Chagsam Bridge

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Chagsam Bridge 1904
Sketch of the Chagsam Bridge, based on information provided by Pundit AK , who traveled on behalf of the British in 1878

The Chagsam Bridge (also Chakzam Bridge , Chushul Chakzam , Chusul Chakzam ) was a suspension bridge over the Yarlung Zangbo near Qüxü (Chushul) southwest of Lhasa in Tibet .

It was built in 1430 by the Tibetan Buddhist philosopher, teacher, doctor, architect, blacksmith and bridge builder Thangtong Gyelpo and was the largest and most important of the eight iron chain bridges that Thangtong Gyelpo built over the Yarlung Tsangpo.

location

The Chagsam Bridge was located just above the mouth of the Lhasa He , which flows from Lhasa into the Yarlung Zangbo , a few hundred meters below the modern road bridge and below the ferry, which also crossed the Yarlung Tsangpo before it was built. It was on the important caravan route from Lhasa to Gyangzê and Samzhubzê and on the way to the passes across the Himalayas to Nepal , Sikkim and Bhutan .

history

According to a legend, the poorly dressed Thangtong Gyelpo was refused passage by the ferrymen concerned about their pay, which angered him so much that he decided to build a bridge next to the ferry and a monastery above its southern pillar. It was initially called Chusul Chagsam after the place , later simply Chagsam or iron bridge . Although it was always free to use, some travelers were so afraid of crossing the violently swaying bridge that they preferred the not harmless crossing over the raging river. Ippolito Desideri had already described in the 18th century that the chain bridges were only used by people who were forced to do so in their need. Nain Singh , who was familiar with dangerous situations, preferred the ferry on his first trip to Lhasa. The Pundit AK alias Kishen Singh used it in 1878 and the sketch was made according to his information. When Wadell came to the river in 1904 with the British Tibet campaign under the direction of Francis Younghusband , only the iron chains were still hanging, as can be seen in the photo, probably because the Yarlung Tsangpo had formed a side arm that prevented further use. It was demolished by the Chinese around 1950.

description

Suspension bridge

The Chagsam Bridge was probably the first real suspension bridge in which - unlike a tensioning ribbon bridge - the path was not attached directly to the sagging chains and thus followed their curve, but with ropes made of yak hair at a distance of just under one meter The two supporting iron chains were attached so that the boards, about a foot wide, inserted into the rope loops, ran in a horizontal line. According to the sketch, the path had no special handrail, for example in the form of additional horizontal ropes connected to the hangers, but was open at the sides. The users had to move from one of the vertical hangers to the next. The path was normally about 7.60 m above the water, but at high tide it was still 4.6 m above the then rapid current. The chains were attached to beams in the two masonry pillars and continued as anchoring until they were fastened in the bank rocks or on the other side in the river floor. A comparison of the photo with the sketch shows that this is only a schematic representation; the bridge piers were not towering towers, but rather compact structures resembling a Chörten . On the south bank the pillar stood on the rocky bank, the north pillar stood on a small rise in the river bed, which was usually dry beyond this pillar. Only at the end of the 19th century does the river seem to have formed a branch to the north of this pillar, which cut the bridge from the north bank.

span

The span of the bridge was estimated by Wadell, who gave the most precise information about the bridge, at 150 yards = 137 m. The 300 steps indicated in the sketch, on the other hand, are puzzling: the pundits had trained themselves to a largely constant step length of 80 cm, which would result in a span of 240 m and a distance between the banks of 800 m. With the chain sag of 1: 11.5 resulting from the sketch, such a span requires pillar heights of 20.9 m above the bridge deck or 25.5 m above the river water at high tide, which is not in accordance with the photo are bring. If one assumes that the stride lengths have been halved on the wobbly bridge, one arrives at a span corresponding to Wadell's estimate, but on the other hand at a width of the river bed that at least cannot be reconciled with today's conditions. In any case, with a span of 137 m, the bridge had by far the largest span of all bridges of that time.

Chains

The chains consisted of about 30 cm long and 5 to 8 cm wide chain links, which were forged from rectangular bars with a cross section of about 1.0 cm × 2.5 cm. Each chain should have had a weight of well over 550 kg. The chain links had to be transported individually or in short chain sections to the construction site and welded together there, whereby the question of how the chains were pulled across the river and tensioned remains open. Even after more than half a millennium, the chain links showed only minor signs of wear and virtually no corrosion.

According to a chemical analysis of a piece of chain from Bhutan, which probably also came from one of the bridges built by Thangtong Gyalpo, the iron contained only 0.2% impurities and had a carbon content of only 0.012%. An investigation of the piece of chain by the ETH Zurich showed that the piece of iron mainly consisted of α-iron crystals ( ferrite ) with portions that contained iron carbide ( cementite ) and thus had a higher carbon content . At the weld seam there was a zone less than 1/10 mm thick with significantly harder iron crystals and with up to 2.6% arsenic . Arsenic iron melts more deeply than the other iron in the chain piece. The microscopic examination of the seam clearly showed that the arsenic-containing surface had melted, which is not the case with normal iron weld seams. It is not known how the thin layer of arsenic was applied to the surfaces to be welded. Such a weld seam has so far only been found in a Roman sword that originated in Germany and was probably forged in Damascus in the first century AD. The weld seam is based on a technology that is currently unknown.

Web links

Individual evidence

  1. ↑ Manfred Gerner: Chakzampa Thangtong Gyalpo , p. 8
  2. ↑ Chagsam means iron bridge ; Chagsam bridge is thus a pleonasm
  3. ↑ Qushui Yaluzangbujiang Bridge : 29 ° 19 ′ 47.3 ″  N , 90 ° 41 ′ 8.4 ″  E
  4. ↑ a b c Waddell: Lhasa and its Mysteries , p. 312
  5. ↑ Gerner, p. 8
  6. ↑ Derek Waller: The Pundits . The University Press of Kentucky, Kentucky 1990, ISBN 0-8131-1666-X , p. 47
  7. ↑ Gerner, p. 83
  8. ↑ A very rough estimate could be as follows: The two bars of a chain link had a cross-section of 2 × (0.01 m × 0.025 m) = 0.005 m². If the round connections of the two chain links are disregarded and the cross-section is multiplied by the chain length rounded up to 140 m due to the slack (without the anchoring chains), the result is 0.005 m² × 140 m = 0.07 m³. A density of 7874 kg / m³ results in a mass of 551 kg.
  9. ↑ Gerner, pp. 44/45
  10. ↑ In the past, welding was understood to mean the production of a firm connection between two pieces of iron, which is created in the forge by hammering the pieces together in a red-hot state.
  11. ↑ Willfried Epprecht: Brief report on the metallurgical investigation of an iron chain bridge piece from Bhutan. In Tom F. Peters: The Development of Large Bridge Construction . 2nd Edition. ETH, Zurich 1980

See also