The Midax House Zinkwazi Beach project entailed the addition of a second level to an existing house situated on the beachfront.
Founding conditions were basically beach sand and the addition, therefore, had serious limitations in structural weight. In addition, the architecture required clear view lines, large open plan areas and quirky overhanging rooms, all which required large spans and cantilever structures. The design had to be exactly matched to the existing ground floor brickwork structure.
This was achieved by using a duplex coated structural steel skeleton structure, lightweight roof, and floor, as well as some composite material cladding.
The following aspects of the project make this an exceptional project and use of structural steel:
- The structural steel skeleton and lightweight cladding allowed acceptable foundation loading and hence, in essence, made it possible to affect the build.
- The structural steel allowed the key architectural elements of open plan areas, uninterrupted views, quirky overhanging rooms, and modern lines to be built.
- The steel structure saved considerable construction time and the use of CAD technology and modern laser measurement techniques resulted in a very successful build on to a complex shape existing structure.
- The property is situated in an environmentally sensitive area (within 100m of the highwater mark) and the steel construction allowed for a clean and neat build process with no damage to protected milkwood trees.
- The structural steel skeleton allowed for proper services planning in a “hollow floor” and also for the integration of add-ons such as facia mountings, gutters, and glass balustrades.
Midax Investments Pty Ltd acquired a beachfront property known as Magai Drive 45 in Zinkwazi Beach in June 2013.
At the time the 1970’s designed house was somewhat run down, but the location right on the beach and the sea views offered warranted the investment.
The plan at the time was to add onto the house so as to create a beach house where a 3 generation family could holiday together. The property is however located within the environmentally sensitive area within 100m of the high water mark. As such footprint extensions were, at the time, limited to 50sq meters. This forced planning to consider the addition of a second level. In addition, the existing cement tile roof was very dilapidated and as it was considered to be unsafe, the plan for replacing the very heavy roof (some 20tonnes) with a second level living area became very appealing.
Structural engineer Rob Young who, at the time was involved in another Zinkwazi project (the 2015 winner) was approached to act as consulting engineer on the re-build of 45 Magai. Following soil testing and inspection of existing foundations, Rob advised that it would be possible to add a second level provided that the design stayed within 25-30 odd tonnes of weight on the existing foundations.
Architect David Mealin, a Zinkwazi resident, was appointed to design the largely “new” house.
He produced a very modern concept which maximised the views, contained large open plan living areas and some quirky features such as overhanging “glass box” bathrooms. He also incorporated the client’s wish for a large veranda at ground level, the roof of which doubled as a large (100sqm) deck for the upper levels.
The structural engineers were therefore faced with the following challenges:
- Engineering a reasonably lightweight structure which would allow the uninterrupted sea views, large open plan areas and quirky design elements
- Engineering a very corrosion resistant structure capable of withstanding the highly corrosive KZN North coast environment
- Fitting the structure to a fairly complex existing ground floor wall layout.
Rob Young suggested a steel structure with a light-weight wooden floor and light-weight aluminium roof for the new upper level.
At this point, it must be pointed out that Piet Coetzer, the director of Midax Investments, is also a major shareholder in the Structa Group of Companies, which is one of the larger manufacturers of steel structures in South Africa. Piet has often advocated the use of more steel in buildings in South Africa and here he was presented with an opportunity to practice what he had been preaching. Being a structural engineer himself he duly accepted the challenge and through Structa Konsult, he participated in the design of the staircase and bathroom boxes, while Rob Young designed the primary structures.
In essence, the structure consisted of the following elements:
- A concrete ring beam cast onto the existing ground floor walls at the rear of the house
- Fabricated girders spanning window openings on the seaward side of the building and serving as the seaward portion of the ring beam
- An array of I-beam floor beams spanning from the concrete ring beam (where they are anchored), cantilevering over the forward fabricated steel girders to form the deck
- Roof support columns
- I-beam front and rear roof carrier beams
- Wing style roof consisting of pointed I section trusses and lip channel purlins
- Thin-skinned box girder 3CR12 sections forming overhanging bathroom structures
- GRP “sandwich” side walls closing off overhanging bathroom boxes
- A “floating” staircase
- A mezzanine portion of the floor suspended from the roof to give clear view lines In the entrance
Most of the seaward fabricated beams were not clad externally so as to express the nature of the structure. Where cladding was used the following materials were employed:
- Plasterboard on internal surfaces
- Brick walls on the rear wall
- Brick infill on some seaward fabricated girders
- Nutec board
The structure was detailed using the TEKLA system. This was performed by C.I.S Engineering (a Structa Group subsidiary), who was appointed as manufacturer. TEKLA details were used as direct input for plasma cutting and beam cutting and drilling.
In order to achieve an “exact” fit to the existing ground floor walls, these were measured up using sophisticated laser measurements (performed by Richard Logan surveyors). The layout model developed from the laser measurements was imported into TEKLA and the structural steel layout superimposed to ensure a proper fit up. An anchor bolt drilling template was developed on the same layout.
The TEKLA model was also used to fully integrate the building, structure, and services. Where possible the structural elements were modified to incorporate add-ons such as:
- Facia mountings
- Service channels
- Gutter recesses
- GRP sandwich mountings
- Channels for the deck and mezzanine frameless glass balustrades
Corrosion protection was achieved by employing a duplex coating of hot dip galvanising and a two-part epoxy paint.
The first component to arrive on site was the anchor bolt drilling jig. This was assembled and put into place on the existing walls and concrete ring beam. Key anchor bolt positions were marked and drilled, thus ensuring proper fit-up of the steel structure.
Construction of the steel structure and logistics had to be carefully planned and synthesized with mobile crane presence on site which had to be minimised (55t mobile crane with 30m reach). The structure was delivered in 3 loads from Gauteng. The loads carried items prioritised to suit the build.
The build was subcontracted to WPM Construction and completed within 14 days of the first steel arriving on site. In total only 3 days of mobile crane presence were required. The long reach of the 55-ton crane allowed sub-assemblies to be lifted in position without damaging any of the protected trees on the site.
From this point onwards the conventional build proceeded up to end March 2016.