polcevera

August 31, 2018 § Leave a comment

aerial 02 onramp

(UPDATE: Sept. 6, see below)

The collapse, two weeks ago, of a span of the Polcevera Creek Viaduct in Genoa is a particularly sobering structural failure.  Authorities put the death toll at 43, and beyond this is the fact that the bridge was literally a textbook example–one of the truly great pieces of structural expressionism that was, for more than fifty years, hailed as a work of structural art.  Its designer, the Roman engineer Riccardo Morandi (1902-1989),  was a near-contemporary of Nervi.  His path took him to bridge design after a similar early career in cinema and theatre roofs.  Morandi’s practice represents a shift in Italian engineering from the lingering economic and cultural influences of autarchy, which emphasized concrete as a locally-produced material, to steel, which had been unavailable in Italy during the fascist era, but which proved itself economical during the post-“Italian Miracle” era of rising inflation and thus the need for more rapid construction.

sectionsThe Polcevera viaduct, completed in 1966, was his masterpiece–a muscular but finely proportioned march of concrete towers across an urban valley that provided a crucial autostrada link between Genoa and the resort town of Savona to the north.  Morandi’s solution to the difficulty of the 1200-meter span was two-fold: a viaduct on the northern half of the valley supported by vee-shaped compression towers, and three cable-stayed, cantilevered spans supported by taller towers on the southern half.  These spans used what would become Morandi’s signature technique, combining steel and concrete into massive pre-stressed tendons.  While ordinary cable-stayed spans rely on multiple, individual strands of steel cable, Morandi’s solution gathered hundreds of these strands into single elements.  Other engineers critiqued this idea, noting that since cable-stayed bridges rely on a deck that can absorb huge compressive forces, this necessitated a stiffer than normal roadway and thus a tremendous amount of extra dead weight.  But Morandi argued for the technique for its elimination of costly cable maintenance.  Wrapped in concrete, the steel strands would not require the near constant painting involved with the traditional fan-shaped solutions, and the resulting visual effect was particularly striking; the simplicity of the structural diagram made the bridge’s structural performance evident even to laypersons.

Const 02 three towersConstruction photos reveal a great deal about the behavior of the bridge.  In the above image, you can see that the decks were actually self-supporting under their own loads.  They are actually supported by diagonal members that frame into the towers’ bases, and were formed by traveling formwork that balanced around each tower.  This shot shows each of the three towers at successive stages–the one in the center shows just how far the decks could cantilever under their own weight, making the tendons themselves responsible primarily for carrying the bridge’s live loads.

Const 01 two towersHere, too, you can see the cables being draped from the tower on the left–not yet carrying any load.  Once these were tightened, encased in concrete, and assisted by further post-tensioned cables in their concrete jacket, the short span between the two cable-supported decks could be placed.  This sequence was much like that of the Firth of Forth Bridge, where the steel cantilever towers were gradually extended, and then the span between them filled with a short, beam-like infill.

Const 03 one towerThis shows the steel tendons being wrapped with their concrete jackets after they’ve been tensioned–the deck is actually warping upwards, a deformation that would be corrected once the load of the spanning element was added to it.

completed viaductThe result was a particularly elegant bit of structural sculpture, but one that did have problems.  In the 1990s, concerns about deterioration of the concrete led to a full survey of the structure, which found that the internal strands in the southernmost tendons had been corroded by water infiltration due to flaws in construction that left permeable voids in the concrete jackets.  In 1996 these tendons were supplemented by steel cables grafted onto the outside of the structure:

retrofit sectionIn this diagram, by Profs. Gentile and Martinez Y Cabrera of the Politecnico di Milanoyou can see both the new ‘jacket’ of reinforcing steel and a new steel ‘saddle’ at the top of the tower.  Recent Google Earth imagery shows the condition of this repair recently:

Screen Shot 2018-08-15 at 11.20.47 AM

The tower that collapsed was the one farthest north, where the span switched from the cable-stayed elements to the pure viaduct.  In the one video of the collapse (available elsewhere), the first few seconds appear to show the tower itself collapsing, and while it’s difficult to see through the driving rain, it appears that the deck has already collapsed.  If that was the sequence, it would make sense that the (now gravely unbalanced) tower would become unstable, too.  Coupled with the 1996 repair of the south tower, this suggests an obvious possibility: on a busy afternoon, with a full live load, cables that had been slowly and invisibly corroding finally failed in tension, leading to the collapse of the end of the deck and then the unbalanced tower.

If, in fact, that is what investigators determine, it raises a much larger set of questions, many of which are already being shouted loudly.  The bridge’s condition had, in fact, been the subject of much concern among the pubic and the engineering community–University of Genoa engineering professor Antonio Brencich went on record in 2016 as saying that the bridge was conceptually “bankrupt” and “a disaster waiting to happen,” a seemingly prescient claim that, notably, didn’t suggest what exactly would cause the failure.   Calls for replacement, however, led to political headwinds; the right-wing Five Star party, now in power, has blamed budget limitations imposed by the EU, but in 2014 the party campaigned against replacing the bridge, on the grounds that such a large construction project would only encourage corruption, calling concerns about its collapse a “fairy tale.”

To complicate the politics of the collapse further, the motorway was privatized in the early 2000s, and the concessionaire, Autostrade, has mishandled the aftermath of the collapse horribly, with embarrassing claims that the collapse was simply a natural disaster (there were initial claims that the bridge had been struck by lightning just before the collapse–but this wouldn’t, on its own, have had any effect at all on the structure).  The company had, in fact, been doing foundation repairs on the span on the day of the collapse, part of an unending series of patches.  (Excavations during a torrential rain might suggest that the foundations were undermined, but the apparent sequence from the video and the initial survival of the tower argue against this as a cause).

In all of this, Morandi’s design has largely escaped blame, though it’s worth noting in hindsight that his revolutionary approach to stayed structures may have contributed to the disaster in at least two ways.  First, collecting all of the cable support into monolithic tendons left the structure with no redundancy; if a cable on a typical, fan-shaped stayed bridge deteriorates, there are dozens of others that can carry its load, at least under emergency conditions until it can be replaced.  That wasn’t the case at Polcevera, obviously.  The loss of one tendon necessarily meant the loss of the span.  Second, the concrete cover meant that there was no way to visually assess the state of the steel itself. Corroded or compromised steel cables can be easily spotted and accessed in traditional cable bridges.  But here, it took a full survey in 1996 to determine that there was even the possibility of corrosion.

Still, Morandi was designing in an era where the expectation was that such a bridge would be fully staffed, and its maintenance fully funded over its lifetime.  Deferred maintenance has become the norm in Italy and throughout the developed world, as governments and voters forget that the cost of large infrastructure is just the down payment on life cycle costs that are necessary to maintain structures’ health and integrity.  Houses need new roofs every twenty years.  Bridges need regular monitoring and, often, invasive, surgical repair of corroded or deteriorated pieces.  The running joke in American politics this year has been “Infrastructure Week,” which keeps getting announced and then trampled by more sensational news.  Meanwhile, the American Society of Civil Engineers reported recently that 9% of bridges in the United States–more than 56,000–are known to be “structurally deficient,” most of them due to lack of maintenance.  40% of American bridges are older, in fact, than the Polcevera Viaduct, meaning that whatever the proximate cause of the next large collapse here, no one should be able to get away with saying it was “unexpected and unforeseen,” the terms used, unconvincingly, by Stefano Marigliani, head of Autostrade’s Genoa bureau, to describe the Genoa collapse.

UPDATE (Sept. 6, 2018):  A good overview on the New York Times website today confirms that the collapse began in the southern pair of cable stays and cites the lack of redundancy as a contributing factor…

Image sources:

Gentile And F. Martinez Y Cabrera (Department Of Structural Engineering, Politecnico Di Milano), “Dynamic Investigation Of A Repaired Cable-Stayed Bridge.”  Earthquake Engineering And Structural Dynamics,Vol. 26, (1997). 41-59.

Prof. Ing. Riccardo Morandi, “Viaducto Sobre el Polcevera, en Génova-Italia.”  Informes de la Construcción,vol. 1, no. 200. 57-99.  May, 1968.  Available online at: http://informesdelaconstruccion.revistas.csic.es

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