Ring thickness

By default, ring thickness is assumed to be constant from springing to crown. In some bridges, the ring thickens to the springing. This situation can be approximated by setting a larger ring thickness at the springing than at the crown.

Archie-M 2.7 changes the way the ring thickness at any point in the arch is calculated from this limited information to avoid some odd results. There are two cases:

• For circular geometry, three points on the extrados are found at the springings and crown by offsetting the relevant intrados points along the normal at that point. A circle is fitted to these points for the extrados.
• For other geometries, the ring thickness is interpolated as a function of the z coordinate of the intrados point between crown and springing.

In Archie-M 2.6 and earlier, the different approaches to interpolation were used for different geometrical forms, and the ring thickness tends to increase too quickly near the crown.

From Archie-M 2.7, varying ring thickness is properly accounted for when a skew transform is applied. All ring thickness measurements given to Archie-M must be on the square line, not as measurement on the elevation of a skew structure. Note that a constant ring thickness will appear to increase to the springing on the skew line.