Towers
In contrast to earlier suspension bridges from the designers, the towers were not constructed in steel plate, due to concerns over the anticipated cost of such construction, but are of reinforced concrete with a small central lift shaft, and were slip-formed, tapering. The setting out point on the saddle atop each tower leg is at +162.5m. The Hessle tower was slip formed over around 4 months, and the four cross-beams were constructed in order, working down from the tower top. The legs each contain a small shaft for a service lift, and this shaft was available to contain the mast of a small tower crane, and a concrete hoist.
To recover some of the schedule slippage due to delayed hand-over of the Barton pier, the tower legs were slip-formed to full height, the top cross-beam concreted, and the tower top handed over to the steelwork contractor, leaving the 3 lower cross-beams to be constructed subsequently. Both towers were constructed by Tileman & Co.
The steel superstructure was constructed by British Bridge Builders, a consortium formed by Cleveland Bridge and Engineering, Redpath Dorman Long, and Sir William Arrol and Co. Cleveland Bridge were, of course, the one UK company with the expertise and plant to ‘spin’ the main cables.
On the up ... a section of road being hoisted into place during the construction of the Humber Bridge in 1980 - credit Hull Live.
The superstructure is an incremental development of the suspension bridge style developed by Freeman, Fox and Partners, on the Severn Bridge and the first Bosporus Bridge. Substantial temporary works were required to create a continuous catwalk hanging immediately below the main cables, and to construct a tramway to unreel the individual wires.
The main cables are formed from 5.0mm diameter, cold-drawn carbon steel wires of 100-115 ton/in2 ultimate strength. The term ‘spinning’ is slightly misleading, in that the many wires are laid parallel to one another, there is no twisting. Each cable in the main and Barton side spans consists of 14948 individual wires, laid two at a time, each wire being adjusted to the correct sag (and hence length), as it is placed. At the anchorages, wires are looped around a cast steel strand shoe, and return to form the next wire. There are therefore no ends, but the construction relies on being able to join wires end-to-end with splice details, which are crimped around each wire. The wires are grouped into 37 strands, each of 404 wires, the wires being adjusted for length, to ensure equal load sharing. A strand is ‘spun’ slightly above its final location, and once complete, the large steel studs on which the strand shoe is supported could be manipulated to allow the shoe to be let out whilst the strand was pulled through the saddle to achieve a final position nestling among the earlier strands. Two wires were ‘spun’ at a time, being drawn out looped around a spinning wheel, hung from, and driven by, a wire tramway system. There has been no detailed account of the speeds achieved in this process, but on the Bosporus Bridge constructed shortly before, Cleveland reported driving the tramway at up to 1100 ft/min, implying that the two ‘live’ wires would be unreeled from their drums at about 25 mph. On the Hessle side span, the span is considerably shorter and therefore steeper. Since the tower top must be in equilibrium horizontally, the side span cable force is greater, and hence an additional 4 strands of 200 wires are added above the usual 37 strands. The cables contain 71 000 km of wire, and total some 11 000 tonnes. Spinning proceeded between September 1977 and June 1979.
Saddles
A fabricated steel saddle is placed at the top of each tower leg. Within the saddles, the individual cable strands are separated, each being deflected in an arc between the cable slopes in main and side spans. The vertical component of the cable tension is therefore transferred as a line load in the bottom of the cable groove. The further function of the saddle is to distribute this loading out onto the top of the tower concrete. To control weights of individual components, this duty is shared between the saddle itself and a steel grillage embedded in the tower concrete. This further allowed the saddle to be initially placed slightly offset to landward, with the saddle and grillage jacked into alignment part way through cable spinning.
Deck
The desk follows the earlier designs in being a steel box section, the roadway running along the top surface of the box, this part being of high-yield steel, stiffened with V-section longitudinal stiffeners below. The remaining faces of the box are in mild steel, with bulb flat stiffeners. Compared with these earlier designs, the box depth has been increased, for greater bending stiffness, principally to raise the critical wind speed at which any deck instability might occur to well above the design wind speed of 45 m/s. Inclined hangers support the deck, a layout originally devised to increase structural damping to prevent torsional oscillation. (for various reasons, this feature appears to have been dropped from later suspension bridge designs). The deck carries two two-lane carriageways, and footways, also providing access for maintenance vehicles, on each side.
The deck was constructed in 18.1m long sections each weighing 130 tonnes. All deck sections were pre-assembled on stallages in a fabrication yard, Priory Yard, on the site of a former railway marshalling yard at Dairycoates, on the west side of Hull. The sections were therefore fabricated to achieve the correct vertical curves for each span. Since a live main railway line runs between Priory Yard and the foreshore, in order to load out each section onto a barge for transport to the bridge site, the transfer across the railway lines was carried out during a rail possession on every Saturday night. At the bridge site, deck sections were winched up to final level, with lifting carriages spanning across the two completed cables, the lifting falls being operated from winches at the tower bases.
Ancillaries
The deck, as initially lifted, lay above its final profile, and as the tension in the main cables built up, the deck profile changed, the deck lowering. Joining of the deck boxes by welding was therefore phased, the deck plate welding being performed early, but the remainder of the perimeter being completed later.
The roadway surfacing is 38mm of hot rolled asphalt, laid on a primer and bitumen underlay. For the first time, the surfacing was machine laid, rather than hand laid.
Crash barriers
The two carriageways run between tensioned wire crash barriers. It has been normal practice to recycle the wires of the temporary catwalks below the cables, cutting and resocketing them to form the wires of the barriers.
Approach roads
Viaducts and approach roads connect the bridge with existing highways, the A63 at the Hessle end, and the A15 at the Barton on Humber end.
Toll area
The toll area and administration building are located at the Hessle end of the bridge.
This photo shows deck box sections for the Hessle side span, ready for lifting, also showing temporary rail tracks on which the boxes were moved at ground level - credit Hull Live