When assumptions made at tender stage about the condition of a critical bridge in Vietnam were found to no longer be valid, the techniques proposed for widening and strengthening the structure had to be revised.

Its city-centre location, hemmed in by traffic, businesses and houses, was another crucial factor the team had to take into account.

  • Name of the owner & client
    Ho Chi Minh City DOT Traffic Management Division No. 1
  • Delivery date of the project
    December 2019
Y bridge in Vietnam - widening works and structural strengthening

A complex structural composition

The Y-Bridge in Ho Chi Minh City was built in 1941 and is named for the fact it has three branches which connect at a central platform, forming a Y-shape in plan. The three-legged structure, which straddles two canals, was to be widened to double its capacity from two to four lanes and repaired to meet current Vietnamese design specifications. The branches of the bridge are between 200 m and 300 m long, with spans ranging from 16 m to 30 m.

Added to the complexity was the fact that the structural composition of the three branches differed, with a combination of concrete beams, steel spans and even spans which had formerly housed equipment to control the original opening span. One of the branches had been rebuilt in 2008 and the remaining parts of the original bridge had been strengthened in 1992 by Freyssinet, by the addition of external post-tensioning.

Widening works and structural strengthening

Key figures of the project

Supplementing the post-tensioning was initially proposed as the conventional way of boosting the capacity of the bridge. But plans had to be revised when tests revealed that the original concrete was not capable of withstanding the increased loading.

Instead, the existing structure was strengthened by the application of a layer of up to 100 mm thick shotcrete to the girders, the soffit of the deck slab and so on, which delivered the same result, but took longer. The extra highway width was provided by the addition of new girders on each side, supported on widened pier heads.

Logistics played a major role in the contract, with the bridge having to be kept open to traffic throughout the project and access for vessels also necessary on the waterways. Freyssinet had to operate within a single lane on the deck, and negotiate occupied houses at ground level, demanding careful planning and a flexible approach.

  • 150
    Micropiles installed
  • 500
    m3 of dry shotcrete applied
  • 2,600
    m3 of conventional concrete poured
  • 480
    Tons of reinforcement
Y bridge in Vietnam - widening works and structural strengthening

Use of micropiles to provide additional capacity – a first for Vietnam.

Meanwhile the existing foundations on the two original bridge spans had been subjected to a thorough analysis, which revealed that they were close to capacity. New foundations were required but the physical constraints ruled out conventional piles. Construction operations had to be kept within the footprint of the existing bridge, but the deck was not strong enough to support piling equipment, and there was not sufficient headroom for standard piling rigs to operate underneath it.

In total more than 150 were installed around the existing foundations, the load from which was then partially transferred by means of flat jacks and reaction blocks which were fixed to the existing piers using prestressed, high-tensile Freyssibars.

Main pier strengthening

One of the key technical challenges was the strengthening needed on the main pier in the canal, which had suffered significant scouring. Although subsequently stabilised, the piles had been left with an 8 m cantilever and were the most heavily loaded on the bridge. Four conventional drilled piles of 1.2 m diameter and 35 m length were installed from a river barge, and a 19 m prestressed jacking beam built to transfer the load to the new piles.

The concrete strengthening work involved spraying approximately 500 m3 of shotcrete, which in the urban environment had the potential to impact directly on local residents. Freyssinet Vietnam worked with Foreva to develop a dry mix that could be produced locally, and to introduce a system for controlling dust and recycling air.

On the Y-Bridge every branch and almost every span was unique and had to be addressed differently, which made the design process very complex and time consuming! The design team had to work in complete synergy with the site team, to be able to provide a comprehensive solution that respected all the local constraints.

Elie Attié  
Former manager – Freyssinet Technical Department - Asia

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Bridges
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