James Parker reports on how an “extraordinarily ordinary” student residence in Vancouver became the world’s tallest engineered timber building almost by accident.
At 18 storeys, Brock Commons Tallwood House, a student residence in Vancouver, is currently the world’s tallest engineered timber building (at least until the 24 storey Ho Ho Tower in Vienna is completed later this year). There is a race on to build tall in timber, driven by the huge sustainability benefits allied to construction efficiencies of engineered timber. However, while timber is the load-bearing structure for 17 storeys of architect Acton Ostry’s building, the really interesting thing about Tallwood House is paradoxically, its relative ordinariness.
As is common in a country with abundant timber resources, building in wood was always the goal for this project, but as architect Russell Acton tells ADF, the aim was also definitively “not to create an international showpiece for the tall wood movement.” It is the first project built under the 2013 Tall Wood Demonstration Projects Initiative by the Canadian Government, offering funding for projects over 10 storeys. The project was launched to help grow the engineered timber industry, and says Acton, “help make the case for mainstreaming tall timber buildings.”
The client for Tallwood House, the student housing department of the University of British Columbia, wanted the project to be “first and foremost be a modest, home away from home for students, similar in form, massing, function and finish to existing student residence buildings on the campus,” says Acton. The scale of the building was more a function of the need to accommodate growing student numbers than part of a grand project to build a tall building from CLT and glulam for its own sake.
It was almost coincidental that the building’s size plus the material of choice being wood conspired to produce a ‘world-record’ project. “Creating the world’s tallest mass wood tower was never a goal,” says Acton, “It was simply an outcome of the fact that student residences constructed on the campus are typically 53 metres tall due to height limitations, and that was taller than any other mass wood building in the world already constructed.”
Demand for on-campus student housing has rocketed recently – the campus sits at the end of a peninsula where Vancouver’s most expensive real estate is located. Acton explains: “If you’re not living on campus, you have to live quite far away.”
The client also wanted Tallwood House to be comparable in cost to a similarly-sized concrete student residence. Beyond that, Government funding would bridge the “innovation gap” of extra money needed due to this being a first-of-its-kind building, as Acton explains. “There’s a bit more design time that needs to be paid for, there are some aspects of construction that need to be examined – we built a very expensive two-storey mock-up.”
The thin, narrow site, on a fairly exposed escarpment, led to a 15 metre x 54 metre footprint. The building is strictly speaking ‘hybrid’ as it includes two equally sized concrete circulation cores, their symmetry being favoured by structural engineer Fast & Epp, but also the contractor, who was able to pour more efficiently.
The cores jut slightly out of the building, to provide a more efficient layout for the CLT timber used for the frame, as well as some visual interest to the facade. Acton explains that discussions with two 20-strong stakeholder panels covering timber’s fire and structural issues (required to meet regulations), meant that a clear view resulted that the exit routes needed to be in concrete. Four members of the fire panel were fire chiefs from Vancouver and surrounding cities. “They recognised a CLT core would be structurally sensible and could be used by emergency responders with confidence.” However Acton adds, “they appreciated that for a first of its kind building we were proposing a concrete core, because they knew a CLT alternative would be a hard sell to the firefighters.”
According to the project’s architect, “once everyone understood and accepted it wasn’t going to be a tourist attraction for architects and engineers, it made a lot of the decision-making very straightforward. It wasn’t about me, it was about ‘them.’” Acton says this anti-ego ethos ties in with the practice’s belief that “not all buildings should be showpiece buildings…the responsibility in designing ‘background’ buildings is to make them handsome, and fit in, as well as having a bit of exuberance. But they shouldn’t be trying too hard.” He summarises the firm’s approach to this project as “very matter of fact and common sense, we understood what was looking for.”
Acton Ostry, like most Canadian practices, is well versed in building in timber – “the first half of our career was spent designing wood buildings, albeit not particularly tall and not in CLT. They have a strong relationship with the university, having previously designed several successful projects collaboratively.
The budget was a “modest” $39m (Canadian dollars), so a primary challenge “was to ensure that the design team always keep the project objectives in mind”. One of the major selling points of CLT construction is of course speed and efficiency, and the 17 floors of CLT panels and glulam posts took only 46 days to erect, with the whole construction completing in 66 days. The project cost $240/ft2 including the 8-9 per cent “innovation gap” funding, however without that it was $221/ft2, and a 2017 comparator for a similar concrete construction was $220/ft2. This was clear validation of the “keep it simple” design and construction approach.
The layout of the building maximises repetition in the interests of efficiency, avoiding obstacles to easy distribution of electrical, mechanical and sprinkler systems. On the ground level (there’s no basement) a concrete podium houses student social and study amenity spaces and building services. Above that are 17 levels constructed from five-ply CLT floors, 169 mm thick, sitting on 265 mm x 215 mm glulam columns (these are 265 mm square at ground floor level). There are four bedroom units at each end with studio units between; all having kitchens and bathrooms.
This construction achieves the key challenge of a thin enough floor assembly to accommodate 18 storeys within the 53 metre height limitation across the campus. Acton commends how the structural engineers at Fast & Epp “rose to the challenge” of creating a relatively thin, strong and simple CLT structure, a two-way spanning CLT slab – which, he notes, “required machine stress rated timber at the outermost laminations.” This application of two-way spanning CLT is thought to be the most extensive ever used and, says Acton, “was a brilliant solution as the floor depth is quite similar to that of a comparable two-way concrete slab.”
Acton says that the collaborative process need to consult all stakeholders and develop the fit-for-purpose solution for the budget was “a huge co-ordination exercise,” adding “that’s where you had to check your ego at the door.” The design achievement has been acknowledged with a clutch of awards, including the Award for Excellence at the 2017 National Council of Structural Engineers Association Awards.
The University of British Columbia performed tests on the structural characteristics of the CLT panels and glulam column system, and it proved to be considerably stronger than anticipated. Acton says that counter to expectations, rolling shear rather than punching shear was where there was a “point of failure” in tests, however even this had 25 per cent greater capacity than had originally been expected.
The panels are simply bolted onto the steel connectors on top of each column, before the next column is dropped into the steel connector and held in place with a steel pin. Infill wall panels are steel stud framed, and the floor panels are screwed to the concrete lift cores, with additional steel drag straps attached to them to transfer seismic forces to the ground.
The wood structure has for the most part been encapsulated with gypsum board, as the budget did not permit the volume of timber that would have been needed for exposed timber to offer the necessary ‘char’ potential to conform with fire regulations. Building Code in British Columbia permits the top floor of a tall building to be faced internally with exposed timber however, so here a student lounge has exposed glulam columns. The only other exposed timber, hinting at the building’s true nature, is a 58 metre CLT canopy.
Acton comments: “As soon as you expose mass wood, the costs go way up. The additional volume of wood to use char for fire protection is extraordinarily expensive, even more so when you factor in the cost of co-ordinating services.”
He says candidly that establishing the cost from other projects was tricky: “I found it very difficult to find other architects and developers around the world sharing their information. Almost no-one wants to say what it costs, because it’s very expensive.
I believe exposed mass wood is at a minimum twice as expensive as encapsulated.”
Interestingly, due to the nature of Canadian CLT, it was 29 mm thicker than it needed to be from a structural point of view, however there still wouldn’t have been enough spare to provide exposed timber – “for each hour of char you need about 40 mm of wood,” says Acton. He adds that if timber had been exposed throughout, two inches would have needed to be added to every floor, “we wouldn’t have been able to get the 17th floor of timber in.” The building would also have needed to be a module wider.
Wind loading “was a consideration, but mainly due to the seaside location and concerns for potential sway at uppermost storeys,” he says. However standard modelling did not throw up “anything remarkable enough to warrant extensive investigation and design allowances.”
Exterior and interior
With the offsite panel construction providing tight tolerances, high levels of air-tightness were achieved. Panels are steel stud framed and measure 8 metres wide x 2.65 metres high, forming the entire wall of two studios. Performing similarly to curtain wall, they arrived on site with two windows pre-installed. Additional insulation was inserted after installation, as well as a vapour barrier and drywall finish.
The building is clad with Trespa panels – “an interesting material as it’s made from 70 per cent wood fibre and resins, so it’s a durable but environmental solution that’s suitable for a high-rise application,” comments Acton. The panels have an appearance similar to timber, and with none arriving broken on site, the substantial surplus was used to clad all lift lobbies. Corner windows “help to dematerialise the mass of the building,” and a metal cornice on the roof echoes 1960s modernist buildings on the campus.
The fast construction process, guided by BIM, delighted the subcontractors, because the facade was enclosed as each storey’s timber frame was erected, so they were working in dry, warm conditions. The construction manager told Acton: “we hate the idea of going back to concrete.”
Internally, the residences resemble their counterparts on the campus, with similar amenity spaces including social and study space on the ground floor. However, Acton explains that to the discerning eye, the accuracy of CLT construction adds a different dimension: “Everything is just so square and crisp and accurate that there’s a sharpness to the units that you don’t sense in a concrete building.”
The interiors are finished to a high standard, with solid wood doors and furniture, following the building’s overall theme, to provide a “very warm, inviting and cosy” feel. The university has made a conscious effort to tell students they are in a timber building, says Acton, and they respond positively. “I have met many students on site and they love the idea of living in a mass wood building.”
The university is using this pioneering but simple building as a ‘Living Laboratory’, using sensors to measure shrinkage, moisture in CLT, and wind and seismic effects. This is being done in the hope the data produced will support future changes to the Building Code for mass wood structures.
The building, with its standard glazed and panelled exterior, plain facades, repeated layout and lack of exposed timber, might be seen as prosaic, but it is exactly what was required by the client to perform the function required. Acton says the ordinariness of the building is its real virtue, as it could help promulgate widespread adoption of tall timber in the mainstream to achieve the greatest carbon gains.
He admits the timber building purists may not approve, but says that it’s important to see the bigger picture: “There’s really been a bit of a disservice by some architects promoting the expression of exposed wood as almost like a mandatory, that you’re doing it wrong if you cover up the wood, it’s a sin, you’re a bad architect.” Acton concludes: “We are going to build tens of thousands of these extraordinarily ordinary encapsulated timber buildings, and this is where we are going to see the benefits of using a renewable resource.”
Project Factfile: Brock Commons Tallwood House
Timber source: British Columbia
Glulam and CLT supplied by: Structurlam
Volume of wood used: 2233 m³
Carbon stored: 1753 tonnes of C02
Greenhouse emissions avoided: 679 tonnes of C02
Total potential carbon benefit: 2432 tonnes of C02 (equivalent to 511 cars off the road for a year)
Sustainability rating: LEED Gold
Number of student beds: 404