When the Ohio Turnpike Commission had to increase the capacity of one of its major crossings, it faced similar problems to the designers of the new Medway Bridge. Nina Lovelace reports on the American solution - and the British-owned contractor turning it into reality.
Interstate 80 in Ohio has much the same capacity problems as the A2/M2 in Kent (See pages 18-19). Widening to dual three lanes was the obvious answer - but this raised the question of what to do with the Cuyahoga Valley Bridge, a 1km long multispan twin steel truss structure dating from 1954. This carried two lanes each way, but was suffering from serious reinforcement corrosion in its concrete deck.
Over the years the Ohio Turnpike Commission (OTC) has had to use vast quantities of de-icing salts to keep the road open during the region's bitter winters. This gave the OTC little choice of what to do when it became apparent that a six lane bridge was needed rather than the existing four. Despite its charm, the old bridge had to go.
'The bridge would never take that kind of refurbishment, from four to six lanes, ' says Mike Myers, project engineer for Balfour Beatty subsidiary National Engineering that is currently constructing a new Cuyahoga bridge for OTC.
Standing on site beneath a blazing sun, Myers explains his point as he gestures up towards nets hanging beneath the old bridge to catch spalling concrete.
But in making the decision to new build, OTC also faced new challenges due to the valley's designation as a National Park 15 years ago. The accompanying plethora of environmental regulations meant that any new bridge would have to have its piers placed more widely apart to avoid several sensitive areas along the valley bottom.
'We've got the Cuyahoga River itself, a canal and a scenic bike route to avoid. The canal dates from the 1800's, and is now an important wetland environment, ' says Myers. He points out a turtle bobbing along on the canal waters, and goes on:
'These issues meant that the new bridge piers had to be 70m apart across the valley.'
But because this length was 14m wider than that maximum practical design span of the standard OTC simply supported beam and pier design, a new solution had to be found.
'OTC decided on a design that would involve erecting precast balanced cantilever beams on adjacent piers followed by drop-in beams, which are spliced to the cantilevers using post tensioning, ' says Myers. 'This type of design is still in its infancy in the US.'
OTC put out a tender for the new bridge in 1999, with two alternative designs using steel or concrete. National Engineering in a joint venture with US contractor Trumbull Corporation, bid for the concrete design and won. 'Concrete was £1.4M cheaper, so OTC elected to go with the concrete alternative, ' says Myers. Construction of the new bridge started in October 1999.
The new 18 span structure is actually two bridges running closely alongside one another, just like the original crossing.
Currently the new eastbound bridge has been built and the old eastbound demolished. The new westbound bridge is being built in its place. Both decks will eventually be 20m wide, carrying three lanes of traffic.
Each bridge is being constructed in three sections as only the central section requires 70m spans. The western 120m section and eastern 580m section are of a simply supported precast concrete beam design, commonly used by the OTC due to its cheapness and ease of construction. For these sections six concrete beams 2m deep are placed onto piers varying 13m to 60m in height. The spans range from 40m to 50m.
In the central 300m long section each balanced cantilever beam is placed in turn onto temporary bearings and attached to the pier hammerheads via posttensioning cables. Only after all of the drop-in beams are installed are the cantilevers lowered onto permanent bearings, Myers explains. 'We had to consider the deflection caused by dropping-in, ' he says.
The drop-in beams, each up to 30m long and 90t, and the adjacent cantilevers, have bulbed ends to allow the installation and anchoring of post-tensioning cables. Eight cables form each connection which is then surrounded with concrete. Once all of the drop-in beams have been stitched together, two 300m long cables are threaded through each beam and stressed.
Myers explains that final post-stressing was a particularly nerve racking exercise on the completed eastbound bridge, largely due to the length of the grout duct. 'Grouting and post stressing over 300m is quite an undertaking. Other projects have suffered voids at high points, ' he warns.
National Engineering utilised a non-segregating grout and installed 'bleeds' at the duct high points to ensure no air would be trapped.
After all of the beams are set in place and post-tensioned, steel diaphragms are laid over to provide a base for the deck and horizontal bracing. These are then filled in with concrete.
The piers are a hollow box construction of reinforced concrete, cast in 13m sections to cut down on weight, explains Myers. Reinforcement used is epoxy coated to protect against corrosion.
Scorching summer temperatures caused problems with concrete curing, says Myers .'We use canal water to keep the concrete cool, ' he says. 'But we've had problems actually getting the formwork out in time to reuse it on another pier, on schedule.
'That's because to remove it we need to send operatives down into the pier - and what with the concrete curing temperatures and the hot weather, it's been unbearable in there.'
The eastbound bridge was completed in October 2001, leading to the demolition of the existing eastbound bridge. The westbound bridge is halfway complete. The whole structure is scheduled for completion in May 2004.
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