One of the world's best known landmarks is under threat from ever increasing traffic loads. Dave Parker opens NCE's bridges special with a report on how engineers are tackling the rebalancing of Tower Bridge.
By the early 1890s the last challenge facing the builders of Tower Bridge was assembly of the two massive bascules spanning the central 61m of the Thames. When complete each would weigh more than 1000t, measuring nearly 49m from tip to counterweight, by 18m wide. Made up of four parallel steel girders topped with a woodblock roadway and two timber footways, at the time these were by far the largest bascules ever built, each needing 350t of counterweight at the shoreward end.
Each turned upon a solid steel trunnion, some 300mm in diameter and weighing more than 25t, supported by eight plain hard faced steel bearings. During the bridge's rapid hydraulicallypowered opening and closing operation - from horizontal to 89degrees in 90 seconds - the entire weight of each bascule would naturally be taken by these bearings alone. But, when both came to rest in the horizontal position and the locking bolts at the centre slipped into place, a degree of weight transfer was planned to take place.
'There are four cast iron resting blocks on each pier in front of the trunnion and hydraulic pawls at the rear of the counterweights', explains High Point Rendel project manager Martin Ray. 'As the set-up is currently configured, only a small proportion of the weight is transferred to the blocks and the pawls, so the trunnions are constantly loaded.
'But there's a 20mm gap between the bearings and the trunnions at the top, so we believe the original intention was for the whole bascule to lift slightly as it came to rest, taking all the weight off the bearings.'
Given that there is no other obvious, straightforward way of removing and replacing the bearings, this is a logical hypothesis, first aired during a major refurbishment in the 1930s.
Why the Victorian engineers did not implement the idea will probably never be known. Perhaps, as they carried out the first live tests on the bascules, they discovered that the flexibility of the rivetted structures meant they never settled into the same position twice running. Hence the loads transferred into the resting blocks vary unpredictably, so much so that forces at the pawl jacks can actually reverse.
These pawl jacks are designed to make sure that the slightly nose heavy bascules line up accurately enough for the locking bolts to function. But in recent years, says Ray, the load variations have caused the overcentre mechanism of the pawls to slip back on occasion.
Coupled with growing signs of excessive wear in the bearings' bottom quadrants, the need to rebalance the bascules is urgent and pressing. High Point Rendel project engineer David Holmes says the answer is to transform the passive resting blocks and pawl jacks into active mechanisms.
He explains: 'We've devised a wedge action resting block replacement that will move into position once the bascules return to the horizontal. These will raise the whole bascule 11mm at this point - and ensure that the load transfer is standard and predictable.'
Raising the bascule in front of the trunnion would produce a compensating downwards movement of 76mm at the rear of the counterweight. This will be countered by the pawl jacks, which will come into action once the resting blocks have done their work and return the bascule to the horizontal.
As a result the trunnion will lift 10mm, removing all loads from the bearings. Removal and replacement then becomes relatively simple.
Equally important, if less dramatic, will be the renewal of the 150mm square nose bolts and a complete refurbishment of the eight hydraulic motors installed in the 1970s to replace the original machinery. Control panels in the two control cabins still operational - originally there were four, staffed 24 hours a day - are also due to be modernised.
The bridge opens fewer than 1,000 times a year. Rebalancing the bascules and renewing the bearings and nosebolts will keep this part of the crossing functional for many years to come.
The real question mark relates to future traffic loadings.
Live load capacity limitations on the side spans means there is a 17t weight limit and 20mph speed limit on the two lanes that cross Tower Bridge, frequently ignored and largely unenforced.
Should moves to introduce congestion charging in central London come to fruition, there is a serious risk that the proportion of overweight, overspeed vehicles on the bridge will increase significantly.
How much more of this overloading the structure can stand is hard to predict. The saga of Tower Bridge is far from over. What seems certain is that its distinctive silhouette will feature in inummerable tourist snapshots for generations to come.