The bridge was designed by Riccardo Morandi. It was similar to his earlier 1957 design for the General Rafael Urdaneta Bridge in Venezuela, except for the stays, which in the Venezuelan bridge are not covered with prestressed concrete.
The Ponte Morandi was a cable-stayed bridge characterised by a prestressed concrete structure for the piers, pylons and deck, very few stays, as few as two per span, and a hybrid system for the stays constructed from steel cables with prestressed concrete shells poured on. The concrete was prestressed only to 10 MPa, resulting in it being prone to cracks and water intrusion, which caused corrosion of the embedded steel.
The viaduct was built between 1963 and 1967 by the Società Italiana per Condotte d'Acqua. It had a length of 1,182 metres (3,878 ft), a height of 45 metres (148 ft) at road level, and three reinforced concrete pylons reaching 90 metres (300 ft) in height; the maximum span was 210 metres (690 ft). It featured diagonal cable-stays, with the vertical trestle-like supports made up of sets of Vs: one set carrying the roadway deck, while the other pair of inverted Vs supported the top ends of two pairs of diagonal stay cables.
The viaduct was officially opened on 4 September 1967 in the presence of Italian President Giuseppe Saragat.
Maintenance and strengthening
The bridge had been subject to continual restoration work since the 1970s due to an incorrect initial assessment of the effects of creep of the concrete. This resulted in excessive deferred displacement of the vehicle deck so that it was neither level nor flat; at the worst points, it undulated in all three dimensions. Only after continual measurement, redesign and associated structural works was the vehicle deck considered acceptable, approaching horizontal by the mid-1980s.
In a 1979 report, Morandi himself recommended: "remove all traces of rust on the exposure of the reinforcements, fill the patches with epoxy resin, and cover everything up with elastomer of very high chemical resistance". In the 1990s, the tendons on pillar 11 appeared to be most damaged. About 30% of the tendons had corroded away. The load of the bridge was 7,000 kg (15,000 lb) per tendon, while the tendons were originally capable of carrying 15,000 kg (33,000 lb) . A single truck can weigh as much as 44,000 kg (97,000 lb). As of the collapse of the bridge, only pillar 11 had been internally inspected in the 1990s, showing severed and oxidized strands. From 1990 onward, the easternmost pillar 11 had its stays strengthened by flanking them with external steel cables. Pillar 10 had the stays at the top strengthened with steel sheathing in the 1990s. Following the collapse many questions have been raised about the stays. In 1979/1980, Morandi's similar bridge in Venezuela suffered one or more stay cable failures with collapse imminent.
The then-minister of infrastructures and transport Graziano Delrio, who was in charge until 1 June 2018, was informed several times during 2016 in the Italian parliament that the Morandi bridge needed maintenance.
In Genoa, in 2017, a confidential university report noted disparities in the behaviour of the stays of the now collapsed pillar 9. The minutes of a February 2018 government meeting reported that resistance and reflectometry measurements had been performed indicating an "average" reduction of the cross section of 10 to 20% of the tendons. A crack in the road had appeared at least 14 days before the collapse, near the south-eastern stay of the subsequently-collapsed pillar 9. The crack may have indicated that the stay had stretched. At no point was there a suggestion to reduce the load on the bridge. Traditionally, bridges were designed only for a 50-year life span; the bridge failed after just under 51 years after its opening.
On 3 May 2018, the Autostrade company had announced a call for tenders for a structural upgrade of the viaduct to the value of €20,159,000, with a deadline of 11 June 2018. The work on the reinforcement of the stays on pillars 9 and 10 would have needed to be finished within five years.
Workers were installing new heavy concrete Jersey barriers on the Ponte Morandi before it collapsed, reducing the already low compressive pre-stress on the concrete of the stays and increasing the loads.
2017 modal analyses
In 2017, Carmelo Gentile and Antonello Ruccolo of the Polytechnic University of Milan studied the modal frequencies and deformations of the stays of the bridge. On pillar 9 they could identify only four global modes, and the deformations of two of these identified modes were not fully compliant. Modal frequencies were more than 10% different, specifically on the southern stays. In pre-stressed concrete beams such a difference could represent the entire effect of the non-linear pre-stresses. As little as a 2% shift could represent severe damage. The pre-stress in the Ponte Morandi has been characterised as relatively small from the start. In contrast, with bare tendons which are relatively under-constrained such as the strings in a piano, the effect of pre-stress is dominant in determining the resonant frequency. Other than pre-stress, changes in geometry, such as corrosion in the tendons could impact the resonance frequency. The effects would be reduced by the composite nature of the stays when observing global modes. Gentile had performed similar modal analyses on pillar 11 in the 1990s. Other related methods were applied on the stays of Ponte Morandi in the 1990s such as reflectometry, which was able to measure the tension but not strength of the tendons.
By the mid-2000s, the A10 route through Genoa and over the bridge had become highly congested. The city council requested proposals for improvement of traffic flow through Genoa, with the Autostrade company in 2009 proposing the "Gronda di Ponente" project to improve flow, by moving traffic to a newly built Autostrada interchange system located to the north of the city. As part of the initial study and report, the Autostrade company measured that the bridge carried 25.5 million transits a year, with traffic having quadrupled in the previous 30 years and "destined to grow, even in the absence of intervention, by a further 30% in the next 30 years". The study highlighted how the traffic volume, with daily queues at peak hours joining the Autostrada Serravalle, produced "an intense degradation of the bridge structure subjected to considerable stress", with the need for continuous maintenance. The study showed that in the option for improving what was termed as the "low gutter", it would be more economical to replace the bridge with a new one north of its current location, and then to demolish the existing bridge.
In the 1990s the bridge had shown signs of weaknesses. At this point material support was added. Rust on metal materials of the bridge had become more evident since then. In 2013, fears were expressed that the structural condition of the bridge was poor. Warnings that the bridge was in danger of collapse were dismissed by the Five Star Movement as a "fairy tale" on their website. In 2016, the bridge was characterised by Antonio Brencich, a professor of Structural Engineering at the University of Genoa, as a "failure of engineering", mainly due to high maintenance costs.