Establishing a Wooden Beam and Reinforced Concrete Floor Construction Method for Wooden Buildings in Medium- to Large-Sized Cities

When constructing medium- to large-scale buildings other than residential structures using wood, one challenge is that the larger cross-sections of the timber can make the space feel cramped. Furthermore, because wood is a lighter and more flexible material than steel or concrete, using it for floors can cause vibrations to be transmitted more easily, potentially compromising living comfort.
The solution to these issues is the newly developed “Composite Beam System Combining Wooden Beams and Reinforced Concrete Floor Slabs” (hereinafter referred to as this system).This method takes advantage of the fact that wood and concrete have similar compressive strengths to create a strong, integrated joint between the two materials. The RC floor slab enhances the rigidity of the wooden beams, making the structure less prone to vibration; as a result, we have achieved a floor with vibration resistance comparable to that of a steel-frame structure with concrete slabs.The beam length has been extended to approximately 12 meters—twice that of conventional beams—enabling long spans and making the system suitable for medium- to large-scale buildings with extensive floor areas. While the beam depth for non-composite beams is typically around 120 cm, this method reduces it to 90 cm—about three-quarters of that—which helps minimize floor-to-floor heights and allows for the construction of additional stories when building taller structures.

We began developing this construction method about six years ago. Since then, we have gone through a process of trial and error and conducted numerous meticulous experiments and verifications. The structural design team at Nikken Sekkei came up with the idea: “Let’s develop versatile wooden beams and reinforced concrete floor slabs capable of supporting larger-scale buildings!”—and thus, a joint project between the two companies was launched.Wood-hybrid construction is still in its infancy, and Kimiaki Harada, a senior expert in the structural design group, reflects, “Until now, no effective technology had been developed for the joint between wooden beams and reinforced concrete floor slabs.”

The team’s first attempt involved connecting wood and reinforced concrete using stud bolts. However, this approach proved costly and time-consuming when attempting to achieve the necessary rigidity. Next, they tested the idea of using the lattice-shaped timber—which Sumitomo Forestry employs as a substrate for wooden exterior walls—as a friction material between the beams and floor slabs. Unfortunately, this did not yield the desired strength.Inspired by this process, approximately three years after development began, the team came up with the idea of a “sawtooth joint”—a method that involves creating ridges and grooves on the wooden beam itself to securely connect it to the reinforced concrete floor.Takashi Fukushima of the Structural Design Group (Director) explains its functionality: “Connecting the elements through compression is the most straightforward and rational idea. It also allowed us to take advantage of wood’s ease of machining.” It is, in essence, a once-in-a-lifetime cross-sectional configuration that efficiently utilizes the properties of these two materials, whose compressive strengths are equivalent.

In the field of construction methods, ideas alone rarely lead to practical application. It took another three years of conducting full-scale experiments and habitability tests before we were able to evaluate the structural performance.

This construction method takes constructability into account when obtaining structural performance certification. It is compatible with a variety of formwork systems, including conventional formwork and deck plates. Furthermore, regarding fire resistance—the greatest challenge in wood-frame construction—this method can be used even in high-rise buildings, which present the highest hurdle for wood-frame structures, by combining it with fire-resistant wooden beams, such as those in Sumitomo Forestry’s “Kigurumi” series (a construction method that enhances fire resistance by inserting non-combustible materials beneath the bottom of the beams).

In 2020, Nikken Sekkei and Sumitomo Forestry announced “W350 Plan,” a 350-meter-tall wooden skyscraper. This project is part of Sumitomo Forestry’s research and technology development initiative leading up to the company’s 350th anniversary in 2041. This construction method is also part of the company’s efforts to develop technologies for medium- to large-scale wooden structures.

W350 Plan—an eco-friendly, wood-based city—is packed with future technologies and ideas, including construction methods that enhance fire and seismic resistance, environmentally conscious technologies, and the development of new types of timber.“ W350 Plan is the embodiment of challenges that take time to solve. We have finally been able to solve one of them,” said Senior Expert Harada. Along with the widespread adoption of this construction method—which is “easy for anyone to use”—Nikken Sekkei’s Structural Group is striving to develop and promote wood-frame construction technologies to achieve the goal of net-zero greenhouse gas emissions by 2050.