How They Built a Giant Bridge, Then Moved It Across a River

This bridge was assembled beside the river. Now engineers have to move the whole structure across it. It's a steel railway bridge 590 feet long and it weighs 2100 metric tons. Right now it sits on the German bank of the Oder, the river that marks the border with Poland, near the town of Küstrin, about 55 miles east of Berlin. Its permanent supports are already built and waiting at the crossing Moving that much steel takes serious machinery, but weight is only half the job. A bridge is designed to rest on a few fixed points. During this move, the points holding it up keep changing, and every change shifts how the load runs through the structure. Keeping the bridge safe means staying ahead of those changes, from the first lift to the last. The bridge it replaces had carried trains over the Yoder for more than 100 years, and decades of heavy traffic had worn it down. The new one is a network arch, a steel arch with a web of crisscrossing hangers holding up the deck, and it's built for trains running up to 75 miles an hour. Building that arch in its final spot would have meant working out over moving water for months, right in the middle of an active rail line. Assembling it on land changes the whole job. Cranes and trucks can reach the steel from solid ground, crews work closer to the ground instead of over the water, and the crossing itself only has to deal with the installation, instead of months of steel going up over the river. The Federal Highway Administration in the U.S. published a full manual on this approach. The idea is to build the bridge where the work is easiest, then carry the finished span to its final position, so the disruption at the site shrinks from months of construction to a short installation window. Mammut, the heavy lift company hired for the Oder job, described its goal the same way. Replace the bridge while disturbing the rail network as little as possible. When we say the whole bridge moves, we mean the assembled span, the arch, the hangers, and the deck that will carry the tracks. The foundations and piers stay put. They're built in place ahead of time, and the span travels to meet them. During assembly, the span rests on temporary supports that hold it up off the yard. Before anything can drive underneath, the bridge has to be raised high enough to leave room. The FHWA manual asks for at least five feet between the ground and the underside of the span so the transporters can fit. At the Oder, Mammut jacked the structure up to about six and a half feet. The next decision is where the bridge will be lifted from. These spots are called lift points, And they matter more than they sound. The FHWA manual says transporters should pick up a span as close as possible to where it will rest permanently, because that's how the structure was designed to carry its weight. In practice, the width of the transporters and the obstacles around them limit where they could go. Once the lift points move away from the permanent support positions, the bridge starts carrying its own weight in a different way. A section that would normally hang between two supports might stick out past one instead. Steel that would normally be squeezed might end up being stretched. The manual calls this a stress reversal and warns that it can be harmful to the structure. So the engineers have to check the bridge twice. The first check covers its working life, with trains rolling across while it rests on its permanent bearings. The second covers the move itself, with the bridge sitting on temporary points and riding over ground that won't be perfectly smooth. FHWA recommends adding at least 15% to the bridge's weight in those calculations to account for bumps along the route. It also says something blunt. The bridge should not be overstressed, even temporarily, while it's being lifted and moved. For a structure this size, that starts with knowing exactly how much it weighs. The manual tells designers to calculate an accurate weight, especially on large bridges, and to brace the structure before it travels, with cross members in place so the span holds its shape while the support points shift underneath it. With the checks done and the bridge raised, the machines can roll in. They're called self-propelled modular transporters, or SPMTs. Each one is a low, flat platform riding on rows of wheels. A single row of wheels across the platform is called an axle line. The modules can be linked side-by-side and end-to-end, and a computer controls them as one machine. At the Oder, Mammut used 96 axle lines. The transporters drive in under the raised bridge with their platforms lowered, then each axle line pushes upward using its own hydraulic suspension. The platforms rise, the bridge lifts off its temporary supports, and for the first time the full 2100 metric tons is riding on wheels. That hydraulic system is what keeps the load under control. The axle lines are connected into three or four groups. Inside each group, oil can flow between the cylinders, so every axle line carries an equal share. When one set of wheels rolls over a bump, its cylinder pushes oil to the others, and the load stays balanced. With three groups, the platform behaves like a three-legged stool, which stays steady on uneven ground. The platform can also rise or drop about two feet to keep the bridge level on a slope. Spreading the load is also why there are so many wheels. FHWA puts the capacity of each axle line at about 24 to 30 metric tons depending on the model. Divide 2,100 metric tons across 96 axle lines and you get about 22 metric tons each on average before adding the weight of the transporters themselves. All of that ends up pressing on the ground. According to FHWA, a single loaded axle line can push down with up to about 2,000 pounds on every square foot beneath it. The manual says the route has to be prepared ahead of time, and freshly placed fill should be avoided, because soft or settling ground under one corner can upset the balance the hydraulics are trying to hold. Steering works differently too. Every pair of wheels can pivot all the way around, so the convoy can roll forward, sideways, or diagonally. Loaded transporters usually move at about walking pace, around three miles an hour. On the older job, that walk ended at the edge of the riverbank. That's where the road ran out. In the water ahead stood the first of five temporary supports, and the transporters stopped with the bridge positioned over it. Getting here had taken about 45 truckloads of specialized heavy equipment, and most of it was meant for the next stage. Mammut had already ruled out the other ways across. Floating the bridge on a pontoon, a barge-like platform, wouldn't work because the odor at this spot is shallow, and its water level keeps changing. A crane couldn't handle the combination of weight, length, and reach this lift would need. So the bridge would slide across instead, a method called launching. The five temporary supports were steel towers, set on the quay edges and in the river. On top of the tower sat hydraulic cylinders and launching plates, sliding bearings the bridge could glide over. The job used 26 launching plates, and Mammut Europe needed everyone it had in stock. The pulling came from two strand jacks. A strand jack grips a bundle of steel cables and pulls them in short strokes. Clamps, pulls, releases, resets, and clamps again. Hooked to the bridge, the jacks pulled it horizontally from one tower to the next. The transporters at the back stayed underneath the rear of the span and moved with it, while the front slid forward across the plates. Then the cycle repeated. Pull to the next tower, land on it, and keep going until the front reached the Polish bank. Each of those steps changed what was holding up the bridge. When the front end slides past one tower and reaches toward the next, it's hanging in open air. Engineers call that a cantilever, like a diving board. The longer that unsupported stretch gets, the more the tip bends downward. Then the front lands on the next tower, and suddenly the load has a new path to the riverbed. Some of the weight that was crowding onto the last tower now flows forward. This is the stress reversal from the FHWA manual playing out in real time. Once the bridge is in service, the arch and the deck work together between two fixed sets of supports. During a launch, parts of the span sit directly on top of a tower or hang out past one, so some of the steel gets bent in ways it will rarely see once the bridge is finished. The design had to handle both. Mamont's lead engineer on the project said the bridge has a large deflection, which is the engineering term for how far it bends. That bending was planned. Steel is supposed to flex under load, and the design calculations set how far it can go during each stage of the launch. The challenge is that the bridge keeps changing its angle as it passes over each support, and the supports have to move with it. Standard launching plates swivel in two directions. For the Odor, the team modified every plate to rotate a full 360 degrees and fitted them with spherical bearings, a ball-and-socket type joint that can tilt any way the steel above it moves. That kept the bridge sitting flat on each plate even as its angle kept shifting. The hydraulic cylinders on the towers handled the other half of the problem. If one support carries too much, the steel above it can be overloaded. If it carries too little, its neighbors pick up the difference. Raising or lowering a cylinder changes how much weight that support takes. According to Mamat, the team controlled the weight on each tower and each cylinder throughout the launch to keep it smooth and safe. Wind was part of the calculation too, since a tall arch catches a lot of air from the side. The FHWA manual shows how this kind of control works on other moves. On one Dutch project, it describes, a measuring crew continuously monitored a dozen points on the bridge, with the deflection limited to four inches. The manual also calls for watching temporary supports for settlement, because a tower that sinks even a little hands its load to everything around it. Pole by pole, the span crossed the river. Once it reached the far bank, the bridge was spanning the odor, but it still wasn't sitting where it belonged. The rear transporters came out first. In their place, crews installed skid shoes, sliding pads that let the bridge move the last distance into its final position. That slide brought it into line with its permanent supports. Then the load moved one more time. Ten climbing jacks took the weight to the bridge, and with the span resting on those jacks, the temporary supports in the river could finally come out. That final step needed precision. The permanent supports and the bottom of the span have to line up within tight tolerances, which is why FHWA calls for surveys that check the real dimensions and elevations of both Before a moved bridge is set down. Even on a much smaller highway bridge in Florida, the FHWA manual notes it took about two hours just to line the beams up on their bearing seats. At the odor, the climbing jacks lowered the bridge in controlled strokes, down to its final height until the steel settled onto its permanent supports. The temporary towers, the transporters, and the pulling cables were gone. For the first time since it left the assembly yard, the bridge was resting on the points it was designed for.