London icon gets an overhaul

Without a shadow of doubt one of the most recognisable landmarks in the nation's capital is Tower Bridge. Along with Beafeaters and red double decker buses, nothing is more quintessentially London. The Dome, the Millennium Wheel and Canary Wharf have all tried to wrestle this mantle but without success, even though the bridge was described by one trade journal, upon its completion in 1894, as "the most monstrous and preposterous architectural sham we have ever known." It is as popular as ever with both tourists and commuters alike, with some 40,000 vehicles traversing it every day and lifting over 900 times a year. With traffic on this scale it was vital that any work performed on the bridge caused the least disruption possible. So, when a recent engineering survey highlighted problems which could cause failures in the long term, it was essential that the bridge was upgraded and kept looking and working as good as ever in the shortest possible time. In 1975 the bridge underwent a much needed upgrade, having its steam power plant replaced with a modern electro-hydraulic system. This upgrade was performed by Bosch Rexroth, who was also chosen as the prime contractor for the time-critical £1.3 million project. The initial design and pre-assembly work was started back in October 2000. Subsequently, Rexroth installed a system of active resting blocks, incorporating electronic load cells, under each bascule; it strengthened the hydraulically-driven nose bolts that interlock the two leaves; upgraded the cam-and-lever pawls at the tail end of each bridge deck; implemented a new programmable logic control (PLC) system, complete with increased monitoring equipment; and replaced the main hydraulic power units, providing 100% redundancy in the event of motor/pump failure. The actual site work was carried out within a tight 39 day, 24/7 timescale and was completed exactly on schedule, the resulting upgrade having a life expectancy of at least another 20 years. The Problems Vibration problems, particularly noticeable when heavier vehicles crossed the bridge, first manifested themselves in the 1990’s. There were also difficulties operating the nose bolts and pawls, and the trunnion shaft bearings were showing signs of wear. A survey carried out by an engineering consulting firm, identified that vibrations from the trunnion shaft were being transmitted directly into the steelwork of the towers. It was also discovered that the dead weight of the bascules and the live load of the traffic were not being carried fully on the resting blocks and pawls, as intended, but partly on the shaft bearings. The pawls were not locking the tails of the bascules in the down position either and the nose bolts were not engaging correctly, due to misalignment and wear. Since the bascules were not sitting evenly on their resting blocks, a new system of active blocks, with moving wedges and load transducers, was proposed; these, together with refurbished pawls, would allow the bascule decks to be lifted off the main trunnion bearings, as in the original design. The ageing hydraulic system also now required all four pumps to operate the bridge at normal speed and replacement parts were not readily available, so a complete upgrade was recommended, together with a PLC-based control system, offering increased flexibility and monitoring capabilities. Nottingham-based Fairfield Control Systems was selected to design, install and maintain the control system which was based around an Allen-Bradley ControlLogix PLC. This was linked Rockwell Software RSView operator stations via a ControlNet high speed fibre optic network and to the hydraulic systems by Allen-Bradley Flex I/O modules. "The control system has been designed with a minimum life of 25 years, so we needed a modern, future-proofed PLC and SCADA package," said Nigel Montgomery, an application engineer at Fairfield. "The ControlLogix PLC makes maintenance easier because an engineer only needs basic configuration software on a laptop to troubleshoot and maintain the system. We are also able to dial into the PLC and RSView systems and diagnose many faults remotely." The bridge can now be operated by a single operator from either of the two control cabins on each side of the river using a simple joystick. Pushbuttons are used to start and stop pumps and operate ancillary systems such as the pedestrian and road barriers across the bridge. "If the situation had been allowed to continue, it would have resulted in serious damage to the pivots and bearings, which would have entailed dismantling the bridge to repair," says the Corporation of London’s principal engineer, Andrew Downes. "Tower Bridge is a national icon and if it ceased to operate that would be a major setback. So it was critical to complete the refurbishment work with the minimum amount of disruption, whilst ensuring it will remain operational for future use." Bosch Rexroth Rockwell Automation Fairfield Control Systems Did you know?Designed by Sir Horace Jones and engineered by Sir John Wolfe-Barry, the construction of Tower Bridge began in 1886. It was built to cater for the increase in traffic across the Thames, which had outstripped the capacity of existing bridges. Finally completed in 1894, it needed to offer 140ft (43m) clearance for tall masted ships but also be shallow enough for horses pulling heavy loads to navigate. Jones, the City's architect at that time, proposed a low-level bridge based on the bascule principle. Reached by two suspension-style spans each 270ft (83m) long, the main section comprises two towers, linked by an overhead walkway and a 200ft (61m) long roadway which is constructed of two counterweighted bascules both of which can be raised to an angle of 86º. It was the largest, most sophisticated bridge ever built, driven by hydraulic power on a scale never attempted before. Interesting facts from Tower Bridge In 1910 the high-level walkways, which were designed so that the public could still cross the bridge when it was raised, were closed down due to lack of use. Most people preferred to wait at the bottom and watch the bascules rise up. In 1912, to avoid an accident, Frank McClean had to fly between the bascules and the high-level walkways in his Short biplane and, in 1952, a London bus had to leap from one bascule to the other when the bridge began to rise with the bus still on it.