The requirement for a pedestrian bridge link between two office buildings was borne out of the need of Allan Gray’s expanding workforce as they out-grow their head office capacity at No.1 Silo building in the V&A Waterfront.
Allan Gray and the V&A Waterfront together with the architect and engineers developed the idea of a linking bridge between the No.1 Silo building and the adjacent Clock Tower building to allow the internal flow of employees between the buildings.
Not far away from the site of the bridge is the defunct Collier Jetty that has a steel gantry structure along its length, historically used to convey coal and later grain. The gantry is constructed from conventional bolted angle trusses – the inspiration behind the truss form of the bridge with matching diagonal angle sections.
The brief was thus to design and construct a bridge that would allow people movement over the 12,5m span. While it is a relatively short distance the challenge lay in designing a structure that would deal with the movement between the two buildings that are built on separate floor plates by catering for differential lateral and horizontal movements of up to 20mm.
The Allan Gray building has a fully glazed double-skin façade supported off long cantilever fins which are sensitive to additional loads. The bridge, therefore, had to be fixed on one side only, and cantilever without physically touching the glazed façade. As such columns were introduced to support the bridge at the glazed façade allowing unimpeded movement against it. The two columns, 219mm CHS, are braced by 254x146mm universal beams forming a ‘portal frame’ on which the bridge rests and on the other side, it is bolted to the reinforced concrete frame of the Clock Tower building by way of a shear key detail.
To maintain the view corridor between the two buildings, toward the working harbour, the architects motivated for a crossing at second-floor level. Limited space between the buildings, the delicate glazed façade and the limited crane load that could be applied on the podium level between the buildings meant the structure could not be erected in its entirety and craned into position – it needed to an on-site assembly, erected on a scaffold deck.
A rectangular girder comprising 200x100mm RHS sections was designed as the most efficient steel structure to span from the Clock Tower building to the ‘portal frame’ and cantilever a further 1,7m to the Allan Gray floor plate. A spliced connection along the length of the top and bottom chords created parts that could be handled on site. Rigidity is provided by expressed diagonal bracing visible externally as a structural ‘exo-skeleton’, painted red.
The design team devised connection details between the various parts that expressed the junction of the steel members, in keeping with the industrial, maritime aesthetic. 100x100mm equal angles bolted to gusset plates brace the vertical SHS posts on the sides of the bridge. 100x100mm SHS brace the underside and the roof level, expressed externally on the soffit and internally below the ceiling finish. Spigot connector splice details were required on the bracing diagonals to cater for the erection sequence.
Where the bridge passes through the double skin façade a flexible EPDM membrane allows for the required movement but also a weather seal. The membrane is protected by overlapping aluminium flashings.
The bridge links offices on two different floor levels – this is achieved by a sloping timber floor within the girder structure. Steel cleats on stub columns welded to beams along the length of the bridge support timber joists at varying heights.
Side wall cladding is Snap Lock profile in Armour Grey colour, selected for its broad pan and narrow flutes with wide rib spacing. Set behind the diagonal bracing, the industrial aspect of vertical sheet cladding further ties the bridge to its maritime setting. Snap Lock is also used for the roof sheeting and in conjunction with a stepped aluminium flashing detail it creates a crisp silhouette of the bridge spanning between the two buildings.
|Completion date of steelwork||05 / 09 / 2018|
|Completion date of full project||14 / 12 / 2018|
|Tons of structural steel used||8.1 tons|
|Structural profiles used||RHS, SHS, CFLC, CHS|
|Completion date of cladding||10 / 10 / 2018|
|Cladding profile/ type used||SnapLock|
|Cladding area/ coverage and tonnage||48m²|
|Nominator||Loudon Perry Anderson Architects|
|Client/ Developer||V&A Waterfront Holdings (Pty) Ltd|
|Architect||Loudon Perry Anderson Architects|
|Structural Engineer||Sutherland Engineers|
|Facade Engineer||Arup Engineers|
|Services Engineer||Solution Station|
|HVAC Engineer||Arup Engineers|
|Quantity Surveyor||MLC Group|
|Project Manager||Principle Agent Architect|
|Main Contractor||R+N Master Builders (Pty) Ltd|
|Steelwork Contractor||Prokon Services (Pty) Ltd|
|Steel Erector||Prokon Services (Pty) Ltd|
|Cladding Manufacturer||Bluescope Steel|
|Cladding Supplier||Youngman Roofing|
|Cladding Contractor||Metro Roofing|