The standard procedure for erecting the deck of a cable-stayed bridge would be followed, starting at one of the pylons and adding single deck units, alternately, first to the centre span and then to the back span, so that the growing section of deck remains in balance over the pylon, in order to minimise the bending forces in the pylon legs. A production yard was established in the main Avon construction complex for the assembly and casting of the deck units, each 7.3 m long. It should be noted that, unlike the twin pairs of deck units used on the adjacent viaducts, those adopted for the bridge were of full deck width.
The structural steel work for the deck was all fabricated in Italy and trial-erected, there, to check the geometry. It was then shipped to Avonmouth and brought to the construction yard with each deck unit in six separate pieces. The steelwork had previously received its first two coats of epoxy primer and micaceous iron oxide. It was then assembled and positioned under the form-work on which the deck slab would be cast. While still in the construction yard, the slab reinforcement was then securely fixed in position and the concrete was poured, leaving room for a 2 m wide lateral concrete stitch to be added later, to link each new unit to the previously erected unit.
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. Each deck unit slab was cast while its steelwork was still attached to the previously completed unit, in front, and the assembled steelwork of the next unit, behind. The previously completed unit was then removed and carefully weighed before being sent to the painting shed. This was because the deck alignment control system needed to know the actual weight within 2%. The final coat of polyurethane paint was then applied to the steelwork in an enclosed and heated shed.
The special deck units that would sit on the lower cross beams of the pylons, on the back span piers and against the viaducts, were all prepared separately so that they could be introduced into the casting sequence at the appropriate time. The deck units were transported to the pylons in the correct sequence, using a motorised barge. The barge was fitted with laser and computer technology that enabled it to maintain its intended position within 500 mm, so long as the tide was running at no more than 2-3 km/hour. As the tide in the estuary could run at up to 10 km/hour, lifting operations were restricted to one hour each side of high tide.
Erection of the deck started at the Avon pylon with the special unit earmarked for the lower cross beam of the pylon. This unit was lifted by a crane mounted on a jack-up barge, into position on the lower crossbeam and temporarily tied down. The next two, standard, units were then lifted by the same crane, one to either side of the central unit to provide sufficient room for two cranes on the deck. After cable stays had been attached to the second and third units and their stresses adjusted, two DSL cranes were lifted onto the embryo deck, one to deal with the centre span, the other with the back span.
Successful erection of the deck units was achieved through the use of two DSL cranes, each comprising a pair of luffing jibs (see photographs/ illustrations) mounted on a common body, with a lifting frame permanently attached. For operational purposes, the DSL cranes would stand on the leading edge of the previously erected deck unit, anchored through the top of that unit, to the structural steelwork below the deck slab. When a new deck unit had been lifted into position, it would be held by the crane while it was fixed to its predecessor and hooked up to the pylon by its own pair of cable stays. 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. 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.
Each deck unit would be transported to the bridge site on a motorised barge as part of a pair, the first for inclusion in the centre span, the next for the back span – always in that sequence. The appropriate DSL crane would pick up the unit from the barge and hoist it to deck level. The new unit would then be bolted to its predecessor in the sequence, using the bolt locations that had been determined during the preliminary assembly of adjacent units in the construction yard. The alignments of the deck units would be checked after bolting. Then, in situ concrete would be poured to provide the lateral 2 m concrete stitch between the new unit and its predecessor. The weight of this concrete was required to balance the deck, but the cable stay installation could get underway as soon as the stitch had been poured.
The weight of the special deck unit that would sit above the first back span pier proved to be beyond the capability of the DSL crane. 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 therefore decided that the two units should be cast together and a crane, mounted on a jack-up barge, was used to lift them into place. Then, when the first back span deck unit had been raised and bolted to the adjacent pier unit, eight cables were used to tie down the embryo deck to the pier.
Erection of Cable Stays
All cable installations were undertaken by international specialists, PSC Freyssinet. Each completed cable consists of a number of parallel seven-wire galvanised strands locked in position by tapering wedge anchorages. The number of strands per cable varied from 19 to 75. The first strand was threaded through the cable sheath and one end of the combination was picked up by a tower crane. The other end was threaded through the lower anchorage and inserted into a mono-strand stressing jack. 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. The mono-strand jack at the lower end then applied a tension equivalent to about 60% of the required final tension, forcing the wedges into the anchorage.
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. This second strand was tensioned using the mono strand Jack until the load measured by the load cell was equal to that measured on the first strand. The process was continued until all the strands in the cable had been installed and stressed to 60% of the required cable load. The additional extension of the cable required to increase the load to 100% of the required value was then calculated and each strand was stressed to achieve this extension. This was because the load applied to the strand could be controlled more accurately by extension than by load measurement. A similar cycle was followed for the corresponding back span cable stay, keeping the lengths of deck, either side of the pylon, in balance.
Some cable oscillations were experienced during the final months of construction and it was decided that the secondary cables, or aiguilles, should be added to provide a lateral link between the cables to dampen these oscillations. Five aiguilles were fitted to each group of 30 main cables. These aiguilles comprise 27 wire strands passing each side of the cables and they are connected to the cable-stay ducts by purpose-made clamps. At the lower end, they are fixed to the deck and tensioned. The damping characteristics of these aiguilles proved to be disappointing so, at a late stage, two dummy longitudinal ‘girders’, made of pressed steel panels, were introduced below the concrete deck between the main outer steel girders. These provided additional aerodynamic damping by breaking up the airflow under the deck and the final configuration has proved effective under all wind conditions experienced to date.
When all the deck units had been erected from the Avon pylon, the DSL cranes were moved to the Gwent pylon, to begin a similar sequence there. The final deck unit of the main span was lifted from the Gwent side. Before this operation could go ahead, the size of the gap between the two cantilevers had to be checked. It was found to be correct within 20 mm but with different gaps between the pairs of plate girders. The splice plates that would connect the pairs of plate girders were drilled according to the measurements obtained.
Adding the final deck unit
In order to insert the final unit into the centre of the main span, it would be necessary to find a means of making small adjustments in the positioning of the existing sections of deck. First, the decks were effectively fixed, longitudinally, to the pylons by modifying the hydraulic circuit on the shock transmission units (described in a paragraph below) to prevent movement. 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. However, when the last cable stays were stressed, there was a difference in level between the two sections of deck, due to the presence of the DSL crane on the Gwent side. The crane was moved back, past the pylon, to equalise the levels. The shock transmission units were then used again, this time by removing some fluid to move the two sections back towards each other, to enable the girders to be spliced together. The modifications to the hydraulics of the shock transition units were then removed to allow the unit to perform as originally intended, i.e. as combined springs and shock absorbers. The final two in-situ concrete stitches, one either side of the last deck unit, were then poured to complete the main span.
Joints and bearings
The movement joints between the bridge and the viaduct, in effect, allow the two elements to move independently of each other and so erection of the final deck units, at the extremities of the bridge, were straightforward. Details of those joints are described under “Construction of Viaduct deck”. Vertical and lateral shear forces are transferred from the ends of the bridge to the viaduct, respectively, by pot bearings and sliding elastomeric bearings on steel beams fixed to the viaduct deck. The completed bridge deck is a single entity, without any internal movement joints.
Hydraulic shock transmission units were fitted to each pylon to transfer live load forces from braking and other minor incidents, and from seismic action, from the deck – equally – to both sets of pylons. These units incorporate elastomeric springs to keep the deck central about the pylons. Vertical forces from the deck are resisted by sliding pot bearings situated on the lower cross beams of the pylons. Lateral wind forces are transferred to the pylons by elastomeric bearings, fixed to the sides of the deck.
On each back span pier, guided sliding pot bearings accommodate longitudinal deck movements. These bearings also transmit vertical loads and wind loads to the pier on which they sit. Four vertical tendons were used at each back span pier to tie down the deck to the pier.
The Bridge was eventually opened on the 5th of June 1996 but, on the 12th of May a charity Event was arranged. Members of the public were invited to walk across the Bridge, and back again, to raise money for a charity of their own choice.
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