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Mountain grown-Hilly terrain contributes mightily to China Bridge



2013-01-10
Yinbo Liu,Guoping C


The Aizhai Suspension Bridge is located in Hunan Province in central China. The bridge is the world’s longest suspension bridge over a mountain valley. The overall bridge length is 1,534 m, including a main span of 1,176 m and two side spans of 242 and 116 m. The project involves the construction of a four-lane, access-controlled expressway from Jishou to Chadong in Hunan Province in central China. The expressway is a section of the national highway from Changsha to Chongqing, which is one of the eight east-west national highways in China. The total project length is 65 km, including 52 bridges and 11 tunnels. The total length of bridges and tunnels is about two-thirds of the entire project length. The total construction cost is estimated at $610 million. Construction of the project started in 2008 and is expected to be completed by September 2012.   
 
The Aizhai Suspension Bridge is one of the key structures of the project. It is located approximately in the middle of the project near the village of Aizhai.  
 
The bridge is a suspension bridge over an approximately 330-m-deep mountain valley. The rise-span ratio is 1-to-9.6.
 
The out-to-out width is 27 m, consisting of 11-m travel lanes (two lanes and shoulders) in each direction. The two towers are reinforced concrete frame structures on spread footings.  The heights of the east and west towers are 130 m and 62 m, respectively. Both towers sit on top of the mountains.   
 
Two different kinds of anchoring systems are used for the bridge main cables. On the east side, a conventional concrete gravity anchor is placed with a total of 70,000 cu m of concrete, while on the west side, two tunnel anchors are used to anchor the two main cables, and the tunnel anchor depth is about 70 m.   
 
The bridge main cable is made of 21,463   September 2012. ROADS&BRIDGES high-strength 5.25-mm-diam. steel wires and the diameter of the cable after tightening of all wires is 86 cm. There are 71 pairs of suspension cables supporting the bridge superstructure.  Each cable is made of 8×41 high-strength 62-mm-diam. steel wire cables. A steel truss frame is used for the superstructure. There are a total of 69 truss sections. Each truss section is 27 m wide, 7.5 m tall and 10 m long. The weight of the truss section is 125 tons. The bridge deck system is made from precast concrete panels with a modified asphalt concrete deck surface.  
 
Rock-hard anchors   
Since the bridge spans over a deep valley, the main span has to be directly connected to the tunnels at both ends.  The direct connection presented challenges to both design and construction due to the requirement for the transition from the flexible bridge superstructure to the rigid tunnel structure. A very tight construction area also was a big challenge to the contractor to carry out the construction. Special considerations and construction planning were given to control the movement compatibility and the construction site preparation.   
 
Unlike a typical suspension bridge,   in which the superstructure is attached to the tower, the superstructure of the bridge is separated from the towers. At  the west end of the bridge, the top of the  rock hill is used as the tower with only  a short concrete tower built on top of  the hill, resulting in a significant savings  on the tower cost. The bridge-to-tunnel connection is at about 70 m below the base of the west tower and away from the tower horizontally. As a result, the length of the superstructure is 1,001 m, which is about 175 m shorter than the main span length between the two towers.   
 
The superstructure at the west end is completely separated from the tower, which imposed another challenge to the design and construction. In order to keep the uniform vertical loading on the main cables, two pairs of the   vertical suspension cables at the west end are anchored to the ground where the superstructure is shorter than the main span. The pairs of cables will provide tension forces to the main cable maintaining their symmetrical shape across the bridge.   
 
Both towers are built on spread footings on bedrock. The geotechnical exploration indicated that the bedrock on the west side is very solid and in good quality, whereas that on the east side is not of very good quality; there are some cracks and faults and even two large caves near the tower foundation.   
 
The construction activities and the dead and live loads from the bridge moving forward will impose considerable extra loading on the bedrock, and it may cause problematic changes to the rock structure and stability.   
 
An extensive investigation and study was conducted and preventive measures were taken. A large number of rock anchors were installed at the rock surface around the bridge towers as well as the sensitive areas to ensure the stability of the bedrock.   
 
A number of permanent monitoring monuments were installed on the rock hill surface, and monitoring for the potential movement and deformation were conducted during the entire period of the construction. The monitoring will be continued even after the completion of the bridge.   
 
An innovative superstructure installation system known as cable track installation system (CTIS) was developed for the steel truss-section lifting, transporting and installation. The system consists of two cable tracks anchored to the rock surface at both ends, supported by the vertical suspension cables from the bridge main cables. Horizontal-moving trolleys with lifting devices are set on top of each cable track. The trolley can move in either direction by pulling cable from   the winches installed at both sides.   
 
During the installation, the truss section is first assembled in the assembling yard near the tower base at each end and moved to the location underneath the CTIS. The lifting device on the trolley lifts the truss section to the elevation right below the cable tracks, and the trolleys move the truss section to the designated location for connection and installation.   
 
Since the quality of the rock on the west side is very good and solid, two tunnel anchors were used to anchor the bridge main cables. The tunnel anchor is a new type of anchoring system for suspension bridge cables developed in recent years. It makes full use of the existing rock mountain as part of the anchoring system and uses much less concrete and other construction materials, resulting in a significant savings for the construction cost.  
 
The anchoring system consists of a large-diameter inclined tunnel in a direction aligning with the bridge main cable, a reinforced concrete anchor placed in the tunnel, and the steel wire anchoring   device connecting to the bridge main cable. The tunnel anchors have been used in several suspension bridges in China recently and proved to be an economical and effective alternative.   
 
Work at the end of the tunnel   
The tunnel anchor construction started with the excavation and stabilization of the construction and assembly yard at the tunnel entrance. Since the tunnel entrance is located in the rock hill slope, a part of the rock hill needed to be excavated to create a working area for the tunnel construction and also was used as the assembling yard for the bridge superstructure later. The slopes after the excavation were stabilized with rock anchors and shotcrete.   
 
The second step was the excavation of the anchor tunnels. The tunnel is about 70 m deep and inclined at about 38° to align with the bridge main cable. In order to create the anchoring system, the tunnel is excavated with varying cross sections with the cross section at the    top smaller than the cross section at the bottom. The cross section at the tunnel entrance is 11 m wide and 12 m tall with a 5.5-m arch top, while the cross section at the bottom is 15 m wide and 16 m tall with a 7.5-m arch top.   
 
                            

(Since the Aizhai Suspension Bridge spans over a deep valley, the main span has to be directly connected to the tunnels at both ends. The direct connection presented challenges to both design and construction due to the requirement for the transition from the flexible bridge superstructure to the rigid tunnel structure.)  

A specially designed explosion excavation  method was used for the tunnel  excavation to minimize the impact to the  remaining rock to ensure the strength of  the rock. After the completion of tunnel  excavation, a cast-in-place, reinforced  concrete with embedded wire anchors  was placed. The length of the concrete  anchor is about 43 m. The remaining  space between the concrete anchor and  the tunnel entrance is used as the transition  chamber to connect the bridge main  cable to the concrete anchor.   
 
In order to verify the design capacity  of the tunnel anchor, a quarter-scale  tunnel anchor model test was conducted  near the actual tunnel site. The test result  showed that the design is adequate.   
 
Building with gravity   
In consideration of the site condition   and rock quality, a conventional reinforced concrete gravity anchoring system is used on the east end. The construction of the gravity anchor consisted of the excavation and stabilization of the anchor site, concrete base placement, cable support and anchoring system installation and final cable installation.   
 
The gravity anchor includes four parts:  anchor mass, cable support and footing,  front chamber and back chamber. Due to  the large concrete volume and the temperature  control requirements, the entire  concrete anchor mass was divided into  four zones and the concrete was poured  by layers in each zone separately. A low- heat cement concrete as well as a cooling  piping system was utilized for the large  concrete anchor mass.The temperature  difference between the concrete surface  and the core was monitored and controlled  strictly to avoid potential cracks  generated by the heat. The construction  started with the concrete pour of the  anchor mass, including the back chamber  and the footing of the cable support,  following the large concrete pouring   procedures described above. The next step  was to build the cable support on top of  the completed support footing, followed  by the construction of the front chamber.  After the concrete reached 100% of the  design strength, the cable was installed  and anchored to the anchor mass. The  steel wires from the bridge main cable  are separated into 103 wire groups at the  cable support and spread out to reach  the concrete anchor mass through the  front chamber. The 103 cable groups pass  through the conduits embedded in the  anchor mass to reach the back chamber.  The wire groups were connected to the  anchor mass by the connectors at the  back chamber. The main bridge cables  were tensioned to the designed tension  forces and lengths.   
 
Cave fill in   
Both towers are reinforced concrete  frame structures on spread footings on  bedrock. Each tower is a two-column  frame structure with two cross beams at  the top and middle.   
 
Before the foundation construction for the towers, the condition of the  bedrock was evaluated and the defects  were treated. At the east tower site,  several underground caves were fi lled  with concrete. There also were some  cracks and rock faults on the east side. At  such locations, the grouting holes were  drilled up to 40 m deep, and cement  grouting was used to fill the voids. At  the west side, no underground cave was  found but some cracks were present. The  grouting foundation treatment applied  on the east side also was used here.   
 
Connecting together  
A steel truss structure is being used  for the bridge superstructure. The main  truss across the bridge is 27 m wide and   7.5 m tall. The top and bottom chords  are made of rectangular box sections,  and the diagonals are I-beams. The  K-type steel truss is used for connecting  the main truss along the bridge direction.  The top and bottom chords for the  K-truss are made of box sections, and  the diagonals are I-beams. The spacing  of the panel points is 7.25 m, and a  standard truss section consists of two  truss panels with a length of 14.5 m.   
 
A total of 69 truss sections and one  middle span section are used for the  entire superstructure. The total number  of truss panels is 169. The truss members  were manufactured and assembled  in the factory to ensure the precision of  the dimensions, and then the members  were dismantled and shipped to the site  by trucks. At the assembling yards on  each side of the bridge, the truss sections  were assembled progressively in such  a way that two truss sections were assembled  at one time. After the completion,  the first section was lifted for installation  and the second piece remained in place  for the next section to be assembled and  connected to the piece.   
 
The assembled truss panel sections  were lifted, transported and installed  using the specially designed CTIS  described earlier. The truss panel section  was first transported to the lifting  pad under the CTIS and lifted to the  elevation right below the cable tracks  and attached to the trolleys. The trolleys  transported the truss panel section to the  designed position by pulling cables from   the winches located on the opposite  side of the bridge. Then the truss panel  section was lifted to its final position by  a crane on the bridge main cables and  attached to the suspension cables and  connected to the truss superstructure  installed earlier.   
 
The installation of the truss sections  started from the middle span and  progressed toward both sides symmetrically  to ensure uniform loading  on the main cables. A total of six pairs  of temporary installation hinges were  placed for the bridge superstructure to  allow the deformation of the superstructure  during the truss and deck  system installation. These hinges were  removed after all truss sections were  securely connected at the completion of  deck-system installation.  
 
Liu is a principal with H&J International PC, Collegeville,  Pa. Chen, He and Zhang are with the Hunan  Jicha Expressway Construction and Development  Co. Ltd. The article is from September 2012 ROADS&BRIDGES.

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