Northam Bridge - Construction and Dimensions

Construction and Dimensions

The parapets of the first (wooden) bridge were 24 feet (7.3 m) apart, as were those of its wrought-iron successor.

The third bridge utilised the latest technology available at the time but the style of the bridge was of the pre-war era. The main deck structure has transverse diaphragms and narrowly spaced beams, which were pre-cast on site using deflected cables. Pre-cast, pre-stressed slabs, known as junction slabs or continuity slabs, were placed between the tops of the beams by transverse stressing over a length where the flanges of the tees were removed. These, together with in situ diaphragms between the ends of the beams, allowed the deck structure to be made continuous for live and superimposed loads. After the junction slabs were in place, the main beams were post-tensioned through the diaphragms.

The cement used to make the concrete in the bridge was Ordinary Portland Cement, which was both cheaper and resulted in less shrinkage than using rapid-hardening cement. It was used in a ratio of 1:11⁄₂:3 - a mix which used more cement than German and British practice at the time - and a water-to-cement ratio of 0.3.

The consulting engineers responsible for the new bridge were Rendel Palmer & Tritton, the same firm used for Waterloo Bridge in London nine years earlier.

At mid-span, the bridge is 44 feet 4 inches (13.51 m) wide, 4.7 metres above mean high water springs and 9.2 metres above chart datum. The bridge is 148 metres long in total, and the supporting piers are up to 32 metres apart.