20 February 2012
A three-tower suspension bridge over the Yangtze River in Jiangsu Province has provided tough challenges for design and construction. Ce Chen and Jianchi Zhong report.
Later this year China’s new Taizhou Bridge over the Yangtze River is set to open to traffic, marking the inauguration of yet another major bridge crossing of this mighty waterway. But the bridge is not just a long-span crossing, it is a multi-span crossing, with each of its two consecutive main spans stretching to more than 1km in length. On completion it will be the world’s longest-span bridge of its type, but designing and building a multi-span suspension bridge, particularly on this scale, is not a straightforward process.
The main structure of the Taizhou Bridge project is a three-tower suspension bridge with two consecutive main spans of 1,080m and side spans of 390m. The bridge is a major element of infrastructure development in the east of China and will play a vital role as a link in the freeway network in Jiangsu Province and the Yangtze River delta region. It is part of a new 62km-long freeway that links the cities of Taizhou, Zhenjiang, and Changzhou, which is being built at a cost of US$1.5 billion – the bridge itself has cost US$400 million to construct.
In general, a multi-span suspension bridge is simply an enhanced suspension structure with an additional central tower; in this case it has been chosen in order to reduce the number of bridge piers in the river, and hence minimise the impact on the water flow. But the characteristics of a three-tower suspension bridge are markedly different to those of a traditional suspension bridge.
At the bridge site, the river is some 2.1km wide and the riverbed has channels at each side with a shallower water depth in the centre, with a w-shaped cross-section. The deepest channel is on the south side and has a water depth of approximately 30m while in the middle of the river this reduces to approximately 17m.
Both banks of the river are typical middle and lower Yangtze River alluvial plains which consist of soft soil and a thick cover layer and the depth to bedrock is generally at least 190m. A three-tower suspension bridge design was chosen in order to minimise the impact on river flow, expedite the development of port facilities and to make navigation easier.
This solution minimises the number of bridge piers in the river, to reduce the obstruction to water current and retain sufficient openness of the river. Multi-tower suspension bridges, which have the advantage of length and sequential spans, are expected to increase in popularity in coming decades as an important option for large-scale sea crossings.
Three-tower suspension bridges such as this can overcome the limitations of single span structures, creating a continuous crossing. Such multi-span systems have often been proposed for use on sea-crossing schemes, but of those which have reached completion, the longest span is just 210m, which is far from the capacity of such a structure.
The Taizhou Bridge is the first attempt to create a long-span multi-tower suspension bridge. Such a bridge is essentially an improved suspension structure with an additional central tower, vertical support of the main cables, at the mid span of single main span suspension bridge, in order to reduce the internal forces of the main cables and anchors. The central tower saddle acts only as a vertical support point for the main cable.
Under dead load, the restraint placed on the middle tower top by the main cable is less than that of the side towers. Three key issues must be resolved to successfully design a three-tower suspension bridge. Firstly, under the most unfavourable loading conditions with all live load on one main span and none on the other, the vertical deflection of the deck must be controlled within a reasonable range. Secondy, the friction between the saddle and the main cable must be such to prevent the cable from sliding.
While meeting these two criteria, the safety and stability of the central tower must also be assured. The longitudinal stiffness of the middle tower must be sufficient to allow flexibility and bending stiffness to suit the characteristics of a three-tower suspension bridge system. Three tower types were considered: an inverted Y-shaped tower, an A-shaped tower and an I-shaped tower. The middle tower was designed to be formed of two A, I or Y-shaped elements, placed one each side of the deck, along the longitudinal axis of the bridge rather than straddling the deck.
The results revealed that the I-shaped tower has a low stiffness, and that deformation of the tower top and deck would be too high under the most adverse conditions. The disadvantage of the A-shaped tower is its low friction safety factor. However the inverted Y-shaped tower offered a reasonable friction safety factor as well as a reasonable value for the tower section stress and main girder deflection. For this tower design, an appropriate stiffness was selected based on extensive calculations of different control parameters, including the tower height, cross-section, the longitudinal distance between the tower feet and the height at which the tower legs join.
For this particular design, when the height at which the legs join is kept constant, and the tower section size is known, the longitudinal stiffness of the tower first increases and then reduces, as the distance between the bases of the tower legs increases. This stiffness is at its maximum when the feet are between 30m and 35m apart. At this range, the minimum axial pressure of the tower bottom increases; when this distance is less than 32m, the minimum pressure is negative at the tower base. The feet should be more than 32m apart in order to avoid tension forces at the bottom of the tower section.
As the bifurcation point is raised, the longitudinal stiffness becomes greater and the anti-slipping safety factor reduces. According to the force transfer mechanism of a three-tower suspension bridge, the middle tower must be suitably rigid in the longitudinal direction with proper flexibility and sufficient bending rigidity. After calculating and comparing multiple structural forms, a longitudinal inverted Y-shaped, transverse portal-type frame steel tower 200m high was selected for the central tower. The tower feet are spaced 34.75m apart and they connect at a height of 69.5m.
The foundation for the central tower, which is located in the centre of the river, is a caisson structure with cross-sectional dimensions of 58m by 44m. It has a total height of 76m, split into two parts each 38m high. The lower part is a prefabricated rectangular steel-shelled structure, while the upper is a concrete caisson.
The first part of the steel-shelled caisson was fabricated onshore, then floated in the water where construction continued, extending it to its full 38m height. It was then floated and tugged to its permanent location, where it was placed on the riverbed by flooding it with water. Once it was in position, the chambers of the steel-shelled structure were filled with concrete, gradually extending it above the steel shell, to the final design elevation.
Underwater concrete casting was used to complete the foundation construction. The connection between the concrete cap and the steel tower needed special consideration. If the tower steel plate had been cast directly into the concrete cap, the axial force from the tower would mainly transfer by anti-shear between the steel plate and the concrete. It would be possible to set shear studs on the steel plate or open hole to put the steel. If this method was adopted, the cap’s thickness is only 6m, and the shear is gradually distributed along the thickness of the cap, transfer process is not clear.
Thus a different method was proposed whereby a thick steel plate was installed between the bottom of the tower and the concrete cap to transfer the axial force. To avoid any friction being caused by the horizontal component of the force, an inclined steel base plate was installed at the bottom of the pillar, to produce a very low shear force and no friction on the anchoring surface under dead load. This connection is designed to be relatively simple and convenient.
The axial force, bending moment and off-centre stress are large at the bottom of the tower section. The stress is transferred to the concrete bearing surface via a 150mm-thick welded steel plate. Prestressing is applied using 34 high-strength screws, each 130mm in diameter, which ensure that the tower section and bearing surface are kept in close contact.
Research was also carried out into different parameters of the central tower, main cable, main deck girder and bearing supports in order to establish the influence of the vertical connection between the main girder and the middle tower. In the end, displacement limiters were installed between tower and main girder, rather than rigid vertical constraints. The upstream and downstream limiters are designed to restrain the torsional vibration to a certain extent and are helpful in decreasing torsional vibration under wind loading. Research into the longitudinal connection between the central tower and the main girder demonstrated that such constraints could be helpful in significantly increasing the friction safety factor and decreasing both the stress in the middle tower and the displacement of the main girder.
Elastic cable constraints can have similarly favourable effects on the structural system, but also they are not as simple to manufacture. Hence longitudinal elastic constraints were chosen and installed between the central tower and main girder. Additionally, engineers investigated whether to install central buckles and if so, how many pairs: installation would be difficult for a three-tower suspension bridge, as it is totally different from a two-tower suspension bridge. If only one pair was installed, it would not be possible to specify the gradient of the cable clamps because if the gradient is small, the stress of the clamp is too high, whereas if the gradient is large, the improvement gained from the buckles is negligible. If three pairs of central buckles were installed, some benefits would be gained but at the same time, the possibilities of failure damage, and the large stress in the buckle cables caused by the buckles would be significant. Thus a scheme without any central buckles was recommended.
Vertical blocks were installed at the central tower to limit the torsional vibration of the main girder and reduce vibration amplitude under the action of wind load. Bearings were also installed at each of the main towers to restrain the transverse displacement of the stiffening girder. In addition, a vertical bearing was installed on the lower cross-beam of the two side towers.
The impact of the cable suspension sag-to-span ratio for the main cable on this three-tower bridge was investigated across a range from 1/7 to 1/13. As the ratio is reduced from 1/7 to 1/13, under live load, the maximum axial force increment of main cable increases by 44%, the anti-slipping safety factor between the main cable and saddle of the central tower increases by 21%, and the maximum vertical deflection of the deck increases by 35%, and it approaches a linear relationship. In order to control the main cable tension and reduce the size of the anchor blocks, the sag-to-span ratio for the main cable was taken as 1/9.
At the site, the surface soil is mainly silt loam and silt layer, with bedrock at around 190m depth and no significant impermeable bearing layer of soil above it. Hence both the south and north cable anchorages were designed as a combination of gravity-type anchorage with caisson foundations. Foundations for both the anchor blocks are designed as caisson structures with the south caisson foundation measuring 68m by 52m and 41m deep, and the north anchor block is has a 57m-deep caisson foundation.
As with the main tower foundations, the lower section of the caisson is a steel-shelled concrete caisson, although here with a height of just 8m. Other sections are reinforced concrete caissons. The base of the caissons for the north and south anchor blocks are founded at 55m and 39m depth respectively, in a dense fine silty sand stratum. The anchor blocks themselves are mass concrete structures.
Construction of the bridge began in August 2007 with the tower and anchor block foundations which took two years to build. Local scour was an important factor in the construction of caissons; the first installation stage was particularly significant. Many different options for the caisson positioning were proposed before the construction, but the chosen procedure abandoned the traditional system of using a positioning ship, a guidance ship and an anchorage system in favour of a new rigid system using rigid steel anchorage piers and anchorages.
The timing of the caisson installation was such that it was carried out during a slack period when the water velocity in the river was low. The bases of the cells were cleaned and sealed asymmetrically during construction. Under the influence of water current and wind, the caisson was at risk of being pushed away from its intended position during installation, or becoming twisted or oblique. To counteract this, an online digital dynamic control system was used to monitor the whole process of caisson sinking including positioning, embedding, sediment dredging, caisson sinking and sealing of the base to ensure the safety and quality of the foundation.
The total height of the central tower is 191.5m and it is constructed of two different steel types depending on the stress each section will bear. The two oblique legs, which have a gradient of 1:4, meet at a height of 69.5m and extend a further 122m above. In the transverse direction, the middle tower has a gantry-type frame. The width of the tower columns is 5m across the full height, and two cross-beams connect the two columns of the tower. In cross-section, the tower columns consist of a single box made of multiple cells, with four edge fillets of 600mm by 600mm which are designed to reduce the wind-resistance of the structure.
The whole column is made up of 21 segments, which are are connected using high-strength bolts. The structural forces are transferred both by the contact surfaces and the high-strength bolts. The tower legs are fixed to the piles caps using bearing plates and anchor bolts, which enable the structure to resist large axial forces and moments because the ends of the columns are under compression with a large eccentricity.
A total of 34 prestressed high-strength bolts of 130mm diameter and made of 40CrNiMoA are installed at each the bottom of each column. The steel tower is formed of chamfered segments with a rectangular cross-section; they have maximum dimensions of 5m by 12.7m and maximum height of 15m. In line with the geometric demands and the predicted stresses, the allowance for longitudinal and transverse dimensional errors is ±2mm; diagonal errors must be within 3mm; the planeness of the whole section must be within 0.25mm; transverse and longitudinal segments must be perpendicular to within an allowance of 1/10 000; the contact ratio of wall and web plates of different segments must be greater than 50% and the contact ratio of longitudinal ribs must be greater than 40%.
These high demands for precision imposed great difficulties for control of welding deformations, marking positioning, mechanical processing and measurement. Each anchor bolt is 10m long and weighs 846.8kg. A special fabrication procedure was needed to manufacture these items, to address such difficulties as achieving accuracy on the threads of such large bolts, heat processing and precise control of axial accuracy for two-end threads of long screws, which had to be within 5mm.
Another challenge was the horizontal shop assembly of tower elements. Some of these segments are up to 15m in length, meaning that shop assembly of two segments could be 30m. For reasons of safety and to avoid the influence of temperature, the plan to preassemble two adjacent segments horizontally aided by computer simulation is proposed.
To control the geometry of the steel tower, a solution incorporating advanced measurements, digital alignment, efficient state control and a normal processing machine was proposed after a great deal of research, and was used during the manufacturing process. Measurements were taken of the errors in perpendicularity of end sections and axial errors of the tower segments during shop assembly, with computer calculations used to predict the installation errors.
Based on these results, guidance for the fabrication of the remaining segments, correcting any height or axial deviation error, and shop assembly of two segments, correcting the torsion error, would be proposed. Furthermore, four adjusting connectors were designed for final geometry correction. Naturally the accurate positioning and installation of the first segment was critical, as its precision would determine that of the whole tower.
Four D0 segment, oblique in two directions, were manufactured with 34 holes of 200mm diameter in the bottom bearing plates and the same amount of holes with 180mm diameter in the top plates respectively. The anchor bolts were installed through the top and bottom plates simultaneously. The steel tower segments were then adjusted to the required gradient in the transverse and longitudinal directions, with an accuracy within 20mm in each direction – a very high demand for the installation. After these segments were adjusted, the precision of single segment as well as the relative preciseness of four segments would be guaranteed.
Furthermore, these segments must be adjustable in three directions. Four connections had to be ensured for segment D4 of the central tower, which weighs 470t. After being hoisted to the right position, the process of making fine adjustments to such a heavy segment was very demanding in terms of the settings of jacks, design of fixtures and the precision of site measurements. The central tower can be divided into three parts – the oblique legs, the curvilinear transition section where the legs connect, and the upper column. The geometry of the tower, in addition to the influences of temperatures, wind loads, differential settlements of the caisson and the forces caused by attached crane arms, is very difficult to control during the hoisting process.
Thus accurate measurements and scientific monitoring analysis were essential to control the installation precision and to ensure the safety and quality of the work. The elevations of top plates of D3 segments were precisely measured over several nights before the erection of the next segment. The horizontal and transverse alignment was adjusted by jacks on the top of segment D3. Limitation plates were set up in either installed or installing segments to make the insert of new segments easier. These plates had be factory manufactured to ensure that the dimensions and positions of holes were correct, and were designd to be removed after installation. Additionally, adjustment brackets which were also manufactured and installed in the factory were used for fine tuning the position.
Every bracket on the D3 segment had one jack with a capacity of 500t, to adjust segment D4 precisely. Segments D6~D20 are standard segments, and these were erected using a Potain MD3600 crane. To decrease the transverse length of the connections and to keep within the capacity of the crane, the segments of the upper tower were divided into two parts horizontally. Construction of the central tower was completed successfully in April, 2010. The longitudinal and transversal perpendicularity of the as-built tower is 1/19591 and 1/50065, much better than the designed standard 1/4000.
The towers at each end of the main bridge spans are traditional concrete frame structures with two cross-beams, partly for aesthetic reasons, and the side towers are also shorter than the central tower by some 20m. Foundations for these towers are supported by 46 friction piles each with diameter of 2.8m, and length of up to 98m on the south tower, and 103m on the north tower. Construction of these towers was completed in November 2009.
Each main cable consists of 184 prefabricated parallel wire strands, each of which consists of 91 high-strength galvanised 5.2mm-diameter steel wires with standard tensile strength of at least 1,670MPa. The unstressed length of each strand is about 3.1km and its weight is 47t. Inside the main cable the void ratio is 18%, while outside the cable clip it is 20%. The friction coefficient between the strands of the main cable and the cable saddle is 0.2 with a safety factor of at least 2 against slippage.
Suspender cables are made of prefabricated parallel high strength 5mm-diameter steel wires, again with a tensile strength of at least 1,670MPa. Suspender cables have a typical spacing of 16m and the tower centre-line is 20m from the nearest suspender cable.
Research was carried out into the type of catwalk that would be most appropriate for cable erection on a three-tower suspension bridge and whether it should be continuous or discontinuous, before the former was chosen, with four spans. This kind of structure can accommodate the characteristics of three-tower suspension bridge and impose minimum influence – for example such as the displacement at the top, the appearance and rehabilitation in the future - on the middle tower.
Cable erection began in September 2010 and was completed in January of the following year. The main girder is a streamlined, closed steel box girder cross-section, designed as a single box structure with three internal sections. The main girder is 3.5m deep and has a total width of 39.1m. In order to increase rigidity of the deck plate and improve support of deck pavement, the top plate has a thickness of 16mm and 8mm-thick U-shaped stiffeners are installed below the vehicle lanes that are intended for heavy vehicles.
A top plate of thickness 14mm and 6mm-thick U-shaped stiffeners with are used elsewhere. Studies comparing whether to hoist deck girder segments from the middle span or from the middle tower symmetrically were used to find a better plan based on several performance indices. These included the deformation of the middle tower, safety factors against slippage, keeping variation of forces in the suspenders to a minimum, and the also keeping absolute values of shear forces and axial forces of guide parts for main girder to a minimum. As a result the team decided to erect segments from the middle of the span. Erection of the steel box girder deck sections was completed last September, and the bridge is expected to open to traffic at the end of this year.
Ce Chen is a doctorate student at Civil & Transportation College, Hohai University, Nanjing and Zhong Jianchi is site director for Jiangsu Provincial Yangtze River Highway Bridge Construction Commanding Department
Bridge owner: Jiangsu Communication Holding Company
Owner’s representative: Jiangsu Provincial Yangtze Highway Bridge Construction Commanding Department
Bridge designers: Jiangsu Provincial Communication Planning and Design Institute, China Railway Major Bridge Reconnaissance & Design Institute, Architectural Design & Research Institute of Tongji University.
Main contractors: CCCC Second Highway Engineering Company (North anchor, north tower, bridge superstructure) China Railway Baoji Bridge Group Company & Jiangsu Zhongtai Bridge Steel Structure Company (steel fabricator for middle tower) CCCC Second Harbour Engineering Company (steel erection contractor for middle tower) China Railway Major Bridge Engineering Group Company (south anchor) Jiangsu Provincial Communication Engineering Group Company (south tower erection contractor)
Specialist suppliers: Baosteel Group Shanghai Second Company, Jiangyin Walsin Steel Cable Company (cable wire) Jiangyin Fasten Steel Products Company (cable strand and hangers) Mageba (Shanghai) Bridge Company (Expansion joints & bearings)