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’.