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Research on General Design and Key Technique of Sutong Bridge



2012-10-11
Zhang Xigang, Yuan H




General Situation

As one key passage cross Changjiang River on the important trunk road from Jiayin, Heilongjiang to Nanping, Fujian, Sutong Bridge, planned by the Ministry of Communications, has one important status in the highway transport networks of the nation and Jiangsu province.

Preliminary work of Sutong Bridge started from 1991, the project was initiated officially in June, 2001, preliminary design work fully commenced at the end of February, 2002, and preliminary design was approved by the Ministry of Communications in March, 2003. Technical design and construction drawing design fully started from February, 2003, and technical design for the main bridge was finished in January, 2004. According to the project schedule, all construction drawing designs were finished in phases from June, 2003 to July, 2004, passed examinations of experts which were organized by the Communication Department of Jiangsu Province authorized by the Ministry of Communications, and all were submit officially for construction.

Main Technical Standard

1. Highway grade: full-closed two-way six-lane expressway on plain and lightly undulate area.

2. Calculated speed of vehicle: 100km/h.

3. Design reference period for bridge structure: 100 years for main bridge, and 60 years for auxiliary bridge and approach bridge.

4. Load grade for vehicle: auto-grade of super 20, trailer-grade of 120.

5. Deck clearance and standard cross section: standard width of bridge: 34m.

6. Longitudinal slope:≤3

7. Transverse slope: 2%

8. Seismic resistance standard: VI degrees for basic seismic intensity at bridge site, seismic resistance standard is shown in Table 2-1.

Table 2-1  Seismic Resistance Standard for Sutong Bridge
Bridge Probability of seismic resistance Requirements of structural performance Target of structural check
Main bridge P1: 10% in 100 yrs
(recurrence period of 950 yrs)
Main structure under limit state of normal service Main structure used for stress check
P2: 4% in 100 yrs
(recurrence period of 2,450 yrs)
Main structure under limit state of bearing capacity, controlling displacement or deformation Main structure used for check of limit bearing capacity
Auxiliary and approach bridges P1: 10% in 50 yrs
(recurrence period of 475 yrs)
Quake durability resistance of structure using Or considering to use durability to check limit of bearing capacity; checking displacement and deformation, main structure used for checking limit of bearing capacity
P2: 2% in 50 yrs
(recurrence period of 2,475 yrs)
Controlling displacement or deformation Checking displacement or deformation

9. Design standard of wind resistance:

Design recurrence interval of 100 years for operation phase and 30 years for construction phase. At 10m height of the bridge site, basic wind velocity of once-in-one-hundred-year is 38.9m/s, basic wind velocity of once-in-thirty-year is 35.4m/s; wind force subject to combination with auto loads is based on deck velocity of 25m/s, no auto load combination is involved excess 25m/s.

10. Designed flood frequency: 1/300.

11. Designed water level is shown in the Table 2-2 (elevation herein is national 85 elevation system).

Table 2-2  List of design water level for Sutong Bridge
Designed flood level for once-in-300-year (m) Minimum designed navigation level for 98% assurance rate (m) Maximum designed navigation level for once-in-20-year (m)
5.29 -1.46 4.30

12. Navigation standard: according to special research result, navigation standard approved by the Ministry of Communications is shown in the Table 2-3.

Table 2-3  List of navigation clearance and number of navigation spans
Name of navigation span Type of channel Typical ship Navigation clearance (m) Number of navigation span
Net width Net height
Main navigation span Single-span double-channel 50,000-ton-level container ship (3,800TEU)
Perspective massive bulk barges fleet of 48,000-ton-level 
891 62 1
Auxiliary navigation span Single-span single-channel massive bulk barges fleet of  9,000-ton-level 220 24 2
Dedicated navigation span Single-span double-channel bulk ship of 7,000-ton-level 220 39 1
Upward span in flood season Single-span single-channel river ship of 1,000-ton-level 70 15 1

13. Standard of ship impact: according to the actual situation of Sutong Bridge, annual impact frequency of the main bridge and continuous rigid structural bridge of the auxiliary bridge may consider respectively: less than 10-4 for general annual impact frequency of the main bridge, less than 10-4 for that of continuous rigid structural bridge of the auxiliary bridge. Standard of ship impact for the main bridge, auxiliary bridge and approach bridge through special research are shown in the Table 2-4.

Table 2-4  Overall ship impact standard
Navigation span Pier Impact force (MN)
Transverse to bridge Longitudinal to bridge
Main bridge North span Transition pier 11.9 6.0
Auxiliary pier of remote tower 13.2 6.6
Auxiliary pier of close tower 40.6 20.3
Main span North tower’s pier 130.6 65.3
South tower’s pier 126.7 63.4
South span Auxiliary pier of remote tower 40.6 20.3
Auxiliary pier of close tower 13.3 6.7
Transition pier 12.0 6.0
Continuous rigid bridge of auxiliary bridge Main span North main pier 41.2 20.6
South main pier 49.8 24.9
North approach bridge, continuous-girder bridge of auxiliary bridge (intermediate approach), south approach bridge Piers of navigable span 2.33 1.165
 
Technical Features and Difficulties of the Project

In view of special construction conditions at the mouth of Changjiang River where Sutong Bridge situates, it uses one solution of double-towered cable-stayed bridge with main span of 1,088m, which is one important project of great influence at home and abroad. The construction of the bridge represents the bridge construction level in 21st century of China.
Technical features and difficulties of Sutong Bridge are defined by a couple of respectives, including relatively complex construction conditions, large technical difficulty in design and construction and so on.

1. Construction condition's feature and difficulty
Construction conditions for Sutong Bridge present four major features and difficulties: viz. relatively poor meteorological condition, complicated hydrological condition, deep-buried bedrock, high standard of navigation.

2. Technical feature and difficulty in design and construction
Technical feature and difficulty of Sutong Bridge in terms of design and construction are defined by: design and construction of massive deep-water foundation for the main bridge, design and construction of high cable tower for the main bridge, and design and construction of long span and long cables' superstructure of the main bridge, etc.

(1) Design and construction of massive deep-water foundation for the main bridge
Deep water foundation of the main tower is one of key projects and difficulties in design and construction because of large scale of the bridge, deep-buried bearing layer for the foundation, large impact load from ships, deep local scouring on riverbed and influence of the tide. It includes bearing capacity of long piles under weak soil layer condition and effective measures for improving bearing capacity, appropriately considering massive pile-cluster foundation's loading feature and cluster effect under joint action of piles and soil, workmanship and quality assurance measures for long cast-in-place piles, and anti-scouring measures for foundation etc.

(2) Design and construction of high cable tower for the main bridge
In terms of design and construction, over-300-meter high tower of Sutong Bridge presents the following technical features and difficulties: design, manufacture and installation of steel anchor box, issue of anti-wind stability of high cable tower amidst construction phase, construction workmanship and monitoring for the high cable tower, etc.

(3) Design and construction of long span and long cables' superstructure of the main bridge
Structural design and construction both face new technical problems because of main bridge using double-tower cable-stayed bridge type with main span of 1,088m and large design wind velocity. Although there were many damping measures for stayed cables before, it still needs to be further researched and solved for vibration mechanism analysis and effective damping means for something like 580-m-long stayed cable of Sutong Bridge. In terms of design and construction, superstructure of the main bridge presents the following technical features and difficulties: issues of structural system, non-linear effect and analysis for thousand-meter-level cable-stayed bridges, static reliability for thousand-meter-level cable-stayed bridges, anti-wind reliability amidst construction and operation phases, superstructure erection and construction monitoring, damping for stayed cables amidst construction and operation phases, etc.


Research on Key Technical Issues

About Construction Conditions

A great deal of paperwork investigation and special research on construction conditions were performed to figure out its features, master reliable data, aiming at the features and difficulties of Sutong Bridge's construction conditions, which provided precise materials for design and construction. Almost 30 items of special researches were carried out over the past years, which achieved large quantities of results, and ensured design and construction schemes being reasonable, safe, and reliable.

About Design and Construction Technique

1. Design and construction of massive deep-water foundation for the main bridge

(1) Research on long piles' bearing capacity under weak soil condition and effective measures for improving bearing capacity
Reasonably analyze and determine the relevant design parameters in combination with detailed geological exploration jobs, test pile jobs and the relevant paperwork investigation and analysis. Research, test and design the pile bottom grouting workmanship after reasonably determining pile foundation's bearing capacity and effective measures to improve such bearing capacity in combination with test piles and theoretical analysis.

 (2) Research on loading feature of massive long pile cluster
Use numerical simulation computation method where joint action of piles and soil are taken into consideration and centrifugal physical model testing method for research, providing reference data for the design.

(3) Research on workmanship for long cast-in-place pile
Reasonably determine and verify workmanship for long cast-in-place piles, formulate special workmanship specification to ensure the requirements for construction quality and bearing capacity in combination with detailed geological exploration job, test pile job and investigation and analysis of relevant paperwork,

(4) Research on foundation anti-scouring
Carry out research on protection engineering tests on local scouring for piers, design and perform special protection engineering In combination with design and construction, which eases and controls scouring effect.


2. Design and construction of high cable tower for the main bridge

(1) Design, making and installation of steel anchor box
Tower anchor mode of Sutong Bridge uses steel anchor box structure that is first employed in China. Thus, in addition to further reinforce structural design, computation analysis and workmanship research, research of stress mechanism analysis and model test research are practiced.

(2) Issue of wind resistance for high cable tower amidst construction phase
With increasing of the cable tower's height, issue of its wind reliability stands out and turns out to be issue of wind-caused vibration of cable tower during operation phase and reliability during construction phase.

As to wind-caused vibration, cable tower of Sutong Bridge employs reinforced concrete structure that has relatively large deadweight and better air dynamic stability compared to steel tower, and undergoes special research and inspection through wind tunnel test. Conduct comparison through computation analysis and set up temporary piers in proper positions, reducing the length of double cantilevers when erecting main girder to ensure cable tower's safety during phase of deck erection.

(3) Construction of high cable tower
So far, the second bridge of Nanjing is the highest concrete tower for cable-stayed bridge in China with the height close to 200m. The highest concrete tower of cable-stayed bridge overseas is Normandy Bridge in France with cable tower as high as 203m and the highest concrete tower of suspension bridge is east bridge of Great Belt Bridge in Denmark with cable tower as high as 254m. Concrete tower of Sutong Bridge is as high as 300m, with relatively difficult construction. Main technical difficulties of construction include: construction control of the cable tower, formwork system and support form for cable tower construction, workmanship for main tower's concrete, technical requirements of key construction equipment, etc. It is technically feasible through research, and also solution of guidance is proposed.


3. Design and construction of main bridge's superstructure

(1) Research on main bridge's structural system
Different structural systems were analyzed and compared, which include vertical bearer provided at longitudinal drift cable tower, longitudinal elastic constraints provided at cable tower, and rigid connection between tower and girder, etc. Through investigation, analysis and comparison, the structural system that Sutong Bridge adopts is such that transverse anti-wind bearer and viscous damper (F=CVa) with longitudinal limit function, rather than vertical bearer, are provided between cable tower and main girder. Viscous damper plays one role of damping and energy consumption for live load caused by fluctuating wind, braking and earthquake, however presents no constraint on slow displacement resulted from temperature and vehicles. When within the designed range of the damper, relative longitudinal displacement caused by still wind, temperature and vehicles between tower and girder does not restrain the main girder's movement; when outside of that range, it would restrain the main girder's movement.

Longitudinal sliding bearer provided between main girder and intermediate pier and auxiliary pier restrains transverse relative movement.


(2) Issue of non-linear and static stability for cable-stayed bridge with thousand-meter-level main bridge
Carry out special analysis and research to research and determine structural non-linear feature, main influence parameters and method of analysis computation for thousand-meter-level cable-stayed bridge. Use dedicated computer analysis program for reasonable designing computation analysis and revision. The result shows that the influence degree which Sutong Bridge's nonlinearity under composite effect has on structural stress lies between 10% and 15%. This means it is feasible for design of thousand-meter-level cable-stayed bridge to employ current design means to control structural safety, and structural design can be totally controlled within the range of safety. Much attention shall still be drawn to the influence of nonlinearity factor on structural deformation and cable stress during construction phase. Detailed analysis and construction control shall be carried out according to nonlinearity theory. To ensure reliability of design result and precision of construction control, computation analysis program is introduced for computation and different programs are used for inter-check; in addition, appropriate design and consultation examination units are authorized to employ different programs and methods for checking computation. One important subject that calls for careful research during design process is the direct effect of structural static reliability on safety of the project during construction and operational phase since Sutong Bridge comes in high tower and long span, with high design wind velocity. Analysis of structural static reliability mainly carries out elastic buckling stability (stability of type one) and elastic-plastic stability (stability of type two). Standard for analysis of stability of type two is not clear in the current code, and different scholars have different ideas about it. Therefore, computation analysis and comparison are performed according to design principles and reference standard for two different computation theories.


(3) Issue of anti-wind stability for the main bridge amidst construction and operational phase
Issue of anti-wind stability is one of the key elements that determine safety of the bridge. Cable tower, stayed cable, and steel box girder's section are selected and compared to reduce wind resistance factor and decrease wind load as much as possible through advanced computational method of numerical wind tunnel simulation. Also, conduct detailed simulation computation analysis, set up segmental model (incl. segmental model of large-scaled high Reynolds number) and perform wind tunnel test for whole bridge model to verify static, dynamic stability and wind load parameters under different unfavorable working conditions during construction and finished-bridge phase. Some structural measures are taken to enhance wind stability, for example, install guide slabs on main girder, build temporary piers during construction, etc. The measures proved to be feasible have been applied in large-spanned cable-stayed bridges at home and abroad. Research and analysis suggests that wind stability of Sutong Bridge's main bridge can be guaranteed with good performance of wind resistance and relatively high safety.


(4) Issue of erection of main bridge's superstructure
Deck of main bridge is 70m above water surface with single block as heavy as 450t, calling for higher requirements in construction equipment. Construction cycle is long, especially for the maximum single cantilever as long as 540m which increases the risk of construction. Due to the existence of nonlinearity impact, cable load and alignment control during erection of main girders are also issues that call for priority research.

Fully investigate the existing equipment at home and abroad, research on multiple construction schemes, and propose reasonable and feasible construction scheme of guidance and technical requirements for construction equipment during design process. For erection of long-cantilevered main girder of the main bridge, firstly avoid construction in windy season in terms of schedule planning; speed up the installation rate of steel box girder through such methods as improving connection mode of steel box girder, using double-armed crane, closing side span ahead of time and improving installation procedure of superstructure. Besides there are other methods such as building temporary piers and deck dampers to ease construction difficulty and risk of long cantilever. The above measures can be taken to ensure closing of steel box girders of the main bridge before or avoiding typhoon season to dodge risks.

For the issue of construction control during installation of the main girders, design and construction control shall be closely connected, using dedicated program for nonlinear analysis, employing various programs for check to ensure reliability of the result. Figure out internal forces and deformation features during construction and perform timely monitoring through repeated analysis. Employ geometrical member control method for construction control according to the features of thousand-meter-level cable-stayed bridge.


(5) Issue of vibration reduction of stayed cable during construction and operational phases of the main bridge
Vibration of stayed cable can directly affect usability and safety of bridge and has drawn attention at home and abroad. Multiple effective methods and products for vibration reduction have been created and developed such as surface treating of cable sleeves, installing different kinds of inside, outside dampers and auxiliary cables. After research on measures of vibration reduction for stayed cables combined with theoretical analysis research and wind tunnel test during design phase, the bridge employs one comprehensive vibration reduction scheme, namely one composite measure covering cable end damper and pneumatic measure. Also, design scheme and technical parameter requirements are raised through performance test and parameter analysis for the relevant products at home and abroad. Sutong Bridge takes efforts to make the solution for vibration reduction of stayed cables advanced internationally, making use of the most advanced techniques in the world and taking measures of combining domestic research and introduction from foreign countries.

Authors from CCCC Highway Consultants CO.,Ltd.
Jiangsu Provincial Communication Planning and Design Institute CO.,Ltd.




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