The gossamer thin Metsovitikos Bridge is to be a vital link in the 700km Egnatia Odos highway, being built to link Greece's western seaboard with Turkey (NCE Roads supplement, 24 June). The route is challenging along its length, but when it reaches the Pindos mountains of northern Greece it has to negotiate steep, unstable slopes and is forced into tunnels and across ravines.
Just below the town of Metsovo itself the highway must exit a tunnel in one hillside, traverse more than 500m west-east across a 150m deep valley, and enter another tunnel on the opposite side. The drama of the setting - one of the most breathtaking in a area full of soaring crags and yawning valleys - called for a structure that would compliment it. Client for the highway, Egnatia Odos AE, with Brown & Root, put the design out to competition. The winning towerless suspension bridge scheme, by Ove Arup and architect Wilkinson Eyre, was selected earlier this year and is now drawing towards the detailed design phase.
'When we visited Metsovo we were struck by its dramatic vantage point, looking at the bridge in elevation,' recalls Arup project director Allen Paul of his first visit with architect Jim Eyre. 'Our initial thoughts were either to hold the bridge up from below, framing the valley with piers or arches, or go for cables and frame the mountains within the catenary.'
The steeply sloping hillsides enabled the partnership to eradicate pylons early on. Cables were to be anchored directly into rock, minimising visual intrusion and focusing attention on the arcing cables.
He and Eyre were also struck by the sense of theatre that would be created by driving from out of the tunnel and across the bridge deck. Working up ideas for a suspension structure they decided to have the cables curving in the horizontal plane, as well as the vertical. 'As you're driving across the bridge you'll experience the cables swooping towards you and then flying up and away,' Paul enthuses. Hangers suspending the deck from cables are 27degrees off vertical, creating a virtual valley in the air.
The area is an earthquake zone and design is being geared to one in 150 years and one in 1,300 years seismic events, equivalent to 0.25G and 0.45G rock head acceleration.
'You need to be able to carry on driving after the first event and the bridge should need only limited damage repair after the second,' says Arup project manager Ian Wilson. The Arup team is working with performance based methods to new Californian bridge design codes.
'The kind of structure we're going for works well in earthquakes: the bridge can move longitudinally, laterally and vertically,' Wilson observes. At each end of the bridge up to+/-600mm longitudinal movement can take place.
With Greek summer temperatures soaring to 35degreesC plus and winters reaching a biting -20degreesC, the all-steel structure is also expected to rise and fall +/-900mm vertically as the cables expand and contract and +/-1,800mm vertical movement is predicted under live load.
The arc of cables on the Metsovitikos Bridge will be 1:16 between abutments and 1:12 between anchors. 'The key to this project is tying forces into the mountain,' Wilson notes.
The Pindos mountains are notoriously unstable and major land slips have been encountered on other sections of the highway. Though aerial reconnaissance of the site indicated rock was competent, more thorough site investigation revealed a major landslip 80m wide and 40m deep where the team had planned to place the bridge's south west anchor.
A solid sandstone footing was found some way forward and downhill. As a result oversail to the south west anchor will be only 63m compared to 110m oversail to the north west anchor, introducing asymmetry to the suspension of the bridge.
Presence of the landslip above and behind the south west anchor has also affected the choice of footing. It is likely a 15,000m3 concrete gravity structure will be used to tie the cable down - it is impossible to tie forces back using rock anchors or a tension socket as they would be placed in the unstable ground, says Wilson.
The importance of access for inspection and maintenance has ruled out use of rock anchors for the bridge's other three footings. Paul says 45m deep tension sockets are being considered.
To minimise loading on the mountain engineers are opting for a steel orthotropic bridge deck. A composite deck was considered but the 230MN load it would place on the suspension cables, compared to the 150MN load for an orthotropic design, was dismissed as too great.
The engineers calculate that with an orthotropic deck £5M is being saved, delivering a final cost of £30M. Weight of steel in the deck will be 5,500t, with the cables weighing 2,500t.
Cables are to be 550mm diameter, each made up of 19 strands. Each strand will contain 450, 5.38mm diameter wires. The 800m cables will be spun in-situ. Once spinning is completed, cross ties equipped with jacks will be mounted and the cables will be pulled towards the bridge's centre line.
Hangers, either 60mm diameter locked coil or spiral strand construction, will be attached at 7.5m centres. To complicate Arup's job further, hangers are to be inclined at 8degrees from vertical at either end of the bridge, becoming progressively vertical towards its mid point.
The deck itself, carrying dual two lane traffic, is to be 565m long, 23m wide and 2.5m deep. Normally cost and weight considerations would dictate a narrower deck, says Wilson. But because twin road tunnels separated by a solid rock pillar give directly onto the bridge at either end, lanes are set 5.1m apart. Egnatia Odos may yet decide to build a single, solid deck, but the central 'gap' in the preferred design 'adds drama from beneath,' says Wilson. The two decks would be joined by steel cross beams at 15m intervals.
The deck consists of a 14mm thick deck plate with 6mm stiffening troughs. Wilson envisages components being prefabricated, flat packed and delivered to a yard near the site for assembly into 35, 15m long sections. Separate halves of the deck could be brought to site by truck and joined with cross beams either within the tunnel portal or at the end of deck already insitu immediately prior to launching.
Prequalification for construction is about to take place, says Paul, and tendering will be next spring. Construction of Metsovitikos Bridge is due to start in autumn next year.
In Greece, Metsovo is fast becoming as well known for its proposed new bridge as for its stunning scenery. Its new spider's web of a landmark should be complete in late 2003.