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Skye Bridge

Introduction


The presentation shall covers the basic preliminary structural perspective of the curvilinear Skye Bridge. Basically, the two buildings with a connecting Skye bridge is the iconic character of the entire development. Crossing over the arterial Anna Salai Road, it connects diagonally opposite office buildings “EVR-Periyar & TNHB-HQ.


Option 1: Simply Supported Skye Bridge


- The Skye Bridge is simply supported on the external bridge piers.


- It is separated from the adjoining towers EVR & TNHB by an expansion gap.


- The deck floor & the roof are supported on 7.8m deep trusses along the outer & inner chords of the bridge.



Option 1: Simply Supported Skye Bridge Challenges


- The Trusses are supported with Spherical Bearings on the bridge pier to facilitate heavy vertical and horizontal loads & its movements. However, due to the plan geometry there is a tendency for outward tilt. Hence, in order to stabilize, cable stays are necessary, and it is suspended from the adjacent buildings in a perpendicular pattern to hold it.


- Since the bridge and buildings are separated from each other with expansion gaps, the floor movements of 3 structures are therefore separated.


- However, cable stays connecting both, the building and the bridge is fixed & hence in the event of building lateral movements, there is a tendency of forces transferring via cable stays. So, the bearings installed under the bridge trusses will stabilize the bridge due to it’s capacity to sustain large rotation about one or more axes.


- There are about 4 to 6 post tensioned cable stays envisaged from each building to carry large forces


- In order to have a separate bridge and building behaviour, the bridge supporting piers are passed though large openings in the basement slabs of the overlapping buildings.


- The opening can be detailed to tackle the water ingress and other service related issues with proper detailing features.


- As the cables are directly connected to the buildings, large lateral forces need to be transferred to the building.


- The bridge construction can only be initiated after completion of the adjacent buildings, thereby it becomes a dependent activity.


- As a result overall construction has to be planned with that perspective.


- The maintenance of the cable stay would be more complex as it is connected to the building, due to non accessibility after office starts functioning.


Tower & Bridge – The Inextricable Structural Behavior


FORCE ESTIMATES

- Significant lateral loads are imposed on the building by the sky bridge through tension cables.


- The lateral loads imposed on the building reach up to 20% of the building base shear.




Preliminary building behavior study:


- TNHB building, due to its non-orthogonal shear walls and concentration of shear walls in south side of the building results in no pure 2nd translation mode.


- TNHB building has a modal time period of around 4 secs, indicating less structural stiffness. The sky bridge behavior can be impacted due to the above points.


Solutions: To increase the building stiffness and reduce drift


- By introduction of shear walls in place of gravity columns, this may impact the functional use of office space or, and;


- By introduction of bracings at perimeter columns at intermittent levels.


Option 2 Pylon system


- In this option, two pylon will support the bridge span uniformly with the cable stays as shown in the image. This option will resolve the complexity of dealing both the buildings and Sky bridge. The buildings and skye bridge will be absolutely independent structures in this case.


- Though with Pylons, the bridge design will be purely cable stayed, however it has more advantages from construction, erection, maintenance and other activities. However, the feasibility of same shall be reviewed from bye-laws front.


- Since skye bridge is separated from the buildings, parallel construction activity can be taken up, thereby leading to an overall faster completion of the project.







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