Did some more googling and I think this is one weird-ass bridge design. I had thought that the second “layer” was a roof to shelter pedestrians from weather (well, rain–snow is probably not high risk) but from concept renderings and models I see that it covers only a fraction of the bridge and is there to anchor the suspension cables.
Then I think of the Liberty Bridge in (nearby for me) Greenville, SC. Also a suspension pedestrian bridge with a single row of cables, but cost $4.5 million and has been in use for years. I don’t know how much it weighs, but the two “masts” are 28 tons each, and I really doubt that the rest of it adds up to an extra 900ish tons. And it actually looks pretty cool.
This article says that for the South Norfolk Jordan Bridge accident,
The article also talks about a makeshift walkway (for use during construction?) they built in Fort Lauderdale-Hollywood International Airport that collapsed and inured a TSA employee.
I used to be a programmer for an engineering company. An engineer told about a project that he worked on, a parking garage, that failed shortly after completion. He said everyone involved with the project was sued, including him as professional engineer. That was designer, engineer, suppliers, manufacturers, workers on site, etc. Then they worked through what happened in order to figure out which of the participants was to blame.
We looked at it in a class I took. What happened was the threaded support rods for the two walkways were originally designed as a single long rod. This made for construction problems as the nuts supporting the top beam had to be turned all the way up the threaded rod. The construction people proposed cutting the support rods in two, so instead of a long rod with one nut supporting the top walkway and a second lower nut supporting the bottom one they’d have two offset rods, with one supporting the top walkway and one supporting the bottom.
They were well within the load factors for the rods, but failed to consider that this modification meant the nuts and beams supporting the top walkway were now also supporting the bottom one.
The engineers took a real beating on that one, and deserved it. Gross negligence, misconduct and unprofessional conduct in the practice of engineering, loss of licenses; the whole works.
See, that’s why engineers are so freaking cautious. If you try a last minute design change and it fails, it’s deaths and injuries and destruction of property in the millions; loss of licenses and civil and criminal charges, but if the last minute design change works, you get a certificate of appreciation in a handsome plastic frame.
This appears to be a solicitation for proposals - it’s titled “Design Criteria” and lists requirements for the bridge. It doesn’t show any specific design.
It contains 18 pages of pictures showing “Design Concept Precedents.” I guess these are what they were hoping to get - they’re all prettier than the one they ended up with.
Also, almost half of this 80-page document seems to be talking about an elevator.
PT slabs can be scary for construction workers. I was warned many times to never cut or drill into a PT house slab. You can get badly hurt.
The slab is usually stamped with a clear warning (in the garage) or they’ll be a sign posted on the wall. PT slabs are used on land with poor structural support. Like a low lying area with fill dirt. The load is spread across a wider area in a PT slab.
I can easily see a bridge failing if one of those rods snapped.
IANACivil Engineer, but this bit seems unlikely to me. Pedestrians aren’t that heavy - how could there be a structure where the design tolerances are such that it’s strong enough to stand alone, but requires an enormous superstructure of supporting cables when a few hundred pounds of people are added as a load. Doesn’t make sense.
Fifteen people can easily be a ton. The bridge goes between the school and a residential area. It is pretty easy to imagine two or three tons of humanity crossing it at some times. And each one of those people is pounding the deck repeatly with their weight. It takes a little more than a few hundred pounds capacity for a pedestrian bridge.
According to the PDF I linked earlier, they were hoping for a bridge that would be Party Central:
[QUOTE=That PDF I linked earlier]
This structure should function as more than just a path for circulation; it should be a place to be and a place to be experienced, and the FIU campus and its students must be proud of it. It should be a destination in its own right where community members might linger, gather, and create an urban social space – a linear park. We expect that the bridge might even be used as an event venue. For those reasons, it should be equipped with furniture, shading, protection from the elements, and state of the art safety features such as LED lighting, video surveillance and emergency call boxes.
[/QUOTE]
When I look at that model, it looks to me like the cables are going through conduits to ultimately anchor to the main deck. So, yeah, I think they were intended to provide support to the whole structure.
When I look at satellite view, I see that the street it was crossing is nine lanes wide (3W, 2LT, 4E) – they easily could have blocked half of that to place temporary support structures under the span until it had at least half of the suspension cables in place.
I feel very bad for the university. They had a serious problem with students from Sweetwater crossing that dangerous road.
The problem won’t go away and now they have to start completely over. That’s assuming the school is even willing to restart the process.
With all the lawsuits, students may be risking their lives crossing that road for many years to come.
The easiest and quickest solution would be a pedestrian crossing light. That could get students across that road at predetermined times during the day.
I’m no structural engineer, but that special self-cleaning concrete with titanium dioxide (which was mixed on site, not premixed elsewhere and delivered in cement mixer trucks, as per usual) would be my guess as to the culprit.
Though, another thing I’m wondering about is that temperatures in South Florida have been particularly cooler than usual and I wonder if that might have factored in somehow.
[QUOTE=Some Guy]
The cables would be needed at a later stage when the rest of the bridge was constructed, prior to opening the bridge for pedestrian loads.
[/QUOTE]
In Florida, the bridge would need to withstand hurricane force winds. Wouldn’t that be a much more significant engineering consideration than pedestrian loads? My thought is that maybe the construction protocol allows it to be assembled outside hurricane season without the supporting cables, with the cables to be installed before the bridge is exposed to high winds.
