The structure submitted is a new house for a young German couple who reside in Cape Town. Having lived in Germany, where floor space is a premium, they were keen to build a very compact house, which indeed the site also required, being only 542sqm. The site is perched on the upper slopes of Tamboerskloof and was accessed through a very tricky pan-handle. The site itself had a severe gradient.
Our client asked us to design a modern house, that took advantage of the wonderful views of Table Mountain and the City Bowl. Precedent images of cantilevered rooms and timber clad steel framed houses on the west coast of America spoke to their brief.
From the outset we were very keen, to look at a steel framed structure. We wanted to use a system that perched or hovered above the sloping terrain, rather than be embedded into the ground (due to the severe slope on the site). This was largely achieved, with only the garages being constructed using conventional construction methods ie bricks and mortar.
Such was the desire to use this system, we actually drew the scheme twice, so as to have it priced both as a concrete and brick structure, and then a structural steel framed structure, with timber and glass panel infills, to prove to the client, that they would not necessarily pay a massive premium for what we believed would ultimately be the best solution, both in terms of construction type and aesthetic outcome.
The structure is a simple exposed structural steel post & beam type. It utilizes a grid system to modularize infill window, wall & roof components. The post and beam structure is laterally braced by mean of the two polished concrete floor slabs. The roof structure is also constructed using structural steel rafters, fixed to stub columns above a 254x 146mm universal ring beam, thus creating a clerestory all round. The steel framed system allowed us to experiment and play with various cantilevered elements, that add to the ‘hovering’ nature of the building.
Generally, the structure uses 254×146 beams to support the exposed concrete floor slabs, and 200×100 IPE columns to support the steel beams. The corner conditions of the envelope were detailed using 150 x150mm equal angles to create a clean negative joint, into which infill window frames and timber cladding can terminate. The upper portion of the 3 storey staircase also uses a 254×146 central stringer to support hidden steel plates that in turn support the cantilevered natural oak treads.
The building’s external walls are infill panels of natural Siberian Larch timber cladding punctuated with aluminium double glazed window and door panels. Generally, both infill types were designed to be full height between floor levels (ie between “I” beams) for simplicity of construction. Special detailing was required in the form of steel flanges to create waterproofing details at the envelops perimeter.
Prior to manufacturing, many design meetings with the steel fabricator, structural engineer, and architect where held, to resolve each structural steel component, how it connected to the adjacent component, and how it connected to non-steel components. Special care was taken to create openings in the universal steel beams (prior to HD galvanizing) to accommodate services in the floor slabs. The construction process was swift due to the simple structural steel frame, which was erected first, with the concrete floor slabs being cast only once the frame was at roof height.
The construction method as described above is not commonly used in the domestic residential environment in Cape Town. Great care had to be taken during construction, as much of the structure was left exposed to create the final aesthetic. To that end, the Project team including the contractor, engineer, and architect, met often in the design development phase to resolve issues that simply do not occur in conventional brick and concrete building methods. Such simple things as linking services through floor/wall junctions where steel I beams exist needed to be thought through and design solutions found.
The project really showed us the great benefits that can be achieved when such a collaborative design process of architect, engineer and builder is undertaken. Working on this structural steel frame building, forced us to consider all aspects and components of the building, prior to manufacture.
|Completion date of steelwork||June 2017|
|Completion date of full project||9th March 2018|
|Tons of structural steel used||unknown|
|Structural profiles used||Universal beams and Universal columns|
|SA content – if this is an export project||All made in SA|
|Cladding Contractor||Team Austin|
|Structural Engineer||ADA Consulting Engineers|
|Main Contractor||Team Austin|
|Steelwork Contractor||Abelia Metal CC|
|Steel Erector||Abelia Metal CC|
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