Stephenson had used this configuration before; he recorded that, "The earliest railway bridge on the bowstring principle is that over the Regent's Canal, near Chalk Farm, on the London and Birmingham Railway". The arch would consist of iron ribs. Fawcett says, "The reasons for not using wrought iron was due to some engineers' distrust of rivetting, the relatively small size of wrought iron plates then available, and the higher cost… On 1 October 1845 when the Newcastle and Berwick Board instructed T E Harrison for their bridges, none of the uses of wrought iron had been developed far enough to be considered as an alternative to cast iron for the High Level Bridge. A tubular bridge might have been considered by Robert Stephenson but the distance between solid and reasonably shallow foundations would have given a span much larger than the Britannia Bridge." The depth of rock in the riverbed resulted in a height of from there to the superstructure. Three river piers were permitted by the Tyne Improvement Commissioners, and therefore four river spans of were decided on; there were additional subsidiary spans on the shore. The cast iron arch ribs are deep at the crown, increasing to at the springing, with flanges; the flanges and webs were three inches thick; in the case of the inner ribs, and two inches for the outer ribs. The rise was , determined by the desired geometry to confine the horizontal thrust within bounds. Each arch was cast in five sections, bolted together. Stephenson described the tie bars: The ties consist of flat wrought-iron bars, 7 inches by 1 inch of best scrap iron, with eyes of 3½ inches diameter, bored out of the solid, and pins turned and fitted closely. Each external rib is tied by four of these bars, and each internal rib by eight. The sectional area of each external tie is 28 [square] inches, and of each internal tie 56 [square] inches, giving a total area of 168 square inches. These bars were all tested to 9 tons on the square inch. The rail deck is supported above the arches by twelve square columns at centres. Suspension rods supported the road deck, and both decks had two layers of diagonally laid three-inch deck timbers on suitable wrought iron cross girders (and rail-bearers in the case of the rail deck). The main contractors for the ironwork were Hawks, Crawshay,