Cable Stayed Structures and Stay Cable Technology

The world’s most amazing structures pioneered by BBR

The idea of supporting a bridge deck with cables from one or two pylons has been around for a long time. To the author’s knowledge, the concept of cable-stayed bridges can be tracked back to the early 1600s when the Venetian Verantius built a wooden bridge supported with chain stays. In 1784 the carpenter C. J. Löscher designed a cable-stayed bridge with an approximate span length of 32 m and where the entire bridge was made out of wood, including the stays.

In the 19th century, the French engineer Navier studied several bridge systems supported by wrought iron chains. The results of his studies show that suspension bridges should in general be preferred over cable-stayed systems. From today’s point of view, it can be said with certainty that Navier’s final conclusions were wrong. However, at the time Navier was studying these different bridge systems, the knowledge and equipment for acheiving an even distribution of the load between all the cables - which is one of the key issues for cable-stayed bridges - were not available.

German engineers pioneered the design of cable-stayed bridges after World War II. They were challenged to find new, innovative, and inexpensive bridge designs to replace most of the river crossings on the Rhine which were destroyed during the war. Dischinger proposed designs, where the central span was supported by a suspension system and stay cables carried the outer parts. Dischinger’s combined solutions were never adopted for an actual bridge, but his studies had a big influence on the development of the true cable-stayed bridge system. It was not until the 1950s that Dischinger designed the first true cable-stayed bridge. The Strömsund Bridge (Sweden, 1955) had a main span of 183 m and two side spans of 74.7 m. Gimsing [1] attributes the increase in cable-stayed bridge designs to the improved structural analysis tools that were available. The Germans further developed the design of cable-stayed bridges in the following decades and built several of them. The series of bridges across the Rhine River near Duisburg are examples of these pioneering German bridges.

Early Design by Löscher
Europe (1784)

Durable and efficient long span cable-stayed bridges require efficient, high strength and high amplitude fatigue resistant stay cable technology. In the past many cable systems have been used, such as locked-coil, wire ropes and bars. From today’s perspective, the above-mentioned technologies only partially fulfill the requirements and nowadays modern stay cables consist of wires or strands. Locked-coil cables still find application in purely architectural structures. Technical information on state-of-the-art wire and strand stay cable technology as well as the required testing and certification regime can be found in [2, 3]

Wire Stay Cables

The first stay cable technology available to provide the required static strength and high amplitude fatigue resistance were parallel wire cables. The anchorage system generally consisted of button heads on the individual wires, which transfer the load into an anchor head and into the supporting bearing plate. The usage of full or partial bond type socket anchorages, in combination with button heads on the individual wires, is a further development and is widely used. The first application of such high amplitude fatigue resistant cables were the BBR type wire cables in the late 1950s on the Schillersteg bridge in Germany.

Today, wire stay cables are made up of a predetermined number of parallel or semi-parallel wires enclosed in an ultraviolet (UV) resistant high-density polyethylene (HDPE) sheath of circular cross-section. The individual wires generally have a diameter of 7 mm, are individually galvanized and are of low relaxation grade steel, with nominal cross-sectional area of 38 mm2 and a minimum guaranteed ultimate tensile stress (GUTS) of 1670 N/mm2 or 1770 N/mm2. The voids between the individual wires and the HDPE sheathing are filled with a corrosion inhibitor. The anchorages used are in most cases still based on the invention of the button head back in the 1950s.

Olympic Stadium Munich
Germany (1972)

Strand Stay Cables

High amplitude fatigue resistant strand stays found their first major application in the Olympic Stadium in Munich (Germany) when BBR pioneered the usage of such cables for the support of the membrane roof structure. Munich hosted the Olympic Games in 1972 and the stadium was also home to Bayern Munich, one of the world’s premier soccer clubs.

Strand stay cable configurations are traditionally anchored by means of the wedges, which bite into the strand and transfer the load into an anchor head and the supporting bearing plate. Epoxy and bond type anchorages have also been used in the past. Modern strand stay cables are generally made up of a predetermined number of parallel strands enclosed in an UV resistant HDPE stay pipe of circular cross-section. The individual strands have generally a diameter of 0.62’’ and are of low relaxation grade, with nominal cross-sectional area of 150 mm2 and minimum GUTS of 1770 N/mm2 to 1860 N/mm2. The strands are galvanized, greased or waxed and individually sheathed with a continuous and wear resistant HDPE coating, providing each strand with a triple protection system.

Comparison

Because wire stay cables are generally prefabricated and strand stay cables are more commonly assembled on site using a strand-by-strand installation method, the choice of the suitable cable system for a particular project depends on many factors:

It has often been considered that prefabricated wire cables are best suited to smaller bridges or those with very long spans. On site fabricated strand stay cables are usually suited to intermediate range spans. However, the designer, together with the specialist stay cable company, should evaluate each project individually, taking structural capacity and the global interaction between cables and the structure into account. Erection requirements as well as the overall economics and availability of the various systems should also be considered.

Cable Vibration

Despite the wide use of cable-stayed bridges, there are still several areas of great concern, especially the effects and elimination of cable vibration phenomena. Even newly constructed cable stayed bridges have experienced quite severe vibrations which may result in failures of cables [4].

Several cable vibration mechanisms have been identified and characterized with the four most common phenomena: vortex shedding, galloping, parametric excitation (deck/pylon and cable interaction), and wind and rain induced vibrations. Excitation mechanisms and preventative design measures are a popular topic in the literature [5, 6, 7, 8, 9, 10, 11]. Additional information on effective countermeasure against cable vibration can be found here…

Future Developments

Storchenbrücke
Switzerland (1996)

 

Both wire and strand type cable systems are today’s solutions of choice for modern cable-stayed structures. Over the years, the ultimate tensile strength of the wires and strands has gradually been increased and will be further increased. Furthermore, the corrosion protection systems have been enhanced. Whereas grout was a widely used corrosion protection system for many years, today’s stay cables employ grease and wax as the primary filler. Although poor or faulty grouting can result in poor performance of the cables, there are many examples of grouted stay cables which have been inspected and found to be in perfect condition. To enhance the long term durability of the stay cables, the use of epoxy coated strands, as well as the injection of the stay pipe with gas to prevent corrosion are more recent developments and potential alternatives for the future.

Engineered in collaboration between EMPA and BBR, the Storchenbridge in Winterthur (Switzerland), crossing the major east-to-west axis of the Swiss Federal Railway Network, was the world’s first bridge to use carbon stay cable technology. Due to its low self-weight, carbon stay cables are a promising solution for ultra long span bridges. Their extremely high fatigue resistance and the fact that carbon is non-corrosive are further advantages of this type of cable. It should be noted that special care should be taken when choosing an anchorage system for carbon stay cables.

Europe

As noted previously, the mother continent of cable stayed bridges and structures is Europe, with the first major applications constructed in the late 1950s. Since the early days of cable-stayed bridge construction in Europe, cable stays have been widely used - not only for technical and economical reasons but also for purely architectural considerations.

Sunniberg Bridge
Switzerland (2005)

The scenic Sunniberg Bridge, in the skiing resort of Klosters in the Swiss Alps was designed by the legendary Swiss Engineer Christian Menn and was opened to traffic in 2005 by Prince Charles. This masterpiece of engineering, which fuses perfectly with the scenic surroundings, employs a button head type wire stay cable system. Characterized by its short pylons and shallow angle stays, it exhibits the essence of a technical and aesthetic solution in a prominent landmark. The stay cable system uses BBR wire cables.

The cable-stayed bridge with the longest main span constructed in Europe is the Pont de Normandie which was completed in 1995. With a main span of approximately 850 m and a pylon height of 215 m, the Pont de Normandie was for a short while the longest cable-stayed bridge in the world, before its main span was surpassed by 40 m by the Tatara Bridge in Japan. Wedge anchored parallel strand stay cables have been used to realize this bridge.

Rama VIII Bridge
Thailand (2002)

Asia / Pacific

Cable-stayed bridges are extremely popular in many Asian countries to help resolve traffic congestion and to improve the infrastructure of the incredibly fast growing metropolitan areas. Japan is probably the country with the highest density of cable-stayed bridges. Among the booming countries nowadays is China, where a series of future world record span bridges are currently under construction. Cable supported structures in Asia have also been identified as purely architectural and landmark structures and are often built to commemorate former or present kings and rulers.

An example of such a structure is the RAMA VIII Bridge in Thailand - it serves the infrastructure network, is architecturally appealing and also beares the name of a former King. Wedge anchored BBR strand stay cables have been utilized in this bridge which is one of many large bridges which span the river in Bangkok.

The longest cable-stayed bridge built in the 20th century, the Tatara Bridge in Japan, has a breathtaking total length of 1480 m and a pylon height of almost 240 meters [12] and features BBR stay cable technology.

The first major application of stay cables in Australia was the unique supporting net structure composed of parallel BBR wire stay cables for the stabilization of the Centre Point Tower in Sydney, which has a height of 230 meters and which was constructed in the 1970s. Today, the Centre Point Tower stands together with the Sydney Opera House and the legendary Sydney Harbour Bridge - other famous landmarks in the city center.