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)