Highways Agency

• M1 Tinsley Viaduct

Introduction

Tinsley Viaduct is situated at Junction 34 of the M1, approximately 3.5 miles north east of Sheffield City Centre. The viaduct is 32 years old, and a unique and complicated structure (a twin deck box girder bridge).

The bridge carries traffic on two levels. The upper deck carries the M1 motorway and was originally intended to carry three lanes of traffic in each direction. It was reduced to just two lanes north and south three years ago following an EU directive on lorry weights, requiring all structures supporting main roads to be capable of carrying 40 tonne vehicles.

The lower deck carries the A631 trunk road. There are roundabouts at each end of the viaduct at the lower deck level with connecting roads to the local road network and the M1. Tinsley viaduct was built in 1968 at a cost of £6m. However, the price of replacing the bridge today would be £200m and the cost to the country for delay and disruption during the construction period would be £1.4 billion.

The Project involved

• strengthening of existing joints
• welding of steel plates
• bolting of prefabricated structural elements
• strengthening of piers with additional welded steel plates
• installation of new torsional stops at abutments
• installation of additional shear connectors
• upgrade and replacement of parapets
• concreting of longitudinal boxes and rocker columns

 

The project involved the removal of lead based paint originally used to paint the insides of the box girders. All the paint was removed so that the strengthening work and future inspection and maintenance could be carried out in a lead free environment. All precautions were taken to carry out this work so that the environment and employees were fully protected from this hazard. Waste from the paint stripping was removed under strict procedures and delivered to a controlled tip.

Facts and Figures

The following materials was used during the £81 million strengthening scheme over three years:

• 2500 tonnes of steel added to the superstructure
• 1400 m3 (3500 tonnes) of reinforced concrete ? approx 250 truckmixer loads
• 100 km (65 miles) of welding
• 53000 bolts
• 75000 shear connectors
• 155000 m2 of paint removed and replaced (enough to paint an average sitting room 3100 times)
• 4 km of parapets

There were a number of challenges associated with the project. Most of the work took place inside the box girders on the underside of each deck of the bridge. The people working inside the boxes were required to undergo Confined Space training. All the equipment, metal blocks, concrete and workers had to enter through holes, which are approximately 1000mm by 600mm wide.

Machined blocks were welded onto the diaphragms of the bridge. The maximum allowable bearing tolerance (which is the amount of space between the machined steel block and the diaphragm to which it will be welded) is 0.25 millimetres over 60% of the area and 0.75 millimetres over the remaining 40% of the area.

In order to work within these limits, the existing welds within the bridge structure were finished as flat and smooth as possible. Using a 3D scanner these welds were then scanned. The information was downloaded into a second computer and then used to machine the steel blocks to suit the close tolerances required by the specification.

During the works, an innovative new access system called Safespan was used to allow work to be done to the underside of the two decks. This was the first time it has been used in Europe, though it has been used for the past seven years to help refurbish famous structures in the USA from the George Washington Bridge in New York, to the Golden Gate Bridge in San Francisco.

Cables to the underside of the bridge suspended the Safespan Platform System. Decking was then attached to the cables creating an enclosed work area, thereby preventing debris from falling on vehicles below and increasing safety for the workers.