Dam analysis example
Dam analysis can be formulated in LS-DYNA according to two-step approach outlined earlier using the INTERFACE_SSI and LOAD_SEISMIC_SSI cards implemented for soil-structure interaction.
Two preliminary analyses are required — a static analysis, and the auxiliary analysis to capture the effect of upstream ground motion.
In static analysis, the foundation rock and water are constrained on the outer boundary, and they are chosen to be sufficiently large to model the static stress distribution. This is not as onerous a constraint as in dynamic analysis, where the outward propagating waves would have reflected back from a fixed outer boundary. Only the dam and the water are subjected to gravity loading, the reasoning being that the current state of the foundation is already its deformed shape due to aeons of gravity acting upon it. The following figures show the model and the "soil-structure" interface, specified using INTERFACE_SSI_STATIC to record the static reactions. In a manner similar to soil-structure interaction analysis — where the nearby non-linear part of the soil was taken as part of the generalized structure — here the near-field water, which may behave non-linearly, is taken to be part of the structure, and the far-field water and the foundation rock are together taken to be the "soil" somain. Of course, any nearby non-linear portion of the foundation rock may also be taken to be part of the structure as necessary.
Auxiliary analysis involves only the far-field water and the foundation rock, with the water domain acting as the structure, and the unbounded water and foundation rock truncated using PML. The earthquake excitation is applied using LOAD_SEISMIC_SSI on a segment set at the water-foundation interface away from the PML boundary, and the motions at the future "soil-structure" interface are recorded using INTERFACE_SSI_AUX for use in the subsequent transient analysis.
In the transient analysis of the entire dam-water-rock system, the direct earthquake excitation at the base of the dam is applied using LOAD_SEISMIC_SSI on a segment set, and the effect of the upstream ground motion, computed in the auxiliary analysis at the "soil-structure" interface, is incorporated using INTERFACE_SSI, which also reads in the static reactions computed in the static analysis.
The corresponding example input files for these analyses are dam-static.k, dam-auxiliary.k and dam-transient.k, using the ground motion files elcentro-x.ath, elcentro-y.ath, elcentro-z.ath. Please also see the README file.
NOTE: Please ignore error messages from LS-Prepost flagging the INTERFACE_SSI cards as invalid.