What is the purpose of the structure/ project?

 The V&A Grain Silo was built in the 1920s to store grain for export out of South Africa. This industrial heritage complex was rejuvenated as the central feature of the world class green Silo District development. Heatherwick Studio was engaged to conceptualise the redevelopment that is now occupied by The Zeitz Museum of Contemporary African Art (MOCAA) and The Silo Hotel.

The most noticeable and significant aspect of the use of steelwork on the project are the 82 pillow windows on the building. These are installed to the rooms and restaurant of The Silo Hotel and on the top floor of the Zeitz MOCAA. The windows on the restaurant level of the hotel are 3.8m wide x 5m high and the windows to the hotel rooms are 3.5m wide and range in height 5m – 3.8m.

On the top floor of the Zeitz MOCAA the windows are 4.9m wide x 5m high and include corner windows consisting of two halves, 3.4m wide x 5m high, which are bolted together and sealed on site. Other significant use of steelwork in the façade packages include:

Zig-zag windows: Folded glass windows built into the silo walls at the museum ground floor, supported by steel flat plate structure.

Trafficable skylights: 12 trafficable glass panels supported by steel structure to the top of the silos over the museum atrium

Skylight: Glass skylight between the  silo building and elevator building supported by steel structure

What was the brief to the architect?

 Heatherwick Studio’s brief was to give new life to the redundant industrial building, repurposing it into something new for the waterfront. Following the studio’s suggestion of installing convex windows into the upper storeys of the building to transform it into a glowing beacon for the waterfront and afar, Arup further developed the concept of the pillowed windows.

Was the project envisaged in steel from the start? If not – why was it built in steel in the end?

For the pillow windows alternatives to steel were explored, such as aluminium. Steel was selected because of:

The capacity of local industry to carry out this work to the required tolerances

Robustness of steel welding, which is not subject to reduced strength in the heat affected zone as is the case with aluminium.

Cost effective for the complex geometry.

Give a brief description of the structural framing. What type of sections were used (e.g. hollow, cellular, I beams etc) and why?  

The pillow windows are fabricated from flat steel plates 50 x 16 that are welded to one another to form the bulging geometry of the pillow and welded to the 160 x 80  rectangular hollow section (RHS) forming the perimeter frame. The choice of flat bars / plate was to minimise the appearance of the framing from inside and maximise the views through the window. The frames are positioned at each facet between the glass panels, with the depth of the plate orientated toward the room and roughly perpendicular to the bulging geometry. Aluminium glazing profiles where mechanically fixed to tapped holes on the outer edge of the flat bars / plates. The flat bars act as domed arching structure for wind loads carrying axial loads and bending moments. The perimeter RHS provided additional width for the perimeter weather proofing and structural strength for transferring the framing wind loads to the four steel brackets used for fixing the window to the concrete structure. The perimeter frame also included a temporary hoisting bracket bolted to the frame during hoisting.

The window includes double glazed triangular panes, structural silicone bonded to the aluminium glazing profile  on the outside face of the steel frames. These window panels were installed in the factory and the windows were transported and installed fully glazed and sealed.

Were there any challenges in the fabrication of the project from the engineer’s design – if yes, please tell?

Achieving the pillow window geometry was the most significant challenge. The steelwork geometry was defined in detail by Heatherwick Studio and Arup using parametric 3D modelling techniques. This allowed the fabrication aspects, such as orientation of framing and alignment of framing at joints mitre lines to be established in a 3d model developed by Arup, while allowing the geometric parameters, such as the geometry of the bulge, framing positions and window size to be fully defined by Heatherwick Studio.

The parametric model was used to generate a 3D model of each of the five window frames, that included the required member size and orientation that was provided to Mazor, the steelwork fabricator, to produce their steelwork shop drawings.

Building this complex geometry was a challenge that was tackled by Mazor through production of steel jigs to define the geometry of each of the window frames. The steel members were assembled into the jig and tack welded into place, removed from the jig and then the welding completed. Careful planning was required in fabrication of the jig and planning of the welding to ensure that all welds could be accessed and welded after the framing was tack welded together. To achieve the neat appearance at the joints welding splatter at these positions was ground smooth and the joints body filled to achieve a high level of quality at these nodes. Mazor had a specialist team responsible only on dressing and shaping each of these nodes on all the windows.

Transporting and installing was another challenge as the pillows were made and glazed as one assembly in a factory (up to 5074mm x 5022mm in size) and transported to site complete. Careful handling was required to ensure that the glass was not damaged in the process. With the window fitting tightly between the concrete beams and columns and inside of the concrete, façade installation was challenging. To reduce the risks it was desirable that the position of the windows during hoisting was similar to installation position of the window and that the hoisting cables did not clash with the concrete structure. To achieve this the design incorporated a temporary lifting bracket, bolted to the perimeter frame,  that aligned  the centre of gravity of the glazed window with the lifting cables, which also positioned the cables outside the concrete face.

What is special/ unusual/ innovative/ aesthetic about the steelwork/cladding in this project?

The use of parametric 3D modelling techniques that respected the detail fabrication considerations for the steelwork assembly and also enabled the architect to shape the desired geometry.

Complex geometry achieved with steelwork fabrication.

Quality of finishing achieved particularly at the nodes as a result of the 3D modelling considerations and the finish quality achieved by the contractor.

Rejuvenating the Grain Silo in such an architecturally astonishing manner has created a unique centrepiece for the Silo District.

How did the project team work together (e.g contractor involved early, challenges/ ease of communication etc.)

Close collaboration with the architect was necessary to realise technically challenging design intent in a pragmatic and buildable manner. Key to this collaboration was combining Arup’s deep design knowledge with market leading parametric capability to create a model that allowed the architects to drive the aesthetic resolution within a technically feasible framework.

Understanding the geometry to high level of detail before tendering ensured that the desired outcome. Communicating the design intent to the contractor and reviewing the contractor’s 3D shop drawing model shortened the shop drawing review phase and allowed for comparison of the design intent model and shop drawings model a 3D virtual environment rather than on 2D drawings, lessening the possibility of errors.

Tons of structural steel used Pillow windows: 82 windows, 5 different types total 63.8 tons

Zig-zag windows: 3.1 tons

Trafficable skylights: 9 units 1.5 tons each = 13.5 tons

Skylight: 2.2 tons

Structural profiles used Pillow windows: 160×80 RHS & 50×16 plate

Zig-zag windows: Flat plate 150×12 & 120×12

Trafficable skylights: 200×100 RHS / 102x203x23 T / 200 PFC

Skylight: 230x133x25 UB

Project Team:

Project Team Role Company
Nominator Arup (Pty) Ltd
Client/ Developer V&A Waterfront
Architect Heatherwick Studio
Structural engineer for façade steelwork in this entry Arup (Pty) Ltd
Quantity Surveyor MLC
Project Manager Mace
Main Contractor WBHO
Steelwork Contractor & erector (pillows, zigzag windows) Mazor
Steel contractor & erector (skylights) Mazor
Cladding Manufacturer Mazor
Cladding Supplier Mazor
Cladding Contractor Mazor
Photographer, Photo competition Arup (Pty) Ltd
Photographer, Other submitted images Arup (Pty) Ltd

 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.