Chapter 13: Problem 3
(a) Can a diagonal coefficient in a consistent mass matrix ever be negative? Explain.
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Chapter 13: Problem 3
(a) Can a diagonal coefficient in a consistent mass matrix ever be negative? Explain.
These are the key concepts you need to understand to accurately answer the question.
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The string shown is under tension \(T\) and has mass \(\rho\) per unit length. Use. [M] and \(\left[\mathbf{K}_{\sigma}\right]\) matrices associated with a cubic lateral-displacement field. Omit the conventional stiffness matrix \([\mathbf{K}]\). Solve for the natural frequencies and mode shapes of small- displacement lateral vibrations. (The exact fundamental frequency is \(\omega_{1}^{2}=\pi^{2} T / 4 \rho a^{2}\).) (a) Use one element. Nonzero d.o.f. are then \(\theta_{1}\) and \(\theta_{2}\). (b) Use two elements and impose symmetry about the center. Nonzero d.o.f. to be used are then \(\theta_{\text {end }}\) and \(w_{\text {center }}\).
Many methods of solving large eigenproblems require factoring either the stiffness matrix or a combination of the stiffness and mass matrices (e.g.) the determinant search and subspace iteration methods). Factoring requires approximately \(n_{\mathrm{eq}} b^{2} / 2\) operations (i.e., multiplications) where \(n_{\mathrm{eq}}\) is the number of equations and \(b\) is the semibandwidth. For full matrices, the number of operations is about \(n_{e q}^{3} / 6 .\) Consider a system of 5000 equations with \(b=500\). If this system of equations is partitioned into \(m\) master and \(s\) slave d.o.f., what must \(m\) be so that factoring the condensed (full) system is no more expensive than factoring the original (banded) system? What if \(b=100\) instead?
Show that the mode acceleration method reduces to the mode displacement method if the structure moves freely-that is, with \(\left\\{\mathrm{R}^{e \times t}\right\\}=\\{0\\}\).
Model a simply supported beam by a single element. Let \(L=1.0 \mathrm{~m}, A=\) \(0.0002 \mathrm{~m}^{2}, E I=300.0 \mathrm{~N} \cdot \mathrm{m}^{2}\), and \(\rho=2100.0 \mathrm{~kg} / \mathrm{m}^{3}\). Impose symmetry (and reduce the problem to a single d.o.f.) by setting \(\theta_{2}=-\theta_{1}\). (a) Determine the fundamental frequency \(\omega_{1}\) if there is no axial force. (b) Determine the axial force that makes the frequency \(347 \mathrm{rad} / \mathrm{sec}\). (c) Determine the frequency if the axial force is \(1200 \mathrm{~N}\) in compression.
Consider axial vibrations of a uniform bar of length \(L\) and mass \(m=\rho A L\), free at one end and fixed at the other. Using two-node bar elements, model the bar first by one element, then by two elements of equal length \(L / 2\). In each case, compute the lowest natural frequency using (a) the consistent mass matrix [m]. (b) the lumped mass matrix \([\mathrm{m}]\). (c) the average mass matrix \(([\mathrm{m}]+[\mathrm{m}]) / 2\). The exact lowest natural frequency is \(\omega_{1}=(\pi / 2 L) \sqrt{E / \rho}\).
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