And when a beam becomes a shell
The late afternoon winter sun dances off the sloping soffit of the first floor concrete slab. The soft curve of the smooth concrete gradually transitions to shadow at the flat portion of the ceiling. Above the floor is a bedroom and bathroom that transitions into an outdoor oasis of greenery screening a view of the distant rooftops to the neighbouring houses and offering privacy to the space.
The two side walls of the house are nearly 9m apart. When you consider this span and the weight of the building over, including the soil from the green roof, the concrete would need to be close to 500mm thick to span between the walls, even if the floor was precambered.
At the first meeting, the architect, Jarad Grice from Sam Crawford Architects, asked us if there was a solution that was reasonable. He knew that the concrete floor slab would need to be thick to span across the building. And he knew that it would likely need to be greater than the 250mm thick he had indicated on his initial schematic sketches. In order to accommodate the concrete, our initial discussions revolved around keeping the area for the planting tiny, the weight of which Jarad knew would impact the thickness of the structure. Jarad also introduced an upturned concrete beam framing the edge of the garden running centrally across the room. But this beam could not span the whole way across the building as it would cut through the middle of the bedroom.
What Jarad was not to know was that the sloping shape that he had created with the architecture, lent itself perfect to a different type of structural solution. A solution that I had never used before.
The concrete floor that was actually constructed is thin; too thin to span the 9m between the walls. But it is not a simple slab as such, it is a beam, or more technically, a shell. The upper and lower portions of the slab act as the top and bottom flanges of the beam, while the sloping portion of the slab acts as the web.
If we compare the shell of the Hidden Garden House to that of an equivalent concrete beam that consists of a top and bottom flange and a web between the flanges, we can start to see how these two are similar. It was only when we stopped talking about the floor as a one way spanning slab, and started to refer to it as a beam, did we realise how we might make it work.
Using the floor as a beam allowed us to use a much thinner concrete profile.
This shell is similar to the Kimbell Museum by Louis Kahn. At first glance the Kimbell museum may appear to be a vault. However, the continuous cut in the top of the roof, that forms a skylight, would indicate this is not a vault at all; a vault requires an uninterrupted curved shape so that the loads arch in compression through the structure. Cutting a slot anywhere in a vault would cause it to collapse. It is in fact a shell: a type of beam that, rather than spanning from side to side, spans the length of the building; spanning between the walls at each end.
Back to the Hidden Garden House, once the concept of the shell was captured, the garden could be deepened, allowing for a more diverse range of plants. In fact the simple geometry gave the structure enough capacity so that the garden was extended the full width of the building and even extended to the front edge of the roof. Greenery could dangle off the edge of the roof. These plants will one day be visible in the ground floor living area, through the highlight windows. They will offer a link to beautifully landscaped rear garden.
Architect Jarad Grice - Sam Crawford Architects.
Builder TOKI
Civil & Hydraulic Engineer: Partridge
Acoustic Engineer: Acoustic Logic
Heritage Consultant: Damian O’Toole Town Planning
Quantity Surveyor: QS Plus
Landscape Design: Gabrielle Pelletier, SCA
Roof Garden Supplier: Fytogreen Australia
