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Future of Bridges | Replacing the Polcevera Viaduct

Polcevera

Operators of the Polcevera Viaduct which collapsed in Genoa in August claim the damaged structure can be rebuilt in eight months. We assess the options.

While investigations to establish why a 200m section of Genoa’s Polcevera Viaduct collapsed in August, engineers’ attention is inevitably turning to options for replacing the structure. Before the failure, around 25.5M vehicles a year travelled on the A10 motorway across the dual two-lane bridge in the Italian port city of  Genoa. It can be assumed, therefore, that either the route will have to get back up and running as soon as possible, or a realistic alternative will have to be found.

So, what are the options? The first is to bypass this congested city centre location altogether. There has long been a plan to build a ring road to the north of the city that would take through traffic off the section of the A10 that includes the Polcevera Viaduct, also named the Morandi Bridge after its designer. This wider project, the Gronda di Genoa as it is known, was originally conceived around 20 years ago, and became something of a political football in the intervening years, before finally getting the go-ahead in September 2017.

Splitting urban and long distance traffic

According to the website of road operator Autostrade: “Some parts of the network register up to 60,000 daily transit, with a high percentage of commercial vehicles. It is therefore of paramount importance to split the flows of urban traffic from those passing through and those related to the port’s activity.”

It says the aim of the Gronda di Genoa project is “to alleviate the section of the A10 most interconnected with Genoa city, transferring the through traffic onto the new infrastructure”. While the section of the A10 referred to includes the Polcevera Viaduct, the plan anticipates that the existing crossing would still be used for local traffic, with the new road acting as an “off-site” reinforcement.

And even if the Gronda di Genoa does go ahead, it is an extremely ambitious plan that includes 72km of roads – of which 54km would be in tunnel – and 13 new overpasses. So, one can only assume that it will be a few years yet before traffic has an alternative to the A10 through Genoa.

In the meantime, then, it seems like some form of bridge will have to be constructed to replace the collapsed section of the Polcevera Viaduct – either as a permanent replacement or to provide a temporary crossing. Soon after the incident, Autostrade chief executive Giovanni Castellucci claimed it would replace the 200m missing section in just eight months, adding that it would be rebuilt using money from a €500M (£450M) fund (New Civil Engineer September 2018). But bridge experts have questioned whether a new crossing could be built in such a short timeframe.

Morandi

Morandi

Independent bridge consultant Simon Bourne says the quickest and easiest solution would be to build a 200m long temporary structure to plug the gap, put traffic back on the bridge, and then build a permanent replacement alongside. But he says there would be a “massive question mark about safety”.

“How on earth could you ever let traffic back onto this bridge?” he says. “Confidence levels would be so low.”

Andrew Bance, technical principal at consultant Mott MacDonald’s metros and civils division, agrees.

“If part of the existing structure is serviceable and can be reused, you could use it temporarily and then divert onto a new structure. But you would have to do a full assessment of the existing structure,” he says. “Technically it could be done, but it wouldn’t save any time, and it’s probably not really feasible in terms of the impression of using that existing structure. You might as well just get on with knocking it down and building alongside.”

Building a new bridge on a different alignment would give more flexibility for locating foundations, as well as enabling construction to proceed while the existing bridge is deconstructed.

“A new bridge alongside the existing one must be the easiest approach, using a deck form that involves maximising offsite fabrication or precasting and that can be lifted or launched in sections,” says Bance. “This approach minimises on site activity and impact at ground level, avoiding the construction problems associated with the old bridge, which was cast insitu with complex falsework/formwork systems and took four years to complete.”

A new bridge alongside the existing one must be the easiest approach

Bance estimates that, using modern construction methods, the new bridge could be built in two years, but he says the design would still take at least a year. “You can’t just rustle up a big bridge like that in six months,” he says. “It would have to be designed in full detail.”

Another option is to build a temporary bridge alongside the existing structure and then rebuild a permanent crossing on the same alignment.

 “If you want a road at that location, and you can’t trust any of the existing bridge, then you have to build a temporary bridge alongside,” says Bourne, acknowledging that this would be a major project.

“A 1.2km temporary bridge is a massive undertaking in its own right. It could be a modular steel Mabey-type bridge, but it would be a massive temporary structure.

“You would need to reduce the spans down to 50m to get any solution that is economically sensible for a temporary structure, which would require a lot of piers to be built. That gives you issues with the canal, the river, the railway and the buildings underneath, but it could be done.”

Temporary bridge

Bourne estimates a temporary bridge could be built next to the existing alignment in around 18 months and would cost around £45M. “It would take 18 months, but it gets the road up and running while you take down the existing structure and put a new bridge in its place.”

As for the new bridge, the design would be dictated by the physical constraints, according to Bourne: “You can’t build something utilitarian. Any structure 40m in the air has to be treated with some elegance.” Once you get to that sort of height, he says, the spans need to be around 100m, as shorter spans make no aesthetic sense.

“With big viaducts, the most economic span is about 100m. That’s your starting point,” he says. “It looks elegant and it makes economic sense. “But because of this location – the railway lines, canals and buildings – this is increased at one end to about 200m or 250m.

“Assuming you still have to cover these constraints, then as soon as you get up to 250m spans, that’s where cable staying would come into the equation.”

Bourne says a new bridge with a mix of 100m standard spans and a 200m to 300m cable stayed span could be built in either steel or concrete using established technology for around £225M.

Questionable timescale

“With €50M [£45M] for the temporary bridge, that gives you a total of €300M [£270M]-plus,” he says. This would fit into Autostrade’s stated budget. But he disagrees with the operator’s proposed timetable. “You can’t get a new bridge that’s been designed and built to last 120 years in less than five or six years.”

So it would be possible to start from scratch and build a new structure either on the alignment of the existing Polcevera Viaduct or alongside it. But that still leaves one problem: demolishing the existing structure with minimal impact on the congested urban landscape.

“It’s a bigger problem to deconstruct the existing bridge than to build a new one” says Bance.

He says there is a precedent: the Newmarket Viaduct in Auckland, New Zealand, which passes over a busy urban retail area and a railway line. The Newmarket Viaduct was built in the 1960s and, like the Polcevera Viaduct, ended up carrying far higher volumes of traffic than originally anticipated. It was replaced with a new structure conforming with modern traffic loading and seismic resistance requirements, built using the balanced cantilever method with a launching gantry. The existing bridge was deconstructed in parallel with the construction, by systematically removing elements.

Bance says the problem with many bridges of this age was that they were never intended to be taken down.

“These days more thought is put into how to deconstruct,” he says. “Part of the whole design process [now] is to go on through how to maintain it and to give a rough idea of how you might demolish or deconstruct it at the end of the design life. I don’t think 50 years ago they thought about that.”

 

 

 

 

 

 

 

    

 

    

    

    

 

 

 

 

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