An arch of interlocking wood
The Kintai Bridge achieves its five graceful spans across the Nishiki River using a nail-free construction method first realized in 1673. The bridge’s total length is 193.3 meters, with the three central arches each spanning 35.1 meters and the two end spans covering 34.8 meters. The entire structure relies on the precise fitting of timber, held together with metal straps and clamps. This technique creates a structure so robust that it becomes stronger as weight is applied from above.
The arches are not single, curved beams. Instead, they are complex assemblies of smaller, straight wooden girders. Each arch rib is composed of eleven main girder members stacked and layered. Builders started from the stone piers, cantilevering the girders out toward the center. Each successive girder was placed to protrude about one-third of its length beyond the one beneath it. The five parallel rows of these built-up beams are bound together by iron straps. A large ridge beam, similar to a keystone in a stone arch, locks the two halves of the arch together at the apex. This method of joinery distributes the load outwards and down into the massive stone piers, creating a stable and enduring structure. A variety of woods were specified in original 1699 drawings, including Zelkova for high-load elements, pine for central beams, and Japanese cypress for the decking and railings.
Surviving floods and time
The bridge's design was a direct response to the Nishiki River's frequent flooding, which destroyed all previous attempts at a crossing. The third lord of Iwakuni, Kikkawa Hiroyoshi, oversaw the 1673 construction that solved the problem by eliminating piers from the river's strongest currents. While the original bridge was damaged by a flood the year after its completion, the stone piers were reinforced, and the design proved sound for 276 years.
A devastating typhoon in September 1950 finally washed the bridge away. At the time, maintenance had been suspended due to World War II, leaving the structure in a weakened state. A reconstruction effort began in 1951, and the bridge was completed in 1953, faithfully replicating the original nail-free techniques. The design also incorporates a clever flood-protection measure: the wooden pathway is designed to "float" on top of the main frame, connected by mortise and tenon joints. This allows extreme floodwaters to lift the walkway off and carry it downstream, sparing the main arch structures from destruction. A periodic rebuilding schedule was also established for maintenance, with the central arches rebuilt every 20 years and the end spans every 40 years.