Motorists face three days of disruption as two giant girders, stretching for almost a quarter of a mile, are gently inched across a busy Glasgow street.

Several roads will be closed when hydraulic equipment is used to push the steel beams over Eglinton Street and the neighbouring west coast railway line.

The girders are an essential part of the new Port Eglinton viaduct which will be an integral part of the M74 extension. The viaduct will lie more than five metres above Eglinton Street and support thousands of vehicles using the motorway once the so-called “missing link” is completed in August 2011.

Once the viaduct’s decking is in place and topped by the motorway, M74 drivers will travel 10 metres above south-side city traffic.

The beams are already on site and supported by heavy-duty scaffolding. But, like a telescope, hydraulic equipment will be used to protrude the beams across the road and railway line. It will take engineers three days fully to extend the beams at a rate of 10 metres per hour. Each beam is 135 metres long and weighs 4000 tonnes.

Motorway project director David Welsh said: “We are endeavouring to minimise the impact of our activities on our neighbours and the travelling public.

“We are carrying these works out at night to minimise disruption to motorists.

“This project will complete a vital motorway link and reduce congestion around Glasgow as well as provide strategic transport links in the west of Scotland.”

Temporary road closures between midnight and 6am will take place from next Wednesday to the following weekend although that may change depending on progress.

Northbound traffic will be diverted off Eglinton Street and on to Turriff Street, Pollokshaws Road and Bedford Street before rejoining Eglinton street.

Southbound motorists will be rerouted along Cumberland Street, Laurieston Road, Cathcart Road, Allison Street, Hollybrook Street, Calder Street, Aitkenhead Road, back on to Cathcart Road, along Pollokshaws Road and Turriff Street before rejoining Eglinton Street. High-sided vehicles will be given different routes.

Transport Scotland today said the work will have no impact on rail services.

Work on the M74 extension which began in summer 2008 ended a 40-year wait, but the new five-mile stretch of motorway from Cambuslang to the Kingston Bridge will cost £692 million when construction ends next year. Building work will amount to £445m, about £200m of which has been spent on land acquisition with the balance meeting other costs such as security and mining works.

However, Glasgow’s business community believe it’s money well spent. The extension will provide three lanes of traffic that will bypass the Kingston Bridge. Manufacturers, especially in Renfrewshire, have repeatedly complained of traffic congestion as they attempt to truck their products along the M8 to the M74.