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PRECAST FACILITATES SHOPPING MALL OVER BUSY RAILWAY LINE
Precast concrete accounts for the major portion of the building materials used in the construction of a shopping mall directly above a 390m section of the KwaZulu-Natal South Coast railway line at Isipingo Station.
Built for Prasa and spanning 20 520m² over two floors (excluding the ramps and existing building), the new building is being erected on either side of and integrated with the existing Isipingo station building, which – in turn – is being modernised and upgraded. The new building comprises two floors, the first to be used for retail space and the level above for parking.
The precast portion of the mall structure, designed by specialist precast concrete contractor Bedrock Group, is being built using precast columns, beams and hollow-core slabs, as well as precast parapet walling for the upper floor.
Unimpeded traffic A major proviso for the construction of the mall was that the line remain open to normal rail traffic during construction. Insitu construction, which involves shuttering and propping, would obviously not have worked and therefore precast concrete proved the only viable building option.
All 141 columns were produced on site and were cast with corbels for beam support. However, a variation order was issued after the first 121 units had been cast, changing the building from a threeto a two-storey structure. This meant the existing columns had to be shortened from 22m to 17m.
Depending on the width of the flooring, the columns were placed in rows of either two, three or four abreast. Cast with starter bars flanking a column foot of 340mm x 600mm x 1 400mm, they were lowered onto structural blindings
PRECAST FACILITATES SHOPPING MALL OVER
of various sizes, cast over pile caps, propped with braces and adjusted into a vertical position with shims. After additional reinforcing was installed, concrete was poured around the base of the columns. The braces were removed after 14 days once the concrete had cured sufficiently.
(Above): A beam supports hollow-core slabs prior to the casting of the structural topping.
(Below): A partially completed section of the mall.
Heavy beams Of the 211 beams used on this project, 132 measured 3-9m and were cast at Bedrock’s production yard in Westmead. Up to 15m long, the remaining 79 beams were cast on site.
The beams were placed between the columns to support the 10m prestressed hollow-core slabs, which were
(Top): A beam is about to be lifted by the 460-tonne mobile crane prior to placement between the western abutment and the central pier.
(Above): Concrete pouring of the pavement.
(Left): The central diaphragm’s reinforcing prior to the pouring of concrete. The Y40s can be seen protruding from the ends of the beams in a knitting formation.
(Right): The completed bridge.
supplied by Echo Prestress. The beams were lowered onto the column corbels by mobile crane and were “stitched” across the columns. The reinforcing involved straddling the beams and the columns with Y32 bars.
The beams were “stitched” together around the internal columns before the 60mm structural topping was poured. The 15m beams in the two-span configuration were partially stressed, in addition to some supplemental non-pre-stressed reinforcing. This offered a better solution and simplified the reinforcement and redistribution of stresses, as well as improving deflection and crack control. It was important to keep deflection to a minimum, as some of the beams were close to the railway’s overhead highvoltage cabling.
Monolithic structure Using the same concrete mix for the structural topping and the “stitching” created a seamless monolithic structure between the columns, beams and flooring. Due to the railway line being used during the day, much of the initial placement of precast elements had to be done at night during line shut-downs. But once the first-floor slab had been installed, lifting could continue during the day because the lines were protected.
“Moreover, hollow-core slab flooring was the only option for this project,” says David Kitching, general manager of the Bedrock Group, “because it doesn’t require propping or shuttering, which wouldn’t have been possible while the line was operational.”
The precast parapet walling was cast on site by Bedrock in sections up to 15,6m long and 1 510mm high. It was attached to column corbels with steel brackets and braced with in-situ stub columns cast onto the slab.
at ground level in order not to overload the pipeline and allow access for cranes and trucks from the casting beds to the building site. Some casting beds were close enough to the building for the precast elements to be lifted directly from the beds into position. However, most units had to be double-handled using cranes and trucks, either over the pipeline, across the “bridges” or from the public road.
Casting beds were used multiple times for various precast units and the sequence of casting was carefully planned so that precast elements could be stacked in the order required.
Surrounding infrastructure A public road abutting the west elevation, the existing station building, the railway lines and a fuel pipeline running parallel to the eastern side of the lines meant that space for on-site casting was limited and the casting beds were placed further from the building site than would normally have been the case.
Three “bridges” were constructed Project team The project’s professional team is as follows: principal agent and civil/structural engineering: Delca Systems; quantity surveyor: Ukuza Consulting; main contractor: GVK Siya Zama Building Contractors; specialist precast contractor: Bedrock Group; structural engineers to Bedrock responsible for the precast structure: Escongweni BPH Engineers; pre-stressed hollow-core slab supplier: Echo Prestress.
(Top, from left): An on-site casting bed; A column foundation trench prior to casting of the concrete blinding over the piles and members of the Bedrock team pose on one of the on-site casting beds: Lungile Phukani, David Kitching, Cathy Paver, Ganee Shaik and Africa Simamane.
(Left, from top): A partially completed section of the first floor prior to the “stitching” process and workers prepare the rebar and shuttering for beam and column “stitching”.
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