Songsu Bridge in Seoul made world headlines in October 1994 when one of the main trusses of the main bridge failed during rush hour, and 29 people plunged to their deaths. Fuelled by an explosion in the growth of car ownership, the reconstructed bridge is one of three existing bridges over the Han River being widened to accommodate rising traffic levels.
The bridge was originally constructed in the mid-1970s to the Korean loading standard DB18 (similar to Japan's AASHTO HS20-44), providing a four-lane crossing over the river. It was 1161m long between abutments and comprised a main bridge of 672m, built using 120m span Gerber trusses, along with 489m of approach spans of plate girder composite construction. Each of the main spans comprised three 48m long suspended trusses, 5m deep, supported by 36m long cantilever trusses, deepening to 11m at the piers.
The cause of the collapse was found to be the failure of the fabricated hanger at the point of the welded connection between the thicker eye plates at each end of the hangers and the thinner plates forming the H section hanger. 'As well as being a poor design detail, the welding had not been carried our properly,' says Mike King, High-Point Rendel's senior structural adviser on the bridge.
In June 1995, SMG appointed High-Point Rendel (HPR), in association with local consultant Yooshin, as supervisers for the reconstruction project, which was due for completion in January 1996. However, High-Point reported that the repairs necessary to welded joints throughout the main truss chords were far more extensive than previous surveys had indicated.
The consultant concluded that the repair scheme was uneconomical and advised that a complete reconstruction should take place.
After a period of weighing up a variety of different solutions, HPR's recommendation was accepted by Seoul Metropolitan Government in August 1995 and work began to replace the crossing in the shortest possible time.
High-Point worked closely with the designers, Chun-Il Engineering and Dongmyeung, to provide more effective solutions. These included a redesign of the hangers to use multiple flat plates and self-lubricating bearings, and a redesign of the steel deck to utilise more structurally efficient orthotropic panels.
In June 1997, the reconstructed bridge was re-opened to traffic and all works were completed by August. While re-establishment of the river crossing was the most urgent requirement, the need for an increase in capacity at some future date was realised, and the reconstruction project was adapted to accommodate widening from four to six lanes.
The US$120M plan called for an extra truss on each side of the existing bridge, widening it from 19.4m to 35m. An agreement was made between Hyundai, HPR and Chun-Il, with HPR responsible for the widening of the bridge and Chun-Il responsible for the interchange designs and approach ramp structures. In May 1998, the Hyundai/HPR/ Chun-Il submission won the competition with the highest mark and largest ever margin in Korea. Construction began on new foundations for interchange ramps in April 1999 and the project is due for completion in 2002.
'I suspect that because of the World Cup, there will be a lot of pressure to get it open for the middle of 2002. They've started on new piers and foundations for the ramps on both the south and north side. They have let the subcontracts for fabrication of the widening trusses and decks, which will be carried out by Hyundai Heavy Industries at the Pusan shipyard, which has a large bridge division,' says Mike King, High-Point Rendel's senior structural adviser on the bridge.
Further upstream, Kayang Bridge is one of three new crossings being built over the Han River in Seoul. It will become one of the key arteries for traffic heading for the stadium being built for the 2002 World Cup. Mott MacDonald is consultant to the client, Seoul Metropolitan Government, with Dong Il Engineering and the Korean Register of Shipping Engineering Department as sub-consultant.
The $125M project started in 1995 and is programmed for completion at the end of October 2000. The bridge is 4,214m in length, including the elevated structures of the main bridge and two sets of interchange ramps, and it will connect Kayang-Dong on the south of the river to Sangam-Dong to the north. It also provides access between two major highways - the Olympic Expressway and the Jayoo Highway. A separate contract will extend the link to give access to the 2002 World Cup stadium.
The main bridge is a twin steel box girder construction supporting an orthotropic steel deck with an overall length of 1,563m. It comprises seven sections and each of the sections has three or four continuous spans made up of continuous 2.5m deep box girders.
The interchanges also include more than 3.4 km of retaining walls incorporating 1,200 steel piles, 2,400 precast concrete piles, and cast-in-situ piles using expanded polystyrene backfill. There are also gravity walls and 340m of reinforced earth walls. A start on the temporary works for the retaining walls began after the end of the flood season last year.
Piling works and the construction of the walls continued throughout the winter, in order to complete all the piled retaining walls prior to the start of the summer monsoon season. Contractor is a consortium of Dong Ah Construction, Hyundai Construction and Samsung Construction.
The main section of the bridge is made up of three spans of 110m, 180m and 110m - the box sections varying between 2.5m and 10m deep. The two main bridge piers are reinforced concrete cellular structures and adjoining sections are reinforced concrete portals. The main spans are supported on 8m diameter caissons, founded in the gneiss bedrock, and the approach sections are supported on caissons or cast-in-situ piled foundations.
Fabrication of the 23,000t of steelwork for the superstructure began in October 1997 and is programmed for completion in October this year. The first sections arrived on site in May 1988 and erection of the first spans began at the end of June last year.
Since then, there have been more than 140 major lifts and 15,000t of steelwork has been erected. Another 7,000t of steelwork for the main span of the main bridge lies ahead, as does work on the 2.7 km of concrete deck slab for the interchange ramps. Surfacing and finishing works will follow.
Steelwork sections are fabricated at Samsung Heavy Industry's works on the island of Koje, south of the Korean peninsula. The main box sections are assembled on the river banks into units of up to 200t, which are then erected using a floating crane. The Han River is only navigable by small craft, so the erection procedure adopted for the main spans includes temporary piers supported in the river by steel piles.
'We have a resident representative in Koje and we go and visit, talk to the contractors and the quality control team, and we also talk to our own inspectors there to make sure everything is running smoothly', says Mott MacDonald's chief bridge engineer Steve Davey.
'I think one of the main things that Mott MacDonald is here for, is to ensure continued, high quality construction. Overseas companies keep people on their toes to continually keep up without cutting corners. That's a priority.'