The Lessons of the Genoa bridge collapse
First publishedin World Highways
Spanning a wide area, the bridge crosses the city of Genoa
The partial collapse of the Polcevera viaduct, better known as the Morandi Bridge, has prompted debate regarding the technical and administrative aspects of maintaining road infrastructures. We discussed it with the engineer Gabriele Camomilla, former Director of Research and Maintenance of the Società Autostrade, who coordinated the only major structural intervention performed on the bridge, carried out in the early 1990s. Interview by Lucio Garofalo
A bridge collapse inevitably has an impact on public opinion. On top of the emotional reaction, media coverage heightens sentiment and the public calls for both answers and justice. Bridges, in effect, shouldn’t fall down; however, as structures that are subject to wear and stresses, they must be maintained. Unfortunately, what happened in Genoa, apart from the specific dynamics of the collapse, should not be considered an exceptional, unrepeatable event. And the problem does not exclusively affect Italy and the 60,000 bridges and viaducts it has. It is an issue for countries all around the world, where since 2000 there have been a recorded 108 collapses, either total or partial, of road and rail structures (source: Wikipedia, List of bridge and bridge failures). The collapse of the A-frame tower of the Polcevera viaduct offers an opportunity to reflect on bridge maintenance and on the materials that 50plus-year-old bridges and viaducts were built with. And it also highlights the relationship between public ownership of the structures and their management on the part of private entities.
The opinion of the engineer who oversaw major maintenance
A few years after it was opened in 1967, the Polcevera viaduct manifested a set of problems that posed no structural threat. There was a noted creep, ie, deformation of the concrete under load, affecting the deck though limited by the box girders, so repairs to the deck were needed in order to improve planarity. In 1992, the technical monitoring group of the Società Autostrade, then a state-owned company belonging to the IRI Group, determined there to be a serious construction defect at the top of Tower 11, the first from the Genoa side and also the first to be built. To solve the problem and to make the stays replaceable in the future, a major maintenance intervention was performed using brand new materials. This structural rehabilitation project was made possible thanks both to these new materials and to the spending power of the Società Autostrade which managed most of the Italian motorway network. The project was put forward and coordinated by Gabriele Camomilla, engineer and Director of Research and Maintenance at Autostrade. We interviewed him to get his perspective on the collapse of August 14th last year and because he has long advocated a different approach to the maintenance of road infrastructures.
Mr Camomilla, from the late 1980s to the early 1990s, you led the maintenance intervention on Tower 11 of the Polcevera viaduct, but before and after you supervised the rehabilitation of many bridges and viaducts of the Italian highway network. What are your thoughts on what happened?
“The first and foremost consideration concerns maintenance, especially of roads with high traffic flows, and generally speaking maintenance of all public works, a problem that has global impact by now. Maintenance is not an activity of secondary importance, intended as a repair due to decay; rather it is a scientific method of predicting deterioration and the preventive elimination of phenomena that lead to more serious problems. The end goal is to achieve the dual condition of maintaining the operation of the infrastructure, even if partial, and of increasing the durability of the structure or even increasing its value with respect to the original. In 1982 the method was adopted in Italy by Autostrade, and was defined as “road terotechnology”, but it didn’t take hold outside the highway sector for several reasons. Often technical and bureaucratic aspects criss-cross and appear as engineering issues, when they are actually more administrative in nature and regard the decision-making process. The partial collapse of the Polcevera may be an extreme example, but it is a glaring and unfortunately tragic one. But the case is not closed by saying the problem is exclusive to Italy: as I said we have to make clear that maintenance is a primary activity in technical and economic terms. I am so convinced of this that terotechnology remains one of my focuses today. That said, the Polcevera viaduct is a special case because the structure is unique, almost visionary, for the times in which it was built.”
When it was first opened, the bridge was seen as one of Italy's engineering marvels of the time
What do you mean exactly?
“I mean that I find it intolerable to take aim at Riccardo Morandi, who alongside Nervi, Musmeci and others, less well-known outside Italy but of equal technical stature, have made contributions to engineering around the world. The real Achilles heel of the Polcevera viaduct is not the design, but the materials and the knowledge at the time regarding their behaviour due to fatigue and the effects of environmental agents. Morandi was almost obsessive about the bearing capacity of his design and protecting the durability of the stays, which were the basic elements of that specific static solution. With adequate maintenance, the viaduct could have been safe and continue to be considered what it had been since its opening: a masterpiece of engineering, at least when it comes to the three large free spans. The Polcevera can be considered the forebearer of all modern cable-stayed bridges with a harp design. When, in order to demonstrate the limits of the statics Morandi used, some cite the collapse of several spans of the Maracaibo viaduct (a structure in Venezuela that he also designed), which shared a similar static design as the Polcevera. But they forget that a V pylon, without stays, was struck by a 36,000tonne oil tanker that was navigating almost at cruising speed. Few structures would have withstood such a stress.”
When you refer to problems regarding materials, are you talking about the stays or the concrete utilised for the structural elements?
“Both. As regards the concrete, which several newspapers erroneously defined as low quality, we have to remember that in the 1960s no criteria existed such as exposure classes and durability, which came about in the 1980s. Not even shrinkage-compensating concrete was a known concept, by the way invented by another Italian, Professor Mario Collepardi, a world-renowned figure in the field of materials. Then we have the development of additives, which several Italian companies export around the world that make it possible to obtain mixes with exposure classes and elastic modules that were previously unthinkable. What’s more, these materials were used for the maintenance of the V pylons of the section left intact after the collapse to protect the outermost reinforcements damaged by carbonatation of the covering. Today, there are also cathodic systems of protecting reinforcements that can guarantee rehabilitations and offer unprecedented guarantees of durability. Morandi in any case was vigilant about the deterioration of the structure and had thought of protecting the 352 strands that compose the bearing part of the stays with prestressed concrete. He wanted to prevent them from cracking over time due to the constant combined stresses from the flow of traffic and other agents like wind and thermal expansion. The concrete used at the time, which contained no additives and was subject to localised deterioration from carbonatation, soon began to appear, first as cracks and then actual detaching of small elements, none of which diminished its bearing capacity. Morandi brought this up at an international congress in 1979, making his position clear that it was necessary to intervene on the protective casing of the stay cables. Instead, the dense protective sheathing of the stays has protected the core of the internal steel superbly. Because of its crack-proof thickness, much thicker than that of normal concrete covers, the cover did not permit direct inspection, which meant that other sophisticated systems of control had to be devised. The same thinking we use about the knowledge of then versus now also goes for the steel used for the cable stays: today’s stays have ultimate tensile strengths and elastic modules far superior than those considered the best at the time for that particular use. Moreover, today they are also covered with polyethylene sheathing and filled with anti-corrosion lubricants that protect them; above all, they are not subject to certain types of stress corrosion cracking, not a known phenomenon at the time.”
When you were Research and Maintenance Director of Autostrade, Tower 11 underwent major works. What was the reason for that?
“As I said, we were constantly monitoring the bridge. The biggest challenge was to inspect the top of the saddles of the masts and the stays, distanced 45m from the road bed and which had no system for scaling them, unlike today; so in order to perform an up-close inspection, we used the first telescopic platforms available at the time. By detaching a thin layer of concrete on Tower 11 that seemed more porous than the rest, we determined that the integrity of the North stay on the Genoa side had been compromised during casting of covering at the time of construction. Near the saddle at the top of the mast, the cables were not spaced well, instead bundled together and not fully covered by the concrete. This meant they were in contact with the air and had undergone corrosion due to the dissolution of about 30% of the cables. When analysing the recent collapse, it’s important to keep in mind that the stay cables were in any case stressed for less than half of their ultimate tensile strength, which provided a considerable margin of safety and made it possible to intervene with caution, but without blocking traffic, except during some of the more delicate phases. The deck, conversely, could be easily inspected via special passages, not a common solution for the time and a design detail that does honour to Morandi.”
The original repairs were carried out on Tower 11 due to problems with spacing of the cables and the modifications alleviated the problem
What was the nature of the maintenance project?
“We ruled out demolition, an idea I was extremely opposed to because it was too complex and risky for the removal of the prestressed stays running over the railway yard. There was also the traffic problem: we could only close the bridge to traffic at night and for very short periods. For the project definition, we sought out the highest experts of the time, because at the time there was no consolidated knowledge regarding the calculation of cables of cable-stayed bridges. It was Francesco Pisani, Morandi’s calculation engineer, who came up with the solution. From the viewpoint of statics, it was a tough challenge because if we had interrupted the load-bearing function of a single stay, even briefly, this would have triggered transversal actions by the other three, which would then have caused the deck to rotate, in turn leading to the torsion and rotation of the saddle, and resulting in the collapse of the entire span – probably similar dynamics of the August 14th collapse. The project consisted of a gradual transfer of the load of the inner original cables onto the outer new cables covered with a new polyethylene resin that had been experimented on more recent cable-stayed bridges. The cables were developed by Alga, a Milan-based company that no longer operates in Italy, but in China, where it has supplied special systems for over 900 bridges along the Beijing – Shanghai high-speed rail line. The project phases are too complex to be detailed in an interview, but can be summarised as the installation of a system of outer cables that gradually replaced the existing ones, and that only at the end of the intervention received all the load of the original. It was a very innovative technique, later presented at an international convention, which restored, and even increased, the static efficiency of the structure, apparently in an excellent condition still today. I am not absolutely certain, but I think it is the type of intervention envisioned for the collapsed A-frame towers.”
Why was no intervention immediately performed on the stays of the other masts?
“Of course we made very careful checks, making traditional carbonatation tests with phenolphthalein as well as a defect detection. To this, we added tests on outer deformation and other types of traditional sensors that then proved insufficient for evaluating stability since they supplied tons of unprocessable data related to the movement of traffic, which in this type of structure are collected for the continuous transfer from one side of the towers to the other. Other more sophisticated sensors were used for periodic measurements, initially every six months and then less frequently since there was no sign of bearing defects in the main cables. These were sensors that aided the reflected waves of the magnetic flow induced in the cable bundle; an increase would have meant potential reduction of the resistance of the stays. This was a deductive system, however, in that the analysis of the signal involved complex activities. We switched systems, still deductive, like the modal analysis that was not very selective at the time. On the basis of all the evaluations, we concluded that no intervention was necessary and we decided to add outer steel plates onto Tower 10, still visible today. Injections to saturate the voids were made to seal some of the surface disintegration of the concrete protection, which could not be treated at that height and in the part below the stay. However, no anomalies were noted on the cables.”
So, what in your opinion caused the collapse of the span with Tower 9?
“Hard to be certain. The experts appointed by the judiciary are still analysing the data available. My conjecture, based on an in-depth knowledge of the viaduct, is that there were concomitant causes that manifested themselves all at the same time, leading to a collapse that could not be predetermined. Essentially, it’s a situation similar to what economists call the “black swan”, that is, unforeseeable circumstances that permanently change the conditions of the market. Mind you, intervention on Tower 9 should have taken place sooner, but it seems the concessionaire was focused on the regular maintenance imposed by new regulations, a factor hardly acknowledged now. A delay in starting works can be one of the contributing causes; there was some generalised cortical deterioration of the concrete, even though I believe that it had no effect on the bearing capacity or on the protective concrete casing of the stay cables. The only problem determined recently was the anomaly on the prestressed reinforcement ties, which were not load-bearing. At any rate, the letters to the Ministry of Infrastructure requesting approval of the project prove that the concessionaire was eager for the project to be started. So I think we have to consider other factors, in particular unusual anomalies not detected by the instruments, and also the fact that there were many lightning strikes in that area.”
Removing the bridge safely poses a major challenge for demolition experts
That theory has been totally excluded by some. Bridges don’t collapse because they are struck by lightning. What do you mean by unusual anomalies?
“I did not say that lightning was the primary cause; I spoke of a potential concomitant cause. The website Lightningmaps.org records lightning strikes all over the world; apparently between 9 and 10am on August 14th, two hours prior to the collapse, two lightning strikes hit the bridge. Lightning can create electromagnetic fields of incredible power also in the area adjacent to where it strikes. I have spoken with a physicist, an expert in electromagnetism, who explained that in these conditions magnetostriction may occur: the steel, stressed by the strong electromagnetic field, can undergo a molecular change in crystallisation that significantly increases its fragility and even make it vibrate uncontrollably, thereby reducing its resistance. We can’t rule out that a lightning strike may have contributed to making even a few strands of a stay cable give way. What I mean by unusual anomaly are the micro-cracks that form in the hot-drawn martensitic steels of the cables; these micro-cracks can develop over time without exhibiting the striction that occurs in normal exfoliation corrosion. The development of the cracks is caused by the high stress the cables undergo; due to this, a phenomenon called stress corrosion cracking, it is not easily detected in its initial phase, but can cause a sudden brittle fracture. If some strands were actually subject to these conditions, the lightning, causing striction and intense vibrations, may have caused the bearing cables of only one stay to break. That would have triggered phenomena of instability enough to imbalance the box girder held symmetrically by the four stays. The unbalancing would have first led to the collapse of the course spans which prevented the torsion generated by the dissymmetry in the bearing capacity of the cables, and then the collapse of the tall A-frame towers, followed by that of the caisson deck, collapsed by the weight of the masts because they no longer had the support of the collapsed stays. This theory, if established by the analysis of the broken steel cables of the stays, is not enough to exclude other causes and, of course, legal responsibilities, but could offer one more element for understanding the technical reasons for the sudden collapse. Among the possible unusual causes there might be another that is easier to understand: a lorry carrying a very heavy load may have lost its cargo, with the load hitting a structurally crucial point, such as the area where the stay cables are attached to the deck. The impact energy of such a load, created by its mass and the square of the speed of the vehicle, could have been devastating enough to break the anchorage, initiating the sequence of collapse that I described. The effects of these types of dynamic loads are always much more disruptive than those considered in the design"