This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The span and rise are frequently predetermined. Fortunately this method is applicable to almost any form of curve, if the change in curvature is not too extreme. Even if the arch is very flat and the curvature very sharp near the abutments, it only means that the virtual abutment is somewhere on the haunches. Therefore, draw the intrados; assume and lay off a reasonable crown thickness; multiply this "thickness by the factors given in the tabular form in Article 427 for the angles with vertical lines made by the various normals to the curve. These thicknesses can be laid off, and the extrados can be drawn through the points.
But since the curve OCB of Fig. 228 does not represent either the intrados or extrados, but the center line of the rib, we should draw a line midway between the intrados and extrados which will represent the center line of the rib, and which corresponds to the line OCB in the figures which refer to the theoretical demonstrations. This also means that the span of the rib, measured between the centers of the skewbacks, will be slightly greater than the nominal clear span. The rise of the center of the rib above the line joining the abutment points O and B will in general be slightly different from the nominal rise of the arch.
Fig. 231. Determination or Position or vm.