[Note that this article is a transcript of the video embedded above.] This is the Washington Bridge that carries I-195 over the Seekonk River in Providence, Rhode Island… or at least, it was the Washington Bridge. You can see that the westbound span is just about completely gone. In July of 2023, that part of the bridge, although marked as being in poor condition, received a passing inspection. Six months later, the bridge was abruptly closed to traffic because it was in imminent danger of collapse. Now, the whole thing has nearly been torn down as part of an emergency replacement project. Rhode Islanders who need to travel between Providence and East Providence have suffered through more than a year of traffic delays from the loss of this important link, and business owners have seen major downturns. If you live in the area, you’re probably tired of seeing it in the news. But it hasn’t had a lot of coverage outside the state. And I think it’s a really fascinating case study in the complexities of designing, building, and taking care of bridges, including some lessons that apply to designing just about anything. I’m Grady, and this is Practical Engineering. The original bridge over the Seekonk River was finished in 1930. Part of that old bridge now serves as a pedestrian crossing and bike link. It’s a nice bridge: concrete and stone multiple arch spans give it a graceful look over the river. In 1959, when I-195 expanded to include this road, it quickly filled with traffic. The old bridge just wasn’t big enough, at least according to the standards of the time. So, a new bridge to carry the westbound lanes was planned, with the federal government picking up most of the bill. Since the feds were paying, they wanted a simple, inexpensive steel girder bridge. But Rhode Island refused. The state didn’t want a plain, stark, utilitarian structure right next to their historic and elegant multi-arch bridge. It took years to come to an agreement, but eventually, they met in the middle with the Federal Bureau of Roads agreeing to include false concrete arch facades between each of the exterior piers, matching the style of the eastbound bridge. But by that time, the field of bridge engineering had shifted. The Interstate Highway system in the US started in 1956 with the idea of an interconnected freeway system with no at-grade intersections. Every road and rail crossing required grade separation, and that meant we started building a lot of bridges. We’re up to around 55,000 today, and that’s just on the interstates. With steel in short supply, a new kind of bridge girder was coming into vogue made from pre-stressed concrete. In simple reinforced concrete structures, the rebar is just cast inside. It takes some deflection of the concrete before the steel can take on any of the internal stress within the member. For beams, the amount of deflection needed to develop the strength of the steel often leads to cracks, which eventually lead to corrosion as water reaches the steel. But if you can load up the steel before the beam is put into service, in other words, “prestress” it, you can stiffen the beam, making it less likely to crack under load. I have a whole video going into more detail about prestressed concrete if you want to learn more after this. If you’ve already seen it, then you know there are two main ways to do it. In some structures, the reinforcing steel is tensioned before the concrete is cast. This “pre-tensioning” is usually done in facilities with specialized equipment that can apply and hold those extreme forces while the concrete cures. Alternatively, you can do it on-site by running steel tendons through hollow tubes in the concrete. Once it’s cured, jacks are used to stress the tendons, a process called post-tensioning. The engineers for the westbound lanes of the Washington Bridge took advantage of this relatively new construction method, using both post-tensioned and pre-tensioned beams. While most of the grade separation bridges on interstate highways were rigidly standardized, this was a bridge unlike practically any other in the United States. It had 18 spans of varying structural types. Except for the navigation span for boats that used steel girders, the rest of the bridge passing over the water used cantilever beams. Rather than having the end of the beam sit on the pier like most beam bridges do, called simply supported, the primary beams in the Washington Bridge were supported at their center, cantilevering out in both directions. The pre-tensioned drop-in concrete girders were suspended between the cantilever arms. Those cantilever beams were post-tensioned structural members. Five steel cables were run in hollow ducts from one end to the other, then tensioned to roughly 200,000 pounds (nearly a meganewton each), and locked off at anchorages on both ends. Then the ducts were filled with grout to bond the strands to the rest of the concrete member and protect them against corrosion. Most of the cantilever beams in the Washington Bridge were balanced, meaning they had roughly the same load on either side. But at the west abutment and navigation span, that wasn’t true. You can see that these beams support a drop-in girder on one end, but the steel girders over the navigation span are simply-supported on their piers. Since the cantilever beams weren’t balanced, designers needed an alternative way to keep them from rotating atop the pier. So steel rods called tie-downs were installed on each of the unbalanced cantilevers. In December 2023, the now 57-year-old westbound bridge was in the middle of a 64-million-dollar construction project to repair damaged concrete, widen the deck for another lane of traffic, and add a new off-ramp, with the goal of extending the bridge’s life by 25 years. One of the engineers involved in that project was on site and noticed something unusual under the navigation span. Some of the tie-down rods on the unbalanced cantilevers were completely broken. The finding was serious, so three days later, a more detailed inspection of the structure was carried out, discovering that half of the unbalanced cantilevers at piers 6 and 7 - the piers on either side of the navigation span - were not performing as designed. The Rhode Island Department of Transportation closed the bridge to traffic that day while the state could investigate the issue and come up with a solution. The closure snarled traffic on a crossing that was already regularly congested. Westbound traffic was eventually rerouted onto the eastbound bridge, with the lanes narrowed to fit more vehicles. The state put up an interactive dashboard where you can look at travel times by route and time of day and view live webcams to try and help travelers and commuters decide how and when to get across the Seekonk River. Still, the closure has had an enormous impact on the Providence area, impacting travel times and economic activity in the area for more than a year now. The state was fully expecting to implement some kind of emergency repair project, essentially a retrofit that would replace the broken tie-downs on the unbalanced cantilevers. The project was designed, and the contractor started installing work platforms below the bridge in January 2024. As they got access to the underside of the bridge, things started looking worse. Deteriorating concrete on the beams threatened to complicate the installation of the new tie-downs, so the state decided to do a more detailed investigation. They tested concrete in the beams, used ground penetrating radar and ultrasound to inspect the tendons inside, and even drilled into the beams to observe the actual condition of the post-tensioned cables. What they uncovered was a laundry list of serious issues. In addition to the failed tie-down rods, there were major problems with the beams themselves. The concrete was soft and damaged, in part because of freeze-thaw action. Like most concrete from the 1960s, there was no air entrainment in the concrete beams. This requirement in most modern concrete mixes, especially in northern climates, introduces tiny air bubbles that act like cushions to reduce damage when water freezes. Without air, concrete exposed to water and freezing conditions will spall, crack, and deteriorate over time. The post-tensioning system was also in bad condition. The anchorages at the end of the beams were corroded, and voids and soft grout were found within the cable ducts. When the inspectors drilled into the beams to reach one of the cables, they saw that the poor grout job had allowed water inside the duct, corroding the cable itself. Most of the damage was related to the condition and location of the joints in the bridge deck, which allowed water and salty snow melt to leak down onto the structure below. If you saw my video on the Fern Hollow Bridge collapse in Pittsburgh, it was a similar situation. When the engineers analyzed the strength of the bridge, considering its actual condition, the results weren’t good. With no traffic, the beams met the minimum requirements in the bridge code. When traffic loads were applied, it was a totally different story. The code does not allow any tension to occur in a post-tensioned member, but you can see in the graph that the top of the beam is in tension across a large portion of its length. Worse than that, the engineers found that the beams were in a condition where failure would happen before you could see significant cracking in the concrete. In other words, if the beam was in structural distress, it likely wouldn’t be caught during an inspection. There could be no warning before a potential failure. In short, this was not a bridge worth widening. It wasn’t even safe to drive on. A big question here is: Why didn’t any of this get caught in inspections? And that mostly has to do with access. Only some of these tie-downs were visible to inspectors. The rest were embedded in concrete diaphragms that ran laterally between the beams. But it’s not clear if any special attention was paid to them, given their structural importance in the bridge. Looking through all the past inspection reports, there’s very little mention of the tie-down rods at all, and only a few pictures of them. The state actually used this photo from the July 2023 inspection, 5 months prior to when it was observed to be broken, to show that this tie-down wasn’t broken then, suggesting that maybe a large truck had caused the damage in a single event. But you can clearly see that, if it were fractured at that time, that break would be obscured by the pier in the photo. Same thing with this one; the fracture is at the very top of the rod, so it’s impossible to see if it was there in July. There’s no easy way to know how long this had been an issue. At least for these outside tie-rods, you have bare steel, exposed and mostly uncoated, directly beneath a leaky joint in the road deck. This is easy to say in hindsight, but if I’m an inspector and I understand the configuration of this bridge, I’m making sure to put eyes on every one of these visible tie-downs, or at least state clearly and explicitly that the access wasn’t enough to fully document their condition. And it’s even worse for the post-tensioned anchorages in the beams. Those drop-in girders sat essentially flush with the ends of the beams, making it impossible to inspect their condition, let alone perform maintenance or repairs. Seismic retrofits installed in 1996 made access and visibility even tougher. And this is a perfect case study in the risks that hidden elements can pose. If you’ve ever done a renovation project on an older house, you know exactly how this goes. You start to change a light fixture, and next thing you know there’s a backhoe in your front yard. The bridge widening project uncovered the situation with the tie rods. The repairs to the tie rods revealed issues with the post-tension system in the beams. Investigation into that problem revealed further structural issues, and pretty quickly, you have a much bigger problem on your hands than you set out to fix in the first place. You’re trying to keep the public informed about what’s going on and predict how long the bridge is going to be closed at the same time that the situation is unraveling before your eyes. The engineers looked at a bunch of options to repair all these issues, but the complexity of implementing any fixes just made it infeasible. Just to get to the beams, you’d have to demo the entire road deck and remove the drop-in girders. Since things have shifted, there was no way to know how the load had redistributed, so even taking the deck would come with risks. Then, with the state of the concrete in the beams, it wasn’t a sure bet that they could even support any external strengthening. And even if you did get it repaired, you would still have all the same issues with access and visibility. The report put it in plain words: the options for repair were “limited, complex, and [did] not completely mitigate the identified risks with the structure.” So, eventually, the state decided to demolish the entire thing and start over. And that’s where it stands (or doesn’t stand) right now. Demolition is well underway, but that’s not the end of the mess. The state put out a request for proposals to design and build the replacement project in April 2024 with an aggressive schedule to finish construction by August 2026. Not a single contractor bid on the job, likely due to the difficult schedule and the inherent risks. The state planned to leave the substructure of the bridge (the piers and piles) intact, giving the replacement contractor the option to reuse it as a part of their design. It seems that no one could get comfortable with that idea, and I don’t blame them, considering how each milestone in this saga has only revealed new bad news about the condition of the bridge. In October, the state decided to just demo the substructure, too, adding it to the existing contract. They started a new solicitation process, this time with two stages, to try and find a contractor willing to take on this project. The two finalists were announced in December, and they expect to award a contract this summer of 2025. But, in the midst of just trying to figure out what to do with the bridge, the fight over who’s responsible for all this chaos started. In August of 2024, the state filed a lawsuit against 13 companies, including firms that did the bridge inspections, alleging that they should have identified these structural issues earlier. At one point the attorney general stopped the demolition work to preserve evidence for the lawsuit, extending the timeline for a month. Then in January, the US Department of Justice disclosed that they’re investigating the state of Rhode Island under the False Claims Act, which comes into play when federal funds are misused or fraudulently obtained. The dual legal battles—one against the engineering firms and another potentially implicating the state—turned what was already a logistical and financial nightmare into a high-stakes showdown, with millions of dollars and public trust hanging in the balance. Then in February, this video came out showing the demolition contractor dropping huge pieces of the cantilever beams onto the barges below, sparking a workplace safety investigation from OSHA. A fellow YouTube engineer, Casey Jones, has been covering a lot of the more detailed aspects of the situation if you want to keep up with the story, and I also have to shout out the local journalists who have done some fantastic work to keep the public apprised of the situation where maybe the State has faltered. This saga is far from over, and we’re probably going to learn a lot more in the coming months and years. Maybe the inspectors really did neglect their duties to identify major problems. Maybe the state has some issues with its inspection and review program. Probably there’s a little bit of both. But also, this bridge had some bizarre design decisions that made a lot of these problems inevitable. Putting critical structural elements, like tie-downs and post-tension anchorages, where they can’t be inspected or repaired is essentially like planting a time bomb. We’re fortunate it was caught before it blew up. And a lot of those design decisions were driven by a roughly five-million-dollar (adjusted for inflation) battle between Rhode Island and the federal government over the visual appearance of the bridge in 1965. Now, it will cost roughly 20 times that just to tear the bridge down, and who knows how much to rebuild. This situation is a mess! It’s an embarrassment for the state, a nightmare for the engineers and contractors who have worked on the bridge in the past, and a major problem for all the residents of Rhode Island who depend on this bridge. Every time I talk about failures, I get so much feedback about how bad US infrastructure is. And I don’t want to sugarcoat this situation, but I do want to put it in context. This is one of roughly 617,000 bridges in the US, and in some ways, it’s a success story: A serious problem was identified before it became a disaster, and the final outcome should be what was needed all along - replacing a bridge that had reached the end of its design life. It’s not a bizarre situation that an old bridge was old. It happens all the time, and although sometimes the roadwork is frustrating, we generally understand that structures don’t last forever and eventually need to be replaced. But just like engineers design structures to be ductile, to fail with grace and warning, we want and need projects like this to happen in an orderly fashion. We should be able to recognize when replacement is necessary, plan ahead for the project, do a good job informing the public, and execute the job on a timeline that doesn’t require panic, chaos, and emergency contracts, and the Washington Bridge is a perfect case study in why that’s so important.

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I spent a portion of today catching up with old friends and colleagues, so fewer highlights, but here are a couple: Like a few hundred other people, I went to the invited talk by Chetan Nayak, leader of Microsoft's quantum computing effort. It was sufficiently crowded that the session chair warned everyone about fire code regulations and that people should not sit on the floor blocking the aisles.  To set the landscape:  Microsoft's approach to quantum computing is to develop topological qubits based on interesting physics that is predicted to happen (see here and here) if one induces superconductivity (via the proximity effect) in a semiconductor nanowire with spin-orbit coupling.  When the right combination of gate voltage and external magnetic field is applied, the nanowire should cross into a topologically nontrivial state with majorana fermions localized to each end of the nanowire, leading to "zero energy states" seen as peaks in the conductance \(dI/dV\) centered at zero bias (\(V=0\)).  A major challenge is that disorder in these devices can lead to other sources of zero-bias peaks (Andreev bound states).  A 2023 paper outlines a protocol that is supposed to give good statistical feedback on whether a given device is in the topologically interesting or trivial regime.  I don't want to rehash the history of all of this.  In a paper published last month, a single proximitized, gate-defined InAs quantum wire is connected to a long quantum dot to form an interferometer, and the capacitance of that dot is sensed via RF techniques as a function of the magnetic flux threading the interferometer, showing oscillations with period \(h/2e\), interpreted as charge parity oscillations of the proximitized nanowire.  In new data, not yet reported in a paper, Nayak presented measurements on a system comprising two such wires and associated other structures.  The argument is that each wire can be individually tuned simultaneously into the topologically nontrivial regime via the protocol above.  Then interferometer measurements can be performed in one wire (the Z channel) and in a configuration involving two ends of different wires (the X channel), and they interpret their data as early evidence that they have achieved the desired majorana modes and their parity measurements.  I look forward to when a paper is out on this, as it is hard to make informed statements about this based just on what I saw quickly on slides from a distance.   In a completely different session, Garnet Chan gave a very nice talk about applying advanced quantum chemistry and embedding techniques to look at some serious correlated materials physics.  Embedding methods are somewhat reminiscent of mean field theories:  Instead of trying to solve the Schrödinger equation for a whole solid, for example, you can treat the solid as a self-consistent theory of a unit cell or set of unit cells embedded in a more coarse-grained bath (made up of other unit cells appropriately averaged).  See here, for example. He presented recent results on computing the Kondo effect of magnetic impurities in metals, understanding the trends of antiferromagnetic properties of the parent cuprates, and trying to describe superconductivity in the doped cuprates.  Neat stuff. In the same session, my collaborator Silke Buehler-Paschen gave a nice discussion of ways to use heavy fermion materials to examine strange metals, looking beyond just resistivity measurements.  Particularly interesting is the idea of trying to figure out quantum Fisher information, which in principle can tell you how entangled your many-body system is (that is, estimating how many other degrees of freedom are entangled with one particular degree of freedom).  See here for an intro to the idea, and here for an implementation in a strange metal, Ce3Pd20Si6.   More tomorrow.... (On a separate note, holy cow, the trade show this year is enormous - seems like it's 50% bigger than last year.  I never would have dreamed when I was a grad student that you could go to this and have your pick of maybe 10 different dilution refrigerator vendors.  One minor mystery:  Who did World Scientific tick off?  Their table is located on the completely opposite side of the very large hall from every other publisher.)