SACS does not allow users to directly input Rayleigh damping coefficients (α and β). In contrast to some finite element programs where you can explicitly define mass and stiffness-proportional damping constants, the Dynamic Response module in SACS takes a different approach.
Instead, SACS represents damping through:
1. Modal Damping (SDAMP line)
Users specify damping as a percentage of critical damping for each mode (e.g., 2%, 5%). This value is applied directly in the dynamic analysis, so rather than deriving damping from Rayleigh constants, you provide the effective modal damping that best represents the structural system.
2. Fluid Damping (FDAMP line)
For offshore applications, hydrodynamic (Morison’s equation) effects can be significant. The FDAMP option allows user to specify fluid damping ratios to capture additional energy dissipation due to water-structure interaction.
3. Default Assumptions
If SDAMP is not specified, SACS assumes default modal damping values depending on the analysis type. Users are encouraged to explicitly define damping values rather than relying on defaults, especially for fatigue and seismic studies.
If the project specifications provide Rayleigh damping coefficients (α and β), the user cannot input them directly into SACS. Instead, they will need to convert them into equivalent modal damping ratios at the frequency range of interest. For offshore structures, this is typically done at the dominant mode(s) of vibration.
For detailed usage, refer to Section 2.1.2 of the Dynamic Response Manual, which explains the SDAMP and FDAMP options and how SACS applies modal damping during dynamic analysis.