Florida Pedestrian Bridge Constructed 5 Days Ago Collapses, Multiple Casualties | Hacker News Reader

Florida Pedestrian Bridge Constructed 5 Days Ago Collapses, Multiple Casualties

jalopnik.com

88 pointsguardiangod26 comments

As one can see in the installation video from last weekend, the bridge in question was the first span of a larger project, in which at least one other span would have to be installed above an adjacent canal to complete the crossing.

A rendering [1] shows the completed crossing to be an asymmetric harp-type cable-stayed bridge, with cables supporting both the portion above the road (the piece in question), and the piece that's yet-to-be constructed to go above the water. In this design, it's usually not possible to (temporarily) install the cables solely on one side of the tower, as the loads on one side of the tower would collapse the tower.

Therefore, it stands to reason that the cables were either to come into play at the very end, or if the bridge were constructed by cantilevering out from the central tower and attaching cables as you go.

In other words, without trying to pass judgment on the cause, the cables are either a red herring (because the sub-spans are self-supporting), or the bridge was assembled in a terribly incorrect sequence.

[1] http://www.miamiherald.com/news/local/community/miami-dade/w...

On the rendering, it seems to me that the part over the water is significantly shorter than the now collapsed part. It also doesn’t look heavier per meter to me, and the tower seems to be perfectly vertical. That would make it impossible to fully ‘hang’ the part over the road from the tower.

=> It is quite possible that the span over the road was designed to be self-supporting.

That would be a non-standard design, but we are so good at building such bridges that ‘being non-standard’ almost has become the standard.

Apparently the elevated walkway was installed in less than 1 day (with major parts of it being pre-fab). But it sounds like it wasn't the complete project, which was expected to be finished in 2019:

> Once it’s finished in early 2019, the new pedestrian bridge will link FIU’s Modesto A. Maidique Campus directly to the small suburban city of Sweetwater, where the university estimates 4,000 of its students live.

http://www.miamiherald.com/news/local/community/miami-dade/w...

Is it common for pedestrian bridges to be so overbuilt? Given the weight and span it sounds like it has a cross sectional area of about 75 square feet of concrete (a little less for the steel components, but still a lot) for a mere 15 foot traffic deck. It's not crossing an especially large span nor need to carry any substantial weight other than its own, so why does it need to be so heavy?

Are they expecting trucks to run into it maybe?

They have to calculate for worst-case scenario. Imagine it were completely full of people. Dancing. Plus, people do ride motorcycles on them if they can get them on.

And Mother Nature.

I have a few friends that easily outweigh my full size motorcycle.

People are surprisingly heavy per square foot - multiple times the weight of a car PSF.

Are bridges collapsing common? Looks like those involved had at least two other bridge related incidents in the past - http://www.miaminewtimes.com/news/fiu-bridge-collapse-constr...

Not major infrastructural bridges, there's a list on Wikipedia: https://en.wikipedia.org/wiki/List_of_bridge_failures

The US seems to have a lot, but there are a lot of bridges in the US. Construction is pretty risky though, there are often elements that are incompletely supported - engineers do plan for this and ensure they are strong enough.

It appears that the intended final design includes suspension cables, but it looks like it was installed without them attached?

It looks like a typical cable-stayed bridge. Often spans are installed in advance of cables by omitting heavy elements, like the deck, so the span is self supporting. The cables are then installed and anchored and the deck is fitted.

>The $14.2 million project was designed as a cable-supported bridge by MCM and Figg Bridge Design. Figg was hired to design the replacement for a bridge on I35 in Minnesota that collapsed in 2007 and killed 13 people.

1 paragraph later.

>“In our 40-year history, nothing like this has ever happened before,” Figg said.

EDIT - As others have pointed out... I misread these statements.

They were hired to design the replacement, not the one that collapsed.

They designed the replacement that was installed in 2008. The bridge that collapsed was designed by a different company in the 1960s.

https://en.wikipedia.org/wiki/I-35W_Mississippi_River_bridge...

Oh I see... I've edited my comment.

Famous video of "Galloping Gertie" Tacoma Narrows Bridge collapse in 1940:

https://youtu.be/j-zczJXSxnw

People died and FIU offered...yes, thoughts and prayers (look at the end of the article). Sigh.

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