February_1971 — Page 37

Road and flyover construction in Hong Kong

PART II

In this second part of a paper recently presented to the Engineering Society of Hong Kong, Mr. R.J. Blake, resident engineer, Scott Wilson Kirkpatrick & Partners, describes stressing methods used for flyover beams and details the materials employed and problems met in road construction.

By R. J. Blake

UDY 12T A

Arrangement of end block reinforcement and cable ducts for precast beam

Stressing

Pre-tensioning

Precast piles are pre-tensioned using both long-line methods and in- dividual beds. Although the latter are not economical in use of strand and stressing time, contractors find them advantageous since the bed construc- tion is much simpler.

Pre-tensioned beams for Chatham Road Flyover were stressed and cast using individual beds. The required loads for stressing individual strands were measured by using a pressure gauge or load-cell and verified by measuring extension at the stressing end. It was also found useful to check full load ex-

Far East BUILDER, February 1971

tension by reference to a nominal 50 ft. gauge length marked on a number of the strands prior to the application of load.

All contractors found the manufac- ture of prestressed piles to be a rela- tively straight-forward operation and, particularly for longer piles, the reduc- tion in handling problems when com- pared with ordinary reinforced con- crete piles was significant. Post-tensioned beams

Following difficulties with single- end stressing at Lai Chi Kok Bridge, subsequent contracts specified double- end stressing for post-tensioned beams.

Individual strands were stressed from each end of a beam without difficulty and the jack operators quickly esta- blished a routine which satisfied the strict supervision requirements laid down. Anchorage slip (Reference 1) was encountered at Lai Chi Kok Bridge and the measures taken on site at that time to overcome this problem were utilised for subsequent contracts. Thus when anchorage slip occurred at Canal Road Flyover, it was quickly possible to eliminate anchorage assem- bly as a possible cause and in this case a consignment of wedges was found to be faulty and had to be replaced.

All jacks were calibrated at regular

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