The practice won a high-profile competition to design the bridge more than a year ago, but due to the council having a 'funding shortfall', the project has not moved on since.
The Department for Transport (DfT) is currently in various discussions about possible funding, but Spence, the lead architect, is backing a bid by opposition Conservative councillors to publish the designs.
Spence said: 'The council is reluctant to issue any images of the scheme, as I don't think it wants to show anything to the public that may not materialise.
'I think they have had their fingers burned in the past, so they are wary of releasing anything. We can understand this, but we just think by releasing the images the scheme may get public support, and public backing, which can only help,' he added.
The scheme is part of a wider regeneration of the area, and the bridge was set to symbolise the surrounding redevelopment.
Spence Associates saw off Frank Gehry to design the project, but more than a year on nothing has happened to the scheme.
The DfT said: 'The department is working with Sunderland on resolving a number of issues before ministers can consider the case for funding.'