Edinburgh North-South trams: Fears over whether South Bridge can take the strain
Edinburgh World Heritage has warned against running trams on South Bridge because of fears over whether it could take the strain.
Edinburgh council is proposing that its new North-South tramline should use the Bridges corridor as its route from Granton to the Royal Infirmary. The plans are currently out to consultation.
But Edinburgh World Heritage (EWH) - the body charged with safeguarding Edinburgh’s World Heritage Site covering the Old and New Towns - said there was not enough information available to show that South Bridge could take the load of trams.
And it said: “Given the evidence that is available at this time, we do not support the use of South Bridge as a route option due to the potentially serious implications and damage to this Category A-listed Georgian “megastructure”, which is one of the most important attributes of the World Heritage Site.”
South Bridge was built in the 1780s and consists of a series of stacked multi-level arches now almost entirely hidden from view by buildings on either side. The only visible span is the one over Cowgate.
EWH said: “South Bridge is a marvel of Georgian engineering. Built over a four-year period, it is one of the largest structures in the World Heritage Site, forming a continuous roadway and street over the steep valley. It was designed as a single entity and is a physical manifestation of the urban ambitions of the city at the time.
“Its importance as a contributor to the Outstanding Universal Value of the World Heritage Site cannot be overstated.”
Engineering consultants Jacobs were commissioned to produce a report for the council on South Bridge which acknowledged a lack of information available about the condition of the structure but concluded the new tramline “can be delivered across South Bridge”.
It said: “It is well established that multi-span masonry arch viaducts can accommodate actions arising from combined light rail and vehicle use. For South Bridge, providing that the structural capacity can be determined, and defects addressed, it is reasonable to anticipate the structure can be safely repurposed for use within a southern extension of the Edinburgh Tram network.”
But it added: “Given that the structure is large and old, significant further analysis is required.” It recommended radar and 3D laser surveys, inspection of hidden vaults in adjacent buildings and surface excavations as well as a survey of utilities and drainage in the bridge.
And it said a mechanisms analysis of the bridge and the forces involved in braking and traction “may show that the current structure is not suitable to carry tram loading without additional works and strengthening”.
EWH said “The [Jacobs] report provides no assurance or technical assessment of South Bridge’s capability to take the dynamic loads of a modern tram, and no details of any reinforcement works to address these were provided. Until such time as suitable evidence that any impact can be mitigated, we advise that South Bridge should not be considered capable of accommodating a modern tram.
“EWH accepts that while it may be possible to mitigate these concerns, the Georgian structure would make this process complicated and expensive.”
And it added that the council’s intention to use battery hybrid trams, which could be lighter and would require less overhead cabling, might also mitigate some concerns.
EWH said it continued to support the expansion of the tram network in Edinburgh and suggested that “other route options, such as Morrison Street or Lothian Road should be kept open”.
The council was contacted for comment.