23 March 2022
Pasta, Pesto and the Bridge-Building Piano
What have we learned about structure, salt and steel? And are they concrete lessons that will stand the test of time?
In Genoa, Italy, a newly erected bridge is safeguarded, 24 hours a day, by four robots that each weigh close to 5,000 pounds. Why, you may ask, should this be of any interest to CAD/BIM designers? Well, quite apart from yet another remarkable design from world-renowned Italian architect, Renzo Piano, or the tasks performed by the afore-mentioned robots, the San Giorgio bridge replaces its predecessor which fell afoul of some design oversights from the 1960s. Sure, we all hate stress, but when it comes to prestressed concrete, how much stress is enough? This turned out to be an epically critical factor in Genoa.
Here, we explain those never-to-be-repeated errors and celebrate the positive impact that modern technology can make.
Genoa, the sixth-largest city in Italy, is sandwiched between the Ligurian Sea and the Apennine Mountains. It is bifurcated by the Polcevera River, creating a need for several bridges to connect the eastern and western portions of this sprawling, ancient city, which covers 94 square miles and has been continuously inhabited since as early as 4,000 BCE. This long-standing habitation may explain the perfection of regional delicacies such as pesto and focaccia, both of which originate in beautiful Genoa, and for which the rest of the world will always be eternally grateful. But we digress.
The pair of inspections robots survey the entire underside of the bridge every eight hours.
Not a Lasting Legacy
The original bridge, nicknamed the Morandi Bridge in tribute to its structural engineer, tragically collapsed in 2018. Although it was undergoing road-strengthening construction at the time of its collapse, it was reportedly in a state of severe structural neglect and attempts at repairs were sadly a case of too little, too late.
Built between 1963 and 1967, and officially opened in September 1967, the Morandi Bridge was considered a significant contribution to Italy’s engineering legacy in its heyday. It soared 150 feet above the Polcevera Valley. It was gracefully constructed of prestressed concrete pylons and cables, and pushed the limits of cable-stayed construction, with only four cables per tower and a deck made entirely of reinforced concrete.
At the time of its construction, it was believed that covering the cables in prestressed concrete would eliminate the need for future maintenance through the concrete’s ability to protect the steel from corrosion.
Sometimes, pushing the envelope results in hitting paydirt. Sometimes, however, it results simply in, well, dirt. Or rubble. Such was the fate of the Morandi Bridge.
Fatal Flaws
At the time of the Morandi Bridge’s design and construction, primary concerns about the structure revolved around the concrete’s ability to protect the steel cables from chemical breakdown in the presence of elevated levels of pollution and increasing loads from vehicle traffic. Prestressed concrete was used to coat the steel cables, but it was only prestressed to 10 MPa (megapascal) or 1,500 psi. This choice, regrettably, turned out to be woefully inadequate as, at only 10 MPa, the concrete was susceptible to cracks, water intrusion and corrosion of the internal steel.
The lights on the bridge are all solar-powered, and sensors throughout the bridge monitor movements such as joint expansion, creating what is essentially a kind of futuristic central nervous system.
The fundamental concept behind the bridge’s design — protectively encasing the cables in concrete — relied on the safeguarding of the steel. Either a miscalculation, the technology of the time, or inexperience ended up being a fatal flaw.
Perhaps, at that time, the engineers didn’t fully grasp the importance of durability, consistent maintenance and resilience the way we do today, or perhaps they weren’t as aware of the impact of fissures and corrosion — particularly in a briny, marine environment. We’ve covered this topic in previous articles, such as this one about the longevity of concrete-encased structural steel in a marine environment, or this one about the use of graphene as a concrete additive, but perhaps much of this data simply wasn’t available in the 1960s.
In 2018, forty-three people died and over 600 were left homeless when a 650-foot section of the bridge collapsed, wrecking vehicles and crushing the buildings below. The people of Genoa were devastated. The Morandi Bridge, of which they had been immensely proud, became a fiercely debated and heated topic, as did the construction of its successor.
The damaged bridge was fully demolished in 2019 and the new San Giorgio Bridge, designed by native Genovese, Renzo Piano, took just 15 months to complete by 2020.
Technology Bridges the Gap
The San Giorgio Bridge does not support the massive pillars and cables of its predecessor. Although some critics believe it lacks “flair,” almost all can agree that it more than compensates with its elegant design and incredibly clever use of technology.
The lights on the bridge are all solar-powered, and sensors throughout the bridge monitor movements such as joint expansion, creating what is essentially a kind of futuristic central nervous system.
Measuring 3,501 feet in length and consisting of 19 spans supported by helical reinforced columns, the San Giorgio sports glass panel barriers that serve two functions. One, they permit beautiful views of the surrounding valley and mountains, and two, they reduce the amount of shadow cast on the citizens that live and work below the bridge.
New technology that provides accurate and immediate feedback for engineers is critical.
There’s a long maritime history emanating from Genoa. During the time of Marco Polo, the Genoese vied with the Venetians for control of the Mediterranean and although Christopher Columbus may have sailed under Spanish colors, he hailed from Genoa. Renzo Piano’s bridge honors this history. “From an architectural point of view, the form described by the deck, which recalls the hull of a ship, is of great importance,” said Piano.
Keeping a close eye on deck — and below — is a quartet of robots that are the first of their kind to be installed on a major structure anywhere in the world. One pair is tasked with the constant inspection of the structure, while the other pair is responsible for cleaning the solar panels on the bridge’s parapet and the clear glass panels that line the San Giorgio.
The pair of inspections robots survey the entire underside of the bridge every eight hours. They send 25,000 real-time images back to the engineers for weekly comparison studies. The cameras and sensors they carry help predict maintenance needs and monitor the overall safety of the bridge.
With the robots in such close proximity to the Mediterranean Sea, “lightweight” and “durable” are key qualities for machinery that must also endure constant exposure to the elements. Additionally, the robots combine elements of industrial automation, robotics, machine learning and aerospace engineering.
Each inspection robot moves on tracks mounted on either side of the parapet and has a retractable arm that can extend all the way from the edge of the underside of the bridge to the center. They monitor paint condition, corrosion and the condition of welds that are subject to fatigue loads. In stormy conditions, the robots retreat to their charging stations.
The robot wash system travels along the edge of the bridge’s glass barrier, evaluating the transparency of the glass and the state of the solar panels below. To save water, the robots only jump into action when the dust and debris accumulated on the glass or solar panels demand it. During drought or times of water shortage, the robots can switch from washing with water to cleaning the glass with blowers. All the water the ’bots use comes from rain or condensation collected on the bridge itself.
Technology: Saving Lives and Money
Globally, as infrastructure ages and needs to be rebuilt or replaced, we clearly no longer need to rely only on traditional methods of inspection and maintenance. The development and implementation of new technology that provides accurate and immediate feedback for engineers is critical. Sporadic inspection, regardless of how thorough, simply cannot provide the detailed feedback and data gained from purpose-built sensors and daily robotic sweeps of the structure. It has the potential to save lives by preventing tragic accidents while also saving millions of dollars in maintenance costs.
Primary concerns about the structure revolved around the concrete’s ability to protect the steel cables.
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It was believed that covering the cables in prestressed concrete would eliminate the need for future maintenance.
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