RS Base Excitation loads are used exclusively in response spectrum analyses to provide excitation in terms of a spectrum. For each spectrum value, there is one corresponding frequency. Use the Boundary Condition setting in the Details pane to apply an excitation to all of the fixed supports that were applied in the prerequisite modal analysis.
Note: Only fixed DOFs of the supports are valid for excitations.
You can also specify the excitation in a given direction (X Axis, Y Axis, or Z Axis).
The user-defined RS data table is created in the Tabular Data window. You can create a new RS table or import one from a library that you have created, via the fly-out of the Load Data option in the Details pane.
Note: Only positive table values can be used when defining this load.
Three types of base excitation are supported:
RS Acceleration
RS Velocity
RS Displacement
You should specify the direction of the RS base excitation in the global Cartesian system.
You can apply multiple response spectrum excitations as long as they are of the same type (acceleration, displacement, or velocity). Typically, you apply the same type of RS excitation in each coordinate direction (X, Y, and Z).
The following additional settings are included in the Details pane of an RS Base Excitation load:
Scale Factor: Scales the entire table of input excitation spectrum for a Single Point response spectrum. The factor must be greater than 0.0. The default is 1.0.
Missing Mass Effect: Set to Yes to include the contribution of high frequency modes in the total response calculation. Including these modes is normally required for nuclear power plant design.
The responses contributed by frequency modes higher than those of rigid responses, specifically frequency modes beyond Zero Period Acceleration (ZPA) are called residual rigid responses. The frequency modes beyond ZPA are defined as frequency modes at which the spectral acceleration returns to the Zero Period Acceleration. In some applications, especially in the nuclear power plant industry, it is critical and required to include the residual rigid responses to the total responses. Ignoring the residual rigid responses will result in an underestimation of responses in the vicinity of supports. There are two methods available to calculate residual rigid responses: the Missing Mass and Static ZPA methods. The Missing Mass method is named based on the fact that the mass associated with the frequency modes higher than that of ZPA are missing from the analysis. As a result, the residual rigid responses are sometimes referred to missing mass responses. When set to Yes, the Missing Mass Effect is used in a response spectrum analysis.
Note: If the Missing Mass Effect property is set to for an excitation, then all excitations must have the property set to .
Rigid Response Effect: Set to Yes to include rigid responses to the total response calculation. Rigid responses normally occur in the frequency range that is lower than that of missing mass responses, but higher than that of periodic responses.
In many cases, it is impractical and difficult to accurately calculate all natural frequencies and mode shapes for use in the response spectrum evaluation. For high-frequency modes, rigid responses basically predominate. To compensate for the contribution of higher modes to the responses, the rigid responses are combined algebraically to the periodic responses, which occur in the low-frequency modes that are calculated using one the methods above. The most widely adopted methods to calculate the rigid responses are the Gupta and Lindley-Yow methods. These two methods are available for a response spectrum analysis under Rigid Response Effect Type when Rigid Response Effect is set to Yes.