As a long awaited £44M bypass takes shape around the Yorkshire town of Selby, Alan Sparks sees a rare and romantic solution to crossing the River Ouse.
Designing and building structures that move pushes engineers' skills to the limit, so working on the 95m span A63 Selby bypass swing bridge is both a privileged posting and a major test for the construction team. 'None of us have been involved in anything quite like this before. It is a unique structure and so everything involved with it is out of the ordinary, ' explains John Harman project manager for contractor Skanska.
The £7.5M cable stay swing bridge will be the dominating feature of a new 10km single carriageway bypass around south Selby. As well as the river crossing this new single carriageway highway crosses two major roads and two rail lines. Due to low lying surrounds of the eastern stretch near the Ouse, much of the road is being built on 9m high embankments won from a 13m deep cutting earlier in the job. Topography around the river crossing was a major factor in the selection of the swing structure.
Although shipping is now rare along this section of the tidal Ouse, occasional vessels still require passage. 'With span and flooding constraints, the only real alternative to a swing bridge was a 30m high level crossing.
But this would have needed too much land take, cost far more, even on whole life costing, and been much more imposing on the landscape, ' explains Highways Agency project sponsor Alan Begg.
Plumping for a swing bridge solution may have been brave, but making it a reality really tested the engineers' mettle. The design team decided on a cable stayed structure with a 55m main span and a 40m back span. The deck would pivot 90 0around a 75t cog system positioned beneath the single pylon to open up the navigation channel when required.
The hollow section concrete pivot pier is built on piles that rake in all directions to resist the hefty torsional forces needed to get the bridge a-swinging. It houses a 20t, 3m diameter slew ring, which will be driven by six motors connected to its outer edge. On top of this sits a 55t transition element that couples with the slew ring cog and provides seating for the 1,200t deck.
Steel deck and tower, complete with cable stays, have already been constructed alongside the pivot pier adjacent to the river. Twin plate-welded girders with plate welded cross beams and cantilevers support an orthotropic steel deck, while the box girder pylon, which will sit over the pivot, towers above it.
Each span will be supported by two pairs of cables at roughly third points, and at the top of the pylon is a control cabin with a view of both river and road.
Fabricating and erecting the structure is subcontractor Cleveland Bridge. Deck segments weighing up to 30t were fabricated at its Darlington base, trucked down to Selby and welded together insitu by a team of up to 30 welders. Piles were installed ahead of delivery to support temporary works needed to ensure the deck was constructed to precise tolerances.
'Once the deck is completed, the whole structure will be lifted on two temporary 70t transverse beams, ' says Cleveland Bridge project manager Malcolm Bell.
'Temporary works are a major feature of this job, with over 500t of steel in the temporary structures alone.'
The 2.5m deep temporary beams will be lifted 3m vertically until level with temporary steel skid rails either side of the pivot pier, mounted on bracing.
Incremental jacking will be used to raise the deck, with brackets fitted beneath each beam bolted to temporary columns after each lift while steel plate packing is used to raise the jacks to the new position, ready for the next lift.
Jacking is due to kick off in early September. Before this can be done though, the finished structure will be weighed using the temporary jacks so that the concrete counterweight on the shorter backspan can be adjusted, to ensure perfect balance.
Once lifted to the right height, stage two of the delicate manoeuvre comes into play - moving the deck 15m horizontally into position above the pivot pier. The transverse beams must be in perfect alignment with the skid rails either side of the pivot pier so that the heavily greased deck can be pushed from one set of temporary supports to another.
As currently planned, this push towards the pivot pier will be performed by a pair of hydraulic jacks, one pushing against each of the temporary transverse beams. However, Cleveland Bridge is carrying out a last minute review of whether to strand jack the deck instead.
'This will be easier to do as we will not have to keep stopping and starting. A constant pull will be more predictable, ' explains Bell. However, to pull the structure across will require installation of even more temporary works on the river side of the pivot pier.
When the structure is positioned correctly, permanent jacks within the pivot pier will lift it off the skid rails so that they and the bracing supporting them can be dismantled. The deck can then be dropped onto the rectangular transition plate, which has pot bearings on each corner to support the vertical force of the mighty structure. A cruciform arrangement of slider bearings sits around halfway along each edge of the transition plate to absorb any torsional forces during bridge opening and closing.
Once in closed position the whole deck is raised, again by the permanent jacks, off the transition plate. It is then transferred to four permanent pot bearings that slide into position on the walls of the pivot pier, relieving mechanical parts of the structure of load. The only time the transition plate will be weighted is when the bridge is opened. Tapered nose pins then extend into the abutments to ensure the bridge is on line, and there the bridge will stand until asked to move again.
To make a return sweep and open the bridge the process is reversed.
The swing movement takes 90 seconds, but could be completed much faster thanks to the high level of redundancy in the structure, assures Begg. Speed is crucial, because opening the bridge for any length of time could quickly snarl up the new bypass.
To prove the reliability of the mechanics, the bridge will swing ten times per day for 30 days. It should be open some time before Christmas this year.
Client: Highways Agency
Client's representative: Parkman
Main contractor: Skanska
Steel bridge fabricator and erector: Cleveland Bridge
Consulting engineer: High Point Rendel
Mechanics manufacturer: AK Heavy Engineering