Seismosoft | Earthquake Engineering Solutions

I am analyzing a single-story single-bay frame element applying a horizontal static force on the beam-column joint.

When I am using the infrmFB element, I notice that the horizontal displacement of the opposite beam node has a slightly larger value, i.e. the beam seems to increase its length.
Is this expected behavior when the beam is under compression?

When running the same model with linear elastic properties, the beam length is slightly reduced as expected.

Could this be attributed to different resulting fiber lengths at the section due to moment-axial force combination, that results in a decrease of the specific fiber length at the centroid where the beam nodes are presumably located?

I isolated this behavior from a more complex problem I am trying to solve that I posted elsewhere in the forum. Sorry if this is some kind of rule violation.

Hi greser,

Your deductions do seem to make sense to me.

Indeed, since the neutral axis depth of a reinforced concrete beam under flexure (flexure caused by the distributed loading on the beam and/or by the end-rotation imposed by the columns) is less that half of the beam's section depth, then one does tend to observe an apparent increase of the beam's length (since the section's centroid is under tension).

Rui

Dear Rui thanks a lot for your response!

Based on your answer, I am therefore correct to assume that the "Structural Displacements" presented in the "Global Response Parameters" refer to the end-nodes of a specific fiber located at the centroid of the beam.

Therefore, I cannot use a simple configuration of two links active only in one direction, in order to impose a compression-only loading on the beam. This concept would have be based on the assumption that beam under axial compression has a reduced length, which is not true in this case.

Thanks again for a truly remarkable program and for the chance to solve/discuss our problems using this forum.

greser, yes, you are correct on all counts:

- displacements at a beam end-nodes must inevitably be related to the deformations at the beam's sectional centroid

- if a RC beam subjected to compression forces is also subjected to bending, then axial extension may indeed be observed at its centroid (depending of course on the level of bending)

- SeismoStruct is a great program, its Technical Support team is very helpful, as are some Forum users, also very knowledgeable..

Rui