Tall Wood

Insights from the FII/BSLC Survey of International Tall Wood Buildings

In the fall of 2013, Forestry Innovation Investment and the Binational Softwood Lumber Council commissioned Perkins+Will to conduct a survey of completed Tall Wood buildings from around the world, and share the lessons learned by others with those contemplating similar projects in North America. The study was published in the spring of 2014, and SABMag took the opportunity to speak with the lead author, Rebecca Holt of Perkins+Will in Vancouver.

By Rebecca Holt of Perkins+Will in Vancouver.

<SABMag: I notice that not all of the buildings in the study are ‘tall’ - in fact some have as few as five storeys. Can you explain the selection criteria.
>RH: Our definition of ‘tall’ was a building that exceeded the allowable height limit of wood construction in the local building code at the time of approval. In most cases that was between four and five storeys. We also understood that the development and design community had expressed reservations with a number of issues including structural stability, construction tolerances, moisture protection or acoustics across building types. The goal of the Survey was to represent the range of construction types, including mass timber panel systems, post and beam and hybrid systems across building typologies, to address these concerns.

<SABMag : Even with the progress being made here in North America, we don’t seem to have developed the ‘wood culture’ that appears to exist in Europe - at least not yet.
>RH: Yes, I agree in reference to solid timber construction. This will come as we gain experience I think, but one of the biggest differences right now is product availability. In Europe there are multiple sources of CLT and other products and each comes with a considerable amount of technical expertise that manufacturers and installers have acquired over time.

<SABMag: You have identified the need to commit to wood as a structural system from the inception of the project. Is this because the properties and performance of wood systems are quite different from those of other structural materials?
>RH: We didn’t address the differences between wood, concrete and steel structural systems specifically, but what we did establish was the need not only for a collaborative team approach, but for the development and incorporation of appropriate details in the construction documents that will capitalize on the speed and precision that is possible when using prefabricated wood components.

<SABMag: Is the use of prefabrication related to the high performance nature of the buildings you researched?
>RH: Yes, prefabrication can ensure very precise cut and fit of elements which helps to create air-tight envelopes and support other essential components of energy efficient construction. For those buildings that had operational data available, the shop fabrication of the envelope appeared to offer the greatest performance advantage. Being a poor conductor of heat, wood reduced the negative effects of thermal bridging. As a material, it seems to be highly compatible with high performance energy standards such as Passive House.
However, it should be noted that prefabrication was not limited to the wood components. Where we looked at buildings with concrete cores, and other concrete components, those that benefited the most were the ones that used prefabricated concrete panels. This was not just a question of precision, but also of speed and the elimination of wet trades on site.

<SABMag: That prompts a question about acoustics. If you eliminate wet trades, and with them concrete toppings, does this compromise acoustic performance?
>RH: There seems to be a lot still to learn about acoustic and moisture issues, so there is a balance to be struck between the desire to eliminate wet trades and the desire to achieve acoustic separation. We did see one or two projects that used a sand layer as an acoustic barrier, and others that focused on separating or decoupling floor and wall assemblies to address transfer of sound between spaces; both appear to be successful. It is worth noting that for Limnologen, a CLT project completed in Sweden in July 2009, which does not include a concrete topping on the floors, a post-occupancy survey recorded only one complaint about acoustic performance.

<SABMag: You raise the issue of moisture protection during construction, Did you find any consensus on this subject?
>RH: There appears to be no single approach to the issue of weather protection. In Sweden buildings are typically protected with a rising tent system, that may have as much to do with creating a better working environment as it has to do with protecting wood components from the weather. In the UK where there are a great number of CLT projects including the Murray Grove and Bridport House apartment towers, and several large schools, they don’t seem to take such a conservative approach. Anecdotally we understand that engineered wood products such as CLT and LVL can handle a fair amount of exposure to weather.

<SABMag: It seems as if architects and engineers are prepared to take on the challenge of building bigger in wood, but what about the owners, occupants and municipal officials?
>RH: Perhaps the most comprehensive response came from the Limnologen project I mentioned previously, where we were able to interview owner/occupiers of one suite, as well as representatives from the City of Vaxjo. The owners were intensely proud of their building and considered it to have been a good investment. They spoke specifically about the sense of well-being and the quality of their space.

For their part, the city of Vaxjo has some very innovative and supportive policies in place. They have been monitoring the Limnologen project since its completion and have committed to building almost exclusively in wood, not just for residential buildings but also institutional buildings for universities and hospitals. Wood is an important component of their municipal carbon neutral policy.

<SABMag: It would seem from the survey that wood is a viable and desirable alternative to steel or concrete, even in larger buildings. Is that a fair conclusion to draw?
>RH: Yes, I would definitely agree with that. However, it was made clear by everyone we spoke to that they are advocates of using wood in the right context, where the material makes sense. Also, that designing large wood buildings requires a holistic and collaborative approach that engages all the stakeholders from the outset. This was identified as a priority, not just by the design teams and owners, but the authorities having jurisdiction as well. Everyone needs to have the opportunity to contribute their expertise, because everything needs to be planned so carefully. This is particularly true right now, when most people are at the very steep end of the learning curve.

<SABMag: Thanks for sharing your insights. I am sure they will be helpful in raising the comfort level for those contemplating a large- scale project in wood.

Several of the considerations identified in the International Survey of Tall Wood Buildings have been explored in two recently completed projects in British Columbia: the Wood Innovation and Design Centre in Prince George, and the Mountain Equipment Co-op Head Office in Vancouver.

Wood Innovation and Design Centre
With a height of 29.5 metres, the Wood Innovation and Design Centre [WIDC] in downtown Prince George, gives Canada its first glimpse of a Tall Wood future. Completed in October 2014, the six-storey demonstration project affirms BC’s growing expertise in the design and construction of large-scale wood buildings. The main tenant will be the new Masters program in Wood Science and Engineering, which will be launched by the University of Northern British Columbia in fall 2015.

The structure is a glulam post-and-beam system with built-up cross-laminated timber [CLT] floor panels. Glulam beams frame into glulam columns using proprietary aluminum dovetail Pitzl connectors. This allows the columns to run continuously from the foundation to the roof, eliminating all cross-grain bearing and shrinkage.

The lateral-load resistance is primarily provided by the elevator and stair core walls, which consist of CLT panels connected together vertically with self-tapping screws. The shear walls are anchored to the foundations using a combination of shear brackets connected to the panels with self-tapping screws and hold-down anchors.

The floor system consists of overlapping 3-ply upper CLT panels on 5-ply or 7-ply lower CLT panels connected together with HSK epoxy and metal mesh connectors, providing a fully composite corrugated structural section. This creates cavities within the structural section that can accommodate services above and below the floor structure, while enabling some of the CLT surfaces to be exposed.

Spanning 6 metres [19.68 feet] between the post and beam frames, the wood-only CLT floor system was selected to minimize the use of concrete and thus weight. The one way cavity chase created by the staggered CLT panels in the floor-ceiling plane are used as a run for the electrical and fire protection system and is filled with loose laid acoustic insulation. The acoustic performance of the floor assembly is further improved by a noise-barrier system lining the underside of the top CLT within the ceiling chase. To provide a flat floor surface, the floor cavity is covered with a removable plywood panel. The ceiling cavity is completed with a removable wood-slat panel. The service chases inherent in the structural system offer flexibility for future reconfiguration. Therefore, the need for secondary ceiling finishes to conceal service runs is significantly reduced.

Rather than protecting the wood structure from exposure to fire by covering it with non-combustible material, the wood is left exposed, except for wood-to-wood connections where fire-rated chalking is applied and fire resistance was demonstrated through the review of historic fire test data, specific fire-rated chalking tests, computer modelling and calculations. The mass timber will char slowly enough to provide the required fire protection.

Metal connectors between building elements were all imbedded to protect them from exposure in case of fire. Structurally, metal if not protected can fail more quickly than wood at high temperatures and extended durations.

Mountain Equipment Co-op Head Office
For its part, the new 4-storey, 12,000m2 Head Office building for Mountain Equipment Co-op in Vancouver offers an intriguing glimpse into the past and the future simultaneously. With a simple heavy timber post-and-beam frame, and nail-laminated solid-sawn wood floors, its construction resembles that of late 19th and early 20th century commercial buildings. This technology is now being rediscovered and relearned by a new generation of architects, engineers and building owners in their pursuit of a broader green agenda.

For more than a decade, MEC has used wood extensively in its new retail stores across Canada because of its environmental, physical and visual properties - but choosing wood for its head office was another matter. With concrete construction being the default choice in Vancouver for a commercial building of this scale, that became the baseline against which other structural options were evaluated.

Analysis identified a small cost premium for building in heavy timber, and likewise a slightly increased level of uncertainty and risk. However, given the client’s expressed desire to create a ‘healthy, fun and inspiring workplace’, and its commitment to sustainability, it became clear that wood was the appropriate choice.

To optimize both solar orientation and wind direction for daylight and natural ventilation, the building adopts a cross plan formed by two narrow, intersecting wings, one of four storeys, the other of three storeys in height.
For simplicity, economy and flexibility, a glulam post-and-beam system was chosen for the primary structure, with the floors being constructed using mass timber panels. The preferred option for the floors was CLT,  but the design permitted nail-laminated timber [NLT] panels to be offered as an alternate at the time of tender.

CLT and NLT have similar structural characteristics and require a similar depth when used as a panelized floor system. The concern with specifying CLT exclusively was one of availability and competitive pricing, given the limited fabrication capacity in BC. When the project was tendered, NLT proved to be the more economical choice.

Each main bay of the building is 60 feet wide and is divided into three equal sub-bays of 20 feet. Thus, there are four lines of glulam columns connected by three sets of paired glulam beams in each main bay. The NLT panels are 4 feet wide and 40 feet long, so that they span two sub-bays of the building. To facilitate diaphragm action, the panels are laid in an overlapping pattern to minimize continuous joints. Plywood sheathing is laid in a similar configuration for the same reason.

The supporting structure comprises glulam beams and columns calculated to provide a minimum one-hour fire-resistance rating. The NLT panels are made up of 2×8 material that [like the CLT panels in the WIDC project] will char slowly enough to maintain their structural integrity for the required length of time  when exposed to fire.

To limit cross-grain shrinkage, the structure has been designed with storey-height glulam posts, superimposed one on top of the other with end-grain-to-end-grain bearing. Constructing buildings of this size in wood presents particular challenges for contractors, some related to the properties of the material itself and others to the aesthetic aspirations of the client and design team. As wood components are are generally left exposed in the finished building design, careful handling to avoid damage is critical.

When building during the rainy season, weather protection is required during transportation, while components are stored on site prior to installation, and then from the time they are installed until the building is closed in until the building is closed in to avoid such things as surface discolouration and water marks.

With our expanding definition of sustainability, the re-imagining of large-scale wood buildings has a significant role to play. In addition to delivering operational efficiencies, projects such as WIDC and the MEC Head Office speak to emerging concerns as diverse as low- carbon construction and occupant well-being.

Article compiled by SABMag Editor Jim Taggart, from material supplied by naturally:wood® [WIDC] and BC Wood Works [MEC]. Rebecca Holt, M.Urb, LEED AP BD+C, ND, is a Sustainable Building Advisor and senior researcher with Perkins+Will’s Research team in Vancouver.

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