Watch: I clambered through a tiny hatch 100ft above the water into a huge chamber inside the Tay Road Bridge

The route into the giant rectangular tubes, or box girders, that form the Tay Road Bridge was totally unexpected, as I accompanied The Scotsman visual specialist Lisa Ferguson and bridge chiefs to capture exclusive video footage inside the structure.

Wearing hard hats, hi-vis jackets and life jackets, we followed it the footsteps of the maintenance crews, which involves riding one of the mobile inspection gantries which move along the underside of the bridge, then boarding a scissor lift to take us up to one of the hatches.

Bridge boat Pier Pressure stood by in case anyone fell into the river.

Inside, and we were in one of a series of 48 hollow chambers, or “boxes”, never seen by the public, which stretch the length of each of the two carriageways of the 1.4-mile-long bridge.

In an indication of the age of the crossing, which will be 60 years old in 2026, there is no wired lighting inside, with the strings of old-fashioned 100 watt incandescent bulbs powered by a cable plugged into the scissor lift as we entered.

The boxes above the navigation channel section of the bridge - used by shipping where the river is deepest, near the Fife side - are the most spectacular. They stretch for 222m (730ft) - twice the length of a football pitch.

They are accessed as part of a continuous series of maintenance checks, including to cracks in the welds, usually conducted between April and October, and when the wind is gusting at no more than 42mph. It normally takes half a day to check each one.

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Seeing the underside of the bridge at such close quarters also showed off its surprisingly slender concrete columns, which are unevenly spaced because the crossing slopes down noticeably towards the Dundee end.

Bridge manager Alan Hutchison said that added to the under-appreciated charm of “our bridge” - as he said Dundonians regarded it.

He told The Scotsman: “It’s a much more elegant structure than it is given credit for. There are a lot of aesthetics there that many people don’t appreciate.

“We always get compared to our ‘noisy neighbour’ at the Forth Road Bridge - they always seem to get the attention.

“The Forth Road Bridge is a suspension bridge and has got that wow factor - like the [cable stay] Queensferry Crossing - and it’s really marvellous how they designed and built it.

“When the Tay Road Bridge was built, they called it a ‘down to water’ bridge, and I think that’s really unfair.

“It is a relatively simple structure design-wise, but when you look at the columns, bridge designer William Fairhurst changed the spacing of the spans,

“So as you come towards Dundee, the columns are actually closer together as they get shorter and squatter because the bridge is on a 1:84 gradient.”

Mr Hutchison said: “He wanted the bridge to look good and I think he achieved that.

“He was concerned about the perspective. If the columns were equally spaced, the bridge would look odd.

“If you look at the bridge from Fife, it looks perfectly spaced and really neat.”

But is there rivalry with the adjacent Tay Bridge, which carries the railway over the river and replaced the original crossing which collapsed in 1879?

Mr Hutchison acknowledged: “The Tay Bridge is absolutely stunning as well. Ours is much more functional.”

A major upgrade of the Tay Road Bridge’s carriageways was completed last month after a total of a year’s work, which involved contraflows.

Next will be replacing the original 1966 bearings at the top of the bridge’s columns.

Disruption caused by the £11 million carriageway works may have held back traffic returning to its pre-Covid level of 27,000 vehicles a day, although at 25,500, it is 95 per cent restored.

But that’s two-and-a-half times what the bridge was designed for, and when the lorry weight limit was increased from 20 tonnes when the bridge opened to 44 tonnes in 2001, Mr Hutchison said the crossing had to be completely design checked and its box girders strengthened.

“Diaphragm” or tie boxes connect the box girders to balance the load on the bridge.

Mr Hutchison said the bridge was a “critical piece of infrastructure”, with a 50-mile diversion via Perth when it was closed, which has included during strong winds of up to 125mph.

However, he said windshielding to protect vehicles would not be not feasible without significant strengthening work. He said: “You’d be as well building a new bridge.”

The bridge is unusual in taking traffic right into a city centre, although it makes for a dramatic entry to Dundee for drivers, especially at night.

Mr Hutchison said that could cause congestion and it would be avoided nowadays, “but back in the 1960s, it was the time of the motor car - it was all new and exciting”.

The bridge also played a role in the creation of Glasgow-based Loganair, now the UK’s biggest regional airline.

Prior to its launch, founder Willie Logan won the contract to build the crossing after submitting his bid 15 minutes before the deadline when flying into Dundee in a snowstorm, according to the airline’s former managing director and chair Scott Grier.

Mr Grier also related in his 2012 book, Loganair - A Scottish Survivor, that the airline’s first scheduled flight between Dundee and Edinburgh in 1963 was spawned by Logan (Contractors)’s regular flights on the route for staff building the bridge.

Mr Logan was killed in an air crash in Inverness in 1966 - seven months before the bridge opened - with his gravestone at Fodderty, near Dingwall, in the shape of the deck and two columns of the crossing.