Carbon Fiber Strands Tested for Seismic Stability

ADDING STRUCTURAL supports to seismically retrofit reinforced-concrete buildings can be done in many ways. Kengo Kuma & Associates, an architecture firm based in Tokyo and Paris, has turned to an entirely new method in which the strength of the solution is concealed within lightweight strands of carbon fiber. Dubbed strand rods, the composite ropelike strands have been used to create latticelike, braced-frame interior bearing walls and an exterior support curtain that appears to delicately drape over the edges of the building.

The strand rods were used to seismically retrofit a three-story, 40 m by 22 m building constructed in Nomi, Japan, in 1968. The rigid-frame, reinforced-concrete structure will now be used by the client, Komatsu Seiren, as a textile factory and will also have an experimental laboratory, according to Kengo Kuma, an architect and the founder of Kengo Kuma & Associates. Kuma wrote in response to written questions posed by Civil Engineering. The building was formerly Komatsu Seiren’s head office.

Carbon Fiber strands

Komatsu Seiren wanted to propose something new to cope with earthquake-induced ground motion, an inescapable feature of Japan, noted Kuma. The carbon fiber strands draw on the rope-braiding techniques that the region is known for, according to the architects.

The strand rods have a carbon fiber inner lining with a synthetic and inorganic fiber outer layer and are impregnated with thermoplastic resin. For this project, seven strands were bundled together to create a rope with a 9 mm cross section, according to Komatsu Seiren’s website. The result is a lightweight, delicate material with a high tensile strength and a robust structure. Indeed, a 160 m long roll coiled for transport can be carried by hand and weighs only 12 kg, approximately one-fifth what metal wire possessing the same degree of strength would weigh, according to the Komatsu Seiren website.

The lightweight, flexible strands of carbon fiber have been twisted together to create ropelike lengths that offer 10 times the strength of steel.

With strand rods, the flexibility of the carbon fiber has been improved while its strength has been retained. “[The] carbon fiber rod has about 10 times more strength compared with steel, but this material has not been approved in Japan so we adopted this for additional reinforcement as outside tension rods and inside walls,” said Norihiro Ejiri, a principal of Tokyo-based Ejiri Structural Engineers, which completed the structural design for the project. Ejiri also wrote in response to written questions posed by Civil Engineering.

The exterior thermoplastic composite curtain is composed of hundreds of flexible strand rods. These extend from a metal arch above the roof of a small penthouse structure and pass over the edges on three sides of the building in descending to the ground. The strand rods for the exterior curtain are connected both to the building and to the ground. “The ropes are firmly fixed (with extra foundation and strengthening of the rooftop frame) at each point, and it is estimated that the whole structure is given an extra strength of 13 percent to resist the quake,” Kuma explained.

Bolts bonded to the strand rods at the top of the building connect them to metal joint pieces, which in turn are connected to a reinforced-concrete parapet. At the ground, bonded bolts connect the strand rods to metal joints connected to an embedded concrete anchor.

The interior partition walls comprise two layers of strand rods held in steel frames with bonded bolts that are bolted to the building’s frame. The strand rods extend along both diagonals in each layer to impart strength during a seismic event. This also creates the visual effect of an open mesh wall that allows sunlight and air to stream through the interior partitions.

As a material, the strand rods have a large allowable stress, according to Ejiri. Thus when the structural engineers designed the joints in conformity with the architects’ design, they took into account the expected stresses rather than the strength of the sections, he said. The calculations and design were based on test results.

The strand rods satisfy seismic performance requirements within Japan, according to the Komatsu Seiren website.

“How it works is quite simple: when the building leans to the east, the rods will pull it back to the west and vice versa,” Kuma explained. “Thus, the building could be sustained as it is,” he noted. The design “is unique in that we take full advantage of pulling (tension).”

Weight also comes into play. “The weight of carbon fiber is almost [a] quarter of that of steel,” Kuma noted. “But its pulling force is 10 times stronger. It eases construction work to a great extent, and there is no need to rely on heavy steel bracing.” Even so, there is one drawback: “Since it is carbon, however, the material could be vulnerable to fire, and that needs to be carefully worked on,” Kuma said.

Kuma’s hope is that when the next large earthquake occurs, the effectiveness of the carbon fiber solution will be demonstrated so that the material can be officially approved as an earthquake-resistant construction material. This is the first time such fibers have been used as a means of seismic reinforcement, according to Kuma.