Old Danjo Bridge (Yahata Bridge)

A Bridge That Lost Its River and Kept Its Name

The Former Danjo Bridge, now called Yahata Bridge (旧弾正橋（八幡橋）, Kyu Danjobashi / Hachimanbashi), is an iron bridge of 1878 standing in Tomioka, Koto City, Tokyo, and was designated an Important Cultural Property on 27 June 1977. It is 15.2 metres long, a single-span tied arch, and it no longer crosses water. The canal it was moved to has been filled in, and the bridge now spans a sunken pedestrian promenade beside Tomioka Hachimangu shrine.

It began somewhere else entirely. In November 1878 it was erected over the Kaedegawa in Kyobashi Ward, in what is now Chuo City, on a road linking Babasakimon with Honjo and Fukagawa. The name came from the nearby residence of a samurai family holding the title Shimada Danjo. The Kaedegawa itself is gone too, filled in after the war and covered by the Inner Circular Route of the Shuto Expressway.

The bridge was made at the Akabane Works of the Ministry of Public Works, a government factory in present-day Mita, Minato City, which turned out machinery and steam engines rather than bridges as its main line of work. Its form follows the patented truss of the American engineer Squire Whipple, and the design is generally credited to Matsumoto Shoichiro, who had studied engineering in the United States, though the attribution rests on secondary accounts rather than the national register.

Two Metals, One Structure

The national register describes what makes this bridge worth protecting in a single technical observation: the arch is cast iron and everything else is wrought iron, a combination of materials no longer found in surviving bridges. That sentence is easy to read past. It is the whole point.

Cast iron is strong in compression and unreliable in tension. Wrought iron behaves the other way round. In a tied arch the curved upper member is pushed, and the tie below it is pulled, so the builders at Akabane put cast iron where the pushing happened and wrought iron where the pulling did. The arch is understood to have been assembled from around five cast segments of roughly three metres each rather than poured in one piece, since welding cast iron was beyond the technology of 1878; the wrought iron tension members were threaded and bolted up.

The result sits at a hinge point in engineering history. Iron bridges in Japan moved from cast iron through wrought iron to steel, and this one belongs to the transition, using both of the earlier metals at once in the places each suited. The Agency for Cultural Affairs calls it the first iron bridge erected in Tokyo and a valuable relic in the history of Japanese civil engineering; it is also widely described as the oldest surviving domestically made iron bridge in the country.

American recognition followed. In 1989 the American Society of Civil Engineers gave the bridge its landmark award, the first time a structure in Japan had received it. The Whipple type is now rare even in the United States, and this example follows the standard design closely. The commemorative plaque sits on the promenade beneath the deck.

What to Look For, and How to Find It

Go to the pin heads at the joints of the lower chord. Each is capped with a chrysanthemum-shaped ornament, said to mark manufacture at a government works. No other bridge in Japan carries the device, though a comparable decoration appears on the cast iron gate of Myohoji temple in Suginami, made at the same Akabane works around the same period.

Note the width, or the lack of it. The bridge was 9.1 metres wide when it carried road traffic over the Kaedegawa. A wider Danjobashi was built about fifty metres to the north in 1913, and the older bridge stayed in use under the name Moto-Danjobashi until the road widening that came with reconstruction after the Great Kanto Earthquake retired it. Relocation ran from December 1928 to May 1929, and in the process the deck was cut down to 2.0 metres. The proportions you see now are those of a footbridge wearing the ironwork of a road bridge, which is why it looks heavier than its span deserves.

The deck itself is not original. It was rebuilt in steel plate welded to steel girders, said to have replaced the timber decking around 1965. In 1976 the branch of the Aburaborigawa below was filled in, and landscaping in 1990 turned the old channel into the promenade and sunken garden the bridge crosses today.

There is no fee, no gate and no closing time. The bridge is a working pedestrian route and you are meant to walk over it. To see the ironwork properly, take the steps down to the promenade and look up at the underside, which is where the material distinction between arch and tie reads most clearly. Monzen-nakacho Station on the Tokyo Metro Tozai Line and the Toei Oedo Line is about five minutes away on foot, and Tomioka Hachimangu, Fukagawa Fudodo and the Fukagawa Edo Museum are all within easy walking distance, so the bridge fits naturally into a half day in the neighbourhood.