The V&A Waterfront (V&A), situated in South Africa’s oldest working harbour, is a mixed-use development for both residential and commercial, offering visitors from near and far a world-class experience when it comes to entertainment, shopping, dining and accommodation.
The V&A already had a swing bridge that was well-used and well-loved. It was an efficient structure that opened and closed up to 60 times a day, carrying up to 2.4 million people per year. It was, however, 22 years old: a lifetime in the context of the V&A and the 2 m wide walkway, which once seemed appropriate, could no longer cope with the rising numbers of pedestrians.
The team was therefore commissioned to design a new wider bridge
The engineering brief
The challenge set by the client was:
- The new bridge had to be equally quick and efficient, effective and reliable as the existing;
- The construction cost had to stay within a tight budget; and
- The works had to limit the disruption to the public, the V&A and the environment.
Both steel and FRP were considered at the early stages of the project as the only materials that were light enough to limit the loads on the moving mechanical parts and to limit the foundation size. However, as the design progressed the use steel was the obvious choice. Its advantages were:
- Its strength enabled the creation of a stiff yet slender pylon as well as a relatively shallow central spine beam.
- The material is robust and can withstand some impact.
- It created a relatively light weight structure that limited the power needed to move the bridge.
- The local expertise to fabricate the bridge was readily available in Cape Town.
- It enabled the bridge to befabricated offsite and easily assembled on site and lifted into place.
- It gave great opportunity to sculpt a beautiful structure.
The new swing bridge has a span of 40 m. The deck is cable-stayed with a single plane of 4 locked coil cables connecting to a central, upstand spine beam. The spine beam is a 500 mm wide fabricated box with a total depth of 800 mm, but only 470 mm protrudes above the top of the deck. The reclining pylon is in the continuity of the main central beam and its stiffness transfers the cable loads into the piled substructure. The steel with timber deck is rotated on a slewing bearing, which is stressed down onto a doughnut-shaped pile cap by 34 vertical Freyssibars.
The steel deck comprises cantilevering cross beams that are fabricated I-sections; a longitudinal edge beam that is a triangular closed section; and bracing members that are standard angle and T-sections.
The end beam houses the bridge wheel assembly and is a box section to resist the loads at the deck tip.
The pylon is a fabricated box that has been sculpted to provide strength and stiffness where required. The stay cable anchorages are discretely housed within the top of the pylon with an access panel at the back to allow for stressing and inspection.
This was a very challenging build. The bridge is over 40m long and 3m wide with the mast section extending over 10m into the air. Once completed it weighed more than 40T. In the workshop the eccentric shapes of the individual sections (some as big as 16T) created big challenges with regards to the moving and rotating for the welding and fabrication.
One of the project objectives was to limit disruption to the V&A. Hence, the bridge was assembled on a nearby jetty. Once completed the bridge was carefully craned onto a barge and towed to its final position. The bridge was then lifted off the barge and mounted onto the slewing bearing. It was an amazing process that took 2 days and careful co-ordination and it was a first for many to see a completed bridge sail away.
The interface between the bridge and the circular slew bearing was a critical joint that required very tight flatness and dimensional tolerances. If not achieved the bearing’s working life might be reduced or worse the fit-up with the bearing might have been compromised. The bridge 3.5 m diameter ring beam that connects it to the bearing was fabricated from heavy plate sections to resist the forces needed to prestress it down onto the bearing. The dimensional control required in fabricating this element was a significant challenge and required all the skills of the welding team.
The architectural intent was for the pylon to be a continuation of the central upstand beam. As such, it is a very slender element. The fit up of the stiffeners and diaphragms as well as the various fabricated pieces of the pylon, ring beam, spine beam and deck elements had to be carefully considered to ensure that the required welding operations were practical. The sequence of closing the hollow box sections and of jointing them on-site also presented several conceptual problems. The 3-dimensional Revit model developed by the design team was essential for testing this aspect of the design. The modelling work done undoubtedly saved time in the fabrication yard and on-site.
Impressive technical aspects of this project
A moving bridge project is unique in itself and for the design team the greatest technical achievement was the combination and integration of mechanical, structural, marine, geotechnical, construction and architectural expertise to create a simple but beautiful structure that moves. The use of a slew bearing was a technical innovation not seen before in Africa and the design team had to undertake detailed research and modelling to validate it could be used for a bridge of this type.
Having to manufacture the bridge on one side of the harbour, ship to across the harbour and offload it onto the bearing on the other side of the harbour introduced elements that we never normally deal with. Ensuring the bridge didn’t fall off the barge and into the sea was always in the back of our minds.
How this project demonstrates the benefit of steel as a material
As mentioned above steel offered the opportunity to create a relatively lightweight and slender structure that could be sculpted into a single attractive form. It’s proven robustness and strength is unrivalled.
The bridge was fabricated into a single sculpted form. It is not a collection of parts; deck, mast, beam. It is rather a bridge with its own identity that can be recognized as a landmark. The use of steelwork has enabled architecture and structure are integrated together. The curves and sculpting of the various box elements create a beautiful bridge that seemingly rotates and supports itself by unseen parts.
How the project team worked together
The core design team: SMEC South Africa, Stefanutti Stocks, Eadon Consulting and the V&A Waterfront came together 4 and a half years before the bridge was finally open to the public. The early contractor involvement, the dedication from the client and the passion from all the designers involved throughout the design and construction processes ensured the success of the project. The vision to create something beautiful for Cape Town to be proud of was shared by all those involved and fuelled good working relationships.
Project motivation editorials are provided by the project nominator. If any technical details, company names or product names are incorrect, please notify the SAISC so that the error can be corrected.
|Physical address of the project Street Address Town Province||V&A Waterfront, New Swing Bridge (connecting the Pierhead Precinct with the Clock Tower Precinct)|
|Google Maps link|
|Completion date of steelwork||August 2019|
|Completion date of full project||August 2019|
|Tonnage and steel profiles used||46.3 tonnes, Ex Plate 25 – 100mm|
|Nominator||Anchor Steel Projects|
|Client/ Developer||Waterfront Properties|
|Architect||Coasite Craft of Architecture|
|Structural Engineer||SMEC Engineers|
|Quantity Surveyor||SMEC Engineers|
|Project Manager||Steffanutti Stocks Coastal|
|Main Contractor||Steffanutti Stocks Coastal|
|Steelwork Contractor||Anchor Steel Projects|
|Steel Erector / Project Coordinator||Anchor Steel Projects|
|Corrosion Protection||MRH Shot Blasting and Corrosion Control|
|Paintwork Contractor||MRH Shot Blasting and Corrosion Control|
|Photographer, Photo competition||Anchor Steel Projects|
|Photographer, Other submitted images||Anchor Steel Projects|
If you were a part of this project, and your company details are incorrect or missing – please notify the SAISC so that the error can be corrected.