The Granger Bay Demountable Parking (GBDP) project arose out of the V&A Waterfront’s urgent need for additional parking. The site consisted of an open area on approximately 12 to 15m of fill material with a gravel surface approximately level with the roads around its perimeter. It was selected for this project due to its close proximity to the Victoria Wharf Mall; however, it had been earmarked for a substantial development at some stage in the future, possibly five years’ time. For this reason, the V&A Waterfront Development Team decided to consider a temporary parking solution such that the land could be used for parking in the medium term and then developed as planned in the future.
Initially, the V&A investigated importing a proprietary system. When considering the full project cost, the proprietary system would have cost more than a traditional concrete parking garage of the same size, although it would have had residual value after five years since it could be re-used elsewhere. The V&A Waterfront then approached LH Consulting Engineers to develop an alternative scheme using local technology. Together with the Quantity Surveyor, Union Steel, NMC Construction and Portland Concrete, this scheme was developed to suit the following requirements:
– The structure had to be easily demountable and able to be re-installed elsewhere
– It had to be modular such that the structure – or parts of the structure – could be reinstalled in different configurations
– It had to be sufficiently lightweight such that shallow foundations would be adequate (the fill material was not sufficiently consolidated to support conventional loading on shallow foundations)
– The structure was to provide two levels of structured parking and be designed to receive a third level
– The project had to be designed, developed and completed within a six month period, including obtaining the necessary Council approvals
The cost for the local alternative came in at approximately 60% of the cost of the proprietary system. This option was therefore given the go-ahead from the V&A.
The structure was set out on a 7.5m x 8.5m grid and consisted of a structural steel frame and precast hollow-core floor slabs bolted to the frame. Primary beams consisted of plate girders with a parallel flange channel section for the top flange. The precast floor slabs were designed to span the 7.5m between the primary beams. Curved plate sleeves were inserted into the ends of the hollow-core slabs to enable an angle section to be bolted to the underside of the slab at each end in the factory. Once the panels had been dropped into position on the structural frame, the factory- fitted angles were then bolted to the sides of the channel section top flange. The joints between the precast panels were then grouted to ensure load-sharing between panels and diaphragm action for stability purposes. No topping was required.
The perimeter barriers spanned between columns and provided a threefold function of balustrade, parking barrier and support to the cladding system.
As part of the development process, two full bays were constructed. After inspection by the Design Team, the mock-up was then completely disassembled and reconstructed. This prototype served to demonstrate that the structure could be taken apart and re-installed without difficulty and without damage to the components.
The choice of materials allowed for off-site fabrication, which got underway while the Principal Contractor prepared the platform and constructed the reinforced concrete column bases. The modular design resulted in a high degree of standardisation of components, which enabled a fast- track detailing and fabrication process. With careful planning of the installation sequence, the two levels of structure were fully installed over the 90m x 50m footprint in 49 working days.
To comply with building regulations, the structure was required to provide 30 minutes fire stability. The precast floor slabs inherently met this requirement, but the structural steel required additional protection. This was addressed by means of intumescent paint, which was applied in the factory to save costs and time.
Although this was intended to be a temporary structure, corrosion protection was a concern since the site is approximately 100m from the sea. A high specification paint application with a five-year warranty was chosen over hot-dip galvanising as this was more economical, met the design requirements and saved time. Drilling and fixing to the steelwork by follow-on trades was restricted and closely monitored to ensure that any damage to the paint system was carefully repaired.
The cladding system had not been developed by the time the installation of the structure had commenced on site, so the support system had to be retrofitted. In light of the abovementioned restriction on drilling into the steelwork, it was decided that the cladding system had to be clamped around the paring barriers rather than relying on conventionally bolted brackets.
Despite delays caused by external factors, the structure was completed in time for the end-of-year tourist season and provided a much needed additional 350 parking bays to the V&A Waterfront. When the time comes for re-development of this land, it is intended that the structure will be relocated elsewhere within V&A Waterfront.
The potential for re-use of the structure is exciting from a sustainability point of view. The modular construction and component detailing allows for flexibility in re-use. The structure can be re-used elsewhere in its entirety, in part, in different formats and even for different occupancy classes if a topping and shear connectors are introduced.
|Project Team Role||Company|
|Client/ Developer||V & A Waterfront|
|Structural Engineer||LH Consulting Engineers (PTY) LTD|
|Quantity Surveyor||BTKM Quantity Surveyors|
|Project Manager||NMC (PTY) LTD|
|Main Contractor||NMC (PTY) LTD|
|Steelwork Contractor||UNION STEEL|
|Structural Steel Detailer||UNION STEEL|
|Steel Erector||L&A Steel Erectors|
|Corrosion Protection Paintwork Contractor||Nu Nation Protective Coating|
The Silo District at the V&A Waterfront in Cape Town is an award-winning example of a sustainable mixed-use development. No.5 Silo, a commercial office building, forms part of the larger Silo Precinct Plan, with the prime requirement to increase the client’s rental stock.
No.5 Silo was built on top of a pre-existing basement. The building sits on two panels separated by an expansion joint. The two portions of the building are independent and the two sides of the atrium are linked by jointed steelwork, facades, structures and bridges.
Key structural steel features of the building include the saw-toothed roof, structural framing for high-level extract fans, fire escape steel staircase, bridges, atrium grillages, double volume arcade, steel support frame to sloping facades, wind lobby and pergolas.
The structural frame is primarily concrete with flat PT slabs and stabilised by concrete cores and walls. The longer span arcade roof over the street and the roof over the plant rooms and rooftops are formed from steel beams with sheeting and bondek slab. Timber has been used for lightweight floor construction.
Hot rolled steel sections were used for majority of the steelwork. As slender members portraying lightweight structures with long spans were envisaged, hot rolled steel sections were best suited due to possessing the necessary strength to achieve the design intention. They are also available in a wide variety of standard section sizes in South Africa.
In keeping with the theme of the building (warehouse type and industrial looking), cladding formed a large part of the aesthetics. Cladded roofs spanned between purlins (a cold-formed section). With a variety of cladding systems were used i.e/ timber, rheizink etc. for which hot rolled steel provided the flexibility when it came to fixing into.
The client design brief called for a functional building with a recurring aesthetic theme throughout: ‘fit for purpose’, ‘workman like’ and ‘dockside warehouse’. Expressed materiality and detailing were considered important criteria to fulfil. This has been achieved through the careful choice of materials for the building. Materials consisted of raw concrete block work, unitised glass and tile cladding, rheinzink cladding, sheer glass curtain walling, rough cut timber cladding, exposed structural steel members, structural steel framing and grillages, cobble and flagstone paving, bold ‘warehouse signage’, ‘street lighting’ and industrial sheet cladding. The brief also called for natural ventilation, as far as was deemed feasible.
Vast extract fans appear at high level at both street ends and signify the intention of the consciously chosen language of the design proposed. The sawtooth roof has sliding joints on one side and fixed joints on the other designed to be able to move laterally over the expansion joint. The design for the steelwork was intricate and the Arup engineers worked closely with the architects to coordinate the process effectively. The use of steel was an integral part in meeting the ‘working warehouse aesthetic’ of this project.
As a 6 star Green Star rated building, reducing the total amount of materials used was an important consideration in decreasing the project’s overall impact on the environment.
A reduction in the consumption of structural steel was achieved by an innovative design informed by the architect’s vision of slender steel members and the client’s request to limit costs.
Further reductions in energy consumption of the building was a direct result of the PV panels used. These were incorporated into the structural steel saw tooth roof. The optimal design angle of the PV panels informed the angle of the saw tooth roof, and weight of panels. Wind uplift and spans of the steel roof informed the sizing of steel members which adhered to the design intent.
The use of hot rolled steel allowed for members to be welded and bolted together during the fabrication process. Fabrication was done in a controlled environment where quality was strictly monitored. Prefabrication allowed for large sections of the steel structure to be made up beforehand, transported to site and hoisted in place, leading to decreased construction time.
As the concrete frame of the building had been completed, it was imperative that the prefabricated steel sections and connections be designed to fit within the concrete frame. As built structural surveys were carried out prior to steel manufacture. Connections with sliding joints were vital when fixing in small enclosed spaces and over the expansion joints to allow for lateral movement.
A challenge experienced was working within the constraints of the as built building to erect the steelwork in place. This was the case for the steel bridges, saw tooth roof and steel staircase. The low level of tolerance of the concrete, meant that measurements had to be taken on site before steel was fabricated.
The prefabrication of the steel bridges and steel staircase led to these sections being lifting through the roof and erected in place. As a result closing the building couldn’t be done until the staircase and bridges were in place.
The client had a requirement for economic and cost saving solutions for the building. To achieve this in a structural steel context, cost savings were managed through the use of a paint specification matrix to the steelwork incorporating corrosion protection. This was done instead of hot dip galvanising the steel. The degree of corrosion protection varied according to exposure to the external environment, considering the building is located in close proximity to the harbour. A set of paint specifications were formulated between the architect and structural engineer to meet both the aesthetics and structural requirements of the steelwork.
Additionally, in keeping with the design intention of the building i.e. open spaces and slender structures, all steel members were optimized in structural design to produce the same/ similar section sizes as required by the architect.
The design brief for No.5 Silo had called for an expansive and open footprint to floors together with a highly economic structural system (post tensioned concrete slabs and columns). Given the pre-existing condition of the structural grid already defined from the basement below and the clients request for no transfer beams, the structural system had to work with the floor space whilst remaining within the predefined conditions.
The inheritance of the expansion joint splitting the building into two independent halves had to be considered carefully during design, as numerous steel structures passed over the joint. These steel structures formed the link between the two building halves. Steel joints and connections were specially designed to allow for lateral movement of the structures over the joint.
Satisfaction/ testimony of the client: ‘The No.5 Silo building is very much defined by the central street splitting the 2 floor plates. The steel elements within this space are a fundamental part of its identity – creating a warehouse type aesthetic which sets it apart from other commercial office buildings in the Waterfront’.