Linear Buckling Analysis Section 7 Linear Buckling Analysis

Linear Buckling Analysis PAGE Theory of Buckling 3 Solution of the Eigenvalue Problem 5 Solution Sequences for Buckling and Stability Problems 7 Examples of Nonlinear Buckling 10 Rules for SOL 105 Buckling Analysis 11 Data Entries for Linear Buckling 13 EIGRL Entries 15 Example – Simple Euler Column 17 Example – Simple Euler Column -- Input File 20 Example – Simple Euler Column – Output File 21

Linear Buckling Analysis (cont.) PAGE References for Buckling and Stability Analysis 22 Workshop 9—Buckling Analysis of Plate 24 Boundary Conditions 26 Applied Loads 27 Partial Input File for Workshop 9 28 F06 Output for Workshop 9 30 Lowest Buckling Mode for Workshop 9 31 Solution for Workshop 9 32

THEORY OF BUCKLING The equilibrium equations for a structure subjected to a constant force system take the following form [ K ] { u } = { P } Include the differential stiffness effects. The differential stiffness is the stiffness [ Kd ] that results from including the higher-order terms of the strain-displacement relations. These relations are assumed to be independent of the displacements of the structure associated with an arbitrary intensity of load.

THEORY OF BUCKLING (cont.) Let l be an arbitrary scalar multiplier for another “intensity” of load. By perturbing the structure slightly at a variety of load intensities, the load intensities can be found that possess unstable equilibrium positions. This leads to the associated eigenvalue problem for buckling.

SOLUTION OF THE EIGENVALUE PROBLEM [ K – lKd ] {f} = 0 The solution is nontrivial (different from zero) only for specific values of l = li for i = 1, 2, 3,…, n That makes the matrix [ K – lKd ] singular

SOLUTION OF THE EIGENVALUE PROBLEM (cont.) To each eigenvalue li, there is a corresponding distinct eigenvector { fi }. { fi } can be scaled by any constant multiplier and still be a solution to Equation 1. The components of { fi } are real numbers.

SOLUTION SEQUENCES FOR BUCKLING AND STABILITY PROBLEM SOL 105 Linear buckling SOL 106 Nonlinear buckling Limitations of SOL 105 In prebuckled configuration: Deflections must be small. Stresses must be elastic (and linearly related to strain).

SOLUTION SEQUENCES FOR BUCKLING AND STABILITY PROBLEMS (Cont.) Example: Three classes of columns (loaded at centroid, no material imperfections)

SOLUTION SEQUENCES FOR BUCKLING AND STABILITY PROBLEMS (Cont.) Note: SOL 105 may be applicable for structures with slight material imperfections or slightly noncentric loadings (i.e., load does not align w ith centroid producing a small degree of bending). Must use engineering judgment Same arguments hold for plate structures.

EXAMPLES OF NONLINEAR BUCKLING Highly Eccentrically Loaded Column Snap-Through of Thin Shell (like the Bottom of an Oil Can)

RULES FOR SOL 105 BUCKLING ANALYSIS (For reference, see section 13 of the MSC/NASTRAN Linear Statics Users Guide) The Case Control must contain at least two subcases. Normally the first subcase is the static solution under loading. METHOD must appear in a separate subcase to select an EIGB or EIGRL entry from the Bulk Data for the buckling solution. If you have multiple static solutions, then use the STATSUB command to select the static subcase for the buckling solution.

RULES FOR SOL 105 BUCKLING ANALYSIS (Cont.) If desired, different SPC sets may be applied in the static subcase and the buckling subcase Output requests may be placed in any selected subcases. Output requests that apply to both the static solution and the buckling modes may be placed above the subcase level.

DATA ENTRIES FOR LINEAR BUCKLING Executive Control Section SOL 105 Case Control Section SUBCASE 1 LOAD = M Defines static loading condition (LOAD, TEMP, DEFORM) SUBCASE 2 METHOD = N STATSUB = i Selects eigenvalue extraction method Selects static subcase to use for buckling solution (defaults to first subcase)

DATA ENTRIES FOR LINEAR BUCKLING (Cont.) The Case Control must contain at least two subcases. Bulk Data Section Static loading condition required EIGB Eigenvalue extraction data entry or EIGLR Eigenvalue extraction data for Lanczos method

EIGRL ENTRY EIGRL Entry - recommended eigenvalue solution method Defines data needed to perform real eigenvalue or buckling analysis with the Lanczos Method. 1 2 3 4 5 6 7 8 9 10 EIGRL SID V1 V2 ND MSGLVL MAXSET SHFSCL NORM 0.1 3.2

EIGRL ENTRY (cont.) Field Contents SID Set identification number (unique integer > 0) V1, V2 Vibration analysis: Frequency range of interest Buckling analysis: l range of interest (V1 < V2, real). If all modes below a frequency are desired , set V2 to the desired frequency and leave V1 blank. It is not recommended to put 0.0 for V1, it is more efficient to use a small negative number or to leave it blank. ND Number of roots desired (integer > 0 or blank) MSGLVL Diagnostic level (integer 0 through 3 or blank) MAXSET Number of vectors in block (integer 1 through 15 or blank)

EXAMPLE - SIMPLE EULER COLUMN Problem Find the critical load and corresponding first mode buckled shape of a solid circular rod.

EXAMPLE - SIMPLE EULER COLUMN (Cont.) Theoretical Solution where Leff = effective column length = 2 x 2" for free-fixed column

EXAMPLE - SIMPLE EULER COLUMN (Cont.) MSC.Nastran Model 7 7 7

EXAMPLE - SIMPLE EULER COLUMN – INPUT FILE

EXAMPLE - SIMPLE EULER COLUMN – OUTPUT FILE First eigenvalue: Pcr = l1 x 10 lbs = 32.18 lbs First eigenvector (buckled shape)

REFERENCES FOR BUCKLING AND STABILITY ANALYSIS MSC Seminar Notes, “MSC/NASTRAN Material and Geometric Nonlinear Analysis”: MSC/NASTRAN Linear Static Analysis Users Guide, Section 13. MSC/NASTRAN Verification Problem Manual (Version 64, January 1986 Edition): Problem 3.0501A, “Lateral Buckling of a Cantilever Beam” Problem 3.0502A, “Simple Frame Analysis with Buckling” Problem 3.7701S, “Euler Buckling of a Simply Supported Beam”

REFERENCES FOR BUCKLING AND STABILITY ANALYSIS (Cont.) MSC/NASTRAN Demonstration Problem Manual (Version 64, March 1985 Edition): Under Elastic Stability Analysis, see Demonstration Problem D0504A, “Flexural Buckling of a Beam” MSC/NASTRAN Application Notes October 1978 “Buckling and Real Eigenvalue Analysis of Laminated Plates” September 1979 “Static Stability of Structures with Nonlinear Differential Stiffness” February 1982 “Elastic-Plastic Buckling of a Thin Spherical Shell” November 1985 “Nonlinear Buckling Analysis”

Buckling Analysis of Plate Workshop 9 Buckling Analysis of Plate

Workshop 9 (cont.) Model description Same plate model as in workshop 5 without the stiffeners The following boundary conditions are applied to the model: Pin at the left end Rollers at the right end Zero vertical deflections at top and bottom edges Apply 100psi compressive loads at the right edge Total loads at right edge = (100) (8) (.01) = 8 Apply 1 lb each at grid points 11 and 55 Apply 2 lbs each at grid points 22, 33, and 44

Workshop 9 -- Boundary Conditions Simply supported Supported in the Vertical Direction Supported in the Vertical Direction Supported on rollers

Workshop 9 -- Applied Loads 1 2 2 2 1

Partial Input File for Workshop 9

Partial Input File for Workshop 9 (cont.)

F06 Output for Workshop 9 Lowest Buckling Load = l1 = 1.7176 x 8 = 13.74 #

Lowest Buckling Mode for Workshop 9

Solution for Workshop 9

Solution for Workshop 9