Kawazu-Nanadaru Loop Bridge, Spiral bridge in Kawazu, Japan

Kawazu-Nanadaru Loop Bridge, formally Nanadaru Viaduct, carries National Route 414 in Kawazu, Shizuoka. Its green steel box girders form two stacked 720-degree loops, 80 meters in diameter, on six tall cylindrical piers, with a total length of 1,064 meters and a 45-meter height difference. A straight elevated section feeds the upper deck; guardrails line both edges.

Construction took place in 1981 following the Izu-Oshima earthquake of 1978, which triggered landslides and damage to the original mountain road. The spiral solution allowed safe restoration of the route through the difficult terrain.

The bridge's formal name is Nanadaru Viaduct, while Kawazu-Nanadaru refers to the seven waterfalls of the Kawazu Nanadaru area along the route. The central space inside the loop is used as a town-run parking lot, and the bridge carries National Route 414, which was renumbered from Shizuoka Prefectural Route 13 in April 1982.

The space inside the central loop is used as a town-run parking lot. The bridge lies about 53 km from the Nagaizumi-Numazu Interchange of the Shin-Tomei Expressway, reached via the Izu-Jukan, Izu-Chuo and Shuzenji roads, then Routes 136 and 414. The speed limit on the bridge is 40 km/h and overtaking is prohibited; bicycles may also cross.

The construction manages a 45-meter (148-foot) elevation change through its double loop form, which is rare among Japanese mountain roads. For its technical design in overcoming the difficult terrain, it received the Tanaka Prize from the Japan Society of Civil Engineers.