Building a dome with extreme mass timber
Building a dome with extreme mass timber
The City of Cape Town commissioned the Greenpoint Urban Park Dome in honour of the First Nation Khoisan peoples.
There is a place few dare to tread from a design and a construction perspective, as there are no guidelines. I call this area of timber construction extreme mass timber.
I am presenting the Green Point Dome because:
- The client was the City of Cape Town, and the brief was to pay homage to the First Nation Khoisan people with an educational structure in the city's beautiful and accessible Greenpoint Urban Park.
- It is the first full-size mass timber dome ever constructed in South Africa.
- It is the world's first perfectly 3-dimensional (3D) mass timber dome.
- It was an epic journey in understanding the engineering, maths, preparation and construction processes.
When the architect Derek Kock presented the plans and renderings, he asked, "Can it be built in 3D using local materials?". I immediately said yes but needed time to work on the details.
There is no handbook or guide on how to build a mass timber dome! Everything on the dome was angled or curved, and every fitting had to be custom-built.
THE BEGINNING
Unsurprisingly, the first question asked by the professional team was, "How do you keep 20 beams in place while attaching them to a ring?
The short answer was "build a tower". We started by discussing how the pieces would slot together. Like an old-fashioned wagon wheel, the hub at the centre is paramount.
We had to decide the size of the support tower, the number of laminates in the ring beam, the weight of the beams, how many could lean against the ring beam, and how to balance the structure during assembly.
TIMBER IS TRICKY
What emerged very quickly as some incredibly experienced professionals put their heads together was how little everyone understood timber, its moods, abilities, strengths and more.
Dr Phillip Crawford at Stellenbosch University's wood science department tested locally sourced Poplar. It did not take long to convince the engineers and architect to adopt the timber.
THE SKELETON
The essence of a 3D dome is having a 3D skeleton, the skeleton being the 3D circular ring beams. How it all slots together is the crux of the engineering design. The engineering must be imagined sequentially before the construction process shapes it into reality.
The dome comprises 20 massive glue-laminated curved beams 8.5m long, weighing 450kg each. They are attached to T-shaped brackets anchored into the concrete base and slotted together in a cross-laminated timber (CLT) ring at the top. The beams must be hoisted upwards and lowered onto a T-shaped base bracket anchored into the concrete floor. At the same time, they need to slot into the centre ring.
THE RING
The consulting engineers specified a colossal ring of steel with bolts and brackets. Instead, we designed a ring with a diameter of 1,5m and weighing 385kg using CLT with walls only 80mm wider than the steel ring but much stronger and versatile.
I opted for massive dovetail joints at the top to link the curved beams into the CLT ring.
There are 20 segments of curved timber per layer. Each layer was shaped immediately after laminating so that when the ring was complete, 20 layers were glued together.
It had to be perfectly positioned as it determined the height of the dome and the correct placement angles of the curved beams 6m above the floor.
PROTOTYPING
The prototyping happened at my factory in Darling, 160km north of Cape Town. We built the test ring beam and six full-size beams from plywood. The ring beam was perched on scaffolding, and the curved beams were fitted, confirmingour calculations. It was a thing of
absolute beauty!
While a team built the tower and a replica of the dome shape with the correct curve and beam spacing, another team began manufacturing the beams.
The beams were clamped with F-clamps and shaped using a router on a sled running in tracks. Everything was done with lightweight machines and craftsman skills. Constant vigilance ensured that every beam was 160mm thick x 260mm wide with a perfect curve.
THE RING BEAMS SHAPE
THE PLY
Sixteen circular ring beams needed to be made for the skeleton of the 3D ply, each beam having a different diameter and set of angles, with placement onto the curve beams every 500mm. The lower seven beams were easy to build as they could be bent and shaped.
Higher up, however, the circles got smaller and the angles more acute. After much experimentation with the eighth circular ring beam, we used CLT to create a ring of the correct diameter. We built an angled jig so a router could cut the angles on both sides of the ring.
Yes, the angled cuts could have been done by a 5-axis CNC machine if the ring was in smaller segments, but we did not want to outsource any aspects of the manufacture. The learning process of building by hand was enriching, and many of our artisans began to understand the 3D process.
The rings were built as complete circles, with the base ring having an angle of 87º and the upper ring having an angle of 11º.
IT'S ALL DONE WITH MATHS
Next came the challenging task of bending marine plywood in three directions. Bending ply in two directions is easy by kerfing lines at 90° and bending it. Using this logic, I played with the mathematics of two kerf cuts and angles to calculate how to bend the plywood in three directions.
Calculus offers the formulas for 3D curves; however, three different formula calculations had to be meshed into one. After a couple of days of experimentation, frustration and stubbornness, I found the solution.
When we added the second marine ply sheet at a 60º cross angle to the first fitted sheet, the curve smoothed perfectly, and no sanding was required.
ON-SITE AT LAST
With all materials and components ready, we began fitting on-site in mid-June 2023 on a cold, wet and windy day. In case you are wondering, we allowed for the Cape's wind shear factor.
The ring was lifted to the top of the tower, aligned to exactly 6m above floor level using a series of scissors car jacks to get it perfectly plumb and screwed in place. The rigging of the 20 curved beams took one day instead of the estimated four days. In the end, the spacing between the beams was out in two places by 4mm - a margin of error with which I can live!
The marine ply strips
HEALTH AND SAFETY
Working access to the dome, 6m in the air, was challenging. South African health and safety regulations do not mention domes. Standard ladders did not work because we could reach up to around 4m in vertical height. Officials were unhappy because it was out of their scope of work and training. We exchanged many words while asking them for an alternative solution.
We tried anchored top ropes with mountaineering harnesses and helmets, but they vetoed it. We then built a curved steel ladder that was the exact shape of the dome. It was anchored on the upper ring with wheels below to push it around. Each tread was carefully angled parallel to the ground, and the handrails got taller as you went upwards from a climbing angle to a walking angle.
However, it was not SABS-approved, and despite the ladder's apparent safety and versatility, it did not follow any known rule book or design. We played this game with health and safety right to the end of the project!
TRUST THE GLUE
Once the beams were positioned, we fitted lateral spacer beams at two different heights. Again, the original spec was to have these beams fitted with steel angle brackets. We chose rebate joints glued together and secured with special pocket-hole screws.
The logic behind the jointing and glueing of the ring and lateral beams was simple. After thoroughly testing various adhesive systems, I trusted the chosen glue to fuse the separate components, thereby increasing the strength of the structure and leaving no weak points.
Two layers of 8mm OSB, cross-laminated, glued and screwed together, cover the beams. The OSB layers are flat on the horizontal axis and curved vertically. The gaps in the OSB surface were filled, and it was sealed with Woodoc exterior sealer. On top of the OSB came the 16 x circular 3D beams and insulation.
The final part of the structure consists of two layers of 9mm marine plywood glued, screwed together and bent over the circular beams in three angles to create the 3D effect.
Because of the curvature, the angles constantly change. We laid the strips of plywood at angles ranging between 65 – 75 degrees and secured it with thousands of zinc alloy screws. No sanding was needed.
OPTICAL ILLUSION
However, the appearance of the curved dome juxtaposed with the buildings in the background caught everyone out. We were scolded for the "dips and dents" on the dome's skin.
The optical illusion was alleviated after we made a curved ruler and ran it over all parts of the dome skin to convince the critics that the curve was pure and correct.
David Marks positions the glass skylight on the steel vent
THE ROOF, FOR THE MATHS OF IT
Initially, the roof was not part of my brief, and I was not asked to comment or provide a quote on the roof covering. However, the maths challenge was something that I could not leave alone, so I did the calculations anyway and filed it away. While we were completing the ply layer, the consulting engineers were asked for a spec for the roof. Suddenly, everyone discovered that the roof was much more complicated than it looked.
A rendering is all we had, but the reality of building a symmetrical yet random roof with different shades of brown that looked like animal skins proved challenging. I held out for three days and then sent the engineers and architect an eight-page spec for comment. I did not receive any, just a message: "Go ahead. We trust you".
CAD COULD NOT COPE
The roof sheeting had to be made from a South African material! So, we chose 3CR12, a stainless steel compound. It needed to be in sheets over-pressed to the dome's curvature to allow for the spring back of the steel. The sheets had to be laid in perfect concentric rings, overlapping the top and bottom but not the sides. It needed to look random, so thes heets were painted with Midas rust paint in different shades of brown.
Various lengths of sheets were fitted in a seemingly random pattern. The reality was pure mathematics. I used spherical geometry, the geometry originally used to map the Earth's surface before the days of aerial photos. This geometry was last taught at Cambridge University in 1936 (thank you to my grandfather's library).
CAD, in fact all drawing software is not programmed with spherical geometry. Not even Rhino 3D could create the roof. So, I drew it by hand.
MAKING THE ROOF
With the help of Marco Grandi, we planned the roof in precise detail. We created positive and negative moulds to press the steel. We designed and built a radial-arm saw slide with an angle grinder attached and a curved rest for the stele. The grinder needed to move upwards and downwards with the curvature of the steel. Hundreds of cutting discs were used as 3CR12 is incredibly hard. Before fitting the roof sheets, we built 3D gutters at the dome's base and above the doors.
WASTAGE
The wastage factors were:
- OSB and ply sheeting: 4%
- Poplar over the entire dome: 3%
- Shavings: Donated to horse owners
- Off-cuts: Laminated to make
architraves and internal filler pieces. - 3CR12 sheets: 5%, with four spare sheets left.
- Steel off-cuts: Donated to two artists who turned them into a sculpture piece.
Even the ladder had to bend
THE VENT
To complete the dome's exterior, we needed a steel air vent at the top to cover the CLT ring and form the base for the curved skylight.
After much discussion, Derek Kock and I agreed on the height of the open-air vent and overhang of the glass roof so that a minimal amount of rain would enter through the vent and create the best airflow within the dome.
Grandi Engineering built the vent. When the day came to fit the ring, I was 98% sure it was perfect. It eased into place with less than a ¼mm tolerance around the ring. The team grinned for hours afterwards.
THE SKYLIGHT
The next challenge was the curved glass dome skylight roof, 2m wide, weighing 155kg and 13mm thick. The glass rood was built, hoisted into place, positioned with careful planning on a windless day and secured with structural glazing glue. The exterior of the dome was now complete.
DOORS
The door frames needed to be angled 9º towards the dome's centre to accommodate the wall so that the double doors could be perfectly square.
THE GREATEST LESSONS LEARNT
There were many paths to completing the Green Point Education Dome, primarily technical and mathematical. I spent hours researching, looking at photos of other mass timber domes to decipher the methodology and developing new techniques.
The unique building and landscaping nears completion
THE FUTURE
Companies and professionals like Christian Hess, Jamie Smiley, MTT, RSB and others continue pushing the boundaries of mass timber. I hope more South African designers, engineers, academics and construction companies will consider designing and building contemporary, carbon-sequestering, sustainable buildings.
Timber is the future of the built environment. Modern architects and timber engineers are explorers going where no one has gone before.
