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New openings for Truss account

Some of the most innovative and exciting developments in steel construction are to be found in the dozens of new pedestrian bridges taking shape throughout the UK. Dave Parker opens NCE's first steel special of 1999 with a report on the design of one of t

Fairport Construction is due to start lifting the first 14m long sections of a pedestrian crossing in Oldham, Lancashire this weekend. The £1.81M King Street roundabout footbridge and Union Street West crossing has main spans of 30m and 42m respectively, and was designed by Robert Benaim & Associates for client Oldham Metropolitan Borough Council.

Benaim based the bridge design on simply supported steel trusses. 'The trusses are modular in construction to standardise and economise on the steelwork components for both footbridges,' says Benaim director Simon Bourne.

'Each truss comprises three discrete segments - two end bays and a central section - through which a top compression chord, made up of two braced tubes, and a bottom tension chord made up of a spine box beam, are continuous.'

Slender vertical posts at the third points along the span act as struts between the top and bottom chords, while centrally positioned diagonal bracing at the two end bays tie the top and bottom chords to the principal vertical support post at the ends .

Support for the 4.5m wide bridge deck comes from slim cantilevered rib beams, connected to the spine box beam, and spaced at 1.9m centres. 'The ribs transfer the pedestrian load to the spine box beam, which transfers the load in bending to the third points of the truss,' Bourne explains.

The two end bay diagonals, which consist of two 85mm diameter solid rods, then transfer the load in the spine beam to the end vertical support post and the top chord.

The 457mm diameter CHS end posts of the Union Street West footbridge are supported on piers through bridge bearing pads, while those of the King Street footbridge sit on a concrete capping beam and bridge bearing pads positioned on top of the retaining wall abutments, on either side of the road.

Project manager and architect is Kit Powell-Williams and the new crossing is due to open by the end of the month.

More than 40 designers entered the 1998 competition to design a new footbridge across the River Leven, the short, rocky stream that connects Loch Lomond in Dumbartonshire to the Clyde estuary. The winner was the Crispin Wride Architectural Design Studio, effectively the architectural division of Gibb Ltd, notching up the practice's first victory on such a commission.

'Our first competition was for the Poole Harbour Crossing, and we came second,' says CWADS architectural director Crispin Wride. 'On the basis of that success we were shortlisted for a number of bridge projects - but we kept on coming second.'

Such was the case in the competition for a bridge at Maidstone, Kent, for which CWADS entered an innovative A-frame cable stay design with deck and balustrades in structural glass. Its design for a new footbridge over the River Kennet at Reading, Berkshire was also placed second.

'This was a single-sided suspension bridge with a counter-catenary cable beneath the deck,' Wride says. 'The basic concept was the inspiration for our submission for the Loch Lomond project.'

Dumbartonshire Enterprise needed the bridge to connect Balloch Castle County Park to a planned new £60M visitor centre to the west. The navigable river can be busy in the summer, mainly with yachts and pleasure boats in transit to and from Loch Lomond, so some form of lift or swing bridge was required.

The client specified a minimum opening of 15m wide with 6m clearance with the bridge closed, and 12m high with the bridge open, Wride explains. 'Most of our competitors went for piers in the river and central lifting spans and the like.

'But we felt that on such a sensitive site a bridge which folded completely out of the way and maintained the view of the loch would be much better.' Avoiding disturbance to the river bed, home to otters and rare species of fish, was another priority.

CWADS' winning design featured two Reading-style catenary suspension structures cantilevered out 30m from each bank and meeting in the middle to create the impression of a classic twin tower suspension bridge.

'The opening should be quite a spectacular event,' says Wride. 'As the bridge 'splits in half', spectators will be able to watch from the elevated approach walkways and from the viewing decks at each mast.'

These 9m diameter circular platforms will remain stationary as the bridge sections swing through 90degrees. In the original design the bridge opened upstream, but this has now been reversed, for safety reasons, Wride says.

'If a boat is blown downstream out of control it is obviously safer if the bridge opens away from it. Reversing the opening also enabled us to simplify the approach ramps and improve the view of the loch.'

The choice of either electric or hydraulic power for the bridge's operation will be left to the successful bidder, but CWADS will specify the speed of opening to maximise the drama of the event.

Motors will be housed in the 7.5m diameter bases that surround the 2m diameter mast bearings beneath the viewing platforms. Originally the out of balance forces from the cantilevers were largely cancelled out by counterweights beneath the backspans. Wride says this feature is now being reviewed.

'When we began discussions with manufacturers with experience of supplying similar bearings for large cranes, they told us the extra vertical loads from the counterweights might be more of a problem than the out of balance forces,' he explains.

Masts are cable-braced 250mm diameter steel tubes, the design influenced by the yachts that will pass by. Initially these were 15m high, a dimension that attracted some criticism at the time, Wride reports.

He says: 'There were claims that the masts would have to be twice as high to take the loads. But since detailed design began we've reduced them to 12m, which fits in better with the trees nearby.'

The programme of detailed design, now past the halfway stage, is being carried out in conjunction with structural engineer Adams Kara Taylor. Still to be worked out is the detailed design of the central joint, but the design team has already looked at, and rejected, the possibility of including a dedicated cycle lane on the crossing.

Development of the design of the slender deck is almost complete. Measuring only 250mm deep at the centre horizontal member and 100mm at the edge, the steel deck was to be topped by a timber walkway. 'But experience on other recent footbridges suggests timber can become dangerously slippery,' says Wride.

'Instead, we will probably use pre-finished boards with an aggregate finish.'

In all, each section of superstructure will weigh little more than 16t. The design concept also simplifies construction logistics - and saves several trees into the bargain.

'The plan is to bring the sections in by barge from a nearby dock on the loch and assemble them on the river banks,' Wride explains. 'Otherwise we would have to remove trees to get in from the nearest road.'

Construction is due to start later this year, with completion scheduled for the start of 2000.

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