Classical response spectrum analysis is based on the idea that you have a primary structure to which some transient excitation is applied. The primary structure forms a filter between the applied transient excitation and some response point on that primary structure, from which spectra (frequency domain) curves may be extracted. There is an interface in Nastran to do this where a virtual vibration table is created (by the software) at the one or more response points that you choose for a set of discrete frequencies that you also choose. When you generate the spectra, you may choose whether they are generated in an inertial frame of reference (absolute) or one relative to the response point. In other words, the spectra you generate, which may be displacement, velocity or acceleration spectra, are output including the movement of the response point or relative to it. These are called absolute and relative spectra.
You may not be the person who generated these spectra, but you should know in which frame of reference they were created. This distinction is necessary because the classical response spectrum method uses the Duhamel integral method to establish the (approximate) relationship between displacement, velocity and acceleration with the corollary that displacement and velocity are relative, but acceleration is absolute; the approximation is too wide of the mark at very low and very high frequencies.
When you move to use these spectra as inputs to a response spectrum analysis, you do not model the primary structure, it is a boundary condition to the response spectrum analysis; what you model are “appendages” to the primary structure where it is assumed that the presence or absence of these “appendages” has little effect on the behaviour of the primary structure. As you now know whether the input spectra are absolute or relative, this will govern your choice of whether to include the rigid body modes in the response spectrum calculation or not. For absolute applied spectra, the rigid body modes must be included (not just computed) in the analysis. For relative applied spectra, the rigid body modes are still computed, but they are filtered out (with a device like PARAM,LFREQ).
If you are using the large mass method for the response spectrum calculation, the point selected for base motion is free to move, but it will only move if you included the rigid body modes and you should be applying absolute spectra in this case.
Now in DDAM analysis, the assumption that “appendages” do not affect the behaviour of the primary structure is rejected. This follows the studies made by the US Navy during and after world war II where certain materiel, which should have survived under water explosion events, did not. It was determined that the degree to which the “appendages” interacted with the primary structure (and other appendages for that matter) was dependent on their location in the sea vessel (bulk head, deck, hull,…) as well as the type of sea vessel (ship, submarine,…) in which the materiel was installed. Consequently, DDAM analysis uses a set of input coefficients which depend on the configuration of the setup being analyzed. The latest set of these coefficients is confidential, and if you want to use them you have to license them from the US Navy and make sure they are installed on a secure server. Coefficients previously considered confidential have been declassified and released in a publication, but they apply to obsolete hardware.
The DDAM coefficients are applied to the input spectra you provide, which are relative velocity and absolute acceleration spectra if I remember correctly, and I think the spectrum it outputs, and that you eventually apply to your “appendage”, is always absolute acceleration; the coefficients will determine from which of your input spectra (velocity or acceleration) the output spectrum to apply was computed and how. Consequently, when using DDAM, you should always retain the rigid body modes in your analysis; if you have a large mass at the base motion point, it should then move with the acceleration of the applied spectrum. The response points in the appendage, when the rigid body modes are retained, will exhibit absolute acceleration.
DG
