3. During the design process, it became clear that plain reinforced concrete pylons would not be able to cope with the design bending moments and so each pylon was pre-stressed vertically. Each four span section has been prestressed longitudinally to form a homogeneous beam.
They were post tensioned to the pylon using seven pairs of stressing cables which were then grouted into the structure. The final two in-situ concrete stitches, one either side of the last deck unit, were then poured to complete the main span. These joints will reduce the effect of an impact on any one pier because it would be resisted by several piers acting in unison. The Government’s tender documents included a detailed “illustrative design” for the whole structure that would provide the river crossing. The crane would then be moved forward by hydraulic rams that were fitted to its body, as far as the leading edge of the new unit, so that the process could be repeated. A number of concessions and amendments were introduced and the Bill received Royal Assent on 13 February 1992. In other words, the longer the cable, the greater the number of strands it would contain. They were stressed from both ends simultaneously and hot wax was pumped up from the bottom of each duct to protect against corrosion. These caissons weighed 2,000 t each which was the maximum weight that the construction equipment could manage. Hot wax is injected into the ducts to exclude air and water and so protect the tendons. The maturity of the concrete was carefully monitored so that the false work could be struck as soon as the concrete was capable of supporting the required loading. On the Gwent side, a protective earth bund was constructed out from the shore, over the alluvium, and large 2 metre (6.5 ft) diameter bored piles sunk to support the viaduct pier bases. Each pylon required 38 separate lifts of concrete, of between 3.5 m and 4.48 m in height, starting at the top of the caisson and using self-climbing formwork. From there, they were loaded onto a special transport barge that was controlled by 4 separate engines, one on each corner, to ensure that the vessel would be able to hold its position, at high tide, to allow the caisson to be lifted off the barge and lowered into its final position.
SRC’s first step was to amend the illustrative design provided with the Invitation to Tender, by standardising all viaduct spans at 98m. Software Update and follow the instructions; Bingo, you should be sorted! 392.48 m apart, to restrict progressive collapse of the structure. Avon casting yard showing Caisson casting beds and a Caisson being transported down ro-ro ramp. The tidal range that occurs at this very point, up to 14m, is the second highest in the world. The reinforced concrete slab varies in thickness between 200 mm and 350 mm. The design of the stiffening truss beneath the bridge deck is dictated by the need for a rectangular space to accommodate the monorail-suspended access train. Unable to swivel under their own power, the DSL cranes had to be positioned accurately on the deck so that the operation of lifting a unit, from a barge stationed 50 m below, could be kept under careful control.
Over 2200 … The source became known as 'the Great Spring'. With this change, the number of blockers or deflector diaphragms within each span was reduced to two, thus increasing the speed of production and reducing cost. The bridge deck is constructed from individual full width units, each 7.3 m long and containing four separate components:- they must remain accessible and replaceable, even when enclosed within a hollow structure – and the free length of any such tendon must not exceed 40% of the span. The task of placing the concrete was complicated by the complexity of the steel reinforcement, especially in the vicinity of the cable anchorage locations.
They had to avoid nearby fault lines and keep clear of the deep and steep-sided channel that had been carved out over time and was now the main navigation channel, known as The Shoots, with its peak currents and turbulence. With the exception of the crossbeams and the anchorage tie beams, they were constructed entirely of reinforced concrete. Last few segments being placed on the Gwent span. SRC’s designs were similar to that of the illustrative design and attention will be drawn to major departures. Later, the upper crossbeams, weighing 900 t each, were lifted to rest, temporarily, on to the previously fixed lower beams and, when the pylons were nearing completion, they were raised to their eventual positions, using strand jacks mounted on brackets fixed to the pylons. Copyright; Neil Thomas of Photographic Engineering Services, Vew of completed structure showing tri-planar soffit to deck.
Transportation of the segment to the Pylon. Intermediate steel platforms, that were suspended from the bridge deck or fixed to the pylons, allow access to the cable stayed bridge. The tunnel is 140 years old and it carries the main line from London to South Wales, so any risk of damage to the structure would be unacceptable. Precast concrete beams were constructed in the yard, to fit transversely across the gap between the box girders.
The crane was moved back, past the pylon, to equalise the levels. All cable installations were undertaken by international specialists, PSC Freyssinet. Copyright; Neil Thomas of Photographic Engineering Services.
Eventually, the first span was stressed to the rest of the viaduct as far as the first movement joint. A second strand was then threaded through the lower anchorage, up the cable stay sheath and through the upper anchorage where it was wedged into place. These protection islands would have been capable of inflicting considerable damage to errant shipping. Also, the presence of the caisson under that pier prevented the transporter barge from approaching close enough to allow the DSL crane lift the adjacent centre span unit from the barge. It was then able to place the first special deck segment in the correct position on the pier but on temporary supports. The two box girders had to deviate from each other to accommodate the road geometry so that the concrete stitch between them had to be made progressively wider. Near the Avon bank, large diameter bored and de-bonded piles were used to support the caisson that would form part of the special foundation in the vicinity of the Severn Bridge Tunnel, see “Design of Viaduct Foundations”. Building the Second Crossing Viaduct Deck. The four year construction period and the much longer concession period both started on 26th April 1992. All rights reserved. High strength friction grip bolts would replace site welding where possible. The tendons stretch from a point below the first cable anchorage, to the slab at the top of each pylon leg. It was then possible to move the two sections of deck apart by pumping hydraulic fluid into one side of each unit, so providing space to erect and secure the last unit from the Gwent side in the normal manner. Then click on the next available link in that text, “For more on Design of the Second Crossing Viaduct Deck”). The Severn Bridges Bill was put before Parliament in the 1990/91 session and details were made public. Copyright; Neil Thomas of Photographic Engineering Services. The alignments of the deck units would be checked after bolting.
Maximum use would be made of pre-casting and prefabricating on dry land, to minimise work done off-shore. The upper end would then be threaded through the appropriate anchorage aperture in the pylon to be wedged and locked into place against a tapered hole that had been machined into the anchorage plate. It provided SRC with two years to refine their designs and complete their preparations for construction before work started.
Lifting the segment on to the Pylon.
Later, the temporary tendons would be removed and replaced by the permanent set. The addition of a 2 m wide in-situ concrete stitch between the top flanges of the twin segments of the viaduct box girders, would enable the in-situ concrete deck slab to be laid right across the width of the deck in a single operation. Not counting the special pier units, 27 segments of equal length went into the construction of each completed box girder span, a total of 54 per span.
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