Around 30 mobile cranes, a dozen tower cranes and a variety of specialised lifting gantries have been used for Thailand’s extraordinary Bang Na expressway, a 55km elevated road running south from capital Bangkok on a 21m-high deck.
Central to the work has been a set of 14 deck segment erector trusses, each more than 50m long. German contractor Bilfinger & Berger had them made in Thailand, says project manager Eckehard Stosch.
Segments come in greater numbers and larger sizes than seen almost anywhere. Biggest are six lane, 27.6m-wide monsters, known as D6s, and there are 21,320 of these.
Then there are 16,250 approach segments, each of which two or three lanes wide (D2s and D3s). Finally there are portal segments, square boxes for beams, supporting 26 approaches and two toll plazas.
Segmental construction allows everything to be built in the air, without disrupting traffic below. “Once stressed you can almost immediately work on a span. There is no need for falsework,” says Stosch.
The expressway runs above an existing six-lane highway to the coast which allowed the client, Thailand’s Express Transit Authority, to obtain rights of way. Compulsory purchase in Thailand is fraught.
Bilfinger is working with Thai contractor Charn Karnchang and fellow German firm Dyckerhoff & Widmann on the turnkey contract, which is valued at Baht 30bn ($508m at 1999 prices). US design consultant Jean Muller from Florida worked on design.
The road uses an elegant, inverted V shaped column, up to 18m high, on which segments sit on two arms. “The column arms need not be so strong because the truss plus the load of segments does not have to sit on them,” says Stosch. The biggest trusses for the main road line, five underslung units, travel from centre column to centre column, and hold the segments above for stressing.
Smaller two and three lane segments are erected by six over-slung gantries, which sit on legs on the piers and hold segments on cable slings until the complete span is stressed. Spans are mainly about 42m and are made by dry segmental construction with no epoxy glue between match cast units.
A third type of gantry has been built for junctions, ramps and toll plazas. Bilfinger creates a wide aerial structure by extending central piers either side into a portal, resting on further outer piers. The portal is made from segments erected with a special two part swinging truss. For this a lower section travels from pier to pier like the main underslung truss. On it is mounted a turntable, carrying the upper truss which rotates through 90O to form portals.
Thousands of assorted segments are made in a single precast yard, covering 650,000m2. The yard employs 2,000 workers to bend steel, assemble reinforcement frames, pour concrete and move, store and deliver units, each match-cast and totally unique. Twelve Liebherr tower cranes do much of the donkey work, lifting forms and steel reinforcement. There are two 78 ECs, eight 154 EC H10 models and two 256 HC-16s.
Four rubber tyred gantries from Paola di Nicola of Italy are used to transport the segments and load them onto low level multi-axle transporters, made in Thailand by DOLL. There are 18 units with 16 axles and four units with 18 axles. Once on site, single arm cranes mounted on the trusses lift the units to deck level. There are also a further 11 RTGs on site – five for handling the D6 segments and six for the D2s.
“We had one other special lifting frame made to lift the trusses themselves to working height,” says Stosch. The frame, designed by Bilfinger & Berger and manufactured locally, was needed because operating heavy crawler cranes, the obvious alternative, was not possible within the narrow central reservation, he explains. Foundation piles too had to be driven within the 5m reservation width.
Columns are cast in special steel formwork. They have not only a V shape at the top but also an inverted V below, formed with a “dome” insert. Smaller mobiles, all of them Liebherrs, lift the forms which weigh up to 100t.
“We have four LTM 1090s, nine LTM 1030s and fourteen LTM 1050s,” says Stosch. There are 16 sets filled by four Schwing pumps in a three stage pour.
The project consortium includes input from French subcontractor GEA Lyons, for toll road systems, and Spanish firm Sainco Traffic for traffic monitoring.
The venture has to hand over in eight phases and progress has been good with more than 25km already opened and a further 15km to open at the end of this month. “We are doing 18 to 20 spans of 42m or 43m length per week,” says Stosch.
But collapse of the Thai Baht in 1997 has hammered the finances of the project. “The fall of the Baht is not good news,” he says.
