New York’s Triborough Bridge which opened in 1936, is undergoing its first major reconstruction. The bridge connects Queens, Manhattan and the Bronx and spans the Hell Gate passage of New York’s East River. In its first year an average of 30,000 vehicles a day crossed the bridge and today it carries more than 200,000.
When joint venture contractors American Bridge and Koch Skanska were contracted to manage the 12-year reconstruction in 1997, a major concern was how to handle the deck replacement on the suspension sections over the East River. Normally this is done by closing traffic lanes and using mobile cranes to haul out the old sections and bring in the new deck panels. Part of the contract with the MTA-Triborough Bridge & Tunnel Authority required American Bridge/Koch Skanska (AB/KS) to keep seven lanes of traffic open during the entire project. To do this, AB/KS needed a different method of removing and installating the deck panels. The method chosen uses three overhead double girder bridge cranes, one for each span, and each spanning the 30m (98ft) of the entire roadway.
To create a new road surface AB/KS is removing all of the existing road surface from the bridge, one lane at a time, and replacing it with new steel orthotropic (having different elastic properties in different planes) deck sections covering more than 25,300m2. The crane supplier Demag Cranes & Components claims it as a first of its kind crane application in the USA. AB/KS describes it as a unique and challenging project. ‘We realised the use of overhead cranes would make the roadway removal a success. Especially with such stringent requirements – a span of 98 feet across moving traffic; a 30 ton capacity for each crane; maintaining specified wheel loads; and the requirement to keep a constant flow of traffic on the bridge while the cranes were in operation – we knew it would be a challenge to make it happen’, says American Bridge chief engineer Stanley Walker. ‘We even experienced difficulties finding a crane manufacturer to bid on the project. It wasn’t until Follansbee & Gruner became involved that the problem was solved,’ said Walker. Follansbee & Gruner, a long time supplier to American Bridge, approached Demag which also has a long-standing relationship with American Bridge. Within two months of accepting the project Demag had delivered the three cranes.
Demag district manager Doug Teeple says: ‘While our components aren’t unique, it’s the way we combined and applied them to this concept that made it a success. However, at first glance, the criteria did seem impossible. We were asked to design a crane system which was to be mounted on a New York suspension bridge, it needed to operate outside, year round, under extreme weather conditions with the ability of forward and reverse variable speed travel on an incline of 3.6%.’
When designing the three crane solution, Demag engineers had to consider wind speed, design a different girder profile, and manage specified wheel load capacities. Finite element analysis was used in designing the girder, which had a reduced profile with eight-wheel end trucks at special wheel centres running on it. The reduced profile, meaning that the girder had to be wider, enabled the crane to operate in windy conditions by reducing the sail effect. As an extra safety feature, Demag installed an anemometer linked to the safety brakes via a PLC. In sustained winds of more than 40km/h (25mph), the PLC stops the crane and automatically engages wind locks to the rail, securing the crane in place.
To operate on a 3.6% incline the cranes are each driven by eight load bearing wheels. Each of these have a Demag cylindrical rotor motor with brake, and are controlled by a central variable frequency inverter to help control acceleration and prevent wheel slippage. Since the crane is running in all weathers an additional encoder feedback system is used to detect wheel slippage. The encoder is between the two end trucks on each side of the crane on two non-load bearing wheels. It compares the rotation speed of the idler wheels with that of the drive wheels and when a difference is detected the crane shuts down. Like the anemometer the encoder feedback device is connected to the PLC so when wheel slippage is detected the PLC immediately sets the magnetic brakes and stops the crane, independent of the friction between the wheels and rail. Each crane also has manual lock downs. In addition to the standard motor brakes, the wind locks and magnetic brakes, these lock downs are set at the end of each day so that the crane does not move when unattended.
The new and old roadway deck panels that are are being moved vary in configuration, which means that the hoists and trolleys, even though they are standard components, had to be specially modified for the application. Each trolley has two standard Demag 15 US ton DH wire rope hoists mounted on a single frame at 5.5m hook centres. Each hoist has a separate inverter for variable speed operation and is supplied with simultaneous and independent operation to give maximum control.
Having two 15 ton hoists allows the operator to pick up lighter loads and using both hoists together gives the added ability to pick up larger deck sections using two-point pick ups for stable transporting and positioning. Demag’s Travel Wheel System is used for trolley travel, with its DRS wheel blocks and gearmotors which are standard on all Demag cranes. The trolleys are also controlled with separate variable frequency inverters for smooth operation and accurate positioning.
Each crane has its own power supply from an on board diesel generator.
‘In addition to the three crane solution, Demag designed their cranes to be erected in a one-time, single night operation, eliminating the usual tedious moves between stages and forcing additional lane closures in the case of traditional gantry systems,’ says AB/KS project manager Kwadwo Osei-Akoto. Each crane was installed in a three lift operation, each lift taking only seven minutes and during the whole process the bridge was never closed for longer than this. Two cranes were installed simultaneously, followed by the third. The girders were set first and followed by the trolleys. Kwadwo put the success down to the design of the end trucks that were already attached to each of the crane girders.
