OS-V: 0290 Lee's Frame Postbuckling

Examines the postbuckling behavior of an L-shaped frame (Lee’s frame) loaded by a vertical point load.

Large displacements are followed by a snap-through, as well as snap-back behavior and the applied load reverses sign in order to maintain equilibrium. Altair OptiStruct is used to capture the deformation of the Lee’s frame and a comparison is made with NAFEMS as a reference.

図 1. Model Details

Model Files


Benchmark Model

The frame was meshed with first order CBAR elements of size 120 mm and a rectangular cross-section of 30 mm x 20 mm was assigned. The z-translational degree of freedom (DOF) was constrained using Single Point Constraints (SPCs) for all the nodes to prevent out of plane motion, while the x and y translational DOFs were constrained using SPCs for the end nodes. The frame was loaded along the negative y-direction with a load of P = 20 kN and the problem was solved using the Arc Length Method.

The material is linear elastic.
Material Properties
Young's modulus
71.74 x 103 Nmm-2
Poisson's ratio


図 2. Comparison of Postbuckling Plot from OptiStruct with NAFEMS
Area moment of inertia of the beam
Vertical deflection

図 3. Deformation Plots


NAFEMS P10 - Finite Element Benchmarks for 2D and Axisymmetric Shells Involving Geometric Nonlinearity