Alignments

An alignment is the basic setting out feature for bridges, defined in terms of chainage and straights or radii. The vehicle paths are then related back to alignments. A structure may have multiple alignments.

Definition

Name

The name is used to identify the alignments where they are referred to. The name is displayed on the tab in the Alignment table. This can be adjusted by double clicking on the tab or right-clicking on the tab and selecting the modify option.

Chainage

Distance along the Alignment from the origin. The origin of the alignment (chainage 0) is at the origin of the axis system of the Grid Plane, see below.

Curvature

Straight, Right Curve or Left Curve (looking in the direction of increasing chainage).

Radius

The instantaneous radius curvature of the curve at that chainage. A circular curve is defined by two chainages with the same curvature, and a transition curve by two chainages with different radii (or a radius and a straight. Curvature varies linearly along the transition as in a standard highways transition). A typical alignment is illustrated below.

The formulae for the transition curves are taken from the “County Surveyors’ Society” Tables.

If a change of curvature without a transition is needed then a very short transition length needs to be used.

Grid Plane

The alignment has to be positioned in space relative to the structure. The grid plane provides this positioning with zero chainage at the origin of the grid plane and the alignment at this point aligned with the positive x axis of the grid plane.