If you are looking for a peaceful place to contemplate your next big thing, pull up a chair at Shellharbour Anglican College. But best to do it out of school time, otherwise you will not be alone. Your view will capture the green rolling hills just south of Sydney.
If you are lucky you may be able to sit in the top seat of the outdoor amphitheatre which was a last minute addition to the structure when the construction cost was less than the proposed budget. Rain is common in this part of New South Wales, so enjoy the roof structure that will protect you from the elements. And as you look up at the roof structure, there are a few things you may notice.
There is no ceiling.
The purlins run over the top of the beams, and you can see how the purlins lap over each beam.
Despite the irregular shape and orientation of the three separate buildings, the beams follow a distinct rhythm, throughout the entire structure.
Every beam has a cantilever at both ends.
The roof is one continuous monopitched roof.
For the keen observer, it is clear that something subtle is going on here. You see, if you were to conceive a building that contained all the elements of structural efficiency, they are all here. This is no accident. Having spent more than 10 years collaborating on more than 25 projects, Cantilever and Terroir formulated a mandate for Shellharbour Anglican College: to design a building that was so cost effective that we would have funds left over in the budget so that we could afford to incorporate the amphitheatre. It would be more effective integrated into this build, rather than sit as a separate structure.
The school had always wanted an amphitheatre to allow their students to take advantage of the views. And we knew this. During most of the design, the amphitheatre sat off in the distance. It was going to be undertaken as a separate project. But the reality is that it was just a dream, and actually being able to raise the funds to build it where unlikely.
But we knew that if we could pull off something special we could pull it into this design. And that is exactly what happened.
Schematic of the roof structure
1. 410UB32.0 2. C20019 3. 12 Diameter rod 4. 200PFC 5. 8 Plate — 1. 11 main roof beams supported off 100 SHS columns; all roof beams are the same size with architectural geometer of building based on capacity of beam: 4.5m cantilever; 14m span; compression element for bracing bay. 2. 1.2m lapped purlins at 900 centres running over top of beams for efficiency from continuous spans. Strength is governing structural solution rather than more demanding serviceability criteria due to absence of ceiling lining 3. Fully braced roof eliminated expensive site welding and moment connections. 4. Structural fascia formed compression elements for bracing bays and architectural trimming to roof edge. 5. Water stop and drip groove for exposed steel elements.
The Design Journey
The first workshop with Chris Rogers from Terroir started with the simple question: “What is the most efficient single story building we can build?”
Over the course of the next 45 minutes the summary was this: “Use main structural steel beams on a regular grid, all with cantilevers at each end; run the purlins continuously over these beams; single, continuous monopitched roof; strategically placed solid walls for lateral stability bracing; and remove select ceiling linings.” These were simple points that are as relevant for this project as they are for any project.
18 months later the building was complete. It never once deviated from these points. And it came in under budget. This allowed the school to build the amphitheatre they had always dreamed about. And this amphitheatre is now the centre of a thriving school.
When this meeting occurred and the question was asked, the project had barely been conceived. It consisted of 3 coloured squares, strategically placed, and slightly apart, on 1:500 site plan. The design was blurry, and over the next few months the design came into focus.
The 'blurred' architectural vision from Terroir at the first workshop
What is it about these points that makes this building so efficient?
The Primary Beams
The building consists of 11 primary beams. They are all the same 410 deep steel beams. Some of the beams have large spans, some have large cantilevers, some are part of bracing systems and some have all three. But the key is that of the 11 beams there are only 4 that aren’t close to 100% utilised. In fact, the dimensions of the spans and the cantilevers were driven by making the beams 100% utilised.
And of course, every one of the beams has a cantilever at each end. This is one of the easiest ways to make an efficient structure.
The Purlins
The purlins run continuously and are lapped over top of the beams. This requires almost 30% less steel than purlins that stop and start at the beams. This takes advantage of the symmetrical capacity of the purlins in both the sagging and hogging directions. This continuity also reduces the deflections purlins.
The Monopitched Roof
The key to simplifying these buildings is by minimising the number of vertical bracing bays that provide laterally support to the building. A monopitched roof means that by bracing the roof plane with simple rods to form a diaphragm, the number of vertical bracing bays can be minimised.
A monopitched roof also means that we don’t double up on structure along the roof edges where there is a change in pitch or a step in the roof. A single beam will do the work of two beams.
Solid Bracing Walls
The building is laterally braced entirely with diagonal bracing elements concealed inside sections of solid walls. This avoided the need for rigid portal frames. Site welded portal frames are common within these building typologies, where large bolted connections would disrupt the architecture.
The entire south elevation of the building offers views to the southern highlands. Minimal disruption to these views was critical to the architectural vision. Hiding the vertical bracing within small sections of walls behind joinery meant the views could be maintained.
The monopitched roof which was braced to form one continuous diaphragm also helped to minimise the number of bracing walls.
No Ceiling Linings
Deflection control, not strength criteria, typical govern the sizes of the roofs structural elements. The criteria used for deflection is direct function of the linings, particularly if they are brittle, or if they are in the line of sight. Throughout major portions of the building there is no ceiling lining. Not lining the ceilings meant that the sizes are dictated by the less stringent criteria, such as strength.
