Portland's Tilikum Crossing shares some similarities to the collapsed Italian bridge

Portland's Tilikum Crossing. (KATU Photo)

Portland's Tilikum Crossing, Bridge for the People, shares similar style components to the Italian bridge that collapsed Tuesday morning, killing at least 26 and injuring 15 more.

The Morandi Bridge, built in the 1960s, stands on the A10 toll motorway, which serves the Italian Riviera and southern coast of France.

It was a cable-stayed bridge, where the vertical elements are trestles, made up of two superimposed Vs: one carried the roadway beam, while the other, upside down, supported the upper tie rods.

At 1,720 feet long, with a main span of 780 feet, the Tilikum Crossing is a hybrid between a traditional cable-stayed layout and an extradosed bridge, with two towers and two landside piers. It's primarily constructed of concrete.

There are four common types of cable-stayed bridges: mono, fan, harp and star. Each vary in design, particularly in cable placement on the bridge's deck or towers. Each style is used in different applications.

The Tilikum Crossing most resembles a harp-style, cable-stayed bridge. In the harp or parallel design, the cables are nearly parallel so that the height of their attachment to the tower is proportional to the distance from the tower to their mounting on the deck.

Cable-stayed bridges may appear similar to suspension bridges, like Portland's St. Johns Bridge, but in fact, they are quite different in principle and in their construction.

In suspension bridges, large main cables, normally two, hang between the towers and are anchored at each end to the ground. The main cables, which are free to move on bearings in the towers, bear the load of the bridge deck.

TriMet owns and manages the Tilikum Crossing.

"The bridge collapse in Genoa is tragic and our hearts go out to the loved ones of all those who died or were injured," a TriMet spokesperson told KATU. "The design and construction of cable-stayed bridges varies significantly from structure to structure. This can be seen simply by comparing photos of the Genoa bridge before the collapse with Tilikum Crossing."

TriMet says the Tilikum Crossing was constructed between 2011 and 2015, to the highest state, national and international seismic and general engineering standards.

Portland State Professor Dr. Franz Rad told KATU the bridge's failure in Genoa cannot be attributed to the fact that the bridge was a “cable-stayed” design.

"Many other types of bridges also fail for a variety of reasons," Rad wrote. "Cable-stayed bridges, if designed and detailed properly, and if maintained properly, can perform a century or longer."

While Italian investigators have not said what caused the bridge to collapse, Rad says it appears the bridge lacked a robust inspection and maintenance program. He says it's also possible that the vehicular loads increased with time, beyond the intended design loads in the original design.

"Concrete cracks can begin as micro-cracks, and eventually get wider and deeper under cyclic vehicular loading. Eventually, fatigue can cause failure," Rad said brainstorming possible explanations.

Rad says routine inspections can detect problems in infrastructure prior to failure and collapse.

Under U.S. federal guidelines, bridges that carry vehicular traffic must be inspected every two years.

"We have very strict inspection requirements that ensures the public that our bridges are safe," Portland Bureau of Transportation spokesperson Dylan Rivera said. "[The Oregon Department of Transportation] handles those inspections for the city of Portland. We work closely with them to address anything that is deficient or anything that is cause for concern."

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