April_1967 — Page 33

Each 14-ton unit was handled by mobile crane

riageway and a 16 ft. wide, 340 ft. long slip road on to Queen's Road East.

It incorporates two 9 ft. wide foot- paths, two staircases from ground level, and a 9 ft. wide suspended below-deck footbridge designed to link the podium decks of future buildings in the two adjoining lots.

The foundation consists of hand- excavated caissons for the main structure and spread footings for the slip road. Caissons were found to be the ideal solution for ensuring the safety of neighbouring old buildings and for the type of soil encountered

an old reclamation with transport- ed material blended with boulders underneath running as deep as 40 ft. below ground level.

Thirty-two caissons of 4 ft. and 4

ft. 6 in. diameters, designed to carry maximum working loads of 354 and 766 tons respectively, were sunk to an average depth of 50 ft. Two cais- sons were test-loaded to 11⁄2 times the design load, the criterion of accep- tance being 1⁄2 in. settlement under full load for three days. The actual maximum settlement was less than 1/4 in.

The foundations contract. finished in May 1966 took 611⁄2 months and cost HK$637,438.50,

Superstructure

The superstructure contract, start- ed immediately after the foundation contract, was carried out by the same constructors. Paul Lee Engineering Co., Ltd.

The structure is a seven-span bridge

with the maximum centre span of 115 ft. running over the tram tracks at a headroom of 19ft. 6ın. Such an 21- ceptional span is achieved by making use of cantilever units supported on adjacent spans of 42 ft.

These units are made of three sec. tions each, one 72 in. deep centre section and two tapering cantilever outhangs. They are separately erected on the piers and subsequently jointed and stressed together to form a com- plete unit of 97 ft.

The piers are essentially of portal frame design with circular columns of 4 ft. and 2 ft. 6 in, diameter for the main flyover and slip road re- spectively. The deck consists

of precast prestressed concrete I-beams and cast-in-situ road slab with a bituminous wearing course. The majority of the prestressed beams were made at a local casting yard.

The Lee-McCall system of pre- stressing was adopted and the size of Macalloy bars used ranged from 1 in. to 1% in. diameter. Thin precast prestressed slabs were used as shut- tering to support the deck slab.

Cost of these units in which the Freysinnet system of prestressing was employed using 7 mm. diameter wires was relatively high but the time gained by the contractor was sub- stantial.

The gradient for the approach ramp is 1 in 10 while that for the remainder is 1 in 200, with a constant cross-fall of 1 in 72 along the whole deck. Guardrails are made of rectan- gular hollow steel sections to improve the appearance of the bridge.

Another feature in this flyover is the design of the expansion joints which are new in the Colony. Precast sealing strips are used between steel angles embedded in the road slab.

Work on the Queen's Road East flyover is due to be finished shortly and the complete Garden Road Com- plex is scheduled for completion by 1970.

Far East Architect & Builder April, 1967

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