Prefabrication and Precision
CLT panels are made from industrial dried lumber. They are built up from narrow planks, about an inch thick, that are laid side by side to form layers, stacked together at right angles and glued over their entire surface. The layers are then pressed and trimmed. Using computer-guided saws and drills, the panels are then cut to the precise dimensions in the architectural plans, including openings for window, door, plumbing and ventilation. Channels for electrical wiring can be cut into the panels. Lastly, panels prefabricated based on the project design, can be shipped to the construction site with windows and doors already installed.
Cross-laminated panels, up to half a foot thick and 30 feet long, are fabricated to precise specifications at the factory, shipped to the construction location and assembled and bolted together on site to form the exterior and interior walls, floors and roof. Even the stairwells and elevator shafts are made from these solid panels. At the construction site, the panels are hoisted into position and bolted together with metal brackets to build up the structure floor by floor. Construction can proceed fairly quickly: two-thirds of the time it would take to construct a similar building in steel or concrete. Prefabrication offers tremendous savings in construction time and cost.
Basically, an architect will design a building using 3-D AutoCAD software. The program will then generate the material specs and sends them to robotic wood or steel routers, which in turn will shape panels with millimeter precision. The result is a set of building blocks that a small crew of workers can assemble together in a matter of weeks.
• Layer: beams of wood, usually spruce, are set down side by side in layers, with each layer perpendicular to the one beneath it, creating a wood board up to a foot thick. A thin layer of glue is placed between each layer.
• Press: the wood boards are placed in a massive press, which squeezes them together.
• Sand: the edges of the boards are sanded down. If longer sections are needed, the edges are finger boarded to create a serrated interlocking end. They are then glued to the matching end of another panel to create sections up to 78 feet long
• Cut: the boards are cut to custom specification using 3-D files sent by the architects or construction team. The panel is processed using numerical control machines in order to achieve openings (doors, windows, staircases, skylights, utilities, etc.) as required in the design phase.
At the frame level, a construction using CLT panels behaves like a monolithic three dimensional structure, characterized by the continuity of the material and the near-uniform distribution of the mechanical stresses on all the available material. The CLT panel allows the design and creation of spaces and architecture in a freer manner, without the traditional limitations of the framed or poled constructions that have failed, especially in Europe, to become an alternative to civil construction based on masonry and concrete. The CLT panel constructions are designed and made to meet high-level performance requirements, which are well-defined and with very high quality standards.
CLT panels are then assembled at the construction site:
1) Whereas steel or concrete structures are skeletal, using columns to carry loads, CLT towers distribute weight over the entire, solid vertical panel.
2) Steel or concrete L-brackets fix the horizontal and vertical panels together.
3) The horizontal spans between vertical CLT elements can be significantly longer than with steel or concrete beams.
4) Interior walls are fireproofed by applying a layer of gypsum paneling on top of the mass timber panels.
5) Layers of insulation to reduce acoustic vibration between floors.
6) Panels can be ordered with windows cut out and sometimes piping and electrical installed. Construction is as easy as screwing the panels together.
7) Elevators have double walls with insulation sandwiched between them for fire safety and soundproofing.
Prefabricated and modular designs can accommodate architectural aesthetics such as building offsets, angled walls, balconies, pitched roofs, and more.