Axial force in forceBeamColumn fiber beams

[ChrisS]

I'm having trouble understanding the behavior of a RC structure I have modeled with forceBeamColumn elements with fiber sections. The beams lie on a rigid diaphragm and I would expect relatively small axial forces to appear. Analysis results show substantial axial forces.

I did a simple test: A single beam fixed at one side, roller support at the other. When I apply displacement controlled moment at the hinge, the roller moves. When I constrain the axial movement substantial axial forces appear.

I run the advanced example 8 (http://opensees.berkeley.edu/wiki/index ... _W-Section). I added the line
recorder Element -file $dataDir/FBeam1.out -time -ele 1020101 localForce;
at file Ex8.genericFrame3D.build.RCsec.tcl. The beam has axial force 338 kips at the end of pushover.

Please share your suggestions.

[fmk]

it is a well documented issue when using the force beam column that if a 0 deformation axial constraint is imposed large axial forces can result for sections in which bending of (typically concrete sections) causes the neutral axis to shift and thus introduces axial deformation. In the unconstrained problem this is fine, in the constrained problem additional forces are needed to impose the 0 axial deformation constraint.

http://opensees.berkeley.edu/wiki/index ... r_Elements

the soln is to use a penalty method to impose the constraint or use a disp based elements for the beams.