The warning indicates that the frequency range of the PSD definition is greater than that which is covered by the transfer functions. Consequently, loads are analyzed in a frequency range of the PSD for which the dynamic behavior of the structure is unknown. This warning can be ignored or modifications can be made to the input data.

But what happens if the warning is ignored? 

At the basic level, a common SPECTRAL analysis is carried out. However, for those frequency ranges for which no sampling points of the transfer functions are available, the neighboring, most recently valid sampling point of the transfer function is taken and used for the additional, missing frequencies.

It is recommended to avoid this error message by modifying input data. This entails checking the PSD definition for correctness in the first step and correcting it if necessary. If it can be assumed that the PSD definition is correct, natural frequency analysis and response analysis must be carried out at least for that frequency range for which the PSD is also defined. Figure 1 clearly illustrates this situation.

Figure 1: Definition range for PSD and transfer function and the consequences

A special point becomes evident in the graphic display of the equivalent stress PSD in the results evaluation if the warning is ignored: the originally defined frequency range of the transfer functions is displayed here. The PSD content from frequency ranges lying below or above this is added to the first or last sampling point, respectively.