Exploration 2: How to design a timber building that’s easy to make in a factory
Once we stabilized the design of PMX at 35 stories, we turned our attention to taking advantage of timber’s potential for use in off-site manufacturing.
Using a computer-directed cutting machine, timber can be spliced into a variety of shapes and sizes, in contrast to concrete and steel, which have to be cast or extruded in custom-built molds. It can then be trucked to a development site, with each truck packed to the brim, as there’s no risk of exceeding the truck’s weight limit. To realize these efficiencies, architects have to proactively consider how to create a building design that lends itself to factory production, an effort known as Design for Manufacturing and Assembly.
The mass timber buildings that Sidewalk is planning will be produced in a factory that turns out standardized parts, which can be combined to form many different types of buildings. Everything in this “kit of parts” has to be modular and interlocking, including infrastructure like plumbing. Standardization of the parts enables the manufacturing process to be faster and more predictable, and their interlocking nature enables easy on-site assembly. Collectively, these traits speed up construction, making buildings cheaper to complete and driving affordability for builders and tenants.
The LEGO Group’s production and design process is a close and helpful analogy. When developing a new LEGO set, the company’s designers must choose from a comprehensive library of bricks and other small plastic components that must click together. Some customization of color, size, and shape is allowed, but creating too many unique pieces causes costs to balloon. In fact, LEGO’s failure to control its piece count in the early 2000s — the number of unique pieces doubled to 12,000 — nearly bankrupted the company.
Optimizing the “floor cassette”
If the basic unit of design for LEGO is the brick, for PMX, it’s what we call the “floor cassette.” The shell of each cassette is made of wood panels with acoustic and insulating layers. The interiors of each cassette house pipes and wires that are part of the building’s mechanical, electrical, and plumbing systems. Cassettes are slotted one by one into the envelope of the building, linking up to one another to form new floors.
Using cassettes to construct floors is very different from how floors of high-rise buildings are constructed today. In a typical site-built building, steel or plywood scaffolding is constructed to form the basis of the floor, with concrete poured on top — a long, arduous, and carbon-intensive process. Even in prefabricated buildings, which are few and far between, floor pieces are usually just basic concrete slabs, and the mechanical, electrical, and plumbing equipment has to be secured in place separately.
For the cassettes to lend themselves to efficient factory production and on-site assembly, we needed to drive down the number of unique cassettes on PMX, as well as the time it would take to produce each cassette in a factory. That meant the cassette-based approach had a couple of substantial implications for the design and the material choices on PMX.
First, the column spans on PMX had to squarely fit standardized cassette modules, or else we would have to create expensive, customized filler pieces. A uniform grid that neatly fits identical cassettes is also useful in construction. When each cassette is interchangeable, you no longer need to keep track of the specific sequence in which parts should be placed into the building.
Second, the choice to use floor cassettes influenced what materials we used. In a typical building, concrete is poured and left to cure on site over multiple days. But producing cassettes in a factory meant we needed to use materials (such as stone wool) that could be cut and applied quickly to keep the process moving. The cassettes in PMX can be produced in 25 steps, with each step taking 25 minutes on an assembly line — and we’re continuing to drive down those figures through process improvements and ongoing R&D.
Standardizing the building’s envelope
Some people still associate the “prefab” aesthetic with the post-World War II housing boom, when factories turned out cookie-cutter suburban homes. One of the goals of the PMX project was to defy that history, showing that prefabrication can still yield interesting architecture.
Much like all other elements of PMX, the building’s envelope needs to consist of a series of panels that can be produced by a factory. But the envelope can be “skinned” with panels of any shape, material, or design, so long as they meet building code requirements and follow a standardized modular pattern.
In the case of PMX, we began with a very basic module, consistent with what one might find on a typical downtown condo in any growing city — a metal panel with 40 percent window coverage, accompanied by a balcony for each unit. (If 40 percent coverage feels low, it’s actually a reflection of energy-efficient building design trends, which call for less window area and better insulation to reduce heating and cooling needs.) The team then explored a number of highly expressive designs, with the intention of showcasing a very wide range of architectural possibilities.
Through using different skins, PMX can transform into countless different buildings, including the four shown here. These skins, or others, can be applied to buildings of different shapes or sizes, all produced in a factory.
Of course, the great promise of manufactured timber is that it should help deliver buildings far faster and cheaper than conventional techniques. With construction costs booming in many major cities, timber is one potential solution for delivering more affordability. But instead of leading to generic high-rises, timber buildings can form dynamic neighborhoods celebrated for their distinct architecture — not just their efficient engineering.
If you’re a developer or timber professional who wants to work together on advancing tall timber building design, contact Sidewalk’s buildings team at firstname.lastname@example.org.
January 24, 2020