We are going to take you on two journeys. First we are going to walk with you through Annandale House by Welsh + Major, taking you from the street, through the front door of the old cottage, into the link, down into the new living space, and then through the garden to the rear garage. We hope that as you come with us you will get some understanding of why this building took out the top residential architectural prize in NSW in 2017 and why it is an important structural engineering story.
Annandale House was not destined to be an architectural masterpiece, but it became one through a truly collaborative design process between architect and structural engineer. And this is the second journey - we believe that, as structural engineers, our jobs are not limited to just providing structural engineering advice. Our contribution should start by understanding the architectural vision. Our contribution should not only support this vision, but should enhance it. Our contribution should also be about empowering the architect, giving them - overloading them perhaps - information about the structure and the options available so that they are able to intelligently contribute to the decisions about the structure. We also believe this enhances a respect for the engineering profession within an industry where respect is often lacking.
This building renovation started out as a timber framed and plasterboard clad, two story extension. And the reason this house started with this premise, along with most houses on a tight budget, is because they are economical and easy to build. The building industry is filled with conservative preconceptions about how the building structure should be built. It is one of our jobs to question that; break it down and see where the truth is. Annandale House now has no timber structure and almost no cladding internally. This house underwent a radical rethink about what could be done to achieve architectural and structural goals.
This house is not large. It is less than 200m2, and less than 5m wide. Annandale House is tiny and innocuous as you enter off the street. The single story weatherboard cottage that faces the street is untouched, only the timber cladding has been painted dark grey. The footings were reinforced as they had failed, but otherwise the structure was perfect, constructed of hardwood framing on brick walls built directly off the bedrock. As you enter the front door and walk through the old wooden floored corridor and past the first two rooms, you may start to wonder where the award winning house has been hidden.
Continue along the corridor and at the end you step out of the old cottage and into the new extension. Pause here for a moment and let your eyes adjust to the natural light spilling through the space. This is the point where you fully experience the full height of the two story building. You are on a bridge with a large void to the right, leading via an open stair, down to the lower level. And now that your eyes have adjusted, look up through the void to the sky. Try to determine now if you are inside or outside - in fact, you are in both – a large retractable skylight opens this space to the outside. The light spills down into the lower levels and you are drawn down the light ribbon stairs. As you descend the stairs you see the concrete extend all the way to the back of the house, to the concrete framing over the rear door and right the way through to the end of the garden. Concrete is the constant motif through this house.
David Welsh from Welsh + Major Architects always had a vision of this open void space - a skylight within the void, and this space being openable to the sky. What he did not imagine was the structure and the cladding required to deal with the durability issues to make this indoor/outdoor space work. Our first of many collective workshops focused on the structural solution for this architectural space. We challenged the presumed notion of timber framing and introduced the option of a concrete structure. And it is this off-form exposed concrete that you now see when you are in this space. You see the concrete bridge leading into the bedrooms, and you see the concrete stair leading down to the lower level polished concrete slab. Using concrete solved the main issue of durability with this indoor/outdoor space. The concrete needed no external cladding to protect it, and could easily be made durable for the external environment. There are other advantages with using concrete but it also has consequences too. It was going to be more expensive, and we would need to find a builder who could build it. Off form concrete is not for the faint hearted. Being a natural material, concrete lives its own life - you pour it in formwork and wait patiently for the formwork to be removed to see what the concrete has become for you. It is a personal experience and is often not what you expect. And that is its beauty. The client must understand that process of revealing, embrace it, and accept it.
As we descend the stairs and approach the bottom, the stairs take a 180 degree turn. At this point your view to the back of the house and yard is blocked by a wall. It appears that this turn has occurred to accommodate this wall. Curiously, in a lower section of this wall is a small section of exposed bedrock. The architect has used this wall to guide us into this space. It is a relaxing and calm space with a small trickle of water running down the face of the expose rock and into a stainless steel grate. In this area there is also a subterranean space underneath the front timber cottage with a ceiling of the timber joists and floor boards of the old timber cottage. Had we not turned 180 degrees we would have completely missed these spaces. The walls at the bottom of the stair are used to define a small living area and are lightly rendered blockwork where you can still see the joints as dark lines. They tell you the story about what they are. These walls are key structural elements. The original architectural concepts did not have these walls protruding into the space.
Passing through this small living area you enter into the unseasonably cool galley kitchen. Above the kitchen is a garden roof, extending the entire length of the kitchen. It is this garden, along with the mass of the concrete slab it grows on, that helps to keep this space an almost constant temperature all year round. The garden offers protection from the summer sun and insulation from the winter cold. Although it is possible to construct these green roofs on a light timber frame, it is far more durable to construct using concrete. The thermal mass of the concrete also helps control temperature of the space. And all the ceilings are off form concrete. These were all additional contributing factor to the use of concrete.
The largest structural challenge with these narrow sites is lateral stability. Without any return walls through the site it is challenging to keep the building stable from lateral wind and earthquake loads. Through another series of workshops with the architect we discussed introducing steel portal frames, introducing large brick return walls, and we even discussed concrete columns combined with concrete beams to form a rigid concrete portal frame. Initially these were all dismissed by the architect, particularly as the detailing of these were difficult and David challenged us to think of other options. We never turn away from a challenge and eventually formulated an alternative solution.
We proposed short heavily reinforced block walls to provide stability. We were sympathetic to the fact that they were going to disrupt the flow of the space and we proposed multiple locations where they could be placed strategically to minimize impact. In all we gave the architect more than 7 options on how to solve the lateral stability challenge. In the end he chose the option of the nib walls and chose a location where the walls could be used to enhance the architecture. We empowered him to make the choice, take ownership, and he believed that blocking the view to the rear at the base of the stair enhanced the architectural experience. When you face a challenge the best thing to do is explore options, always question, always explore, always explain. At Cantilever we have dozens of anecdotes where it is this exploration that has led to extraordinary architecture.
It is probably appropriate now to introduce you to the builder. If you are going to deviate from a conventional timber framed building, and stick to a budget, by introducing off-form concrete, you need to think about the builder. We have done dozens of buildings with SFN Constructions. And when we explored the use of off-form concrete we thought it was important to recommend that they contribute to the design and offer some cost guidance.
We introduced the architect to SFN very early on in the design process – even before off form concrete was the firm choice for the build. SFN met with us and discussed the project and they, from time to time during the design phase, contributed to the project. We believe this project would not have been a success without their early involvement.
As you stand in the cool summer kitchen your gaze will eventually be drawn out through the large sliding doors to the long floating concrete bench in the rear yard. It is difficult to see the small steel brackets supporting the bench, but it was desirable to make these supports invisible. We analyzed the bench and the brackets using finite element software, Strand7. Perhaps bizarrely, this was the only element of the building done using this sophisticated structural engineering package. It took two days to analyze and document these tiny supports and the concrete bench. Like a perfectly composed photograph this bench has long leading lines which draw you out of the kitchen, into the yard, onto the brick checkerboard patterned paving, down the brick stairs and right to the door at the rear of the site. Even as you stand in front of the rear garage door you are looking at the structural theme that cements the entire build together: off form concrete.
Although we have said that it was our collaborative approach that made this house a success there is a key aspect that we need to reflect on here. Collaboration only works when all parties are open and humble, devoid of ego. It is when everyone listens with respect and ideas discussed openly and without fear that collaboration can occur. There is an additional idea at play here which makes this house a success. It is the harmony between the structure and the architect. At no point do you see these two elements fighting each other. Nothing seems forced. Utilitarian expressed structure compliments and enhances the architecture. The structure becomes ornament. As structural engineers, this is our number one priority.
Structural Engineer: Cantilever Consulting Engineers