Seismic Input in LS-DYNA
Effective seismic input has been implemented in LS-DYNA, with INTERFACE_SSI cards used to identify the soil-structure interface, and LOAD_SEISMIC_SSI used to specify the ground motion on such an interface. Typically, only ground acceleration histories are required to specify the ground motion, but if ground velocity and displacement curves are also available from signal processing of the accelerograms, then the ground motion may be specified using DEFINE_GROUND_MOTION.
The variations on the INTERFACE_SSI cards (_AUX, _AUX_EMBEDDED and _STATIC) are meant for different stages in the analysis. All the INTERFACE_SSI cards (except for _AUX) create a tied-contact interface between two specified segment sets, the master surface being on the soil side and the slave on the structure side.
Soil-structure interaction analysis under earthquake excitation may then be carried out in LS-DYNA using these cards as follows:
0. Carry out a static analysis of the soil-structure system (e.g. using dynamic relaxation; see *CONTROL_DYNAMIC_RELAXATION), with the soil-structure interface identified using *INTERFACE_SSI_STATIC_ID.
Optionally, carry out a free-field analysis to record free-field motions on the future soil-structure interface, using either *INTERFACE_SSI_AUX or *INTERFACE_SSI_AUX_EMBEDDED, for surface-supported or embedded structures respectively.
1. Carry out the transient analysis as a full-deck restart job (see *RESTART), with only the structure initialized to its static stress state (see *STRESS_INITIALIZATION), and the same soil-structure interface identified using *INTERFACE_SSI_ID with the same ID as in static analysis:
- The structure mesh must be identical to the one used for static analysis.
- The soil mesh is expected to be different from the one used for static analysis, especially because non-reflecting boundary models may be used for transient analysis.
- The meshes for the structure and the soil need not match at the interface.
- Only the structure must be subjected to static loads, via *LOAD_BODY_PARTS.
- The earthquake ground motion is specified using *LOAD_SEISMIC_SSI, and/or read from motions recorded from a previous analysis using *INTERFACE_SSI_AUX or *INTERFACE_SSI_AUX_EMBEDDED.
NOTE: Please ignore error messages from LS-Prepost flagging the INTERFACE_SSI cards as invalid.