> 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/coordinate-systems-fea/alignments-fea.md). # Alignments \[FEA] An Alignment is the roadway geometry the bridge is built to: a horizontal path, a vertical profile, and a cross-slope definition. Objects assigned to it are positioned by **station and offset** rather than by absolute coordinates, so the whole model follows the alignment and updates when it changes. Alignments can be imported (see the LandXML import page) or built from OpenBrIM objects. An Alignment is a **geometry reference**. It adds no stiffness, mass or load — but because it positions elements, it determines every length, orientation and lever arm in the parts of the model driven by it. {% hint style="info" %} The horizontal path, vertical profile and cross-slopes are held in the alignment's child objects; the fields on this sheet set the alignment's **starting reference**, its display, and report what the geometry resolved to. {% endhint %} ## Alignment **Start Station:** The station value at the alignment's start point. *Engineering influence.* This sets the origin of the stationing, so every station in the model — pier locations, tendon start and end stations, live-load surface extents — is measured from it. Changing it **shifts the entire station reference**, which moves every station-positioned object at once. Matching it to the design drawings is what allows model stations to be checked directly against the drawings; a mismatch means every station in the model is offset by a constant, which is easy to overlook precisely because the structure still looks right. **Start Azimuth:** The bearing of the alignment at its start point. *Engineering influence.* Sets the direction the alignment initially runs, and therefore the orientation of everything positioned along it — including the skew of piers and abutments relative to the global axes. The default is π/2 (due east in the usual convention). An incorrect azimuth rotates the whole structure in plan, which is obvious in a site view and invisible in a local one. **Latitude / Longitude:** The geographic position of the alignment origin, used to place the model on a map and to relate it to survey and GIS data. They do not affect the analysis. ## Offset **Transverse Offset / Elevation Offset:** Offsets applied to the alignment as a whole. *Engineering influence.* These shift the reference laterally and vertically, which is how a girder line or a parallel structure is defined from a single roadway centreline rather than by duplicating the alignment. Because every object referencing the alignment moves with the offset, a change here relocates a whole family of elements together. {% hint style="warning" %} Neither of these parameters is declared in the object type. They remain writable from the agent API and `set_parameter` — those resolve any field the object exposes in its sheet — but they carry no unit category and cannot be authored from a library template. {% endhint %} ## Display **Major Station Interval:** The spacing of major station labels in the 3D annotation. **Minor Ticks per Major:** How many minor ticks appear between major stations. **Annotation Tick Size:** The drawn size of the annotation ticks and text. **Show Geometry Points / Show Curve Data / Show Bearings / Show Cross Slopes / Show Elevations:** Toggles for the annotation layers. **Cross Slope Vertical Exaggeration:** Vertical exaggeration applied when displaying cross-slopes. *Engineering influence, all of these.* They are **visualization only** and change nothing in the analysis. They matter for review rather than for results: cross-slopes and vertical curves are small relative to a bridge's length and are effectively invisible at true scale, so the exaggeration is what makes a profile error visible at all. Turning the annotation layers on is the practical way to check an imported alignment against the drawings. ## Alignment Info **Horz. Station Range / Horz. Segment Count:** The extent and number of segments of the horizontal geometry. **Vert. Station Range / Vert. Min Elevation / Vert. Max Elevation / Vert. Point Count:** The extent, elevation range and number of points of the vertical profile. **Cross Station Range / Cross Max Left Offset / Cross Max Right Offset / Cross Section Count:** The extent and limits of the cross-slope definition. *Engineering influence, all ten.* These are **read-only output** summarising what the alignment actually resolved to, and together they are the most efficient way to validate an import. The characteristic failure of an imported alignment is that one of the three components is missing or truncated — a horizontal path imported without its vertical profile, or a cross-slope definition covering only part of the station range. That shows up immediately as a station range that does not match between the three, or a count of zero. An elevation range that is implausible, or a station range that does not match the drawings, points to a unit mismatch in the import. {% hint style="warning" %} All ten of these columns are derived and cannot be edited, but they are not flagged read-only, so typing into them appears to work and the value is discarded on redraw. {% endhint %} ## Verification * Compare **Horz., Vert. and Cross station ranges** against each other and against the drawings. They should cover the same extent; a mismatch means a component is missing or truncated. * Check the elevation range is plausible for the site — an implausible one usually indicates a unit error on import. * Confirm the segment and point counts are non-zero for all three components. * Turn on the annotation layers and compare stations, bearings, curve data and elevations against the alignment drawing. * Use cross-slope exaggeration to check superelevation transitions, which are invisible at true scale. * Confirm a known station in the model lands where the drawing says it should — this validates the start station and azimuth together. * After changing the start station or an offset, re-check the positions of station-driven objects. ## Common mistakes * **A start station that does not match the drawings**, offsetting every station in the model by a constant while the structure still looks correct. * **Importing horizontal geometry without the vertical profile**, leaving the model flat — visible in the Alignment Info station ranges and almost nowhere else. * **Unit mismatch on import**, giving an implausible elevation or station range. * **Wrong start azimuth**, rotating the structure in plan. * **Changing a transverse or elevation offset** without realising it moves every object referencing the alignment. * **Reviewing cross-slopes at true scale**, where superelevation errors are invisible. * **Assuming the display settings affect the analysis.** They do not. --- # Agent Instructions This documentation is published with GitBook. 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