Abstract
The problem of free vibrations of the Timoshenko beam model is here addressed. A careful analysis of the governing equations allows identifying that the vibration spectrum consists of two parts, separated by a transition frequency, which, depending on the applied boundary conditions, might be itself part of the spectrum. For both parts of the spectrum, the values of natural frequencies are computed and the expressions of eigenmodes are provided. This allows to acknowledge that the nature of vibration modes changes when moving across the transition frequency. Among all possible combination of end constraints which can be applied to single-span beams, the case of a simply supported beam is considered. These theoretical results can be used as benchmarks for assessing the correctness of the numerical values provided by several numerical techniques, e.g. traditional Lagrangian-based finite element models or the newly developed isogeometric approach.
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Abbreviations
- \({\mathbf{A}}\) :
-
Coefficient matrix for the homogenous system
- \({\mathbf{X}}\) :
-
Unknown column matrix for the homogenous system
- \({\mathbf{0}}\) :
-
Right-hand side column matrix for homogeneous system
- \({A}\) :
-
Cross-sectional area
- \({A_{1}}\), \({A_{2}}\), \({A_{3}}\), \({A_{4}}\) :
-
Integration constants for \({V}\), first part of the spectrum
- \({B}\) :
-
Cross-sectional depth (and width)
- \({B_{1}}\), \({B_{2}}\), \({B_{3}}\), \({B_{4}}\) :
-
Integration constants for \({\varPhi}\), first part of the spectrum
- \({C_{1}}\), \({C_{2}}\), \({C_{3}}\), \({C_{4}}\) :
-
Integration constants for \({V}\), transition frequency
- \({D}\) :
-
Constant factor [see Eq. (B.3)]
- \({D^{\star}}\) :
-
Differential operator d/dx
- \({D_{1}}\), \({D_{2}}\), \({D_{3}}\), \({D_{4}}\) :
-
Integration constants for \({\varPhi}\), transition frequency
- \({E}\) :
-
Young’s modulus
- \({E_{1}}\), \({E_{2}}\), \({E_{3}}\), \({E_{4}}\) :
-
Integration constants for \({V}\), second part of the spectrum
- \({E_{1n}}\), \({E_{2n}}\), \({E_{3n}}\), \({E_{4n}}\) :
-
Integration constants for the \({n}\)th eigenmode
- \({F_{1}}\), \({F_{2}}\), \({F_{3}}\), \({F_{4}}\) :
-
Integration constants for \({\varPhi}\), second part of the spectrum
- \({G}\) :
-
Shear modulus
- \({H}\), \({K}\) :
-
Amplitude of eigenmodes for double eigenvalue
- \({I}\) :
-
Cross-sectional mass moment of inertia
- \({L}\) :
-
Beam length
- \({\tilde{L}}\) :
-
Special value of beam length
- \({M}\) :
-
Bending moment
- \({T}\) :
-
Shear force
- \({T^{\star}}\) :
-
Differential operator d/dt
- \({V}\) :
-
Vibration mode for transversal displacement
- \({a}\) :
-
Shear stiffness
- \({b}\) :
-
Transversal inertia
- \({\hat{b}}\), \({\hat{c}}\) :
-
Coefficients of biquadratic wave-numbers equation
- \({b^{\star}}\), \({c^{\star}}\) :
-
Coefficients of biquadratic frequency equation
- \({c}\) :
-
Bending stiffness
- \({d}\) :
-
Rotary inertia
- \({f_{\lambda}}\) :
-
Space frequency associated with wave-number \({\lambda}\)
- \({f_{\lambda_{n}}}\) :
-
Space frequency associated with the \({n}\)th vibration mode
- \({K}\), \({k_{1}}\), \({k_{2}}\) :
-
Integer values corresponding to wave-numbers of vibration modes
- \({t}\) :
-
Time variable
- \({v}\) :
-
Transversal displacement
- \({x}\) :
-
Space variable (beam abscissa)
- \({\hat{\varDelta}}\) :
-
Discriminant of wave-number equation
- \({\varDelta^{\star}}\) :
-
Discriminant of frequency equation
- \({\varPhi}\) :
-
Vibration mode for section rotation
- \({\hat{\alpha}_{1}}\) :
-
Coefficient of eigenmode for generalized wave-number
- \({\alpha_{1}}\), \({\alpha_{2}}\) :
-
Eigenmode coefficients for first/second wave-number
- \({\tilde{\alpha}_{2}}\) :
-
Eigenmode coefficient for second wave-number at transition frequency
- \({\kappa}\) :
-
Shear correction factor
- \({\hat{\lambda}_{1}}\) :
-
Generalized wave-number (first part of the spectrum)
- \({\lambda_{1}}\) :
-
First wave-number (second part of the spectrum)
- \({\lambda_{2}}\) :
-
Second wave-number (first and second part of the spectrum)
- \({\tilde{\lambda}_{2}}\) :
-
Second wave-number at transition frequency
- \({\lambda_{1}^{\star2}}\) :
-
First root (squared) of wave-numbers equations
- \({\lambda_{2}^{\star2}}\) :
-
Second root (squared) of wave-numbers equations
- \({\nu}\) :
-
Poisson’s ratio
- \({\xi}\) :
-
Dimensionless space variable (dimensionless beam abscissa)
- \({\rho}\) :
-
Beam density (mass per unit volume)
- \({\phi}\) :
-
Section rotation
- \({\omega}\) :
-
Angular frequency
- \({\tilde{\omega}}\) :
-
Angular frequency at the transition value (cutoff frequency)
- \({\omega^{\star}}\) :
-
Limiting value (upper/lower bound) for angular frequency
- \({\omega_{n}}\) :
-
Angular frequency (theoretical value) for \({n}\)th vibration mode
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Cazzani, A., Stochino, F. & Turco, E. On the whole spectrum of Timoshenko beams. Part I: a theoretical revisitation. Z. Angew. Math. Phys. 67, 24 (2016). https://doi.org/10.1007/s00033-015-0592-0
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DOI: https://doi.org/10.1007/s00033-015-0592-0