SAP2000 - Nonlinear Buckling Analysis

Linear Buckling Analysis predicts the theoretical buckling strength of a structure which is idealized as elastic. For the undeformed structural configuration, structural eigenvalues are computed from boundary conditions and a specified set of loads. Linear buckling analysis produces a set of buckling factors. When loading is multiplied by these factors, the resultant scaled loading conditions represent those which induce buckling.

In a real structure, imperfections and nonlinear behavior keep the system from achieving this theoretical buckling strength, leading Linear Buckling Analysis to over-predict buckling load. Therefore, to predict the “real” buckling load, we recommend Nonlinear Buckling Analysis.

During Nonlinear Buckling Analysis, the load is applied incrementally until a small change in load level causes a large change in displacement. This condition indicates that a structure has become unstable. Nonlinear buckling analysis is a static method which accounts for material and geometric nonlinearities, load perturbations, geometric imperfections, and gaps. Either a small destabilizing load or an initial imperfection is necessary to initiate the solution of a desired buckling mode. Stiffness and response are evaluated at each load increment. Between each step, stiffness may change due to the following effects: 
- P-Delta: effect of large tensile or compressive stresses on transverse bending and shear behavior.
- Large-Displacements: deformed configuration is considered when assembling the equilibrium equations.
- Inelastic behavior: material nonlinearity of layered-shell objects, frame hinges and T/C limits, and non-linear behavior of link elements.

The results of Nonlinear-static buckling analysis can be indicated by a plot of deformed configuration against load application.