What is the cable-stayed bridge?
The first bridge was invented by some unknown ancient man who threw a flat stone or tree trunk through a stream so as not to enter cold water once again. So there was a simple beam construction, which is still used where you can get by with a short span. For example, the river is narrow. Or the river is wide, but small, and many supports can be placed along the bottom so that the spans between them remain small. But if the bridge is thrown at high altitude or over a deep strait, where each new support is new costs and new engineering difficulties, additional measures have to be taken. Indeed, two forces act simultaneously on the bridge span - extension from below and compression from above. Each span has a tensile strength, and if this strength is constantly increased, the weight of the stiffener will also increase, and the bridge will once collapse under its weight. Designs that allow you to redistribute the load from the beam to various other devices (without additional supports) have appeared long ago and exist in a multitude. This can be openwork trusses rigidly connected to the beam (this is the way most railway bridges in Russia are arranged) or an arch that transfers the load from the center of the span to the side supports. Another brilliant idea that has existed for centuries is suspension bridges. There, the main supporting structure is made in the form of a flexible element - a cable or chain, and the beam of the bridge is suspended from it. Such are the Crimean bridge in Moscow or the Golden Gate bridge in San Francisco, and there is no number for them. The longest span in the world (1991 m) between the two supports is held by the Akashi-Kaykyo suspension bridge connecting the Japanese islands of Honshu and Awaji.
Cheaper and more sustainable
However, many high bridges or bridges with extra-long spans are made according to another scheme - cable-stayed. The cable-stayed bridge is considered a kind of suspension, but it has one important difference: a flexible supporting structure is not there. The load on the beam is transmitted to the high supports (pylons) through the cable-stay system. There are two main schemes for attaching guy cables to a pylon - fan style and harp style. In the first case, the cable bundle is attached to one point, and then, like a fan, it diverges to connect at different points with a stiffener. If the bridge is made in the style of a harp - the cables are attached to different points of the pylon and go to the stiffener almost parallel. From the point of view of structural stability, a “fan-shaped” option is preferable - this way the overturning moment transmitted to the pylon is minimized, but ... if there are too many cables, it is quite difficult to take them out of one point from an engineering point of view. In this case, an intermediate option is chosen - closer to the fan, but the cables are mounted on the pylon at a small distance from each other.
Today, Russia holds the championship in the length of the span between the supports of the cable-stayed bridge: the Russian bridge, thrown over the Vostochny Bosphorus in Vladivostok, defeated the main Chinese giants of modern times. The span of the former record holder - Sutun Bridge - is 16 m shorter (1088 versus 1104 m).
Guys are not so simple
The cables used in the Russian Bridge vary in length from 135.7 to 579.8 m, and the latest figure to date is also a record. There was no such cable-stay length for bridges, and moreover, on the eve of the construction of the bridge, doubts were expressed in the expert community about the advisability of creating such long and heavy structures. Nevertheless, the French company Freyssinet, which is considered one of the world leaders in the production of cable-stayed bridges, went on to manufacture these structures.
Despite the fact that the creation of giant cable-stayed bridges as a whole is a very difficult engineering task, especially when you consider the significant wind loads in areas where land meets the ocean, the cable stay is perhaps the most high-tech moment. The Freyssinet guy is not a simple metal cable, but a complex, "multi-trend" design, consisting of individual thin cables (strands). The trend is formed of six galvanized wires entwined around the seventh central one. The number of strands can vary from units to hundreds. For example, in cables that support one of the TV towers in Spain in an upright position, there are 205 strands. For the Russian bridge, this number turned out to be more modest - from 13 to 85. The advantage of the multi-trend design is that when attaching the pylon and stiffener to the anchors, each "thread" is separately attached and separately stretched in the anchor structure. And what is especially interesting, a separate trend can be pulled out of the guy and replaced if necessary. Inside the cable, the cables do not touch each other: in addition to galvanization, each of them is protected from corrosion by an additional braid of high density polyethylene. In addition to this, the entire bundle of strands has a double shell: the inner layer is made of the same black dense polyethylene, and the outer shell is made of a softer color that is defined by the design of the bridge. For example, the shell may be silver-metallic, creating the illusion that the cables are made entirely of metal. Decorative lights can be mounted on the cables, and then the “fan” or “harp” of the bridge looks especially spectacular.
As soon as it comes to the design, exposed to the elements, aerodynamics begins. And of course, the guys, like other elements of the bridge (or models of these elements) undergo aerodynamic tests. As a result of such studies, Freyssinet engineers came up with the idea of applying a small spiral-shaped rib to the outer shell of the guy. This "snake" removes rainwater, which, given a smooth surface, would flow freely along the cable, increasing the load on it. In addition, the rib introduces turbulence in the oncoming air currents and thus reduces the negative moments of wind and rain.
In any case, multi-ton cables (the total weight of cables for the Russian Bridge is 3, 700 tons) is strongly affected by the elements, which causes serious vibrations. So that they are not transmitted to the stiffness beam (or the bridge itself), dampers are used, which are hydraulic cylinders with pistons.
Growing from the bottom
The other most impressive structural component of the cable-stayed bridge is, of course, the pylons. For giants such as the Russian Bridge, the construction of these supports is comparable in technology with the construction of skyscrapers (which we wrote about in detail in the March issue of PM).
In any case, the base of the huge pylon (the A-shaped pylons of the Russian bridge have a height of 319.6 m) is carried out according to a similar technology: first, wells were drilled to the rocky soil at the bottom of the strait, where concrete was then poured - this is how bored piles were obtained. On top of the piles, a grillage was also formed from concrete - a platform to which the structures of the pylon itself were already attached. The pylon for the Russian bridge was created by the method of self-lifting formwork: after the next section of the pylon was formed in the formwork, the formwork “left” up to build up the next section. To facilitate the design, the converging racks were made hollow. They were given a variable section, and the thickness of the concrete walls varied from 2 to 0.7 m. Several jumpers were installed between the pillar racks. The lower capital bridge made of reinforced concrete became a support for the stiffener, other prestressed metal jumpers additionally pulled the pylon racks.
The Russian bridge, due to its scale, belongs to unique structures, however, among the cable-stayed bridges of a smaller size, there are other rather original designs. For example, the central span of a bridge can be held by one pylon, which can be seen on the example of the Zhivopisnoy bridge in Moscow. Here the only pylon is made in the form of an openwork arched structure, with the cables diverging in a fan on both sides of the pylon.
Another spectacular example of a one-pylon construction can be seen in Mexico - a cable-stayed bridge crosses the Santa Catarina River, with a sloping pylon holding the stiffening beam, and the guys from it run in parallel in the form of a typical “harp”.
Millau Viaduct has no record-long spans, but it holds another record, not only among cable-stayed bridges, but also among bridges in general. This is the world's tallest bridge structure. One of the seven pylons (P2) rises above the base to a height of 343 m, which exceeds the height of the Eiffel Tower in Paris. Such giant pylons were needed in order to throw a highway over the Tarn River in southern France.
In conclusion, it is worth noting that many large cable-stayed bridges (and Millau Viaduct, and the Russian Bridge, and the bridge in Mexico) often found themselves in the center of public criticism for the high cost. Of course, in any country, taxpayers have the right to make their own judgments about the effectiveness of spending public funds, but still, when political passions subside, the bridges will remain standing and will surely come in handy for future generations.The article “Fan and Harp” was published in the journal Popular Mechanics (No. 4, April 2013).