The Italian group ASTM, which deals with the construction and management of infrastructure, announced on Monday, November 2 that its US subsidiary Halmar International will carry out a part of the seismic adjustment work of the Golden Gate Bridge, the suspended bridge connecting San Francisco to Marin County, California. The contract has a value of 863,9 million dollars, about 770 million euros, and it concerns above all the strengthening of the two large towers and the side spans of the bridge.
The intervention is part of a program launched in the nineties to protect the Golden Gate Bridge from earthquakes that periodically affect California. The bridge is located just a few kilometers from the San Andreas fault, one of the most important active faults in North America, responsible among other things of the devastating earthquake that struck San Francisco in 1906.
The Loma Prieta earthquake of 17 October 1989, which caused dozens of deaths and seriously damaged several Bay Area infrastructures, was evident. The Golden Gate Bridge suffered no damage, even because the epicenter was about a hundred kilometers away, but the verifications subsequently showed that an earthquake just as violent and closer to the bridge could seriously compromise the structure.
Since then several consolidation measures have been implemented. The first were mainly concerned with access viaducts and structures at the ends of the bridge, initially considered the most vulnerable. According to the agency that manages it, the works already completed have eliminated the risk of a collapse of the infrastructure in case of a strong earthquake, but remains the possibility that some parts suffer damages such as to make the bridge unusable for a prolonged period.
It is precisely on this aspect that the last phase of the program focuses. The goal is to ensure that the Golden Gate Bridge can be reopened quickly after a large earthquake, allowing the passage of the rescue and the restoration of the connections between San Francisco and the locations north of the city.
Umberto Tosoni, CEO of ASTM Group, commented: “The Golden Gate is an engineering and cultural icon. To be called to realize its modernization and seismic adaptation is for us reason of great pride, but also a responsibility that we feel deeply. This important award strengthens our position as an international reference partner for major high-tech and strategic infrastructure. ”
Umberto Tosoni, Amministratore Delegato del Gruppo ASTM
The main problem concerns the different behaviour of the structures that make up the bridge during a shock. The central span, supported by large steel cables, is designed to swing and absorb a part of the movements. The towers and the sides are much more rigid. During a particularly intense earthquake, these differences can cause stress and displacement capable of damaging links between the various parts of the structure.
The work entrusted to Halmar International therefore previews the strengthening of the bases of the towers, the beams that support the road and the metal structures side. Some joints will also be replaced, so as to allow larger movements during the shocks, and installed 28 seismic energy dissipation devices, a sort of large shock absorbers designed to reduce the forces transmitted to the bridge’s bearing parts. The contract also includes the repainting of the southern tower above the road level.
These are complicated operations also for the conditions in which they must be performed. The Golden Gate Bridge, inaugurated in 1937, is about 2.7 kilometers long and is crossed daily by an average of over 95 thousand vehicles. Much of the work will be done while the bridge continues to be used. For this reason, from the beginning of October some lanes are closed during the night, generally from 20 to 5 in the morning, leaving at least one passage in each direction.
The conclusion of the interventions entrusted to Halmar is scheduled for September 2031. The overall programme, which also includes the strengthening of the central camp through a second contract, should continue until 2036.
Halmar International is part of Itinera, a construction company controlled by the Piedmontese group ASTM, which operates in different countries and also manages thousands of kilometers of highways. In the United States it is already engaged in other infrastructure projects, including some railway and metropolitan interventions in the New York area.
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