| PROJECTS |
profile we found in the original design – but, of course, using pre-tensioned, rather than post-tensioned beams,” says Kritzinger. “It was essential that the structural behaviour of the new beams mirrored the structural behaviour of the existing post-tensioned beams as closely as possible, because construction tolerances were very tight. This meant we had to model and analyse the deflections and the induced movements of the pre-tensioned beams to match those of the existing beams as much as we could.” Allowing for creep and shrinkage in this alternative beam design proposed by the contractor entailed some specialist eng i neer i ng i nput , wh ich wa s why consulting engineers, Mowana Engineers, was appointed to design the pre-tensioned concrete beams. Creeping bridge Edward Smuts, Mowana Engineers bridge eng ineering director, says there was evidence that considerable creep had taken place on the bridge over its 50-year lifespan. “Armed with the concrete and section properties of the old bridge structure, we were able to simulate theoretical deflections in order to match the behaviour of the new beams as closely as possible with the performance of the original beams. Part of this exercise involved calculating instantaneous and long-term creep. Based on these calculations, we compiled a report which was submitted to AECOM in the first instance and, once approved by them, to Cape Concrete,” says Smuts. Cape Concrete factory manager Johan Nel says that as a result of Mowana’s calculations, some additional detailing was required in casting the river bridge beams and this aspect was handled by Mowana. “We’ve done numerous post-topre-tensioned conversions with Mowana Engineers and they get involved with the pre-stressing and checking procedures. “The river bridge beams were specified as W40MPa and the rail bridge beams as a W50MPa mix. However, we used the W50 mix for the river bridge beams as well because it also covers ASR durability issues. We’ve used this formula historically on ot her br idge project s, w it h ver y satisfactory results. “Steam-cured, the road bridge beams
40
PRECAST | ISSUE ONE | 2021
“The extension of the abutments and centre pier required approximately 30 working occupation permits.” reached the desired de-tensioning strength after 18 hours. However, we only detensioned on the third day because earlier de-tensioning leads to greater hog and deflection issues. It was essential that we controlled the deflection of the beams very carefully so that they matched the camber alignment of the existing bridge beams,” explains Nel. Work-in-progress Empa Structures was appointed by Martin & East to handle the actual construction of both bridges. Work on the river bridge involved the installation of bearing pads, beam placement, cross-bracing with insitu cast diaphragm beams and casting a reinforced concrete deck on reinforced concrete permanent shutters. It also involved demolishing the old balustrade above the
midpoint of the new bridge structure and replacing it with an in-situ cast safety barrier between the two sets of traffic lanes. Unlike the river bridge, the abutments and centre pier of the rail bridge had to be extended under extremely constrained conditions. Once this was accomplished, the precast beam/balustrade units were placed on the bearing pads and attached to the existing bridge structure. Work still to be done at the time of writing includes demolishing the two original balustrades, constructing new pavements and resurfacing the road. José Dos Reis of Empa Structures says the river beams were placed on the bearing pads using two mobile cranes. (Above): Workers involved with attachment of a precast concrete beam/balustrade unit to the rail bridge.
