A Load at a Fixed DOF

If you apply a nodal load in the direction of a fixed DOF, will you get a reaction? The answer depends on which structural analysis software you use.

Let’s see what happens in OpenSees with a minimal working example.

All we need is one fixed node and an applied load, P. This is one of those rare cases where no element is required for an OpenSees analysis.

The script is simple, just be sure to use the UmfPack solver because there are no equations to solve.

import openseespy.opensees as ops

ops.wipe()
ops.model('basic','-ndm',1,'-ndf',1)

ops.node(1,0); ops.fix(1,1)

P = 10

ops.timeSeries('Constant',1)
ops.pattern('Plain',1,1)
ops.load(1,P)

ops.system('UmfPack')
ops.analysis('Static','-noWarnings')
ops.analyze(1)

ops.reactions()

R = ops.nodeReaction(1,1)

assert R == -P

Run the above script and the assertion should be True, i.e., a reaction is developed at the fixed DOF, equal in magnitude and opposite in direction to the applied load.