MASSIVE PRECAST BEAMS USED IN HYBRID BRIDGE CONSTRUCTION

October 22, 2020  980  980   

Built by the main contractor Empa Structures, a subsidiary of the Raubex Group, the structure differs from traditional utilitarian bridge construction in that considerable emphasis was placed on its aesthetic attributes, and as such it is referred to as an ‘architectural’ bridge.

The original concept was based on a stone bridge with multiple spans reminiscent of medieval bridges over streams. However, the Department of Water Affairs and Sanitation (DWS) would at first not allow any construction activity within the full width of the riverbed. At this point there was little choice other than a more modern steel-and-concrete composite arch bridge.

Permission was eventually granted by the DWS to construct a midspan single pier in the riverbed with the proviso that formwork or any form of staging would not be allowed within the primary river course (under the western span) and stringent guidelines regarding spillage into the river were imposed. Water quality up and downstream had to be monitored weekly to ensure that construction activity did not result in any contamination.

“Given the restricted access, we had no other option but to use as much precast concrete as was feasible,” said Strydom. “However, we were permitted to use in-situ construction for the central support pier, the two abutments and several other elements such as the reinforced deck and the concrete balustrades. The area under the eastern span forms part of the flood plain and construction access to the pier was originally only possible from the east.”

In order to facilitate easy access for workers and supervisory staff from the main site camp to the pier construction area, Empa Structures pre-fabricated and installed a temporary steel pedestrian bridge over the main channel.

Supplied by Cape Concrete, the precast concrete elements included huge prestressed beams, large facing panels, coping and culverts. Permanent shutters were precast on site by the Empa Structures.

The revised design had a heavy but shallow arch appearance with the stone elements being used where possible. However, the community was not entirely happy with this revised  proposal once renders had been circulated and an alternative design submitted by the estate’s architect, Boogertman and Partners, was implemented.

In harmony with the architecture in other parts of the estate, the bridge has a single shallow arch rising to the central pier between the abutments and was built with a combination of steel and concrete balustrades to afford visibility of the river and surrounding countryside. One of the ‘architectural’ requirements was the use of smooth precast facing panels which were mounted on the sides of the bridge to accentuate the shallow arch.

The abutments were generously proportioned and were fringed with arched and embellished walling to create a prominent entrance to the estate. They also provide viewing platforms for people on foot and for the tethering of horses. Additional viewing coves were constructed on cantilevered platforms situated over the central pier.

The initial precast design was based on the use of post-tensioned beams that could be cast on site. However, the contractor opted for pretensioned beams constructed off-site for commercial reasons.

Numbering 18, the Val de Vie precast beams are 35.62m long and two metres high. They are Western Cape’s largest precast beams to date and could well have set a new record for the entire country. Weighing 64 tonnes apiece, their manufacture required the construction of a dedicated stress-bed and an 80 tonne gantry crane at Cape Concrete’s production yard.

 “We entered unchartered territory with this project and were obliged to operate to extremely tight deadlines,” said Cape Concrete managing director, Darty Louw.

“Everything happened simultaneously; the construction of the laying bed, the erection of the gantry crane, the manufacture of the moulds and the preparation of the beams’ reinforcing frame assemblies. The latter comprised a combination of rebar as well as 38 pre-tensioned 15.7mm diameter strands at the bottom of each beam. An additional six five-metre long Y40 reinforcing bars at both ends of each beam were also required.

“The pre-tensioned strands situated at the beams’ bottom ends neutralize mid-span bending moments, whereas the Y40s restrain the tension forces at the top of the beams caused by the moments at each beam end. In addition the Y40s protrude asymmetrically from one end of each beam. When laid end-to-end as the beams were, the Y40 extension bars overlapped with the Y40 bars of the facing beams, thus creating a knitting effect. In order for the trouble-free execution of this link-up, casting tolerances had to be extremely tight and the Y40s had to be positioned with great accuracy prior to casting,” said Louw.

Concrete pouring commenced once all the beam strands were evenly stressed.  A combination of external and poker vibration was used to compact the concrete. This provided a very good surface finish with minimal blow holes.

As soon as the pouring was completed, a tarpaulin was placed over the mould and steam was injected under the cover to accelerate the hydration process. This meant that a strength of 40MPa could be achieved in 20 hours and enabled Cape Concrete to produce one beam a day. The final strength required was 60MPa which was achieved within 28 days.

The de-tensioning of the prestressed strands took place once the concrete had reached 40MPa using four 250 tonne hydraulic jacks

Retarder paste was applied at the end of each beam after casting and was washed off with water the next day to create a rough exposed aggregate finish. This provided a good bonding surface for the assembly of the bridge.

The facing panels were also steam cured, the largest units measuring 4m x 3m. These panels were cast with a glass-smooth finish and attached with cleverly designed fingers that eliminated the use of visual fixing on the front face. Some of the panels were curved, for example those which were used for cladding of the central viewing cove.

A juggernaut was required to deliver the beams to site by road, one at a time. And a mobile crane company, Teemane Cranes, was contracted to offload and place the beams.

Due to the drought conditions being experienced at that time in the Cape, a dry area suitable for the mounting of two of the three mobile cranes used for off-loading and placing the beams was put to use.

The Western Cape’s largest mobile crane, a 460 tonne behemoth was the only mobile crane which could lift the beams off the delivery vehicle unaided. The first nine beams were hoisted off the truck onto the ground close to the river’s west bank. They were then lifted from each end by the 460 tonne crane and a 220 tonne crane positioned on the other side of the river and placed into position between the western abutment and the central pier.

The operation for the positioning of the second nine beams on the eastern side of the bridge was somewhat different and entailed the use of the third mobile crane, also positioned on the eastern side of the river.

Once all 18 precast beams were in position, five diaphragm transverse beams were cast in-situ between the precast beams. They comprised a central beam which binds all 18 precast beams together, two mid-span beams and two beams above each abutment. The central beam was the largest, being 1.5m wide and 2m high. Empa’s construction team had to create working platforms on either side of the diaphragm sections by fixing anchors into the pier and abutments. This gave them working space to erect the vertical shuttering.

In addition to normal reinforcing, the central diaphragm beam was further reinforced with the installation of four post-tensioned cables, two at the bottom and two at the top.

Permanent deck shutters were cast in Empa’s site yard and were used to close the remaining gaps between the precast beams. Measuring 1.2m x 300 x 50mm they rested on recesses which had been cast into the beams for that purpose. Concrete was then poured onto the shutters to create the deck.

It comprised a 250mm thick road section and a 335mm pavement section. Heavily reinforced, it included a substantial quantity of Y32s. The casting of the pavements was especially challenging because provision had to be made for the attachment of the precast cladding on each side of the bridge. This involved the use of box-outs to create holes into which the panels’ concrete fingers could be inserted. Once the fingers of a panel were inserted and tied temporarily to the deck, final lining adjustments were made using jacks before final grouting took place.

The paving of the road and pavements was done by Highland Paving using clay pavers laid in a mortar mixture.

Empa Structures contracts manager, Gareth Stander says that time saving was another benefit of using precast concrete.

“The beams were cast in parallel with the construction of the sub-structure and were completed more or less at the same time and this saved at least nine weeks in construction time.”

Strydom added that before construction began a hydraulic assessment of the river course and the flood plain was undertaken.

“Top water levels for a one-hundred-year flood was calculated as well as the approximate scour depths around the pier and abutments during a one-hundred-year flood. The bridge was constructed above the one-hundred-year flood line and the panels on the side of the bridge are all positioned above it. So even in a worst case scenario the bridge should stand clear and proud of the water.

“Moreover, the piles on which the entire structure is supported, were carefully designed to ensure stability of the bridge even during deep scouring.

“Not being a national thoroughfare, the bridge is unlikely to bear unusually heavy traffic. Therefore our design incorporated the standard NA loading and an NB24 loading, the latter being the lighter loading for abnormal loads. However, although it was not specified, the bridge can actually take the NC loading (described as a super load in the design code). Because of the bridge’s span lengths, the NA loading resulted in the largest design moments,” concluded Strydom.

See YouTube link for construction video - https://www.youtube.com/watch?v=EJ1Rgl3f9cc .

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