By: Hans-Erik Blomgren, PE, SE, P.Eng., Struct.Eng., Director of Testing and Certification for Structural Products
Written for Wood Design & Building, published by the Canadian Wood Council
Providing flexible timber solutions for commercial long-span floor and roof applications
The application of CLT panels as floor and roof structures for commercial buildings in North America is advancing rapidly, driven by the aesthetic appeal of exposed wood, faster construction times and carbon storage benefits. But the gap between market interest and defined market solutions often can be significant. In order to be competitive in the marketplace compared to established concrete and steel options, the CLT architect and engineer are challenged to bring a high level of optimization to floor plate designs that reduce cost – without compromising structural, acoustic or fire performance. This is especially the case for open office and education buildings, where the program requires floor spans between column lines that are larger than traditional timber framing approaches can efficiently achieve.
CLT FLOOR & ROOF SPANS
Katerra opened the highest-capacity CLT factory in the U.S. in 2019 and began delivering CLT for use as floors and roofs on multi-family residential projects and office buildings. For typical multi-family buildings, CLT spans ranging from 12–20 ft. are required; for this project type, three- or five-ply Katerra CLT panels are well suited to span between bearing walls; however, Katerra’s first office and education mixed-use building, the Catalyst Building in Eastern Washington State, required a 30-ft. floor span between column lines. A CLT flat panel is not capable of spanning this distance, even when increased in thickness to seven- or nine-ply.
RIB PANELS DEFINED
Posed with this challenge, along with a desire to maximize the use of its CLT product, Katerra saw an opportunity to increase the span of its product beyond 20 ft. by introducing glulam beams or “ribs” to the underside of each panel, spanning parallel with its long (strong) direction (Figure 1). The efficiency of the resulting product is achieved when a rigid and strong structural connection between the CLT and glulam ribs is provided, which enables the full cross-section to work compositely.
Calculations demonstrate the strength and stiffness of a rib panel can increase nearly four-fold when full composite action is achieved (Figure 2). This optimized use of wood fiber is the primary advantage of a rib panel. As an example, the Catalyst Building rib panel has a span-to-depth ratio of 15.5:1, which is equivalent to a benchmark composite steel floor beam for the same span and load, while being 40 percent lighter in weight. It is clear to see the potential rib panel products have to expand mass timber floor spans well beyond the 20-ft. limit of five-ply (Figure 3).
With the project benefits of the rib panel identified, Katerra’s engineers and manufacturing team began a product development effort to define the most suitable materials, method and process of compositely fastening glulam ribs to the underside of CLT. The selected connection method is achieved by: 1) applying an adhesive to the top of each rib; 2) laying the CLT panel on top; and 3) installing partially threaded screws through the joint to clamp the surfaces closely together and to spread out the adhesive, covering the full surface area of the mated parts (Figure 4). This process has been dubbed the “Screw-Press-Glue” method and has undergone prior research and testing in Europe . The resulting manufacturing process allows for a rib panel to be fully assembled in less than 20 minutes (Figure 5). The production for the Catalyst Building was able to stay ahead of the construction site schedule by producing up to 18 30×10-ft. rib panels in a single factory shift.