Award of Excellence: Rama 8 Bridge

CATEGORY: TRANSPORTATION

BUCKLAND & TAYLOR

The Rama 8 cable-stayed bridge is the most recent crossing over the “River of Kings” in central Bangkok. Named as a tribute from the King of Thailand to his late brother King Rama 8, the bridge is part of a new east-west link through central Bangkok. It relieves the nearby Pinklao Bridge of traffic, reducing congestion in the historic Ratanakosin Island area where the Grand Palace and many Bangkok landmarks are located. The bridge is seen as a gift from the King of Thailand to the Thai people.

Rama 8 Bridge is one of the world’s largest asymmetric cable-stayed bridges. The structure, a majestic, golden-hued, main span, cable-stayed from a single tower, surpasses both monumental and practical objectives. It marries technical ingenuity with a profound respect for traditional Thai culture. Architectural details that reflect Thai culture and customs include the lotus bud finial atop the tower, elephant foot shaped enclosures at the base of the legs of the pylon, a Buddha outline between the legs of the tower and the lotus motif of Rama 8 incorporated into handrail and pier details.

The bridge is a 475-m long structure supported by a 160-m tall inverted Y-shaped tower. The main span is 300 metres and uses a light composite steel and concrete deck section enclosed in an advanced composite enclosure. The land spans consist of two 50-m back spans and a 75-m anchor span. The anchor and back spans consist of single cell, post-tensioned, concrete box-girders, measuring 9.25-m and 2.5-m deep respectively. These box girder sections support transverse concrete ribs carrying the concrete deck slab.

Buckland & Taylor’s scope of work included the design of the bridge and the provision of construction engineering for its erection. The firm developed innovative features, such as:

The main span is enclosed with an advanced composite skin to give the appearance of a closed box girder without the associated weight and cost. As well as providing the desired appearance, the enclosure allowed the use of a more cost-effective structural framing and provides an excellent aerodynamic response. It also gives access for inspection and maintenance of the main span superstructure and eliminates the need for an expensive maintenance traveller.

The plenum created by the enclosure is dehumidified, thereby protecting the structure from the aggressive Bangkok atmosphere. The added protection meant that a less costly paint system could be used for the steel superstructure. The enclosure on the land spans also functions as an energy saver by reflecting night-time light to provide extra brightness under the viaduct.

Longitudinal ribs on the underside of the precast concrete deck panels of the main span add strength and stiffness while minimizing weight. The added stiffness of the ribbed deck significantly reduced the second order bending effects in the axially loaded deck panels. This is the first application of longitudinally ribbed precast deck panels on a composite cable-stayed deck system. The first use of precast deck panels on a composite cable-stayed bridge was on the Alex Fraser Bridge in Vancouver, also designed by Buckland & Taylor, in the 1980s. On Rama 8 Bridge, the design was taken a step further through the addition of the longitudinal ribs.

The deep post-tensioned concrete anchor span was progressively filled with ballast concrete to balance the weight of the main span during construction. This unique solution eliminated the need for a large number of piles, which would have been required to carry the full final ballasted weight of the anchor span structure before the relieving uplift force of the cable stays was applied.

Both the main span stay cables and the back stays were anchored in the main span wall of the concrete tower to minimize the post-tensioning in the upper pylon. The stay cables pass through and are anchored into a single tower wall. The horizontal force from the two planes of main span stays directly opposes the horizontal force from the single central plane of back stays. The load is transferred via two diagonal compression struts through the concrete wall with a relatively modest tension tie between the two struts.

In order to minimize the demands and the number of piles used for the main tower foundations, the pile groups were located eccentrically to the tower legs. The added eccentricity proved to be an efficient means of resisting permanent load effects in the tower legs resulting from dead load over time.

The Rama 8 Bridge was designed in a competitive design-build format. It was therefore necessary that the solution be economical and cost-effective in order to be the winning bid. The resulting structure, begun in 1998 and opened in May 2002, is a unique mix of composite steel and concrete, post-tensioned segmental concrete, state-of-the-art cable stays and advanced composite construction.

Name of project: Rama 8 Bridge

Client: CBP Joint Venture (Thailand)