Civil Engineering October 2022 | Vol 30 No 9

Civil Engineering October 2022 11 on the old bridge during construction of Phase 1. “Once the piles had been sunk, seven in Phase 1 and six in Phase 2, they were trimmed to the bottom of the foundation level before the first-lift abutments were cast. The second lifts act as diaphragm beams, tying all the precast beams together at either end of the bridge. Moreover, we cast concrete jockey slabs between the second abutments and the feeder road sections at each end to act as an interface between the solid concrete of the bridge deck and the more flexible road surface,” he explains. When the first-lift abutments had reached the requisite seven-day strength, the precast beams were lowered onto temporary steel bearings which had been cast into the first abutment lifts of both phases. In addition, matching bearing steel plates were cast into the beams’ soffit sides. After being lowered into position the beams were tied together at the bottom of the lower beam flanges with transverse reinforcing which was covered with a 150 mm layer of in-situ concrete. The installed beams were capped with a permanent Nutec formwork and reinforcing for the 175 mm to 250 mm cast-in-situ bridge deck. The deck was cast with protruding rebar to tie into and support the walkways’ cantilevered reinforcing. “Apart from some minor staging at each end of the bridge, the walkways are being constructed without ground-sup- ported staging due to environmental con- cerns. This meant we had to come up with an alternative method of supporting and constructing the cantilevered platform. “We opted for inverted steel-girder T-shaped suspension brackets, designed and manufactured by Form-Scaff, from which the support work and formwork could be hung. The brackets, which extended 2.4 m off the edge of the deck, were mounted on two pivot-point jacks,” says De Sa. In addition, the brackets’ horizontal sections were secured to the underside of the deck with Dywidag bars, and once all 24 T-brackets had been attached to the deck they were linked together with steel girders and poles to form an integrated support unit. Girder sections were also attached to the underside of the deck to support the Econo-Form panels used for casting the walkway’s sloped soffit surface. Six transverse cantilever beams sup- port each walkway slab. They include voids in the concrete to save weight and make provision for future service pipes. The cantilever beam reinforcing was spliced together with the Y20 rebar rods which had been cast into the top deck. They were also spliced with R20 inclined pull-out rebar which extended from the bottom of the edge beams. Additional formwork support was provided by tension cabling which was attached to the top of the vertical bracket sections and the lower support girders on the one side, and from the top of the bracket sections and the end of horizontal girders on the deck side. Once the walkway concrete attained an early strength of 60% of the required 40 MPa rating, the cabling was de-tensioned. After all the support work had been removed, a further 20 mm of concrete was T-shaped suspension bracketing at the Phase 1 walkway mounted on pivot-point jacks Apart from some minor staging at each end of the bridge, the walkways are being constructed without ground-supported staging due to environmental concerns.