Raising the Hangers,

Hangers-to-Deck Connection (after strengthening)

The design of the hangers, which suspend the deck from the cable, was based on a good deal of research and development. The aim was to dampen vibration through hysteresis and the novel inclined pattern into which they were hung was intended to add to the dampening effect. Each hanger was of a predetermined length and 2 ins (50 mm) in diameter. The construction was of 178 individual 0.133 in zinc coated wires made into a long spiral with each successive layer being wound in the opposite direction to its predecessor. This countered any likely untwisting effect that would have occurred if they were all wound in the same direction. The hangers were attached, at 60 feet intervals, to clamps around the main cable by clevis pins and also similarly attached to the deck. This made it possible to produce the hangers to predetermined lengths and socket them before delivery to site. While they might have looked slender, each had a specified breaking load of 225 tons against a working load that was not expected to be greater than 100 tons.

Suspending the Deck

Deck boxes being assembled and matched on slipway

Meanwhile sections of the bridge deck were being formed at the slip-ways on the Wye at Chepstow.  They were each match-cast against their predecessor, ensuring that adjacent sections accurately fitted together in the erection sequence. The deck sections are 60 feet (18 m) long and about 105 feet (35 m) wide, weighing about 120 tons.  Following application of a protective coating, the sections were launched onto the river and moored, until being towed to the bridge site when required for erection.

First deck box being lifted from river

They were then positioned using a special pusher barge and accurately located by multi-pulley block and tackle operated by winches from the tower tops. Small winches controlled swinging and rocking as the first section was lifted into place in the middle of the main span. With the deck unit lifted to the correct height, the hangers were attached and the weight of the unit was transferred, via the hangers, to the main cable.

Erection of boxes adjecent to the towers

Further units were lifted, first to one side of the centre and then the other, to keep the overall weight balanced across the centre span.  Each new unit was fixed to its predecessor, initially, using temporary clamps on the top flange. This was because a considerable number of units had to be erected to each side of the centre, before the shape of the hanging deck began to resemble its final profile. At that stage, the erected units could be permanently welded together. Erection continued, keeping the sides in balance throughout the main span. The erection of the side spans then commenced, from the anchorages to the towers, keeping the Aust and Beachley side spans in balance. The final two units erected, each side, were those adjacent to the towers in the side spans.

These operations were carried out without significant problems or delay. This was no mean feat given the unpredictable forces of nature and the strength of the tidal flows. Once the 88 sections of deck were in position and welded together, only minor additions such as the deck furniture remained, apart from the roadway expansion joints either side of the towers and the surfacing of the deck.

Surfacing

The aim was to minimise the deck dead load. Therefore concrete surfacing, as was common in America, was not used. Experiments were carried out by the Road Research Laboratory followed by site trials at a number of locations around the United Kingdom. It was discovered that a 1 1/2 inch (40 mm) layer of hand-laid, stone-filled, mastic asphalt would prove satisfactory. This was laid on the zinc-sprayed top surface of the boxes, coated with a thin adhesive, over which a thin layer of rubberised bitumen had been spread to seal and prevent corrosion of the underlying steel.

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Before work commenced on the permanent main cables, a catwalk together with a system of high level ropeways, had to be put in place along the line of each main cable, carrying free-wheeling groved metal wheels capable of moving from anchorage to anchorage, via the tower tops.

Cable spinning – showing four wires (two bights)

A barge was used to carry the first catwalk cables across the river. A wire mesh floor and sides formed the catwalk from which the cable spinning would be carried out. The apparatus for the “spinning” process was brought down from the Forth Bridge, where it had completed the main cables for that structure during the previous two years. At Severn, it was decided to take only two bights of wire on each wheel, instead of the four used at the Forth where there had been some problems, in control, during construction of the cable.

Spinning wheels passing each other at mid-span

The construction of each cable, involved hauling 8322 galvanised wires of 5 mm nominal diameter, two at a time across the river from anchorage to anchorage. The wires were then made up into 19 strands, of 438 wires each, and the strands were arranged in the form of a vertical hexagon, being held in this shape over the saddle at the top of each tower. The ultimate tensile strength of each wire was specified to be 110/120 tons per sq in (about 1550 N/mm²).

Cable compaction device

The completed cable was finally compressed into a circular shape and bound tightly with a single layer of wire applied under controlled tension. When the spinning was complete, cast-steel cable clamps were positioned along the cables at the predetermined hanger suspension points about 60 feet (18 m) apart and carefully bolted up to a specified minimum clamping force.

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The contract for the provision and erection of the whole steel superstructure, including the towers, was awarded to a consortium, Associated Bridge Builders Ltd (ABB), on the 11th May 1962.

The novel design for the towers, by Freeman Fox and Partners, was based on the fabrication of many lightweight stiffened steel panels up to 60 feet (18 m) long, and varying in width up to about 12 feet (3.7 m), these sizes being the maximum that could be transported by rail. These relatively thin panels were stiffened longitudinally and transversely by rolled section stiffeners, welded to the inside face of the tower panels. This stiffening can be seen in the photograph below, as a new side panel is being fixed on the previously completed section of the tower.

The towers were fabricated by Arrols at their workshops in Glasgow.  Both were fully trial-assembled, lying on their sides in the workshop, before being transported by rail to Chepstow as individual panels. All fabrication, welding and protective treatment took place in workshop conditions.

Tower construction showing stiffened side panels

The Consortium’s first job on site was to erect the two steel towers 400 feet (125 m) high from the completed piers. The thickness in the four outer stiffened plates in the panels that formed the towers, reduces from 1 in (25 mm) at the bottom to 9/16 in (14 mm) at the top. This novel form of tower construction resulted in a significant reduction in steel weight in the towers themselves, compared to the previous traditional multi-cell construction used on previous large suspension bridges.

The heaviest sections at the bottoms of the towers, 55 feet 6 in (17 m) in height, were placed in position by derricks erected on the temporary staging at Aust and Beachley. They were fixed to the steel bases which had been cast into the concrete of the pier tops and were held down by pre-stressing strands. A specially designed climbing derrick crane was used to place the remaining sections of both legs. The four panels, making up each lift of the tower legs, were bolted together, internally, by vertical rows of bolts.

Horizontal diaphragms were fitted across the interior of each tower leg, at vertical intervals of 14 feet (4.2 m). Four 2 ins (50 mm) dia. screwed rods, one at each corner, were used to bolt the leg sections together as they grew free-standing. Three horizontal portals were built between each inward-looking face of the towers, as they grew, one just below the intended deck level, one at the tower top, and one between these two. The final lifts for the climbing derricks involved hoisting up the pre-fabricated, all-welded, steel cable saddles, weighing 25 tons each, which had to be set in position on the tower tops. They were the heaviest units lifted by the cranes.

Following the construction of the main cables and before erection of the deck commenced, the top of each tower was pulled back from the main span by some 2 feet 6 in (750 mm). To achieve this, a separate single cable was stretched from the top of each tower leg to the nearer anchorage. Each of these cables was then pulled downwards from its mid-point by a separate cable secured to the foot of the tower leg. As the deck units were being lifted and fixed in position, starting in the middle of the main span, the towers were progressively released until, at a maximum, they would be leaning some 15 inches (375 mm) into the main span. Then, as the side spans were constructed, each tower would return to the vertical position as the deck construction was completed and the main cable carried the full dead weight of the suspended structure.

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Severn Bridge. Construction of Foundations.

The taking off point for the John Howard contract was the 135 foot (42 m) high Aust Cliff. A 25 feet (7.5 m) deep cutting was excavated through the Aust Cliff to create the approach to the main deck level at 120 feet (37 m) above mean sea level, at centre span. The Aust Pier was founded on a tidal limestone outcrop known as the Ulverstone Rock. It was about 1600 feet (500 m) offshore, separated from the land by a deep channel and exposed only for a brief period at low water. Access to the location of this pier was provided by building a temporary viaduct to a working stage above high water at the Ulverstone Rock.

Pre-stressed Concrete units bolted down to Ulverstone Rock showing

A mobile drilling platform was set up adjacent to the pier location and used to construct the working stage, made from prefabricated steel sections. The staging was sufficiently robust to bear the weight of the sections of the main tower which were, later, off-loaded from barges. The access viaduct was also used to transport the concrete and reinforcing steel used in constructing the East anchorage and, later, the pier. In order to prevent over 15500 tons of pier construction being swept away by fierce tidal forces, steel rods were grouted into the surface of the rock below while it was exposed between tides, and used to secure a ring of pre-cast concrete blocks that would form the base of the outer shell of the pier foundation. Each of the higher levels of precast concrete blocks were set with gaps between them to allow the water to flow out at low tide. The gaps were then closed between high tides and that sequence was repeated, layer by layer, to form a permanent cofferdam. In time, the top of the dam was above high water level and so could be pumped out and filled with concrete.

The Beachley pier differed from the one at Aust in that there was no convenient outcrop of hard rock available at this location, some 600 ft (200 m) offshore. The river bed level at this point is close to average low water level and so it was possible to gain access to the site at low tide.  New boreholes indicated that the river bed in this location consists of about 30 ft (10 m) of Keuper marl, before hard limestone would be reached. The Keuper marl in this location has a tendency to soften when exposed and so it was decided that the foundations for the tower should be taken down to a depth of 33 ft (10 m) below the top of the marl. Two circular concrete shafts would be provided to this depth, each 60 ft (18 m) diameter. The shafts were 78 ft (24 m) apart, centre to centre, to accommodate the twin legs of the Beachley tower. The concrete shafts would be built up to river bed level and pier protection elements would be added later across the tops of the two shafts. A complex series of operations was required to bring the plans for this foundation to fruition.

Two separate circular sheet-piled cofferdams were first constructed in the required locations for the circular concrete shafts. Unfortunately, the Keuper Marl proved to be too hard for the sheet piles to be driven very far but the cofferdams proved to be invaluable in allowing work to continue within them at all states of the tide. The sheet piles were bolted to a concrete anchor ring which was cast on the surface of the marl and anchored down into it by dowel bars. Subsequent excavation stages within the cofferdams were lined and supported by prefabricated concrete circular tunnel segments, bolted together.

Closer view into the south caisson for the West Pier construction

The image shows the concrete ring-beam at the river bed level at the bottom of the vertical sheet piles. The excavation continued top down below this level and can be seen lined with concrete pre-fabricated tunnel segments. A small digger can be seen in the bottom of the excavation continuing to break out the less good red marl.

As the excavation continued, the cavities between the segments and the rough walls of the shaft were grouted with neat cement. However, the limestone bands, on which it was intended that the circular foundations would rest, proved to be too thin and broken to provide a satisfactory bearing for the pier. The excavation continued until a non-conformity was reached, exposing the top of a steeply inclined, strongly consolidated, strata of hard carboniferous mudstones which were deemed to afford adequate bearing for the pier in conjunction with the frictional side support offered by the walls of the shafts grouted into the marls.

When completed, the excavation was filled with concrete and the two parts of the cutwater pier were built inside the cofferdams up to high water level. The cofferdams were then removed and the central portions and pointed ends of the pier were built as a tidal operation.

One of the final operations of Contract 1 pier construction was to cast pre-fabricated steel tower base units into the tops of the main piers. These had machined top surfaces ready to receive the steelwork of the bridge towers. In setting these to level an allowance had to be made for the curvature of the Earth over the c.1 km main span distance; this amounted to about 3/8th of an inch (c.1 cm) apparent difference in level.

Severn Bridge Cable Anchorages

The cable anchorages at both Aust and Beachley are essentially enormous blocks of concrete. They are keyed into the rock to resist the horizontal pull of the cables and their dead weight prevents them from overturning. The Aust anchorage is located 500 feet (150 m) from the foot of the Aust Cliff and was founded on rock exposed at low water. The top of the completed anchorage is at the level of the roadway. Besides anchoring the main suspension cables, the constructions at both Aust and Beachley were used as the foundations for the two main abutments of the Severn Bridge deck.

Externally each anchorage appears to be a solid block of concrete. In fact both are to some extent hollow with a central void between the two outer chambers into which the main cables are anchored. The anchorage at Aust only needed to be keyed by about 3 meters into the limestone bedrock exposed at low tide. That at Beachley had to be constructed down through the sediments of the peninsula. The initial excavation was by open-cut in a pit with battered sides about 10 meters deep over the whole plan area of the anchorage. Two parallel trenches then continued the excavation downward for another 20 meters or so to the underlying solid limestone into which a series of steps were keyed. As the excavation was performed top-down the higher levels of looser sediments needed to be supported by the arched concrete cast in-situ walings each about 2 meters deep. These were propped at the third points with pre-cast beams spanning across the trenches. The final depths of the excavation were in good rock which stood without support. Once the bottom had been prepared the trenches were filled with mass concrete with an open matrix of reinforcing bars. See image below.

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