Riccardo Morandi

Riccardo Morandi: The Bridge Builder and His Warning

In 1979 an Italian engineer in his late seventies published a technical paper with a deliberately dry title: the long-term behaviour of viaducts subjected to heavy traffic and situated in an aggressive environment. It was about a bridge he had designed himself, over the Polcevera valley in Genoa, and it said that the structure was corroding faster than he had expected. Riccardo Morandi had been raising the problem since the early 1970s. Thirty-nine years after that report, on a wet August morning, a 250-metre length of the bridge fell into the valley and killed forty-three people.

Rome, and Concrete in the Rubble

Morandi was born in Rome on 1 September 1902 and graduated as an engineer in 1927. His formative work was in Calabria, rebuilding in reinforced concrete among earthquake damage — an apprenticeship in a material that was still young and in a region that punished anyone who used it carelessly. He then opened his own office in Rome and spent six decades pushing concrete further than most of his contemporaries thought it could go.

He taught as well as built, holding professorships in bridge design at the universities of Florence and Rome. He was made a Fellow of the Royal Society of Arts in 1963 and given an honorary doctorate in architecture by the Technical University of Munich in 1979.

The Idea: Concrete Cables

To understand Morandi, you have to understand what a cable-stayed bridge is. The deck — the road you drive on — is hung from diagonal cables that run up to a tower. In almost every such bridge built today, those cables are numerous, thin and made of steel: a fan of many stays, each carrying a modest share of the load.

Morandi did the opposite. He used very few stays, often as few as two per span, and instead of leaving them as bare steel he encased the steel in a sleeve of prestressed concrete. The reason was stiffness. A concrete-encased stay is roughly five times stiffer than prestressing steel alone, so the deck sags and bounces far less under heavy traffic — a real advantage for a motorway viaduct. It was the first time anyone had used prestressed concrete tendons as stay cables.

The economy was extraordinary. The Polcevera viaduct used only about a third as much prestressing steel per square metre as the contemporary Bendorf Bridge in Germany. Morandi designed for cost and speed as much as for elegance, using prefabricated elements and cantilevered construction with temporary tendons.

The drawback is now obvious and was not then. Once you wrap the steel in concrete, you cannot see it. Inspection becomes guesswork. If the concrete cracks, or if the grout injected into the ducts is incomplete, water and salt reach the steel and eat it invisibly. And with only two stays per span there is no redundancy at all: lose one and there is nothing to carry the load.

Maracaibo

His largest work is the General Rafael Urdaneta Bridge across Lake Maracaibo in Venezuela, completed in 1962 — 8,678 metres of cable-stayed concrete, built in a record forty months. It made his international name and served as the template for what came next.

It also gave the first warning. Exposed cables on the Maracaibo bridge snapped from rapid corrosion, and every stay cable had to be replaced eighteen years after construction. Morandi's other major crossing, the Wadi el Kuf bridge in Libya, was closed in October 2017 after inspections identified potential fractures. He built other things too — Rome's Fiumicino airport in 1970, an underground car showroom in Turin, cinemas, and work on the power line crossing of the Strait of Messina — but the bridges are the legacy, and the bridges corroded.

The Polcevera Viaduct

Built between 1963 and 1967, the Polcevera viaduct carried the A10 motorway more than a kilometre across a valley of factories and apartment blocks, about forty metres up, on three A-shaped pylons ninety metres tall with main spans of roughly 220 metres. It was, by any measure, a spectacular object and the symbol of Genoa. Morandi could have finished it in twenty-three months but for administrative delays.

By 2016 it was being described as a failure of engineering, with maintenance costs escalating. Two of the three pylons had been strengthened in 1993. The western pylon — pier 9 — had not, despite documented corrosion damage.

14 August 2018

Pier 9 collapsed. The technical cause was the rupture of a stay cable that had suffered severe corrosion, itself caused by inadequate and in places entirely missing grout injection in the ducts. The steel had been rusting inside its concrete sleeve, unseen, for decades. Forty-three people died.

The engineering criticisms are specific and fair. Morandi underestimated the effects of concrete creep, which caused loss of prestress and cracking. The duct diameters he specified were too narrow for the grouting to be done reliably. The concrete cover over the steel was small, offering little corrosion protection. And the structure had no robustness: a single cable failure was sufficient to bring down a span.

Why Riccardo Is Called a Genius

Morandi's gift was conceptual originality combined with severe material discipline. He did not refine other people's bridges; he proposed a genuinely different structural system — the concrete-encased stay — and solved the stiffness problem it was designed to solve, while using a third of the steel of comparable structures and building faster than anyone expected. Walter Kaufmann, who holds the chair of structural engineering at ETH Zurich and has examined the collapse in detail, concluded that judged in the context of his own era Morandi undoubtedly remains the pioneer of bridge design that he was recognised as before the Genoa tragedy. Kaufmann's argument is that robustness was simply not a design objective in the 1960s, and that the redistribution of forces by concrete creep had not been sufficiently researched at the time. By that standard, Morandi was not careless; he was working at the edge of what was known.

The counter-case cannot be softened. Forty-three people are dead because of a structure he designed. Even setting aside the standards of a later era, two of his choices were his own and were consequential: burying the tension steel where nobody could inspect it, and providing so few stays that the structure had no second line of defence. A design that is uninspectable and non-redundant transfers the entire burden of safety onto maintenance, and maintenance failed — the owners never strengthened pier 9 as they had strengthened the others twenty-five years earlier. Morandi's own warnings from the early 1970s and his 1979 report are to his credit as an engineer and are also, uncomfortably, evidence that he understood the vulnerability and that the system he designed offered no margin once it began. It is worth noting too that his few-stay approach proved less economic than multi-stay bridges and had limited influence on other engineers; the profession admired him and then went a different way. The honest verdict is that he was an original of real stature whose signature idea contained a flaw that killed people, and both halves of that sentence have to stand.

Legacy

Morandi died on 25 December 1989, twenty-nine years before the collapse, with his reputation as one of the great concrete engineers of the twentieth century intact. Genoa replaced his viaduct with a new bridge. The Polcevera disaster has since become one of the most studied cases in modern structural engineering, and its lessons — design for inspection, design for redundancy, assume the maintenance will be imperfect — are now taught as fundamentals. That they had to be learned this way is the tragedy; that they were learned at all is the one thing that can be salvaged from it.

Achievements

• Notable work: Polcevera Viaduct
• Notable work: General Rafael Urdaneta Bridge
• Affiliated with University of Florence and Sapienza University of Rome
• Educated at Sapienza University of Rome
• Worked as civil engineer