Conceptual design of cable supported bridge starts with determining the type of the cable supported bridge to be designed and its suitable location.
The location of the cable supported bridge plays an important role in this regard. For instance, in situation where the location of the bridge is expected to experience earthquake, it might be beneficial to take seismic performance into consideration.
Mostly, the type of the cable supported bridge specified is based on factors other than seismic criteria and then seismic design is considered. This might be because, there are not exact techniques or methods which can be employed to compare seismic performance of different types of cable supported bridges and consequently choose the best option.
Seismic design of cable supported bridge is used to determine whether the selected type is practical and suitable or not, in other word it is employed as checking computation.
The conceptual seismic design of a new bridge, which is discussed in the following sections, may be used for seismic retrofit design of existing bridges with the exception that the design in the latter is start with investigation, then bridge diagnose and finally the design.
Finally, it should be said that, seismic retrofit design of old bridge is more difficult compare to new ones because situations are fixed and hence options are fewer in former compared to the latter that has broader options to choose.
Fig.1: Conceptual Planning of Cable Supported Bridge Design
Fig.2: Cable Supported Bridge Conceptual Design
Cable Supported Bridge Conceptual Seismic Design
The conceptual seismic design of cable supported bridge might include the following:
- Determine seismic response feature of cable supported bridge
- Design to avoid collapse
- Design to remove seismic vulnerability of different bridge components
Determine Seismic Response Feature of Cable Supported Bridge
Commonly, the span of cable supported bridges is long and it has specific seismic response feathers includes long vibration period, low damping, complex vibration modes, sensitive to multi-support ground motion, and large expansion joints.
Most cable supported fundamental vibration period is between 2 to 8 second. This not only make the bridge response force considerably small in comparison with short span bridge but also detrimental effect of P-delta will be much greater. The cause of larger P-delta effect in long span cable supported bridge is due to larger deflection of the span.
Damping ratio of cable stayed bridge is between 1-2% and suspension bridge is 1.5 to 2% and these values are well below 5% critical damping used for bridge seismic analysis. So, when the bridge experiences an earthquake, its vibration need long time to disappear.
As far as vibration mode is concerned, cable supported bridge vibration is substantially complex because each component of the bridge has its own vibration period and shape of vibration mode. These will affect each other and consequently the bridge vibration modes will very complex one.
Since the distance between supports of the bridge is quite long, therefore the not only does the time of receiving seismic vibration by each support is different but also soil types could be different and hence vibrations will not be the same.
These will lead to different seismic ground motion response at each supports and eventually the distance between supports will have influence on the bridge seismic response behavior.
Another parameter that affect bridge seismic response behavior is the large expansion joints. There are various sources that lead to initiate movement in bridge structure. For example, temperature fluctuations, seismic forces, and services loads. Therefore, there will be considerable movements in the bridge that needs to be accommodated and dealt with.
So, cable supported bridges need large expansion joints to make rooms for bridge movements. Of course, this will be huge issue that designers have to deal with properly. Rio–Antirrio bridge in Greece can accommodate movements of up to 2.5m under normal conditions and 5m in extremely sever situations.
Finally, because most of cable stayed bridges are constructed in strategic locations and need substantial budget to build, therefore these bridges should be designed in such a way that their life spans will be larger compared to normal bridges.
Design of Cable Supported Bridge to Avoid Collapse
There are number of methods by which bridges are equipped to resist earthquake detrimental and hazardous effects. These techniques may include energy dissipation, multiple articulation, base isolation, improve ductility, redundancy provisions and strengthening. These will be discussed in the following sections:
Energy Dissipation in Cable Supported Bridge
The ability of cable supported bridge structures to resist earthquakes may be increased substantially by providing energy dissipation tools. These devices improve the seismic performance of the bridge structure by absorbing hits or energy generated by earthquake.
The major and most usual and applied kinds of energy dissipation tools or dampers are fluid viscous dampers (Figure-3, Figure-4, and Figure-5), friction damper (Figure-6 and Figure-7), and metallic yielding dampers (Figure-8).
Not only do energy dissipation devices decrease movements but also decline force demand.
Displacements can be decreased by installing large fluid viscous dampers between stiffening trusses and end piers or stiffening trusses and towers whereas small size fluid viscous dampers are fixed at suspended span strategic positions to decrease vibration force.
Fig.3: Viscous Damper Parts for Energy Dissipation in Cable Supported Bridge
Fig.4: Viscous Damper
Fig.5: Viscous Dampers Applied in Rio–Antirrio Bridge in Greece
Fig.6: Friction Damper for Cable Supported Bridge
Fig.7: Friction Damper
Fig.8: Metallic Yielding Damper
Multiple Articulations in Cable Supported Bridge
The need for multiple articulations raised by the incompatibility of seismic performance and wind performance requirements. If the bridge structure is designed to withstand earthquakes, the bridge needs to be flexible in order to be able to resist the seismic forces whereas the structure must be rigid to resist large wind loads.
When multiple articulations are applied, rigid structural members, which join bridge towers to stiffening trusses, are used to resist wind loads and prevent tower and stiffening truss displacement.
The rigid structural elements can resist wind loads but they cannot withstand seismic forces. That is why, when an earthquake shock the bridge, the rigid structural elements will break and damper devices will activate.
Consequently, the structure obtains flexibility and dampers absorb portion of the energy and decline displacement between joined components of the bridge. This strategy has been employed in Rio–Antirrio Bridge in Greece. The bridge resist wind load during normal conditions, and it was tested in 2008 for earthquake and the bridge resist seismic forces successfully.
Figure-9 shows the installed viscous damper used to resist earthquake. The black color material in the figure prevent the dampers from working and consequently the bridge can resist wind loads but in the case of earthquake the black color material indicated in the figure will break and the damper will activate.
Fig.9: Viscous Damper of Rio–Antirrio Cable Supported Bridge
Base Isolation in Cable Supported Bridge
Base isolation considerably improves seismic resistance of cable supported bridges. Frequently, base isolations are accompanied with viscous dampers to decrease detrimental effect of inertia force.
Sliding bearing is the most common type of base isolation employed in cable supported bridge for instance it is employed in Rio–Antirrio bridge and Golden Gate bridge.
Rocking and uplifting of bridge foundation is another type of base isolation that is utilized in number of bridges in the past. Figure-10 shows the foundation of Rio–Antirrio bridge in Greece, layer of grave is laid under the base on which the base can move horizontally.
Fig.10: Foundation of Rio–Antirrio Bridge in Greece Supported by a Layer of Gravel and can Move During Earthquake
Improve Ductility of Cable Supported Bridge Structures
Enhancing the ductility of cable supported bridge components like towers, piers, and pylons is a suitable option. If the components are constructed from steel, then ductility can be increased by providing lateral stiffening.
However, lateral confinement is commonly employed to improve ductility when the bridge component is constructed from concrete.
Redundancy Provisions in Cable Supported Bridge
Redundancy is seriously significant for cable supported bridges because the response of the structure during earthquake cannot be precisely determined and earthquakes have random nature.
Redundancy can be obtained by creating alternate load path. The alternate load path can be achieved by installing restrainers, shear key, and catch blocks. Not only do these devices are cheap but also, they are easy to be designed and fixed on the intended location.
Strengthening of Cable Supported Bridge Superstructure
This method is used for superstructure components of cable supported bridges. It is employed for the connections of bridge elements that carry most loads need to be designed as a capacity protector of the element.
Design to Remove Seismic Vulnerability of Different Bridge Components
Cable supported bridge are composed of various components. Some of these parts possess seismic vulnerabilities, such as, shaft of the tower may buckle during earthquakes, connections and expansion joints may damage when the movement of bridge component exceed their capacity, and foundation of the bridge might suffer because of soil liquefaction.
Therefore, seismic vulnerability of components of cable supported bridge need to be considered at conceptual design stage. Different design and materials could be considered to choose the best and suitable option.