Folded Plate Roofs Are Back, and Now in Mass Timber
Folded sheet structures were invented in the 1920s and became all the rage in mid-century architecture around the world. Capable of very long spans, they have their own beautiful aesthetics, but they can be finicky and difficult to engineer and build.
In his 1964 essay The aesthetics of folded plates, wrote Clovis B. Heimsath of Rice University:
“Who is really responsible when a good design of folded sheets is produced – the architect or the engineer? Both should take credit. The architect is technically unfamiliar with folded sheets, although he tries to use them correctly. The architect turns to the engineer, should look to him for guidance, not only in calculating a structure, but also in its meaning…A beautiful building is a tribute to everyone who worked on it, and real teamwork between engineer and architect is achieved too rarely, but an ending leaves much to be desired. When general design and structure come together in good work, it is always worth the extra attention invested in it.”
Take the new Southwest Library in Washington, DC, which is dominated by its beautiful folded sheet roof made of laminated wood (DLT). It really is, as Heimsath described, a testament to the architect, Carl Knudson of Perkins & Will, and the engineer, Lucas Epp of StructureCraft in Abbotsford, British Columbia. Indeed, Epp tells Treehugger that he and Knudson “worked super closely throughout the project, from concept to build.” It is unusual for StructureCraft to be the Engineer of Record for the project and have performed the construction of the structure.
Epp tells Treehugger that there were numerous technical challenges and complications.
“The ‘wrinkled’ folded sheet roof presented technical challenges in how it would be detailed, fabricated, installed and erected. Connection details, prefabrication, the mounting mold and mounting planning were the keys to the speed of mounting and successful installation. The unique shape of the long span folded sheet roofs created a particular challenge for both the structural engineering and construction of these complex elements. The use of dowel laminated timber in a folded sheet structure was a world first.
The panels were assembled at the StructureCraft factory in Abbotsford (see Treehugger’s tour of it here), in a process that involves assembling wood on a giant custom rig, drilling holes and then pushing dried hardwood dowels into them. As the hardwood absorbs moisture from the surrounding softwood, it swells up a bit and seals it all together. In this case, the panels were “sheathed with glued plywood to create the necessary membrane stiffness. The plywood was secured in the StructureCraft shop and glued to the folded sheet of glulam cords at the ridge and trough.”
Before shipment, the cords were split in half. Once connected on site, tie bars the four trough corners, as the two halves of each gable were joined into self-supporting “trusses” of the folded sheet that weighed more than 15,000 pounds each. These up to 70 ft. long and 20 ft wide facades were then lifted into place. From initial shape and design to engineering, prefabrication and installation, the entire team came together around this complex challenge and successfully delivered the unique architectural form.”
Writing in 1964, Heimsath noted, “support for the sheet roof can be effective or disastrous. In buildings in which it is effective, it is fair and clear. The roof itself must “read”, so the structure supporting it must be known Here the support is definitely fair and clear, especially outside with those exposed wooden canted columns.
The overhang on the outside is dramatic, but also has an important function as a shadow. As Heimsath points out,
“Interestingly, with the application of so many of the well-designed folded sheet metal roofs, the roofs overhang the front and rear enclosing walls. Visually, this accomplishes two things: it lets the viewer know what’s going on and prepares before him in front of the space.The overhangs cast shadows on the enclosing walls making them less of a visual barrier.If the enclosing walls are glass it is good to have a cantilever as glass reflects sunlight during the day and can appear more impenetrable than marble. A roof that appears to float at night when the lights are on the inside making the glass disappear can appear an ominous block when the glass reads like a wall during the day.”
And that’s exactly what Knutson and Perkins&Will have done here: extend the roof well beyond the front wall because it looks so nice, but also makes the glass disappear inside and out.
Heimsath even had something to say about enlightenment, noting:
“In many buildings, the simplicity of the interior form is destroyed by the lighting system, whether plastered as small warts on the form itself or hung in a random manner with little regard for the overall effect. A well-chosen lighting fixture can illuminate the roof with discount, and the few dollars invested in the fixtures can be the difference between chaos and a remarkable space.”
Lighting is certainly an area of building design that has evolved significantly since 1964, and here it does two jobs: illuminating the work surfaces in the library, but also highlighting the wooden ceiling above.
In the mid-20th century, glazing was often integrated into the folded sheet roof design. light from skylights is difficult to control, but the angled panels on the roof are used very well in the 21st century version: solar panels. The 100-year-old roof concept plays nicely with modern technology.
There are many reasons to get excited about this building. Folded sheet structures are efficient in the use of materials and can cover very long spans, in fact very deep beams on a slope, leaning against other very deep beams. They are complicated to design and build, but StructureCraft has put their modern tools to work, with “complex nonlinear finite element analysis to enable prediction of stresses and structural behavior of the folded sheet.” They used BIM (Building Information Modeling) and “a 3D fabrication model with a high level of detail that can be used by designers and constructors alike – the team on site has used it extensively.”
Epp tells Treehugger:
“BIM also enables proactive clash detection and penetration coordination across all industries, which is especially important in a prefabricated highly exposed wood structure. The BIM model controlled the CNC manufacturing process of all Glulam, DLT and steel, producing detailed part-work drawings for each part.”
Perkins&Will and StructureCraft have designed and built a beautiful hat on a beautiful building, demonstrating the principle Clovis Heimsath wrote about in 1964 about what can happen when talented architects and engineers actually work together as a team. His conclusion, and ours:
“Folded sheets are so beautiful in themselves that every effort must be made not to destroy their integrity, a job that requires the best that both professionals have to offer.”