68 m free-spanning, centre-supported bridge trusses at Koeberg
Upgrading of Koeberg interchange, Cape Town, South Africa
The complexity of the new flyover proved to be challenging for Paarden Eiland Joint Venture, a Group Five and Power Construction JV. The in-situ concrete bridge is made up of a box section with a total depth of 3,5 m and width of 12 m and is cast in 3 sections of 22 m each. To maintain consistent concrete characteristics throughout the 22 m casts, the top/road deck had to be cast within 10 days after completion of the box section, bottom deck and walls.
Each 22 m section requires approximately 250 m3 of concrete. The entire deck is supported by the structural steel girders until the post-pensioning is completed, which allows the deck to support itself. In order to perform the post-tensioning, the soffit to the bridge deck had to be able to adjust and to achieve this, the QUICKSHORE system is used on top of the girders, in combination with PERI MULTIFLEX.
The large structural steel girder support arrangement will allow the bridge deck to be constructed without interrupting main rail traffic into Cape Town. The special formwork trusses and temporary false work were designed by PERI and the single, slender central support, at 68m long truss mid-span, allows the railway to continue operating during bridge construction while still maintaining the required 2 m inside clearance to the nearest railway line.
The four boxed trusses across the 12 m width of the bridge are 56 m in length and 3,10 m deep, each weighing 74 t. They allow a clearance of 5,2 m above the tracks and support the underside of the bridge at a height of approximately 10 m above track level. The design was in accordance with BS5975 Code of Practice for Temporary Works, and the girders were designed to withstand wind speeds in excess of 250 km/h, as required by the Highways Code.
The total weight of each completed bridge is 2 200 t and this is fully supported by the truss system, PERI HD200 supports at both ends and the central support. The girder trusses are tied into a pier at one end and are supported by elastomeric pads, at both the centre and opposite end of the truss assemblies. These pads allow lateral movement of the system as it is progressively loaded during the construction process.
The centre supports, carrying 480 t each, support the majority of the loading with the end supports bearing 140 t each. There are two bridge spans being constructed (A and B), and after construction of Bridge A, the truss system will be rolled out sideways, on a specially designed roller system, and supported on temporary travelling bogies utilising a purpose-designed, modular 1 000 kN/leg shoring system. This will allow minor adjustments to the truss system, prior to final positioning in order to support Bridge B, which is of a slightly different orientation.
A 550 t mobile crane is used for the erection and dismantling of the very large truss system. The crane is also used to lift and place precast ‘U’ beams for other sections of the contract. lead designer Peter Potgieter, said the sheer size of the steel girder was due to the fact that only the single support was allowed within the railway servitude, ensuring that a total truss span of the 68 m servitude was achieved in accordance with Railways’ requirements.
Potgieter added that the two 68 m long in-situ concrete box sections are the longest single span bridges over rail tracks in South Africa. One has been constructed and is already in place while the second is now under construction. They are cast in-situ and the concrete is post-tensioned. The box sections lie some 30 m apart and facilitate A and B ramp traffic access across the railway area.
Heavy Duty Shoring for Ramps
In addition to the in-situ rail crossing bridges, two further in-situ bridges are required at the ramp ends. Ramp A is approximately 130 m long and spans the existing roads of the Koeberg interchange for traffic flowing from the N1 to the M5 towards Muizenberg.
Ramp B (also about 130 m long) is required to span the N1 as the M5 joins the N1 in the direction of Paarl. The ramps are complex and arched, sloping in two directions from the soffit of the bridge. The vertical walls of the bridge therefore change shape and a formwork solution using the PERI VARIO system catered for the changing profile of the vertical walls. This was achieved by adding extensions to cater for the variation in profile, which meant that no modification was required to the main shutter.
A heavy duty, QUICKSHORE 80 kN/leg shoring and support system was recommended and designed by PERI. QUICKSHORE was selected for its light weight, support strength, easy handling and transportation as well as its suitability for heavy duty civil structures.
John Colman (Group Five)
Jeffares & Green, Bergstan, ASCH
Stewart Scott International (SSI)
Western Cape Provincial Government
|PERI Systems In Use
VARIO GT 24 Girder Wall Formwork
MULTIFLEX Girder Slab Formwork
HD 200 Heavy Duty Prop
PERI (Pty) Ltd, South Africa