Pure cable stayed bridge
Pure suspension bridge
Besides conventional large span bridges such as cable stayed bridges and suspension bridges, a hybrid bridge has a cable system that is combined cable stayed and suspension, is a new selection for design engineers. These bridges have a long history, and they were built in USA, such as Brooklyn Bridge, Niagara Bridge, Covington-Cincinnati Bridge, etc. Recently, in Japan and China, some structural researchers have mentioned about this type of bridge. The hybrid bridge will be given exceeded advantages more than conventional large span bridges, especially long span length, short construction period as well as ability of aerodynamic stability. The aim of this paper is to analyze structural behavior of the hybrid bridge in comparing with other large span bridges by numerical calculations to emphasize the benefits of this type of bridge.
Cable stayed bridge and suspension bridge are the long span bridges. However, these bridges will have certain drawbacks when their main span length increase. In cable stayed bridge, value of cable tension force to support beam will be large in area of middle span, and will be very small in near pylons. So, displacement of main beam in middle span will have a very large value. On the other hand, the pylons are very tall that will cause a lot of dificult during construction time. For suspension bridge, value of tension force of main cable is very large, and value of main beam moment in two sides of pylons will be very large that this will make a lot of difficults to keep pylon stability during construction. The anchorage abutment will have a big dimensions and its construction is also very complex.
Suspension bridge will promote ability to support main beam in area of middle span and cable stayed bridge will keep to stable pylons during construction through balanced cantilever installasion technology.
Therefore, suggestion of a new model of long span bridge to overcome the disadvantages of traditional cable stayed bridges and suspension bridges is very necessary. This model is called the bridge combined between cable stayed bridge and suspension bridge. It is also called hybrid bridge. Recently, the types of these bridges were extensively studied in Japan, China.
The hybrid bridge will have characteristic of cable stayed bridge and suspension bridge. Specific:
– For structure side: hybrid bridge is a system of bridge which cables are arranged at two sides of pylon, and main cable and hanger cable are arranged in middle span.
– For capacity side: beacause of combined between cable stayed and suspension, so hybrid bridge have advantages of cable stayed bridge and suspension bridge.
- Displacement in middle span is reduced due to main beam supported by main cable in middle span.
- Cable stayed system is arranged symmetrically through the pylon so value of tension force of cable stayed are distributed better. These values in hybrid bridge are also less than in cable stayed bridge.
- Reaction force value of anchorage abutment is reduced due to a part of main beam is supported by cable stayed.
– For architecture side: combination between cable stayed bridge and suspension bridge will make many types of bridge structures which have high aesthetics. Hybridge bridge reduces height of pylons and dimesions of anchorage abutments.
II. HISTORY OF HYBRID BRIDGES
The hybrid bridge include a system of suspensions and cable stayeds which supported by cable stayeds which was built in 19 century such as Brooklyn bridge (Figure No. 1) connected New York city and Brooklyn in USA. Brooklyn bridge was designed by John Augustus A. Roebling (a bridge engineer was borned in Germany and moved to USA when he was 25 years old). This bridge was completed in 1883, total of bridge length is 1825m, and main span length is 486.3m, width is 26m.
Figure No. 1. Brooklyn bridge was completed 1883 in USA
Before started building Brooklyn bridge, John A. Roebling attended design of other suspension bridges and they were very impressive. Among of these bridge there was a railway bridge. Its name was Niagara bridge with main span length was 250m (Figure No. 2). Niagara was operated in 1855. This was a hybrid bridge.
Figure No. 2. Niagara bridge
Next bridge was Covington-Cincinnati bridge which was a combined bridge structure between cable stayed and suspension. Covington-Cincinnati bridge was built on Ohio river in USA with main span length was 322m. This bridge had largest span length in Easthern. Covington-Cincinnati was completed in 1867 (Figure No. 3).
Figure No. 3. Covington-Cincinnati bridge in USA
After disaster of Tacoma bridge in 1940 due to the wind, the study of aerodynamics has become an important part of the design process for most of the suspension bridges. Especially, the constructed suspension bridges were concerned to survey aerodynamic stability issues. One of the surveyed bridges was Bronx – Whitestone bridge (Figure 4) in New York and it was decided to strengthen in 1946. This bridge was designed by O.H. Amman, and was completed in 1939. Among the strengthening works of this bridge, there was a problem of strengthen for cable stayeds at the top of the tower with main beam near the tower. According to Roebling’s point of view, the combined system with cable-stayed suspension bridge will increase to resist to vibrations.
Figure No. 4. Bronx-Whitestone bridge in USA
At that time with John A. Roebling, there was a other designer engineer also designed hybrid bridge. He was Charles Ellet. He designed Wheeling bridge (Figure No. 5). Wheeling bridge had the main span length 308m, started in 1847 and completed in 1849.
Figure No. 5. Wheeling bridge
The idea of combining the suspension system with stays to achieved more efficient structural systems had not been completely forgotten after the days of Brooklyn Bridge. Already in 1938, Dischinger proposed a system in which the central part were carried by stays radiating from the pylon top. This system was intended for a cable supported bridge with a 750m main span across the Elbe river in Hamburg – Germany.
In connection with the reconstruction of German bridges after the war, the Dischinger system was proposed on several occasions but it was never used for actual construction (Figure No. 6). One othe reasons is undoubtedly the pronouced discontinuity of the system both with respect to the structural behaviour and to the appearance.
Although Dischinger adopted the idea of combining the suspension system and the cable stayed system he did not appreciate the Roebling original solution with the much more continuous lay-out. In the publication of his own system Dischinger simply stated that the stays of Roebling’s bridges had proved to be completely inefficien. Although the system proposed by Dischinger was never adopted for actual construction, undoubtedly it had a considerable influence on the introduction of pure cable stayed bridge.
Figure No. 6. Proposal scheme of Dischinger
In 1873, England completed Albert bridge (Figure No. 7) across Thames River in London. This was a bridge system of suspension and cable stayed with the main span 117.27m.
Figure No. 7. Albert bridge in London
Until 1977, the bridge system of suspension and cable stayed was proposed in premilary design of Great Belt bridge in Denmark. The main span of this bridge was up to 1500m.
For a long time, and up to now the hybrid bridge were studied and applied again. Nagisa viaduct in Japan (Figure No. 8) was a span bridge with combined structure between suspension and cable stayed which was built and completed in February, 2002 in Aomori district. This was a combined structure with two pylons in two sides, main beam near the pylons made by prestressed concrete structure and supported by cable stayed, and main beam in span middle part made by box steel girder and supported by vertical hangers on main cable.
Figure No. 8. Nagisa Viaduct in Japan
The project of hybrid bridge will be built in the furture across Tsugar sea in Japan. It a combined bridge of suspension and cable stayed with two main spans about 4 km.
Author: Master of Engineering Ha Van Han