# Arch \[TB]

An arch rib that spans between two truss-bearing insertion points, forming the primary load-carrying element of an arch or tied-arch truss bridge. The arch curve is generated parametrically between the two bearings using one of four shape types, and its members are designed under AASHTO LRFD Article 6.14.4 (arches).

## General

**Insertion Points (Arch Bearings):** The two insertion points that anchor the ends of the arch rib. Pick exactly two from the picker/list — these must be insertion points whose **Reference Object** is set to **Truss** so their **Elevation** values feed the arch bearing elevations. Required.

**Arch Type \[Parabolic/Circular/Elliptical/Catenary]:** Shape of the arch curve between the two bearings. **Parabolic** uses a second-order polynomial (`4·h·t·(1-t)`) and matches dead-load thrust efficiently. **Circular** builds a constant-radius arc sized to the span and rise. **Elliptical** uses a half-ellipse with the major axis along the span. **Catenary** follows a hyperbolic cosine, which is the natural form for a self-weight-dominated arch.

**Arch Rise (Height at Crown):** Vertical distance from the chord connecting the two bearings to the crown of the arch. AASHTO LRFD 6.14.4 typical rise-to-span ratios for steel arches fall between 1:5 and 1:8; the Rise-to-Span ratio is exported and can be reviewed against the chosen arch type.

**Additional Crown Transverse Offset:** Lateral shift applied at the crown, distributed parabolically along the arch so the bearings remain unchanged and only the crown is displaced transversely. Positive values move the crown in the -Y direction (to the right when looking up-station along the PGL, matching OpenBrIM's sign convention). Use this to warp the arch into a curved plan footprint.

**Section:** The Section library object used as the arch rib cross-section. Picks a previously defined entry from **Sections**. Required. Drives both the 3D geometry and the AASHTO 6.9 compression / 6.14.4 arch-member checks applied later.

**Orientation Angle:** Rotation of the section about the arch's local tangent (beta angle). Use to roll I-shaped or box sections so the strong axis aligns with the arch's bending plane.

**Maximum Elevation (Crown):** Computed absolute elevation of the arch crown, combining the mean bearing elevation, the Arch Rise, and the section's top-fiber offset. (Read-only)

## Offsets

**Vertical Offset (point-1):** Vertical shift (Z) applied to the first-bearing end of the arch, in addition to the insertion-point elevation. Positive is upward.

**Vertical Offset (point-2):** Vertical shift (Z) applied to the second-bearing end of the arch. Positive is upward.

**Horizontal Offset (point-1):** Transverse shift (Y) at the first-bearing end. Follows OpenBrIM's convention: positive offsets move the point toward the right when looking up-station (internally applied as -Y so the stored coordinate is to the right of PGL).

**Horizontal Offset (point-2):** Transverse shift (Y) at the second-bearing end. Same sign convention as point-1.

**Longitudinal Offset (point-1):** Shift along the alignment (X) at the first-bearing end. Positive moves up-station.

**Longitudinal Offset (point-2):** Shift along the alignment (X) at the second-bearing end. Positive moves up-station.

## 3D Detailing

**Number of Segments:** Number of straight chords used to discretize the arch curve for 3D rendering and FEM meshing. Higher values produce a smoother visual arch but increase mesh and render cost. Twenty segments are typical for a smooth display.

**Start Face Skew Angle (X-Axis):** Rotation of the start-face cutting plane about the local X-axis, used to match the arch end to an inclined bearing or a skewed gusset connection.

**End Face Skew Angle (X-Axis):** Rotation of the end-face cutting plane about the local X-axis.

**Start Face Skew Angle (Y-Axis):** Rotation of the start-face cutting plane about the local Y-axis.

**End Face Skew Angle (Y-Axis):** Rotation of the end-face cutting plane about the local Y-axis.

**Arch Opacity:** Display opacity of the arch rib in the 3D view, from 0 (fully transparent) to 1 (fully opaque). Useful for seeing hangers and deck elements through the arch.

## FEA Settings

**Maximum Mesh Size:** Longest allowable finite-element line length along the arch. Each subsegment between hanger/bracing connection points is subdivided so no FE line exceeds this length, which controls accuracy of axial, shear, moment, and buckling results obtained from AASHTO 6.9/6.14.4 checks.

## Quantities

**Min Section Area:** Minimum cross-section area encountered along the arch path — useful when the section is variable. (Read-only, may be slow on recompute.)

**Max Section Area:** Maximum cross-section area along the arch path. (Read-only, may be slow on recompute.)

**Total Surface Area:** Total exterior surface area of the arch volume, including the two end cross-section faces. Feeds painting / coating quantity take-offs. (Read-only, may be slow on recompute.)

**Min Cross Section Perimeter:** Smallest section perimeter encountered along the arch. (Read-only, may be slow on recompute.)

**Max Cross Section Perimeter:** Largest section perimeter encountered along the arch. (Read-only, may be slow on recompute.)

**Volume:** Total concrete/steel volume of the arch rib. (Read-only, may be slow on recompute.)

**Min Length Path:** Shortest path length (inner fiber). (Read-only, may be slow on recompute.)

**Length @COG Path:** Length measured along the center of gravity of the section. (Read-only, may be slow on recompute.)

**Max Length Path:** Longest path length (outer fiber). (Read-only, may be slow on recompute.)

**Weight:** Computed weight, equal to Volume × section-material density. (Read-only, may be slow on recompute.)

**Material:** Name of the material assigned to the selected section. (Read-only)

**Section:** Name of the section assigned to the arch. (Read-only)

## IFC

**Type \[IfcBridge/IfcBridgePart/IfcBeam/IfcColumn/IfcSlab/IfcFooting/IfcPile/IfcBearing/IfcReinforcingBar/IfcReinforcingMesh/IfcTendon/IfcMechanicalFastener/IfcAlignment/IfcPositioningElement/IfcKerb/IfcGuardrail/IfcSurfaceFeature/IfcElementAssembly/IfcCableSegment/IfcSensor/IfcDistributionElement/IfcPlate/IfcMember/IfcDiscreteAccessory/IfcBuildingElementProxy]:** IFC entity class used when the arch is exported. Default is **IfcMember**, which is the standard class for primary structural line elements.

**Predefined Type \[Brace/Chord/Collar/Member/Mullion/Plate/Post/Purlin/Rafter/Stringer/Strut/Stud/Arch\_Segment/Cable/Suspension\_Cable/UserDefined/NotDefined]:** IFC predefined-type attribute for the exported entity. Default **ARCH\_SEGMENT** identifies the element as part of an arch in the IFC model.

**Parent \[Bridge/Superstructure/Deck/Substructure/Foundation/Approach\_Structure/Retaining\_Structure]:** Structural-hierarchy parent under which the arch is nested in the IFC model. Default **SUPERSTRUCTURE**.

**Properties:** Custom IFC property entries (text, numeric, or object references) that are attached to this arch when exported. Use the custom-IFC-properties editor to add rows.

## Referencing Objects

**Arch Bracings:** Arch Bracing objects that attach to this arch. (Read-only)


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