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Marine spans viewed from north
with it by the mechanical lock-off.
The strands were tensioned to about 77 per cent. of the ultimate stress which, allowing for anchorage set in and friction losses, produced 70 per cent. of ultimate stress in the middle section of each beam. In computing frictional and wobble effect loss, values of μ
0.3 and k 5 x 10-4/ft. were assumed. Other calculated losses amounted to about 23 per cent.
=
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The specified transfer stress was 5200 lb/sq. in. and no stressing was allowed to take place until concrete control cubes reached this stress and/or 7 days had elapsed after casting. Stress was applied by manually- operated hydraulic jacks capable of developing a 16-ton pull. All loads were measured on a load-cell meter linked to the hydraulic system of the jack and giving readings to ±1% ac- curacy. Extension of each strand was also recorded and, where this was more than 5 per cent outside the theoretical value, the strand was re-lifted later and stressed again to its original load. Re- lifting was done by fitting a purpose made 'bridge' over an anchorage and inserting shims between barrel and anchor plate as necessary.
Stressing times varied from 1% hours for four tendons in a 60ft. I-beam to three hours for 10 tendons in a 90ft. box beam. Jacks were calibrated against a proving ring at least twice in any one stressing session.
The beams were grouted in their new positions with 0.5 w/c grout applied at a pressure of 100 lb/sq. in. The anchorage recesses were then con-
36
creted and later the ends were painted with four coats of bituminous paint.
Beam handling
The method of handling beams (Fig. 4) was similar to that used by the contractor on another job some years ago. Steel joists fixed to each end of a beam were arranged to cantilever about 4ft. and to act as supports during handling (Fig. 5). Beams were jacked up on to railway bogies and transported into place between two specially-built towers each capable of lifting 35 tons.
Bogies running on tracks laid on the capping beams of the superstruc- ture carried each beam from the towers to its final position where it was jacked down on to prepared bearings.
The towers were moved from one span to another when placing beams over land but a steel lattice girder was provided for launching beams over the water. This girder could be lifted into position between marine span capping beams by the floating derrick crane.
Beams were travelled out using it as a support until they could be trans- ferred yet again to another system of track and bogies for traversing them into final position. In-situ diaphgrams were provided at both ends of each span to tie the precast beams together.
Roadway
The spacing of the prestressed beams was designed to produce the least out-of-balance force on the columns at the worst loading con- ditions and to fit symmetrically about the centre line of the bridge. In the
long spans the spacing is 7ft. 6in., while the shorter beams are 6ft. 4in. apart. The corresponding deck slab thick- nesses are 71⁄21⁄2in. and 7in. The slab was cast in 4500/%4in. grade concrete and reinforced with high-tensile steel bars.
The design envisaged the use of precast concrete planks which would be placed between the longitudinal beams to form shuttering for the deck slab and later form a composite part of it. Early in the contract, the con- tractor proposed to dispense with the planks and to cast the slab using conventional shuttering. The proposal was accepted provided there was no increase in cost.
Shuttering was devised to use the bottom flanges of the I-beams as sup- ports with special arrangement at the edge box beams. Stripping was carried out from walkways hanging from the I-beam flanges. The system worked satisfactorily and a good concrete finish was obtained.
PVC waterstops were cast into each joint between the deck slab and the capping beams and further waterproof- ing was effected by painting the top face of the slab with three coats of bituminous paint.
Finally, two layers of hot-rolled asphalt, each 2in. thick, were laid by a mechanical spreader and compacted by a 10-ton roller to form the carriage- way surface.
Surface water is drained through ported kerb inlets at regular intervals along the bridge and discharged through downpipes cast in the columns into the ground drainage system.
Far East BUILDER, April 1969