Kintai Bridge, a spectacular carpentry
Kintai Bridge is a remarkable example of the Japanese system of traditional wood
construction, revolving around centuries of craft tradition and knowledge that has
been passed from generation to generation.
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The crossing was first built in 1673 and was extensively replicated three times
over the space of three centuries - in 1686, 1950 and 2005. Between the main rebuilds,
it was constantly repaired. The structure itself is truly unique — making
it difficult to give a definitive description of bridge type, which features wooden
arch spans on masonry piers. Its derivative reaches back to the intricate wood joinery
found in Japanese temples, and translated, Kintaikyo (Kintai) means Brocade Sash
Bridge, from the Japanese ‘kin’ (brocade), ‘tai’ (sash)
and ‘kyo’ (bridge). The Kintai Bridge, as a wooden arch bridge structure,
is rare in the world, and the original appearance has been kept in almost perfect
condition since its construction in 1673. However, being a wooden artifact, corrosion
and wearing of the wooden members are conspicuous, and repairs of the wooden members
and reconstruction of the whole bridge take place every few decades.
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The bridge spans the Nishiki River in a series of five wooden arches. The second,
third and fourth bridges are 35.10m in span, while the first and fifth bridges are
34.80min span, and total length is 193.33m, 4.60min width. More than 20,000 parts
combined construct this bridge. Repairs of the wooden members and reconstruction
of the whole bridge take place every few decades.
Background of bridge construction
For nearly three hundred years, the many versions of the bridge stood without the
use of metal nails. This was achieved by the careful fitting of the wooden parts
and by the construction of the thick girders by clamping and binding them together
with metal belts. The main wooden parts of the bridge were covered by sheets of
copper for additional durability.
Structurally, the first and fifth bridge spans on either side are called ‘Hashirabashi,’
that is a girder bridge structure with bridge pillars and the second to fourth bridge
spans in the middle are called ‘Soribashi’, an arch structure bridge
Wooden arch bridges with such a structural type are rare in the world, even in comparison
with other Japanese wooden bridges(Kobayashi, 2010, 1), Chinese wooden arch bridges
(Watanabe, Kobayashi, Ito and Jojima, 2010, 15), and European wooden arch bridges
(Honda, Watanabe and Kobayashi, 2011, 85), no wooden arch bridge of the same kind
exists at present.
Each arch of the bridge consists of smoothly curved skeleton lines, its end support
points restrained so that when a vertical load is applied to that curved surface
of an arch, a horizontal reaction force is generated in the support points. When
subjected to free vibration, each arch shows the symmetric and asymmetric modes
of deformation inherent to an arch structure, as well as the asymmetric mode of
deformation. In view of these characteristics, each of the central three spans of
the Kintai Bridge is considered to have an arch structure, and is the prototype
of an arch bridge made of large-section, glue-laminated timbers. Each arch, retained
at the end support points, provides different rigidities against small rotation
and large rotation.
Technology behind bridge engineering
The three central spans of the Kintai Bridge are arch bridges, and the other two
end spans are girder bridges. The span of each arch bridge is 35.1m; that of each
girder-bridge is 34.8m. The girder of each span comprises 1st through 11th girder
members, a large ridge beam and a small ridge beam. The rear ends of the 1st through
4th girder members are inserted and bolt-clamped in the iron shoe mounted on the
upper part of the substructure. The 5th through 11th girder members are longitudinally
staggered so that each member protrudes by approximately one-third of its length
from the girder member immediately beneath it. Girder members are sequentially installed
in this way from each end of the span.
The problem of the passing on of the traditional carpentry techniques a carpentry
technique called ‘Shikuchi’, which is used for joints of the wooden
members in Kintai Bridge. However, outstandingly difficult and complicated carpentry
techniques used for example in temple building, are not used for the Kintai Bridge.
Only the general carpentry techniques for traditional Japanese house building are
used in the construction. Therefore, since the time of completion, the Kintai Bridge
has been a common site for the daily training of carpenters. In March 2007, the
Kintai Bridge Future Plan Consideration Committee of Iwakuni-shi made a plan for
regular bridge reconstruction every 20 years. As a result of which the systematic
problems of passing down the carpentry techniques were resolved, but quite a few
specific methodological problems still remain.
The arches feature a network of interlocking wood supports and laminated beams based
on the form of reversed catenaries. It is considered one of the country’s
most outstanding contributions to wood-frame craftsmanship. Even by modern engineering
standards, the structure of this bridge is considered to be extremely advanced.
One test placed a uniform load of 60 tons on the center span of the bridge before
it was disassembled. Despite then advanced age of the bridge material, the center
of the bridge sunk only 27mm under the load and this result satisfied the present
standards for pedestrian overpass.
Each arch of the bridge consists of smoothly curved skeleton lines, its end support
points restrained so that when a vertical load is applied to the curved surface
of an arch, a horizontal reaction force is generated in the support points. When
subjected to asymmetric uniform loading, each arch prominently shows the asymmetric
mode of deformation inherent to an arch structure. In view of these characteristics,
each of the central three spans of the Kintai Bridge is considered to have an arch
structure, and is the prototype of an arch bridge made of large-section, glue-laminated
timbers.
Research to evaluate strength of Kintai
A research program to evaluate the behavior and structural integrity of the pedestrian
bridge under operating environment has been conducted every five years over past
50 years. Field tests and an inspection of the bridge in 1998 highlighted minor
problems such as water penetration from the roadway and localized decay of woods.
An assessment verified the findings of the inspection and showed the arch ribs to
be slightly deteriorated. A conservation recommendation of the superstructures was
developed bearing in mind the static and dynamic field tests of the bridge, which
led to the reconstruction of the entire superstructures. The Kintaikyo Bridge was
considered to be safe from the strength point of view for its remaining desired
life as of 1998. A revised basis for the real safety of the bridge was performed
from 2001 to 2003. The inspection and assessment report concluded that the Kintaikyo
Bridge was approaching to the end of serviceability life of the floor surface. The
report also concluded that reconstruction of the superstructure was recommended
to provide a waterproof floor and prevent unpleasant vibration of the floor.
Restoring of the bridge
Since the construction of the first Kintai Bridge, the Bridge has been rebuilt and
maintained by local people. To long sustain the traditional bridge building technology,
it is best that such technology be passed down to local people. With this view,
in selecting builders the local authority adopts a single tendering method rather
than a competitive tender. Although the use of the single tendering method is rare,
in the case of the Kintai Bridge projects, it is most effective in transferring
traditional technology.
The Kintai Bridge having been designated a national cultural property, the City
of Iwakuni, home of the Kintai Bridge, bears the responsibility of preserving the
Bridge for present and future generations. Despite the Bridge’s unique five-span
arch structure, which is designed to enhance durability, the Bridge is vulnerable
to natural disasters such as floods and earthquakes. As a way of long sustaining
the Bridge, the City of Iwakuni decided to establish a unique system: instead of
reinforcing the existing bridge structure, the City decided to guarantee the succession
of bridge building technology, so as to ensure repeated rebuilding of the Bridge.
Facts:
Type: Arch Bridge
Place: City of Iwakuni, Yamaguchi Prefecture
First construction: 1673
Reconstruction: 1686, 1950 and 2005
Iwakuni, Iwakuni-shi, Yamaguchi-ken
Teruhiko Yoda, Waseda University, Tokyo, Japan (www.bma.arch.unique.it)