The Blue Downs swimming pool was developed in response to the City of Cape Town’s wish to provide a high-quality indoor public swimming pool to serve the Blue Downs community. It is intended that the pool should serve as both a general community facility as well a sporting facility for use during swimming competitions.
The design of the structural steel roof comprises a main roof over the swimming pool area, as well as adjacent subsidiary roofs over two separate public seating areas, an administration block and a lifeguard tower. The requirements of the architectural design were furthermore that the main roof should be supported only at the four corners, standing 35 metres apart on circular concrete water storage ‘silos’ (the silos are for storing rainwater run-off from the roof for use in the pool). Due to the architectural requirements and the large spans involved, it was considered that no other structural medium other than structural steel would be feasible.
The original architectural design called for a rounded double barrel-vault roof over the main swimming pool area, in accordance with which a thin shell-type roof structure was designed. Due to financial constraints and difficulty in engaging suit-able local steel fabricators prepared to carry out the work, which required extensive use of curved members, the design was subsequently revised during construction to the current pyramid shape. In carrying out the re-design, the new design was constrained to an extent by the configuration of the supporting reinforced concrete structure already constructed.
Because of the highly corrosive swimming pool environment, a duplex system of corrosion protection consisting of both hot-dip galvanising and painting was adopted. As the components of the toblerone trusses were too large for the galvanising bed, zinc metal spraying was used for these elements in lieu of hot dip galvanising. Inspection and testing of the zinc spray application was carried out by the Hot Dip Galvanizers Association in order to ensure compliance with the speci-fications.
The main roof as per the revised design and as constructed comprises the follow-ing main structural elements: Pitched ‘toblerone’-type trusses forming the four corners of the pyramid shape, edge girders, and sloping secondary trusses.
The toblerone trusses are the primary structural elements, spanning diagonally across the pool between the supporting silos, and intersecting at the roof apex. The trusses work in combined bending and axial loading and rely on the lateral resist-ance provided by the four support points. The trusses support the ends of the purlins, the ends of the secondary trusses, and the translucent sheeting forming the facetted corners of the roof. Because of the potential for buckling in the slender axially-loaded trusses, diagonal bracing was provided in the plane of the roof to provide lateral stability.
The edge girders are placed along the perimeter of the main roof. Besides support-ing the side cladding and the outer sections of the roof, the edge girders assist in resisting the lateral support reactions from the sloping toblerone trusses. The edge girders on two sides of the structure also support the top edges of the subsidiary roofs over the seating areas.
The secondary trusses support the roof purlins, and are in turn supported at their top ends by the toblerone trusses and at their lower ends by the edge girders. In addition to supporting the roof loads, the secondary trusses are also used to provide lateral support to the chords of the edge girders.
The architectural requirements were for the structural steelwork to be exposed and to make use as far as possible of circular tubular members in order to be aestheti-cally pleasing. All main structural elements with the exception of purlins and sheeting rails, and including all knee braces and diagonal bracing, have therefore been fabricated using circular hollow sections. With the numerous intersecting members, welded connections between intersecting tubular members were in some cases rather complex, requiring careful fabrication.
A number of technical challenges had to be overcome in both the design and the erection of the roof structure. Due to the large span of the main roof, vertical midspan deflections of almost 60mm could be expected in the four edge girders. The interface between the main roof and the side roofs therefore had to be designed to accommodate this relative movement. In the case of the roofs over the administration building and lifeguard tower, this was accomplished by separating the side roofs from the main roof, and accommodating the movement by provision of sliding flashing joints in the cladding.
The erection sequence was established in consultations between the engineer and the steel fabricator, taking cognisance of the requirements of the design. The edge girders and toblerone trusses intersect at common corner elements, and these elements had to be installed at the beginning of the erection process.
The corner elements had to be accurately placed in order to ensure proper fitting of the various trusses and girders. Because the toblerone trusses rely on the edge girders to withstand the lateral support reactions, the four edge girders had to be erected beforehand. This meant that the toblerone trusses had to be installed while working over the edge girders. One of the toblerone trusses was installed, remaining temporarily propped until the intersecting truss could be installed to provide stability.
Once these main members were in place, the secondary trusses and other structural elements such as bracing members and purlins could be installed. Because the design allows little tolerance for dimensional discrepancies, a high level of input was required by the steel fabricator in order to achieve the required accuracies.
|Developer/ Owner:||City of Cape Town|
|Structural Engineer:||Bergstan South Africa|
|Quantity Surveyor:||LWA Quantity Surveyors
(R/A Waterson & Hoosai cc)
|Project Manager:||ARG Design|
|Main Contractor:||Tempani Construction (Pty) Ltd|
|Steelwork Contractor:||Mazor Steel (Pty) Ltd|