Collapsed Miami bridge was lifted into place without central tower

Engineering experts say investigators looking into the collapsed ‘instant’ bridge in Miami will want to know why a central tower which is usually built to support a suspension bridge was not in place when it collapsed onto Tamiami Trail on Thursday afternoon.

Last week, Florida International University’s official Twitter account posted a rendering of the bridge in its completed form as envisioned by the planners.

The rendering shows a tall central column with cables connecting it to the main span.

Engineers say the design is known as a ‘cable-stayed bridge,’ which is a kind of suspension bridge, according to USA Today.

The FIU bridge was not yet open to the public when it collapsed onto the eight-lane highway below, killing at least six people and injuring at least 10.

Engineering experts say investigators looking into the collapsed ‘instant’ bridge at Florida International University will want to know why a central tower which is usually built to support a suspension bridge was not in place when it collapsed onto Tamiami Trail on Thursday afternoon

Last week, FIU’s official Twitter account posted a rendering of the bridge in its completed form as envisioned by the planners. The rendering shows a tall central column with cables connecting it to the main span. The city of Sweetwater also released a rendering above

The FIU bridge was not yet open to the public when it collapsed onto the eight-lane highway below, killing at least six people and injuring at least 10. It is seen above after it was installed last Saturday and before its collapse on Thursday

The bridge did not have the central tower in place, even though experts say it is usually placed at the early stages of construction.

In the absence of a tower, there is usually a temporary support, though in this case it is unclear what the builders were using in the absence of a central structure.

‘Whoever is going to investigate, they will ask the fundamental question: shouldn’t the tower be there, and the cables ready to connect to the structure, when you lift it?’ said Amjad Aref, a professor at University at Buffalo’s Department of Civil, Structural and Environmental Engineering.

‘That’s a question for them to answer.’

Cable-stayed bridges are built in stages. First, planners will locate a clear piece of land with stable ground and a good location.

They will then conduct a subsurface investigation which involves lab testing on the soil to make sure that any proposed structure would be supported by the geological conditions at the site.

If those tests permit, engineers would then erect piers – the upright supports for a structure like a bridge or an arch – and the support span.

After building the main piers, engineers usually begin construction of the central tower.

After the tower is built and the stay cables are installed, engineers then begin work on extending the central span.

Cable-stayed bridges (like the new Kosciuszko Bridge connecting Brooklyn and Queens as seen above during construction in 2017) usually have central towers and cables in place before the main span is completed

When asked about why there was no central column built before the span, the head of the National Transportation Safety Board, Robert Sumwalt, said: ‘That’s part of our investigation.’

The NTSB is an independent federal agency that probes transportation-related accidents.

Andrew Hermann, a former president of the American Society of Civil Engineers, said it appears the engineers who built the FIU bridge didn’t follow the proper sequence.

‘When you’re doing staged construction like this, what you have to make sure is that at each stage that the structure is strong enough for the loads that are on the bridge,’ Hermann told USA Today.

‘The engineering, both design and the construction engineering, should have taken that into account with the bridge in that condition.’

Engineering analysts are also vexed as to how a cable-stayed bridge built to carry pedestrians could buckle under no weight – even though a number of larger structures which can withstand cars and trucks remain in place.

‘I wish I would be on that kind of investigation, to be honest with you, because in this country we build so many cable-stay bridges for carrying trucks, not pedestrians, and all of them work fine,’ Aref said.

‘The spans, from one end to the other, is much larger than that.’

The most popular examples of cable-stayed bridges are the newly constructed Kosciuszko Bridge, which connects two New York City boroughs along the Brooklyn-Queens Expressway.

The Sunshine Skyway Bridge in Tampa and the John James Audubon Bridge in Louisiana are other examples of cable-stayed bridges.

The partially built 950-ton bridge had been assembled by the side of the highway and moved into place Saturday to great fanfare.

The Sunshine Skyway Bridge in Tampa (above) and the John James Audubon Bridge in Louisiana are other examples of cable-stayed bridges

The span stretched almost 200 feet to connect FIU with the city of Sweetwater.

It was expected to open to foot traffic next year.

An accelerated construction method was supposed to reduce risks to workers and pedestrians and minimize traffic disruption, the university said.

Robert Bea, a professor of engineering and construction management at the University of California, Berkeley, said it’s too early to know exactly what happened, but the decision to use what the bridge builders called an ‘innovative installation’ was risky, especially because the bridge spanned a heavily traveled thoroughfare.

‘Innovations take a design firm into an area where they don’t have applicable experience, and then we have another unexpected failure on our hands,’ Bea said after reviewing the bridge’s design and photos of the collapse.

Aref said he expects this to be a quick investigation.

‘I don’t want to speculate. From a structural-engineering point of view, the forensic engineers won’t take long to figure out what happened,’ he said.

‘I think it is not a long investigation. There are glaring things.’ 

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