Results Interpretation: Initial vs Update Beam Length

Description

Consider the cantilever beam below, undergoing thermal contraction while being loaded by a force at the tip (Point B):

The thermal contraction is significant as it is equivalent to 20% of its initial length.

The new length of the cantilever after contraction is 80 mm.

In geometric nonlinear analysis, depending on the Beam Length option set under SOLVERS Parameters: GNL, the resulting bending moment at the fixed end of the cantilever (Point A) can be different:

The difference between the two options is not in the different node coordinates but in the different value of beam length used in the element stiffness matrix. The length term appears on the diagonal of all the moment terms in the stiffness matrix, as well as in the terms that couple axial and bending terms. This affects the force/moment equilibrium.

Initial

The beam length will not be updated, therefore the equilibrium is calculated based on the initial beam length. The moment will be 1 N × 100 mm = 100 N.mm.

Update

The beam length will be updated, therefore the equilibrium is calculated based on the expanded length. The moment will be 1 N × 80 mm = 80 N.mm.

The effect of updated beam length is particularly of interest in large axial strain problems such as those with telescopic arms, hydraulic actuators, etc.