Nonlinear buckling may be evaluated in SAP2000 through nonlinear analysis with using Nonlinear static analysis. This procedure takes an iterative approach while implementing P-Delta and Large-Displacement effect. The displacement of selected joints may be plotted against loading configuration to reveal softening behavior and the onset of structural bucklingStructural response is shown by plotting selected joint displacements against load application. A softening behavior may be observed in this plot, indicating the onset of buckling, and the condition of instability which follows.
Modeling tips
- Symmetric structures. When analyzing symmetrical structures, users may need to introduce either a geometric or loading imperfection should be introduced to initiate buckling.Request multiple output steps in the load case definition to improve convergence and
- Subdivision. Structural objects should be subdivided into lengths small enough to capture geometric nonlinearity. Four to eight frame or shell objects are typically necessary per span.
- Convergence. In the load-case definition, multiple output steps should be requested to improve convergence, and to better indicate buckling response. Convergence tolerance may need to be tightened, possibly to the order of 1e-6.
- Displacement control. If a structure loses load-carrying capacity, use displacement control should be implemented, rather than load control. This refers to the load-case control definition, and not how the load is actually applied.
- Users may need to tighten convergence tolerance, possibly to 1e-6.
- Elements should be small enough to capture nonlinear geometric effect. Four to eight frame or shell elements are typically necessary per span.
- When buckling-case Extreme conditions. When instability is severe, nonlinear static analysis should be converted to direct-integration time-history analysis.
Difference between
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Linear and Nonlinear buckling
- Linear (Eigenvalue) analysis. During Linear buckling analysis, perturbations are applied to the undeformed structural configuration
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- . A specified set of loads are observed for which deflections could induce instability under P-Delta effect. Linear buckling analysis produces a set of buckling factors and corresponding mode shapes.
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- When loading is multiplied by these buckling factors
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- , the resultant scaled loading conditions represent those which
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- induce buckling. Similarly, the mode shapes are normalized
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- displacement sets which indicate the configuration of the buckled
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- structure
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- .
- Nonlinear analysis. During
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- Nonlinear-static buckling analysis, the total load is applied incrementally. Stiffness and response are evaluated at each increment. Between each displacement step, stiffness may change due to the following effects:
- P-Delta effect, which involves large tensile or compressive stresses on transverse bending and shear behavior.
- Large
- -Displacement effect, in which deformed configuration is considered when assembling the equilibrium equations.
- Nonlinear material behavior,
- in which performance
- incorporates inelastic
- response. SAP2000 implements material nonlinearity
- using frame hinges and nonlinear layered-shell
- objects.
- Comparison. Because
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- Nonlinear-static buckling analysis considers material nonlinearity
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- while generating buckling response, results are often more realistic than
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- those of Linear buckling analysis. The results of
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- Nonlinear-static analysis are indicated by a plot of deformed configuration against load application. This plot displays the softening behavior which indicates the onset of buckling.
The CSI Analysis Reference Manual is an excellent resource for information on buckling. Users are We recommended to review the following chapters:
- Load Analysis Cases > Linear Buckling Analysis, page 315
- Geometric Nonlinearity > Overview, page 365
- Geometric Nonlinearity > P-Delta Effect, page 369
See Also
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- Nonlinear buckling analysisarticle