Motorway traffic flowed across the UK's new Medway Bridge this month.
Is this the end of the story? Far from it, as Dave Parker reports.
There could have been just one new superbridge across the navigable River Medway upstream of Rochester in Kent, southern England. Or one new and one old, according to the Highways Agency's structural engineer FaberMaunsell associate Alan Toms. 'When it became obvious that the widening of the A2/M2 meant increasing the capacity of the crossing to four lanes in each direction, we looked at a number of options, starting with straightforward widening of the original Medway Bridge.
'But what made this project unique was the fact that the Channel Tunnel Rail Link would be crossing the valley on almost the same alignment.'
Widening the old bridge was soon rejected on the grounds of foundation inadequacy. Several options involving a combined motorway/CTRL crossing were also considered.
In the end a triple crossing won the day, featuring a strengthened and remodelled old bridge, a new bridge for the CTRL and, slotted in between them with just 20m each side to spare, a new road crossing carrying four lanes of London-bound traffic.
Both the new crossings would be essentially similar to the first;
box girders with related pier spacings and decks on virtually the same level. The illustrative design prepared by FaberMaunsell for the 950m long bridge featured post-tensioned concrete construction with 12 spans, the main river span measuring just over 152m. Atkins, structural engineer for winning JV contractor Costain-Skanska-Mowlem, produced a final design which was very similar if somewhat simpler, says CSM project manager Tony Scutt.
'We went for a very straightforward single twin cell box girder right through, with minimal haunches and chamfers to simplify formwork. At 20m wide, this is a pretty large box girder, which is one reason for choosing insitu construction.'
Another was logistics. By the time the CSM team arrived on site in January 2000, Nuttall was well advanced with the CTRL crossing alongside. Two railway lines ran along the valley side, one a busy commuter route into London's Victoria Station. Building above these would pose the usual challenges to the site team.
For environmental reasons foundations for the river piers had to be bored, not driven.
These were massive; 2.4m diameter and going down 25m to the underlying chalk. On top were equally massive pilecaps, each containing 2,000m 3of high strength concrete.
'We asked a lot of the concrete mixes on this job, ' reports CSM construction manager Charlie Ball. 'The foundation mix has 80% OPC replacement by ground granulated blastfurnace slag.
'On the superstructure we were looking for a compressive strength of 38MPa at 36 hours from a C50 mix - which meant a 'summer mix' with 60% ggbfs replacement and a 'winter mix' with 50%. And as we were pumping up to 150m horizontally and 40m vertically there was lots of water reducing admixture in there as well.'
Balanced cantilever construction was chosen for the main river spans, with simulataneous pours of 80m 3at each end of the growing cantilever. One special design feature here was the encastrÚ joint formed by continuation of the river piers right up to deck level, creating a 'zero section' from which the cantilevered boxes could spring.
Viaducts were largely constructed on falsework - except above the two rail lines. Here the working platform was provided by two enormous trusses weighing up to 1,000t each, which had to climb up the piers each side.
The inevitable deflection of the trusses under load was to cause problems, as Scutt reports. 'We were only allowed 1MPa of tensile force in the box girder before stressing, which was hard to achieve. The trusses had to be set with 140mm precamber to keep the crack width below a design value of 0.1mm'.
Given a free choice, CSM might not have opted for external prestressing tendons. Hendy says the key element was the design of the deviator pipes, 1,600 in total.
'There's always a risk of spalling around the deviator pipes. We minimised this by using medium density polyethylene pipes, which are corrosion resistant and easily bent.'
The result is the UK's - and very nearly the world's - longest post-tensioned bridge with external tendons. With all but surfacing work effectively complete, the CSM team is now gearing up for the next stage.
Delicate balance The new bridge may be nearly complete, but a major refurbishment of the old bridge is just entering its most complex phase. Described by the project team as 'a delicate balancing act', the work involves radical surgery as well as conventional strengthening and repair. Transverse cantilevers will be cut back, extra diaphragms implanted and the central 30m suspended spans will be removed section by section and a lighter, all in one steel deck inserted.
Under an earlier, separate, contract Nuttall has already carried out a major strengthening exercise involving the installation of post-tensioning tendons inside the two deck boxes. But there was a limit to what could be done while the old bridge remained open. Once traffic was fully transferred to the new crossing in September the CSM team should be able to get to grips with the final stages of rejuvenation - starting with the removal of the deck surfacing and a detailed inspection of the underlying concrete surface.
This will reveal any signs of de-icing salt attack. Meanwhile, the cantilever on the northern side of the bridge will be trimmed back by 1,600mm to match the southern cantilever, which was cut back while the old bridge still carried four lanes of traffic.
This will help compensate for the extra loads from the relieving slab, a wedge-shaped topping up to 550mm thick that transforms the cambers on the crossing into the correct profile for a one way, four lane carriageway. But before this slab can be completed come some delicate operations.
First, slots will be cut into the deck to allow at least six transverse insitu concrete diaphragms to be cast under each approach viaduct, linking the existing precast beams together. Then two 500t capacity cranes will be stationed at the end of each balanced cantilever span, where they will pick apart the existing suspended central spans and lower the 125t precast beams individually to barges on the Medway below.
Then comes the balancing act. As the cranes are dismantled and driven away, counterweights must be added to compensate for the missing 1,500t of suspended span - and these must be tuned so that each of the paired balanced cantilever boxes has virtually identical longitudinal profiles. This will allow the site team to link together the eight pairs of transverse diaphragms already installed inside the box girders by Nuttall, and finish the connection with longitudinal link slabs.
However, before the central gap between the now monolithic twin box girders can be bridged with a single steel span weighing 400t less than the original twin spans, a final piece of surgery could be required. De-icing salt damage to the post-tensioned half joint nibs that support the suspended span is suspected.
If this is confirmed, the nibs will have to be destressed, repaired and restressed before the new span can go in.
Tendon moments 'Some people say post-tensioned structures with external tendons are easier to design than those with internal tendons, ' says structural engineer Atkins special structures group manager Chris Hendy. 'But they're wrong.
'By the time you've ensured a clear walkway through the box and access to the drainage, finding room for all the deviators and anchorages is far from easy.'
With the tendons closer together vertically the efficiency of the post-tensioning operation is reduced. And with no opportunity to spread the tendons across the deck cantilevers, a double layer of tendons was needed in the main balanced cantilevers, leading to very complex anchorages.
The UK Highways Agency also specified that the bridge should be safe under dead load with 25% of the tendons at any section ignored or two cables removed. It should also be possible for one cable to be removed and replaced with the bridge open to traffic. The design team had to maximise efficiency of the prestress by all means available.
'For example, BS5400 only allows you to design for 70% of the ultimate tensile strength of the tendon after lock-off, ' says Hendy.
'By contrast, the new Eurocode allows 73.1% and the French code 80%. So we went for73.1%'.
A similar philosophy was applied to the tendons themselves.
Adopting draft Eurocode limits of 15.7mm strands to 1860MPa, Hendy reports, a saving of 700t of box girder reinforcement was achieved.
Sophisticated analysis was also needed on the massive anchor beams that transfer compressive forces from the tendons into the cross-section of the viaduct box girders. Measuring up to 2.5m wide and 1.5m deep, the most heavily loaded has a design load of no less than 10,000t. A finite element brick model was used to check the design - but buildability also had to be considered, Hendy points out.
'The reinforcement cages weighed up to 20t. It made sense to design them to be prefabricated at ground level and lifted into position by crane.'