play with the alternative ways of applying the load yourself.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[ss2013]

I appreciate the advice Doug. Cheers

 

[ss2013]

I'm still having trouble applying the end moments. I have tried to apply them as plate point moments and node moments, and in different load cases. When I view the results in the load case containing only the moment there the model ignores the moment and there is no stresses in the beam. I have compared the results and it seems that the presence of the moment is still having no influence on the results.

 

[IDS]

When I view the results in the load case containing only the moment there the model ignores the moment and there is no stresses in the beam. I have compared the results and it seems that the presence of the moment is still having no influence on the results.

That's why I suggested you separate out the moment loads in a second load case

Unlike beam elements, the in-plane strains of plate elements are defined by the translational deflections of the nodes. They do not have an in-plane curvature, so you cannot apply in-plane moments as node loads or point loads. You can apply a moment as equal and opposite nodal loads, but this will cause large local strains if you apply them to the web. You could apply equal and opposite loads to the flanges, or create rigid links to connect the node at the centre of the web to the nodes at the centres of the flanges, and apply the moment to the central node. The rigid links will then create a couple with loads applied to the top and bottom nodes.

With your units, your moment of 1000 Nmm is equivalent to a 1000N load with an eccentricity of 1 mm, so the effect of the moment will be very small compared with the axial load.

Also note that your resultant axial load (in load case 1) is applied at mid height, but the beam is restrained at the base, so the axial load will generate a moment.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/