> For the complete documentation index, see [llms.txt](https://docs.openbrim.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.openbrim.org/templates/openbrimfea/loads-fea/dynamic-loads-fea.md). # 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. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.openbrim.org/templates/openbrimfea/loads-fea/dynamic-loads-fea.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.