Dynamic Loads [FEA]

Dynamic Loads holds the objects used for modal, seismic and transient analysis.

Eigenvalue & RSA Cases compute natural periods and mode shapes, and - when response spectrum curves are assigned - combine the modal responses into peak seismic demand. Response Spectrum Curves define the design spectrum those cases look modes up on.

Time History Functions hold a tabulated excitation record, and Time History Cases integrate the equation of motion step by step, which is what you need when the timing of the response matters or when nonlinear links dissipate energy.

Support Motions prescribe a displacement record at an individual support joint. Several of them on one Time History Case give multi-support (spatially varying) excitation - different ground motion at different supports, which is what a bridge long enough for wave passage, incoherence or differing site conditions actually sees. Uniform excitation cannot produce differential-support demand at all, since moving every support together is rigid-body translation.

Nodal Mass adds concentrated mass at a joint for inertia that exists on the real structure but is not carried by any modelled element - barriers, utilities, equipment.

Mass is the thing to get right first in this branch: every period scales with it, and every dynamic result follows from the periods. Note that nodal mass values are entered as weights, which the solver divides by gravity.