Identifying Mixed Modes |
 
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Identifying Mixed Modes |
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The “rules” described in the last section enable Modes to exclude the majority of non-bending modes from SHEAR7 output. However it is difficult to ensure categorically that all modes routed to the common files are bending modes. This is particularly true when you request a large number of solution eigenpairs, and specify that most or all should be considered for SHEAR7 output, which is often the case as VIV typically excites the higher modes of vibration. The modes which Modes cannot easily identify are called mixed modes, because they are combinations of, typically, bending and axial or bending and torsion. For example, in-plane SCR modes may be pure bending or may combine bending and axial displacements; while out of plane modes may combine displacements and rotations due to torque.
Modes does produce output to help you decide if SHEAR7 output includes mixed modes. This is based on the fact that a plot of mode number v maximum modal curvature will be monotonically increasing if only bending modes are considered, but that mixed modes will cause local maxima or “spikes” in such a plot. So when you request SHEAR7 output, Modes automatically produces one (TTR and User options) or two (SCR option) plot files graphing maximum curvature as a function of mode number.
It is generally advisable to also visually inspect the modal response manually. The procedure is relatively straightforward – you simply open two views of the structure simultaneously in the Model View. One should be a front elevation (i.e. a view normal to the plane of the SCR) and the other an end elevation (i.e. a view parallel to the plane of the SCR). Pure in-plane bending modes will be visible in the front elevation view, and pure out of plane bending modes will be visible in the end elevation view. Additionally, if there are any combined modes present, these will contain some combination of bending, axial and torsional deformation. In a pure bending mode, all of the nodes of the mode (as opposed to the nodes of the finite element model) will be stationary, so a standing wave effect will be exhibited. In a coupled mode, there will be modal nodes which are not stationary, as there will be an axial or torsional response component present also.
If on examining the modal results you conclude your SHEAR7 data does include mixed modes, then an obvious question is what to do about them. One option of course would be to manually edit the SHEAR7 file or files to remove them. A better solution though is to use the options provided by Modes for excluding specific modes from SHEAR7 output. In the case of a TTR, you simply list the modes you want to exclude. For the case of an SCR, you nominate in-plane and out of plane modes to exclude separately.
Modes includes two further options in relation to manipulating SHEAR7 output. The first of these is an option to “include” modes that Modes would otherwise have excluded (in the case of an SCR, you nominate in-plane and out of plane modes to include separately). This facility is provided because very occasionally the rules described above which Modes applies to identify mixed modes lead it to excluding a genuine bending mode or modes. The second is an option to “replace” modes. Normally when a mode is excluded from SHEAR7 output, both the natural frequency and the modal displacements and curvatures are eliminated. However you have the option of replacing rather than excluding a mode; the natural frequency of a replaced mode is retained in the SHEAR7 output, but its mode shape and curvature distribution are replaced by those of another mode you specify.
•*EXCLUDE MODES ($MODES section) is used to specify modes to be excluded from SHEAR7 output.
•*INCLUDE MODES ($MODES section) is used to specify modes to be included in SHEAR7 output.
•*REPLACE MODES ($MODES section) is used to specify modes to be replaced in SHEAR7 output.