IANAE but that seems like a weak design (also a week design). It looks like a Howe Truss but concrete’s strength is in compression and not tension. Using the tensioning system described in the video above doesn’t seem to make up for the loss of tension across the length of the bridge. Any shifting of the load is going to make it snap like… concrete.
Most concrete bridges have some part of it in tension and some part of it in compression. A standard box girder bridge, for example, has the bottom part in tension and the top part in compression. That’s why they are made of prestressed concrete, not pure concrete.
Following a link in that wiki, their entry for Prestressed concrete covers both pre-tensioning and post-tensioning very well, with excellent pictures and easy to understand diagrams.
Was this bridge done with stressed concrete? If so, Why the lateral tensioning system?
Updating this thread the Miami Herald contacted some engineers for their analysis:
http://www.miamiherald.com/news/local/community/miami-dade/article212571434.html
Thanks to PastTense for linking to a very interesting article.
I’d read a fair amount about the bridge collapse and, like many, wondered why a 950-ton bridge would be put it place (over a road that carries lots of traffic) without its support cables in place. Turns out, those things that look like support cables in the drawings of the finished bridge were in fact simple pipes, intended as aesthetic elements - they were not structural.
The aesthetics further required that the diagonal concrete beams be set at varying angles, to align with the pipes. This produced an unusual distribution of forces, with the highest stress at the base of beam #11 - which is where the failure happened.
Another decision related to aesthetics seems to be the choice of concrete that included a lot of titanium dioxide - which may have lower than normal strength.
In the end, a structure with a number of unusual elements appears not to have been the subject of any unusual amount of design review. It thus may have been destined to fail, with a high probability of killing a number of people (since traffic controls routinely caused cars to be stopped below).
“The pylon was designed to be 109 feet tall to mark the location at 109th Avenue, a height that helped set the angles for the pipes and trusses, Beck said.”
That right there might be the stupidest decision among quite the passel of stupid decisions in the conception of that bridge.
It’s a good thing they weren’t metric.
IOW, the fake structural elements set the parameters, both constraining and complicating the key, non-redundant, single-point-of-failure, actual structural elements. And making it worse, the parameters of the fake structural elements are basically hinged on something that was supposedly symbolic*, but nobody would have ever noticed or cared about in the least.
- Symbolic of a little wanna-be college town main street in a little wanna-be municipality that was never going to have much of a chance to distinguish itself with all the competition in the very near area. The road the bridge was to cross, Tamiami Trail, is AKA Calle Ocho just down the road!
I wouldn’t blame the college for wanting a really nice pedestrian bridge with a socializing area.
They had the money to pay for it.
The engineers fucked up. There’s no nice way to say it. They should have taken in account that this was an ambitious design and been extra cautious in testing it with modeling & simulation.
It’s still to be determined if the construction team screwed up too. But, the latest news indicates the bridge design was badly flawed.
Function follows form.
I teach statics in an engineering college. As every sophomore engineering student knows, you are supposed to load a truss only at its joints - never in the middle of a structural member. That guarantees that the individual members only experience tension and/or compression, never bending. Concrete is very strong in compression, and pre-stressed concrete can be very strong in tension. But it is NEVER strong in bending.
I’m pretty sure I saw a picture of that bridge right before the collapse where it was resting on some temporary pylons. One of the pylons was under a truss joint, as it should be. But the other one was in the middle of a horizontal member, in between two joints. This could have been a screwup on the part of the construction and installation crew, and led to the demise of a perfectly safe design during a temporary phase of the installation process.
The article says the bridge failed as workers were tightening the #2 and #11 (the one that failed) support rods. The reasons for this were unknown but someone surmised they were trying to close up the cracks that had appeared before the move and worsened afterward.
“Hmmm. Looks like things are pulling apart in there. Let’s tighten this up to pull it back together.”
Sound like someone needs to have his license checked.
NTSB releases new photos of cracks before it fell.
It’s even more shocking that all the involved parties had prior knowledge of these cracks. They had a meeting to discuss what to do.
Yet, they didn’t recommend closing the road?
What the fucking hell were they thinking?
Imho all parties involved need to be severely punished. Anyone who saw these new photos should have recognized that bridge was compromised. Closing the road to traffic before touching those rods, should have been automatic.
Sure seems like a dead-to-rights case of bad design – the preliminary comments by the NTSB indicate the concrete was sound and the bridge conformed to blueprints (possibly flawed blueprints, mind you – but that would make it an error by the designers rather than the construction crew). Since the investigation is taking a while, I hope the authorities are tracking the assets and the travel plans of the design firm’s executives.
Looking at the video of the bridge collapse, I am struck by the number of people who immediately jump out of their cars and run toward the collapsed structure, presumably trying to help. Good on you, Florida Man!
I consider the flawed design, and the decision to tighten those rods with the multilane road open, two separate issues.
The flawed design could have been corrected. The bridge redesigned & rebuilt if needed.
The response to the cracked bridge structure is appalling and unforgivable. Florida’s DOT was at that morning meeting. They should have insisted on public safety and closed that road for a few hours. Perhaps even rescheduled the work on a weekend when there’s less traffic to disrupt.
I have a bad feeling all the blame will fall on the engineering company. The decision to leave that road open to traffic won’t have any consequences.
The companies involved with buildiing this thing have been fined $86,658. For the record, with a construction cost of 14.2 million, that is around 6/10[sup]th[/sup]s of 1 percent of the contract cost, otherwise known as “the coffee budget.”
I’m sure these fines will make these corporations re-evaluate their woefully inadequate worker safety practices and lead to meaningful changes in the culture of safety they display…
This is just the OSHA fines for violations of worker safety rules.
That’s just the beginning. The civil suits will be where the real money goes out.