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.

Figure 1. Model Details

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


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

Figure 3. Deformation Plots

Model Files

Refer to Access the Model Files to download the required model file(s).

The model file used in this example includes:



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