Flying saucer interchange: a masterpiece of incremental launching accuracy
This article was published in “Concrete Trends” (Official Journal of the Cement and Concrete Institute) VOL 12 No4 November 2009 edition. We thank Concrete Trends and the Cement and Concrete Institute for allowing us to republish it.
Anyone who has experienced Gauteng’s peak-hour logjam will agree that an upgraded traffic infrastructure was long overdue and an urgent necessity. The South African National Roads Agency Ltd (SANRAL) thus embarked on the extensive Gauteng Freeway Improvement Project (GFIP), to improve the existing freeway network and provide additional infrastructure by May 2010. Stefanutti Stocks Civils, a local leader in the incrementally launched construction technique, was awarded the construction of Bridge B0041, the 240-m bridge spanning the N1 and R21 interchange near Pretoria. In addition to relieving traffic congestion, the interchange, now affectionately called ‘the flying saucer’, will improve access from the R21 (from the airport) on to the N1 North.
Site manager, Werner Pretorius, explained why the Incremental Launch (IL) method was chosen for this bridge.” Conventional concrete bridge construction involves either constructing the bridge in situ, or using precast sections, lifted and placed into position. This approach requires heavy lifting equipment, an abundance of support and falsework, and large working and lay-down areas to store materials, maneuver plant and demarcate and protect the support work. On Bridge B0041 however, none of these criteria could be met as the bridge had to be constructed 17 m above very heavy live traffic without disrupting its flow; the sub-structure had to be positioned in the very limited space between existing lanes and the bridge needed to swing the traffic 90° within 240 m.
“The IL method chosen was ideal, as it allowed the bridge to be constructed in long sections, to cross wide spans, be constructed off-site and work to take place within the small area available to us,” he explained. The IL system involves casting sections of the bridge deck behind an abutment in a unique casting yard and, once the concrete has achieved the required compressive strength, the segment is stressed and launched forward, using hydraulic jacks, to clear the yard. The second segment is cast behind the first, tensioned, and the two segments are then launched forward again. This process is repeated until the whole superstructure has been launched into its final position. The bridge is being constructed in 18 segments, each weighing about 323 tons. Sections are cast in the casting yard and then launched forward, using specialised hydraulic equipment, into the final position. With a 240-m span and the curved geometry, tolerances are critical. The geometry of the superstructure adds to the technicality of the project. The Flying Saucer project is cramped with a composite geometry consisting of a global curve (accomplished by twisting a circle on both the x and y axes) and a bevelled superstructure pivoting along the centre line of the soffit.
Theoretically, the superstructure is viewed as an ellipse from plan and the soffit of the superstructure is conically formed. By constructing a circular bridge and launching the superstructure forward, additional horizontal forces are imposed on the substructure. The temporary substructure is thus designed to cater for the biaxial horizontal force as well as a vertical component. Extensive temporary works designs were carried out to cater for the powerful launching forces, the great spans between piers and the dead weight of the superstructure in the construction phase. The temporary works designs include a launching girder, temporary piers (temporary substructure), structural guides and a casting yard facility capable of achieving construction tolerances of 1 mm. The superstructure slides along temporary bearings, prepared and installed to an accuracy of 1 mm.
Pretorius says the success of the project depends on tailoring all incremental launch activities and advantages to maintain a cycle in order to deliver the project on time. “Areas that drew great emphasis in the post-tender planning stage included temporary works designs, parallel work activities, health, safety and quality, working within factory conditions and overhead lifting”, he explains. “The target was to deliver a section every seven days. Complicated temporary works and formwork had to be simplified to allow for faster assembly and dismantling. A sophisticated concrete had to achieve early strength together with greater and longer workability to address potential delays from traffic and equipment breakdowns. Labour needed to be accustomed to specific tasks and to operate in a ‘conveyer belt’ system. Preparation of a future section’s reinforcement and formwork involved programming parallel activities to sustain the cycle. To achieve our cycle, concrete had to reach a compressive strength of no less than 35 MPa in 60 hours. This allows Stage 1 post-tensioning to be carried out. We thus poured the concrete on Friday afternoons to have elements ready for launching after the weekend.”
The sophisticated concrete mix had to meet the following criteria:
- Early strength of at least 35 MPa within 60 hours – even in cold weather
- Allow for congested traffic conditions and plant breakdown
- Greater and longer workability in a pump mix that remained within the specified slump envelope
- Not be susceptible to short-term shrinkage
- Conform with durability and project specifications
“All the above criteria were achieved by having a cement:water ratio of 2,49, increasing the stone content and using a Chryso superplasticiser. The superplasticiser contributes to a concrete with a greater workability and a working life-time of 5 hours. In addition, it also enhanced the early strength gain damped by the fly ash content. The high cement: water ratio is the driving factor to achieve an early strength.” The role of reinforcement in the bridge is considerable. Having a thin deck compared with other IL bridges, the reinforcing bars are large and spaced densely. This produces an extremely stiff superstructure. Apart from working in compression and tension, the reinforcement caters for the large torsion forces in both the temporary and permanent phases.
In addition to the reinforcement, post-tensioning is also used. First-stage post-tensioning is applied to the individual bridge segments, while second-stage post-tensioning will effectively place the entire superstructure in compression to cater for large axial live loads onto the superstructure.The substructure is founded on both layered backfill and unweathered shale. Bases positioned on backfill material are piled using reinforced concrete piles 900 mm in diameter with both end- and friction-bearing capacity. The piles are also designed to cater for seismic activity. Piles are commonly socketed 2,4 m into the unweathered shale layer.
Experts were consulted in the fields of incremental launching, movement monitoring, hydraulic equipment and surveying, and their advice proved invaluable. Great effort was invested in achieving the specified 1-mm construction tolerances, and involved use of precision survey equipment capable of surveying to the fourth decimal. The bridge, 240 m long and weighing 4 606 tons, requires a launching force of 3 700 kN to move forward. The movement is activated by hydraulic equipment supplied by Enerpac. This system also includes an automatic hydraulic power-pack.
The high launching forces applied to the substructure, and in particular to the temporary 15,5-m high concrete columns, made monitoring movement in the columns critical. Tilt-tech, the leader in movement monitoring, was consulted and a system capable of monitoring movement to as little as 0,005° in a biaxial direction was installed. This system provides accurate measurements on a nearly real-time basis. The design of temporary works, including the launching girder, was carried out by GOBA. Although Stefanutti Stocks engineered the launching system and carried out much of the temporary works designs, Wiehahn Formwork & Scaffolding played an integral part in detailing and supply of formwork for the structure’s complex geometry.
Fabricating the structural steelwork was undertaken by Ferro Eleganza, and both the structural steelwork and the formwork had to be manufactured to the specified 1-mm tolerance – another factor contributing to the project’s success. AfriSam was contracted for the tricky and risky task of delivering the readymixed concrete to an interchange characterised by high traffic volumes and congestion. This posed a challenge for both quality and process control, but Pretorius says that AfriSam have been equal to the task. All materials to be used in the concrete underwent rigorous testing to ensure they met the strict durability requirements.
The post-tensioning system is supplied by Freyssinet who also carry out the post-tensioning activities. The tensioning of the stage 1 cables – straight cables in both the top and bottom flange of the deck – is undertaken when the concrete compressive strength has reached 35 MPa. Grouting of the cables takes place once the cable extensions are approved by the engineer.
“For me,” concludes Pretorius, “one of the greatest achievements of this project to date is seeing how everyone is pulling together to manage all the activities involved in
ensuring our success. Everyone on site understands what is necessary and is actively working to meet our targets.”
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