ETABS에서 TMD를 Link요소로 모델링하고 해석해보니 Ritz해석에서 저런 경고메세지가 나오는데 무슨 말인거죠?

[Warning]
For starting load vector: Link/Def = ~6/U1
Load is applied to one or more massless degrees of freedom,
corresponding Ritz modes may be unavailable or inaccuate for dynamics.

[Ref]
https://wiki.csiamerica.com/display/kb/Modal+analysis+FAQ

[Ans]
What is the meaning of the warning message: Loads are applied to massless degrees of freedom?
Extended Question: During Ritz-vector modal analysis, I received a warning message which states: Loads are applied to one or more massless degrees of freedom, corresponding Ritz modes may be unavailable or inaccurate for dynamics. What does this mean, can it be ignored, and if not, how should I revise the model?

Answer: This message indicates that load is applied to a joint which either has no mass or is restrained. Gravity load from structural self-weight creates a set of fixed-end forces which includes shear and moment. These forces will also impose load on massless degrees-of-freedom (DOF). In a structure with frame or shell objects, each joint has up to six DOF, including translations (UX, UY, UZ) and rotations (RX, RY, RZ). Mass from the material density of structural objects is assigned to translational DOF. Additional mass may be assigned to any DOF, including rotational DOF such that rotational inertia is captured.

Ritz-vector application during dynamic loading is most effective when all applied loads act on DOF with mass. While earthquake loads automatically act on DOF with mass, mass should be assigned to DOF subjected to other types of load, and to nonlinear NLLink elements, which are treated as external force generators.

It is not necessary for loaded DOF to have mass when Ritz vectors are generated for gravity and other static loading conditions applied during time-history analysis because load is applied so slowly that dynamic behavior is not important. In this case, either the warning may be ignored, or a small mass may be assigned to DOF in question.

Additional information is available in the CSI Analysis Reference Manual (Modal Analysis > Starting Load Vectors, page 331).