Combining strength and beauty—Features of the Danjiang Bridge
The world's longest-span single-pylon asymmetric cable-stayed bridge
The main river-crossing section of the Danjiang Bridge has a length of 920m, a pylon height of 211m, a main front span length of 450m, a back span length of 175m, and a maximum deck width around 70m. It is the world's longest-span single-pylon asymmetric span cable-stayed bridge. In view of the sunset location and the depth of bedrock under the Tamsui River, the pylon is located toward the Tamsui side from the center of the river chan-nel, and a boat navigation channel with a width of 200m and clearance of at least 20m will be retained in the river trough area under the bridge.
Taiwan's first co-constructed highway and railway scenic bridge
In response to the construction and use of the Danhai Light Rail system, the plans for the Danjiang Bridge also call for the retention of deck space for the future construction of a Bali extension line (the space for the pro-posed Bali extension line will be used as a dedicated bus lane until the light rail line has been opened
The Danjiang Bridge will be a new landmark in northern Taiwan, and will provide an important traffic route. Apart from the bridge's basic transportation function, it will also serve as a safe escape and emergency response route in the event of a major natural disaster. As a consequence, the project's earthquake resistance and wind resistance design, hydrological and hydraulic analysis, anti-corrosion and durability design work have been con-ducted on the basis of the common standards for large bridges around the world, the design and review work has been performed in an effort to ensure that the bridge satisfies the environmental conditions of the bridge site to the highest standards.
Earthquake resistance design
Taiwan is located at the junction of the Eurasian and Philippines Sea plates, and has frequent earthquakes. It is considered one of the world's six most seismically active areas. Apart from performing earthquake response spectrum and earthquake time-history analysis in accordance with the "Highway Bridge Earthquake Resistance Design Standards" issued by the Ministry of Transportation and Communications in June 2009, the earthquake resistance design of the Danjiang Bridge also included "site earthquake hazard analysis" by using fault seismic source parameters and actually recorded earthquake datas for the region within 50km of the project site, with the goal of ensuring that the bridge's earthquake resistance is sufficient to withstand the strongest earthquake under consideration. By adjusting the distribution of structural load, and establishing seismic isolation and energy dis-sipation facilities, including HFR and FVD dampers and friction pendulum bearings, in appropriate locations, the engineers responsible for the bridge's structural system effectively controlled the bridge's vibration frequency, allowing it to withstand earthquakes of over 7th grade, ensuring structural safety, and safeguarding the bridge's function as an escape route following an severe earthquake, as mentioned above.
Location of fault seismic sources
The Danjiang Bridge's bearing system and earthquake loadings
When an earthquake occurs, the forces on the bridge can be divided into horizontal and vertical components, and the horizontal components can be further divided into forces in vehicles' direction of motion and perpendicu-lar to it (i.e., in the longitudinal and transversal directions relative to the bridge), so forces in three directions must be taken into consideration.
Vertical seismic forces
The bearings mounted on the top of the piers can directly transmit vertical forces to the foundation ground, and the deck stays can transmit tensile forces to the main pylon, which transmits the forces to the foundation ground in the form of vertical comprehensive force pressure.
Horizontal seismic forces
Longitudinal: Longitudinal forces are chiefly reduced and transmitted to the foundation ground employing hydrau-lic pressure via 7 HFR dampers mounted on the main pylon and 4 auxiliary FVD dampers mounted on the two ends of the bridge.
Transversal: Steel bearings mounted on the top of interior piers and synthetic rubber pads mounted on the both transversal sides of main pylon leg of Bali side employed friction and collision energy dissipation methods to reduce forces and transmit them to the foundation ground.
Wind tunnel testing
The bridge is situated in a wide-open location at the mouth of the Tamsui River, in northern Taiwan. Generally speaking, the northeast monsoon wind blows in winter, and the southwest monsoon wind blows in summer. Fur-thermore, when typhoons approach in the summertime, heavy winds may blow from the northwest. In order to understand the behavior of the Danjiang Bridge—including flutter, buffeting, and vortex shedding—under different wind speeds and directions, wind tunnel model testing and numerical modeling analysis were performed during the bridge design stage to simulate the bridge's responses when wind forces are present. Tests included a "sec-tional model test" and "full-bridge model test," and test content included bridge stability during the construction stage, stability of the completed bridge, stability of vehicles on the bridge, comfort of pedestrians on the bridge, and assessment of the vibration response of cables under windy conditions, etc. The results of these tests indi-cated that the Danjiang Bridge would not give rise to aerodynamic or aeroelastic instability, and the resulting wind force response data for structural components was included in structural analysis and design, ensuring the safety of the bridge structure.
Independent pylon model test (1:200 scale model)
Hydraulic model testing
The Danjiang Bridge spans the mouth of the Tamsui River. In order to understand the effect of the construction of the bridge's pylon and piers on scouring and silting in the river, as well as the effect of scouring by the river flow on the piers and the nearby riverbed, a small-scale model (1:81 scale) was employed during the erarly de-sign stage for hydraulic model testing, which simulated the scouring of the foundations of the piers under river flow conditions. Testing revealed that the elevation of the pile caps of the pier foundations was lower than that of the current riverbed, which can reduce scouring around the pier foundations, and the final test results indicated that the amount of scouring around the pier foundations will not affect bridge safety.
State of nearby scouring and silting after pylon foundation(seen with no cofferdam) scouring testing
Main pylon localized scouring testing
The results indicated that scouring occurred to a maximum depth of 8m on the Bali side of the main pylon
Full bridge general scouring testing
The results indicated that apart from the main pylon, none of the remaining pier pile caps were exposed above the riverbed.
Detailed assessment of impact on the river
Riverbank scouring and silting analysis: After the construction of the Danjiang Bridge, localized scouring of the riverbank on the Tamsui side will increase slightly (indicated by black dotted lines), but there will be almost no effect on the riverbank of the Bali side. An overall assessment concluded that the bridge will have no impact on scouring and silting at the mouth of the Tamsui River.
Before construction of the Danjiang Bridge
After construction of the Danjiang Bridge
Anti-corrosion and durability measures
The Danjiang Bridge's location at the mouth of the Tamsui River will expose it to severe salt damage. As a con-sequence, the concrete type, mixing ratio, protective coating, thickness of the rebar protection cover, and pre-vention of steel structure corrosion all constitute key aspects of the bridge's durability design.
In accordance with the environmental conditions of the bridge site, the thickness of the rebar protection cover met 120-year service life requirements.
A pure polyurea coating will be applied to the surface of the concrete piers in the river channel from the top of the pile caps to 1m below the water surface, and a general fluorocarbon resin anti-corrosion coating will be applied to the piers from 1m below the water surface to 10m above the surface.
The main rebar elements, stirrups, and crossties in the outermost layer of the main pylon and piers in the river channel will consist of galvanized rebar.
A heavy-duty anti-corrosion coating will be applied to exposed steel parts of the bridge.
Type II Portland cement or IP water-hardening concrete will be used to reduce the probablity of concrete cracking.
The cables, bearings, dampers, and expansion joints are designed to be replaceable, and the steel coating and deck waterproof coating must be maintained and replaced after a certain number of years.
Inspection trusses will be installed to facilitate regular inspection and maintenance. In addition, important structural elements, such as the pylon, cables, steel box girders, and piers P120 and P140, and the natural environment around the bridge site, will proceed 24-hour monitoring.
Monitoring system configuration