To determine whether a structure is rigid or flexible, the fundamental natural frequency can be either calculated based on approximate equations given in Section 26.11.3 or it can be calculated from an Eigenvalue analysis.

Equations (26.11-2), (26.11-3), (26.11-4), and (26.11-5) are not implemented. But calculated values based on these equations can be directly entered to the program.

Limitations imposed on calculated values of approximate natural frequencies as mentioned in Section 26.11.2.1 are not implemented.