I don’t know if this was cable tensioning:
They put some weights to hold down the popped up section and reopened it to traffic.
Looks like these were improperly sized bolts that failed under tension. The bridge I was referring to carries subway trains and has a bike route attached to it. Of course, none of those things were present when the bridge was standing without tensioned cables.
If you ever found yourself wondering why indemnity clauses exist in contracts, this, right here, be a prime example of the reason.
A good discussion about the status of the bridge at the time of collapse - it was not complete, and was still under construction. The author has identified “post-tensioning” hardware which is a method by which reinforcing steel is put under tension for its final state. The tensioning of this reinforcement may or may not have been underway.
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I have this…bridges, not tunnels. (I don’t love driving in tunnels, because lanes tend to be narrow and pavement slick, but it doesn’t turn my knuckles white.)
As for bridges, I’ll usually choose a driving route to avoid them if I can, though sometimes it is difficult (I drive in and out of San Francisco pretty often, for example). Once I had to drive across the Humber Bridge on a day that was so windy that they closed it a few minutes after I’d crossed. That was delightful.
Honestly, as phobias go I think this one is pretty rational.
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Given that a pedestrian bridge currently going up here in Spokane is projected to cost $12m, it doesn’t seem like a particularly whopping number.
(There is, of course, a councilman questioning why a pedestrian bridge costs almost as much if not more than a major automotive bridge, but it’s not like a lengthy bridge is any less complex because it’s not carrying cars, and isn’t there always That Guy?)
Yes, good read, thanks for the link.
From the link:
If you look at the image of the deck being placed, you can see that the end of the bottom ‘flange’ has a line of small gray cylinders sticking out of it. These are ducts for post-tensioning cables, ‘super-reinforcing’ that, once tightened, would take the huge tensile force in a bridge deck acting, temporarily, as a beam across its span. These may have been tightened before the deck was put in place, or the bridge may have been waiting for the tower and backspan to be installed, so that cables could be run through the entire length of the bridge and tightened at once, holding all of the pieces together. If that’s the case, then the deck would have been particularly vulnerable to failure along its bottom, tensile flange. Another possibility is that the top flange could have failed in compression. From the images of the collapse, there appears to have been buckling there, but it’s hard to tell whether this occurred before or after the deck impacted the ground.
This isn’t more than speculation from a gut feeling, but I’d say that the tensioning cables were tightened before the bridge segment was put in place. Possibly the idea was to have the lower flange act like a beam that would carry the whole segment until the pylon and cables were assembled. Thing is, this is in effect a structure in prestressed concrete - the tensile force in the tensioning cables has got to go somewhere. It also means that an element that would have to bear tensile forces in the finished bridge would have to bear tensile and compressive forces temporarily. Looking at the cross-section of the flange in the picture, the ducts for the tensioning cables take up quite an area (or rather volume) of the flange. Now the anchor points of the cables would be the critical points of the construction; but if the ratio of massive concrete (which has to bear the pressure) to cable duct is, let’s say competetive, any imperfection in the flange’s concrete is potentially a point of desastrous failure.
So maybe too much tension in the cables and either tension failure in a cable or compression failure of the lower flange.
Or maybe not enough tension in the cables and compression failure in the upper flange.
Or maybe something else entirely.
In any way, if you put a half-finished structure over a highway that is in use, you should be damn sure that this works.
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Well, that sure doesn’t make your life easier.
At least I don’t live across the Humber from the airport anymore. The Richmond and Golden Gate bridges are a piece of cake by comparison.
My impression of public works engineering has always been that up to the 1950s the engineers were a conservative lot who overengineered everything. In the post-Reagan era public expenditures (outside of the military) have been so heavily scrutinized that trying to meet the specs and not an iota more seems to be the rule. I think that this is an issue not only for immediate functionality (as in the FIU bridge) but also for long-term durability and maintenance.
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Dash cam of the span collapsing. It breaks on the left side right where there is a crane. Crane appears to be supporting something just over the junction of the diagonal members.
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Horrifying. Instant failure. I’ve only ever seen prestressed structures fail that fast (not live, videos). And somewhat frustrating as you still can’t really see what exactly happened. It seems like the lower flange failed first, but I’m far from sure.
What was the crane doing; lifting up/lowering down? Hooking on/unhooking?
Why wasn’t whatever the crane was supposed to do not done while the highway was closed to traffic?
I think if they were adjusting tension on the post-tensioning tendons as I’ve read reported, it would seem that they were working on the steel bars in the diagonal members that met at the upper deck/flange where the crane was operating. If they were adjusting tendons in the upper or lower deck/flange I’d expect them to be at either end.
If something failed relative to one of the diagonals, the span would loose its truss configuration and the upper and lower deck would likely snap or fold like cardboard. Thats what the collapse looks like, breaking right at the place where the second diagonal met the lower deck/flange.
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It is interesting to note that the design renderings for the bridge include a tower and lots of diagonal supports:
If not decorative, then this would seem to be an element to be installed early in the construction process.
Just an anecdote here, not data.
I lived for a few years in a Miami-area house that was built in the late '30s. There were no building codes when the house went up, but I am sure that the old dudes who built it had lived through a couple of hurricanes.
The roof timbers were the size of railroad ties, fastened with massive nails every 3-4 inches, attached to the walls with long, heavy duty bolts, and the whole covered with thick concrete tiles. Hipped roof at all four faces to deflect the wind. This was built based not on regulations, but on experience.
This house was about four miles from the dead-center track of hurricane Andrew in '92. We lost a patio, a rooftop vent exhaust, and a couple of trees. That’s it. Houses a mile and a half from us, built to the 1980 building codes, were literally flattened. Yes, I mean literally. Do an image search on “Hurricane Andrew Country Walk” to see what I mean.
The houses which went down were built to code. If the code said “one nail every eight inches” that’s what they put in. Not one nail every seven and five-eighths. If the code said “roofing material shall survive a 100 knot wind” that’s what they built to. If the code did not say “you can’t put a framed gable end with a great big window in it facing east” hey, I think we can get a lot of morning sunshine in the master suite!
So yeah, you can have all the regulations you desire, but if they are caught up in regulatory capture (and they will be) I don’t think they will never be a match for good experience based workmanship.
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What’s the engineering standard to measure the hubris required to do that sort of adjusting when there is stopped traffic under the bridge?
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I think you can expect that to be regulated out of existence, and a great deal of the economic advantages of “advanced bridge building” will be nullified.
Sounds plausible.
TL;DW for the theory (don’t blame you)
When transporting the bridge the construction company, with permission from the engineering firm, altered the configuration of the temporary bridge support from what was originally planned.
The theory is that under this new configuration, the bridge was placed under a different stress load than what was planned for, causing a failure of some sort, compromising the structure of the bridge. Like an overloaded shelf, this didn’t cause the bridge to collapse immediately, but to deform over time before completely giving way.
So, the theory continues, this failure grew, manifesting itself in messed up tension measurements in some of the supporting members. One of these supporting members was a rod running in one of those diagonal steel tubes separating the roof of the bridge from the bottom of the bridge. Apparently, the tension measurement was too low and, to correct this issue, they opted to apply more tension to the member, not realizing that it was already in its plastic deformation phase[*], causing it to snap and bring the bridge down. This accounts for the fact that, in photos of the wreckage, you can see a rod attached to what looks like a large version of the tension machine the guy uses in the video to demonstrate the various regimes of a rod under tension just sort of dangling there.
[*] when you pull on a wire or rod, at first, the force required to stretch a rod a certain length is proportional to the change in length. In this regime, known as the elastic deformation regime, when you stop pulling on the wire or rod, it returns to its original length. If you stretch the wire or rod beyond a certain limit, it enters the plastic deformation regime where the wire or rod no longer returns to its original length when you stop pulling on it. In fact, it takes less and less force to cause the wire to stretch more and more until it snaps.
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Old-school engineers would over-build, over-design things - because they didn’t have simulation software to tell them “make this little change, and what happens to the overall result?” They had to think “Hmmm. The math says X should be enough. OK, then put in 2X to cover my ass.”
This was true in just about everything - you could push American engineering designs, be they in electric motors, mechanical equipment, bridges, you name it - to wild over-loads, and the design would survive, Truckers and farmers have taken 3X overloads over two-lane bridges - just idiotically taking 30 ton loads over 10-ton rated bridges - and the bridges (built 40 years ago) survived without so much as groaning. The engineer who built that bridge had no CAD software, no simulation, and perhaps not even an electronic calculator - so he over-built it up the wazoo. Just like my Miami house.
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Even when you have a good idea of the stress levels that will result in failure, there is the issue of what safety factor you use. There was, a century ago, plenty of fairly elaborate stress calculations for joints and beams made of wood, and factoring in the species and dryness of the timbers. It was still engineering, but done manually.
I think one factor is that when the calculations become easier to accomplish, and more reliable, there is a tendency to trust the numbers, and cut the safety factor down.
A similar thing happens in diving. Nitrogen blood saturation levels is a serious limiting factor in dive depths and times. When we did simple manual calculations, using maximum depth times total dive time, dives were shorter, but there was a built in safety factor, as it was not possible to actually spend the whole dive at the deepest depth. The newer dive calculators use the whole profile of your dive to determine safe dive times at multiple depths. It allows for longer and deeper dives, but at a cost of moving closer to that invisible edge. The effects of any error are compounded.
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Thanks for posting that vid. (Or V-jay-o as the guy keeps saying on it.) What I found fascinating is that the explanation implies they only measure tension in the rods of a structure. They do not measure total length of rod under tension from time A to time B. If they had a length measurement, as well as the tension measurement, they would know instantly that the rod had entered a plastic deformation phase.
I also found it interesting they they thought it would be acceptable to move a lift on one side, as well as not distribute the load between lifts.
And on top of it all… did not stop traffic during retensioning.
!!! idiots!!! (On that last one!)
Engineering is fascinating.
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