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WEB EXCLUSIVE ARTICLE: Prefabrication in wood

Almost a century ago, the Modern Movement began with a strong social agenda and the belief that the new technology of mass production could be harnessed to improve the circumstances of ordinary people. For the last several decades, technology in architecture has become an end in itself.

By Jim Taggart, SABMag Editor

Early Modern architects shared the conviction that industrial production, with its virtues of economy and efficiency, could be harnessed to better the living conditions of ordinary people. Thus in their vision of a new and more democratic world, they saw technology as a means to a greater social end.  However, in the decades that followed World War II, Modernism lost much of its sense of social purpose, as those same virtues of economy and efficiency were subverted into cheapness and monotony, in the pursuit of narrow commercial goals.

In the 1960s amid the growing disillusionment that gave birth to Post Modernism, Moshe Safdie made a last courageous attempt to redirect technology toward that sense of social purpose. His vision for Habitat ’67 was that it would make the amenities of suburbia – including daylight, fresh air, views and private outdoor space – available to city dwellers at an affordable price. That vision proved to be a mirage, impossible to realize at a reasonable cost given the materials and production methods of the time.

Fast forward almost half a century and we are beginning to see a resurgence of interest in prefabrication as a means to realizing the artistic and social ambitions of architects. This time around, most of the attention is being given to prefabricaition in wood.

In British Columbia, in particular, new mass timber structures such as the Earth Sciences Building at UBC by Perkins+Will and the Wood Innovation and Design Centre in Prince George by Michael Green Architecture are composed almost entirely from prefabricated components. Similarly the new generation of six-storey light wood frame residential buildings, of which about 50 have now been completed, also employ either site or shop fabrication of floor, wall and roof components.

Less common is the use of prefabrication for smaller multi-family wood-frame buildings, but two recently completed projects demonstrate that there are considerable advantages to be gained even at this scale.

Bloc 10, Winnipeg – [5468796 Architects]

Bloc 10 is a multi-family housing project that strives to re-imagine and re-invent the market-driven condominium. Situated on the site of a former gas station in Winnipeg’s River Heights neighbourhood, the developer wanted a modestly priced building
designed and constructed within 12 months. The resulting three-storey, 10-unit condominium project was modelled after the ‘white-box’ concept, in which each buyer purchases an unfinished unit with basic plumbing, heating and electrical systems. Buyers can decide which rooms they would like distributed on each level and they can personalize their finishes to their own taste and budget.

All of the units [sized between 900 and 1,300 sq.ft.] rise over three levels and most have two balconies. As the apartments ascend, they cross from one side of the building to the other, resulting in 10 unique, interlocking layouts that feel like a hybrid between a condominium and a house. This arrangement provides views to the north and the south, and eight of the10 apartments have views in three directions.

To take advantage of the maximum allowable mass for the development, cantilevered projections expand rooms, create balconies and provide support for the wooden, vertical-slat privacy screen that wraps the building’s exterior. The screen serves multiple purposes. It provides each homeowner with privacy and shade and yet at the same time offers glimpses of neighbours, so encouraging a sense of community.

The building is constructed entirely of prefabricated light wood-frame floor, wall and roof panels – and in this case, the technology serves the design ambitions of the project. Although the choice to prefabricate was the culmination of a variety of factors that did include cost, speed of erection and quality control, there was more to it than that.

As Colin Neufeld of 5468796 Architects observes, “During the design process we also realized that the project would benefit greatly from prefabrication. Being very complex and compartmentalized, with many double walls and floors, it would have been very difficult to achieve using traditional site-based stick framing methods. Conversely the same design fits explicitly with how prefabrication works.”

Although not immediately visible through the veil of cedar slats, the geometry of Bloc 10 is extremely complex, with long spans, double-height spaces and significant cantilevers in three directions. Similar conditions in a previous project had required scaffolding and a considerable amount of work having to be done overhead. This situation made the control of cost and schedule difficult.

With bloc 10, all the components were shop prefabricated by Holz Construction in climate controlled conditions, before being labelled and shipped to site. When lifted into place off the truck, double walls, insulated exterior cantilevers and other elements of the building that would have been difficult to build in the field were already complete and protected from the weather.

Neufeld adds, “Prefabrication also meant that workers did not have to be on ladders or scaffolding, both of which would increase the safety risks. Prefabrication renders the entire building safe from the moment workers step onto the site.”

In addition to providing a Habitat-like living environment, the developer and the architect were both committed to giving purchasers real choice in the way their units are organized internally. To this end, servicing takes the form of vertical risers that are stubbed out at every floor. The architects then developed various layout options to demonstrate to potential purchasers that it was indeed possible to locate a bathroom or kitchen pretty much anywhere they wanted.

Delivering on its promise of ‘white box’ living, Bloc 10  provides tangible evidence that the cookie cutter condominiums that have dominated the market for decades are the product of a narrow and shortsighted approach to design.

Within the tight economic parameters that govern this building type, technology can be a liberator.  Using prefabrication in a creative way, it is possible to sculpt more interesting and more flexible spaces, and to give those spaces more daylight, fresh air and a stronger connection to the surrounding community.

Monad, Vancouver – [Lang Wilson Practice in Architecture Culture]

One of the most important aspects of sustainability is affordability. With urban infill projects it is very difficult to estimate and control costs accurately, largely because of the complexities of site staging. According to architect Oliver Lang, there is also a gap of expertise at this scale of building. “Our industry executes small projects in light wood frame consistently well, and likewise large commercial projects, but at the scale of Monad [a four-storey building on a 33-foot urban infill lot] there are few contractors with the necessary experience.

Monad attempts to eliminate some of the unpredictability by introducing a systematized approach to design, fabrication and construction. This not only reduces the risks associated with time delays and cost overruns, but introduces a level of quality that goes beyond what is achievable in site-built work. Prefabrication in wood offers a level of quality and affordability that makes it an attractive alternative to building in concrete or steel.

Prefabrication overcomes many of the limitations of wood-frame construction, such as its vulnerability to water damage, its dimensional instability and the necessity to accommodate the generous tolerances of site construction; and instead takes advantage of all its positive qualities, such as low carbon footprint, and the local availability of materials and skilled labour. With the range of engineered wood products now available, much larger projects are possible.

Monad uses a spatial approach to prefabrication, in which open-ended and open-sided volumetric components are designed with a floor, one long wall and a ceiling, the other side and the ends of the module being left open – although braced for transportation and handling.  A ‘left hand’ and a ‘right hand’ module can be placed with their open sides opposite one another and, according to the width of the site, connected with infill pieces of the appropriate dimension.

While there is a rigour to the modular design, it does not impose itself on the experiential quality of the building.  In fact, with its courtyard plan, apartment units with dual exposure and the potential for wide open spaces, there is little to indicate that the building is of modular construction.

Among the challenges to be overcome in a prefabricated wood infill project in which storey height modules are stacked on top of one another, is that of demonstrating the continuity of the firewalls that are required along the lot lines shared with the adjacent properties. With Monad comprising three storeys of wood above a single-storey concrete podium, it would have been necessary to construct three-storey freestanding concrete masonry firewalls on either side of the lot prior to the placing of the modules. Instead a solution was devised whereby the side wall of each module is fire rated, and as the modules are stacked these walls interlock.

According to the scale of the project [and LWPAC is talking to the City of Vancouver about a 12-storey version], the choice and combination of materials used will vary. For this three-storey version, a combination of conventional wood framing and engineered products are used. Floors are made from PSL panels and I-joists, walls are 2×6 studs with Microlam top and bottom plates [to eliminate cross grain shrinkage], and the ceiling panels are 2×4 framing with services pre-installed in the voids. These ceilings are non-structural when installed, but designed to brace the modules during shipping and installation.

The ceilings do include PSL perimeter header beams for the exterior walls, to ensure dimensional accuracy for the installation of windows On the long open side, PSL columns are used for stability during shipping and installation, and to carry vertical loads when the modules are stacked. Tapering steel shoes on the tops of the columns allow the modules to seat themselves accurately as they are dropped into place.

Retrofitting and densifying our cities is an increasingly important component of sustainability, and prefabricated wood structures could play a valuable role in providing solutions that combine high quality with affordability. As these examples illustrate, there is also an opportunity for technology to be applied in a way that enhances liveability and increases customer choice.