Basically the bridge is a truss design, while the cable stays are both for aesthetics and vibrational dampening only. The cross-section profile is basically an I-Beam.
My amateur questions below …
QUESTIONS:
Why is the deck of the bridge composed with less support mass than the roofline? Due to self-supporting bottom-heavy mass, PT rods would need higher tensioning along the roofline, with fewer rods embedded in less concrete.
Why are the truss diagonals staggered in such a way as to shift the static load to one end of the bridge? Typically, sag would occur in the middle of a symmetrical set of diagonal braces. In this case, asymmetry would shift the compressive forces from the middle of the span toward the end near the pilings. This would mean that approximately 1/4 of the diagonal braces would need to support 3/4 of the cumulative load.
Why was there increasing spacing between braces without reinforcing vertical members? This would increase the compressive forces on the bottom deck with fewer load-spreading members to dissipate stress more evenly.
Why do the counter-diagonal supports meet at a singular point above the height-line of the cable-stay tower? Although it’s for aesthetics rather than support, the cable-stay tower mimics the evenly-spaced attachment points of a cable-stay bridge versus a typical suspension bridge. This dissipates force along the tower rather than a single point at the top. However, if we imagine the counter-diagonals cable-stayed in the opposite direction, we can see that support would not be dissipated properly. This would suggest that even without a support-tower, the angles of force should be adjusted, especially if compressive tensioning has to therefore vary inside each beam.
DIAGRAM OF THEORETICAL REASON FOR FAILURE (enlarge to read):
VIDEO OF COLLAPSE:
This appears to be a design flaw or design oversight. Here are some video still shots showing the collapse path. The collapse appeared to be partially due to over-tensioning and snapping a PT rod. But why were they overtensioning?
Video stills:
CONJECTURE:
From video pictures you can see there was an abrupt, shattering single point of failure. As the deck collapsed it was mostly intact until hitting the ground. It was reported that the failure-point occurred where sagging had loosened the PT rods.
The compressive forces are roughly analogous to a ladder propped up against a building at an extreme angle, while you jam your foot against the feet of the ladder to act as a brace. Remove your foot or simply shift your body-weight back as you straighten your leg, and the ladder is going to slip and fall. Your center-of-gravity shifts and your leg will have less strength to keep the ladder in place, because your leg is acting as a kind of diagonal cantilever with less to buttress against the length of the ladder.
It’s possible they actually needed to tighten PT rods on the opposite end of the bridge to try and re-distribute the load tensioning. Instead, tightening the sagging rods simply removed the static support on the counter-diagonal members?
The author of the video in the original post had a really good follow-up post that explained that (benzbanana posted it above). Basically, his hypothesis was that the PT rod was damaged when lifting the bridge into place, possibly because one of the lifting supports was not positioned as designed. So he thinks they overtensioned because they misdiagnosed why the stress on the beam wasn’t coming up to the expected level, and they just kept on trying to tighten the PT rod until it failed.
I don’t know all the details in the particular case but you’d be surprised how often pylons, cables, and other structural-looking features are added to beam-type bridges for purely aesthetic reasons. The process goes like this: elected officials solicit proposals for an iconic bridge design to bring prestige to their local region or municipality. Architects come back with a number of amazing, innovative designs that would also go well over the allotted budget. Civil Engineers are then tasked to “value engineer” the design and come back with a solution that can be built in the allowable budget and still preserve as much of the creative intent as possible. The solution is often to build a boring beam bridge (often the most cost effective these days) then slap a bunch of unnecessary features on it.
I’m not going to say that never happens, but in my experience, the “value engineer” eliminates as much of the creative intent as possible, and unnecessary features (which shouldn’t be there in the first place, if you ask me) are the first things to go.
Yeah, I think it’s possible there was damage in transit, but …
SUPPORT NEAR FAILURE POINT?
But you’ll note that when they moved the bridge, the lifting rig was already supporting it where the failure occurred.
So it was positioned near the point of stress failure already, and I don’t know how that would cause damage unless the deck beam was canted when lifting it.
SHEAR FORCE LIKELY?
It appears likely to me that shear force along the length of trusses punched through the deck, kind of like creasing the outside of a soda can.
FLEXING?
As even grade-school kids can show you with models and simple software, without symmetrical truss-spacing, a bridge will not flex evenly in the middle of its span.
There would have to be countervailing force inside each diagonal member tensioned differently.
VARYING TENSIONING?
This would likely require either thicker trusses toward one end of the bridge (to provide more mass for both more tensioning rods and static support).
Or it would require additional support trusses. Unless the PT rods were rated at different strengths and gauges, like bicycle spokes.
TEMPERATURE!
However, one thing I haven’t seen mentioned is temperature control:
I know for a fact, when they install welded steel rails in long (quarter-mile?) lengths, they put them in during warm temperatures, so that the rail is expanded along its length.
Then during winter time, the rails contract by pulling against each other at the expansion joints.
Problems arise when there are big swings in temperature from nighttime to daytime: if the rail expands sufficiently under high heat in summer months, it will literally butt against itself.
This will bow the rail out-of-alignment, forming a heat kink that will derail a train.
If “heat-kinks” formed in the PT rods while the bridge was being cast, the variable-tensioning would be completely “out of alignment” as the concrete set.
This could create tortion that would twist the rods inside the concrete, leading to failure.
The FIU bridge was a truss bridge, its designers, the FIGG Bridge Group, confirmed after the collapse. Many have assumed it was a suspension bridge because renderings of the finished structure show a mast with pipes or cables connecting from its tip to the bridge in a sail-like pattern. Observers, including some engineers, have posited that had the mast been in place, the bridge might not have collapsed.
But in fact the mast would have provided no vertical support. FIGG advertised that as a “cable-stayed” bridge, and plans and other materials on the FIU website say the mast was there mostly to dampen vibration, provide some “stiffness” and create dramatic aesthetics.
After studying the engineering drawings on the website with colleagues, Verrastro confirmed the mast had no role in holding up the bridge.
“They definitely didn’t need it,” he said. “It’s there mostly for looks.”
Because the FIU bridge would see no motorized traffic that could strike a truss, the decision to go with that design by itself raises no red flags, Verrastro said.
Well, yes, if the requirement is to build the cheapest possible functional structure that is exactly what the value engineering process ends up doing. But as I mentioned, for many projects there are local politics or other concerns that make creative intent one of the REQUIREMENTS of the project. I have personally been involved with a number of such projects where engineering the cheapest possible functional solution would never have been considered acceptable, so the engineers have to go back and forth with the creative types to come up with a compromise that results in a structure that’s supported in an entirely different way that it appears at first glance.
Several others in this thread have already linked to articles with information explaining that the pylon and cables were decorative on this bridge, and maybe you still don’t believe that. But I’ll bet I can show you some examples of fake cable suspension bridge that you’ll admit are just there for cosmetics:
Cable-stayed bridges are a thing. The Margaret McDermott bridge in your first photo is one of them, though Wikipedia does say that the outer arches are indeed “decorative.”
Non-functional decorative fake structure…that’s just nuts.
I think someone noted the lack of spreader plates in use when the bridge was being transported… that too could have been a factor since plates tend to spread the amount of stress on the supported structure out. And you have to tension the rods properly to account for the redistributed stress… if the rods were tensioned under the assumption of there being spreader plates when there aren’t any, then things could have gone wrong.
That whole bridge seems like a ridiculous mess. I can’t believe they’d go to all the trouble of building decorative cable stays and then having those cables rip up the concrete because the rods used to attach the cables to the bridge are underspecced. Then again, there have been a lot of engineering boondoggles with regard to cable-stayed bridges, like giant chunks of ice forming on cables and falling to the bridge deck…
After reading through some of the engineering websites-- and familiarizing myself with the technical language that I don’t have-- I realized they’re were talking about similar things that people have mentioned here.
Basically, the engineering forums are now focusing on truss member #11, and it being strained past capacity, due to less structural redundancy. Conjecture around the failure appears to be the following:
lack of longitudinal PT tendons attaching vertical truss-end #12 to deck
lacked of reinforcement caused by drainpipe and other fittings through the deck and the end of truss #12 (increased chance of stress fractures)
angle of truss #11 at approx. 30-degrees, thus increasing horizontal load strain (compare photos of thicker truss #2 with thinner truss #11)
potential eccentric load (that is, load unevenly applied through truss-members due to asymmetry)
retrofitted ducts in truss #11 for the use of temporary PT tendons to stabilize cantilevering due to SPMTs not being able to maneuver and support the bridge-ends
FDOT redesign request to extend the length of bridge to make room for additional traffic lane
north pier undersized with soft footings due to canal work and delays by Army Corps of Engineers
emphasis on making trusses visually line up with cable-stays, despite increased instability (disregard for design safety in order to aesthetically promote future nearby gentrification projects; early conceptualization had real cable-stays dropped from design but kept for “vibration dampering”)
no redundancy in trusswork (high failure-rate with single row of trusses and no vertical web)
torsion strain (trusswork subject to weak cross-section twisting from wind and over-height truck collisions)
lack of node revolution plates for flexion (joint between truss-ends and top and bottom chords was fixed and brittle)
use of materials lacking ductility (concrete has rapid failure in tension, whereas steel fails more slowly)
change of compression elements without accounting for mid-transport readjustments (tightening compression in non-compression members for transport, but creating tension counter-effects when deck was lowered onto piers without timely readjustment)
push for rapid project completion in order to receive Federal funds
loss of design focus: over-weight, over-budget, under-built, over-promised multi-purpose bridge for social gathering place (not just transit, but superfluous fittings requiring extra cable ducts for circulation fans and wifi with cafe seating over a road with heavy traffic)
And that’s just what I could recall off the top of my head! This was a bad design just waiting to fall down. Period. They’re just lucky it didn’t fall down with hundreds of people on it.
Here’s a truss simulator where you can practice what went wrong:
