A VITAL elevated section of Birminghams M6 Spaghetti Junction will stay shut for at least another nine months, it emerged this week nearly doubling the originally estimated closure.
The Highways Agency risked traffic chaos when it closed the 20,000 vehicle/day sliproad for 10 months last March, to allow inspection of the structure and for repairs to be carried out.
Design complexities of the one-off structure, plus problems finding a repair solution which avoids further weakening, have been given as the reason why work, due to finish last month, has so far failed even to start.
Tenders for the proposed 0.5M strengthening of deck crossbeams with stressed reinforcing bars will now be sought within a fortnight. And it is hoped repairs will be sufficiently advanced by autumn to reinstate full traffic flow.
Problems have been caused by weakening of a 70m long V-shaped section of sliproad. This normally carries two converging lanes of traffic on to the M6 northbound. One lane contains local suburban traffic but the other is the main feeder route from Birmingham city centre.
Last March the structure failed the HAs standard stage two bridge assessment check for 40t vehicle loads. Local traffic was immediately removed, and flow from the city centre routed across the strongest sections of the viaduct.
The structures three span deck, carried on concrete column supports, consists of three distinct sections. The 20m long central span, which crosses a local electrified rail line, is formed with precast concrete beams, while trapezoidal shaped side spans either end have insitu concrete slab decks.
The problem lies with the two large crossbeams either end of the central drop-in span. These 2m deep beams incorporate a shelf to carry the central precast deck, but were also built as insitu downstand beams integral with adjacent side span decks.
The Agencys project manager David Cropper said: We have no design drawings and it has proved far more difficult to analyse than anyone envisaged.
It took us some time to understand the live load flow paths through this section, he added. Sophisticated three-dimensional finite element analysis suggested the structure could barely support its own weight. But in reality this was clearly wrong as the deck has carried full, and occasionally abnormal, loads for years without showing any signs of distress.
Engineers now think the beams contain inadequate reinforcement for the several load paths that must be allowed for. Repair solutions are equally complex. The Agency and its structures consultant Maunsell is still finalising a safe installation sequence for the two rows of 2.5m strengthening bars. They will be drilled through downstand beams close to existing reinforcement and then lightly stressed.
We must not interrupt traffic flow overhead so have to carefully co- ordinate vehicle routing and reinforcement installation, said Cropper. If we introduce additional forces into the structure there is a danger of further weakening it. Downstand beams were taking some tensional forces without distress, but they could not risk adding to them.