After months of frantic effort, Millennium Bridge structural engineer Arup has come up with a solution for the opening day sway problems which led to the £18M structure's embarrassing closure only three days after it opened in June.
As reported exclusively in NCE last week, Arup has gone for a unique combination of viscous and tuned mass dampers tucked up under the deck, plus viscous-damped diagonal bracing at the piers. A complex viscous-damped tie will also tether the landward end of the southern span.
These proposals, which have been with the client's professional advisers since early September, appear to meet all original objectives. The landmark 'blade of light' will look much the same, with access for pedestrians unrestricted.
The wobble - particularly the lateral accelerations - is expected to be reduced to ultrasafe levels. But at an estimated £5M, the cost of the retrofit will startle some, and there is still no agreement on who is going to come up with the cash. Last Friday, Arup chairman Bob Emmerson would only predict that 'this matter will be resolved before Christmas'.
In an attempt to avoid further delay while the interested parties wrangle over the bill, Arup has decided to fund the £250,000 cost of trials on the bridge itself.
Emmerson said: 'Our top priority is to get the bridge fully open as soon as possible. The client's advisers have approved our proposals in principle and agreed that we can go ahead with prototype trials which should be complete by mid-January, weather permitting.'
The trial will assess the performance of two sets of tubular steel 'chevron' braces and viscous dampers, plus a temporary tuned mass damper. Arup's detailed proposals for the total solution show around 30 of these chevron/viscous damper sets facing each other in pairs along the full length of the northern and central spans and the outer end of the southern span.
The outer ends of the braces are connected to the deck edge tubes. As the bridge sways and deforms, the resultant lateral movement of the point of the chevron is taken, via a horseshoe-shaped link, to the outer end of a viscous damper. This is mounted on the central transverse arm back to back with its equivalent on the adjacent bay.
'These US-manufactured dampers have already been independently tested and shown to work with piston displacements as small as 0.25mm, ' Arup project engineer Roger Ridsdill Smith explained.
'In practice we expect displacements of up to 5mm, ' he added. 'These dampers were developed for the aerospace and military markets, and get round the normal piston sealing problem by enclosing the whole unit in metal bellows.'
As a result the dampers come with a 35 year guarantee. Arup has combined these with 30 tuned mass dampers (TMD), each weighing around 2t, of which all but four are designed to damp out any excessive vertical movement (see box). The other four, beneath the main span, will provide additional lateral movement damping.
For the trials, Arup will fit a single, temporary 5t TMD which will work both laterally and vertically if required. There will be no attempt to try out the proposed diagonal viscous dampers at the piers.
Arup intends to test the prototype set-up by marching up to 300 volunteers across the structure in a larger scale repeat of earlier experiments (NCE 10 August). If the bridge behaves as predicted - and the funds are forthcoming - Arup believes that work on the complete retrofit could start as early as February.
Installation should be relatively straightforward. The deck planks can be lifted individually to give access to the main bridge structure from above. Some form of temporary access platforms will be needed, and the successful contractor may prefer to hang a full length access deck off the existing structure.
In all, Arup reckons, it will take three months to install the dampers and tiedown. Provided the all-important validation and testing period goes well, the bridge could be re-opened close to its first anniversary.
Those who venture on to it in 2001 will find it a much tamer ride, Ridsdill Smith predicted this week. 'On opening day the worst effects occurred when there were around 2,000 people on the bridge - that's around 1.3 pedestrians per square metre.
'At the quarter points of the main span the bridge was swaying about 70mm each way with alarming lateral accelerations of nearly 0.4g. After the retrofit we calculate that even with as many as two pedestrians per square metre the biggest deflection will be less than 20mm and the maximum lateral acceleration 0.04g.'