May_1966 — Page 68

Shorter T-shaped columns beneath the north approach

beams was supplied by the contrac- tor's own mixing plant, but the out- put from the plant was limited so that the contractor switched to ready- mixed concrete. The strength of the concrete was checked at three, seven and 28 days in order to achieve and maintain the standard of workman- ship required. This was maintained throughout the whole contract.

Lee Macall System

The prestressed beams were made at a casting yard specially set up at Hung Hom. Using steel shuttering. the beams were cast in pairs at the rate of two beams per day. They were given a small initial stress after 24 hours but the main stressing was not applied until the beams had achieved the predetermined strength requirements of 5.250 lb./sq. in.

The Lee-Macall system of pre- tressing was adopted by the contrac- tor and, after the first four beams had been stressed and the early "teething" problems sorted out. the remainder of the casting and stress- ing proceeded without a hitch at a rate of four beams every two days.

Beams were transported from the casting yard to the site by means of a low-loader and lifted direct from the transporter on to the piers. At first the beams were erected using two "gynns" driven by a single diesel engine and a two drum winch. This method, though relatively slow, was required for the higher piers. On the lower piers, i.e. below about 20

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22 ft., the contractor preferred to use two 19 RB cranes. This proved very efficient with skilled operators and was subsequently adopted for all except the higher piers.

Only the spans over the road cross- ings were erected at night, the re- mainder being done during normal daylight hours. The average erection time for a beam was 12 minutes dur- ing the day and 20 minutes at night. No prestressed beams were damaged

during erection and the piers were only touched once by a beam during erection causing negligible damage to the corner of one column leg.

Originally, it was intended to use timber shuttering to support the deck slab, but with the co-operation of the contractor it was decided to use thin, precast, prestressed units on the Freysinnet system of prestressing with 7 mm. diameter wires. The units were cast separately in two lines, 165 ft. long. The actual cost of the units was relatively high but the time gained was very substantial.

The cast-in-situ edge beam was constructed from timber bearers supported from and carried by the prestressed longitudinal beams.

Concreting of the slabs and edge beams was carried out to a carefully laid down schedule: the centre line of slabs first with the outside slabs and

edge beams following up about four days later. The whole of the deck slab was laid in 40 days, a very cre- ditable performance.

The records of tests taken of the concrete on the slabs showed that the concrete results at seven days in many cases equalled or exceeded that required at 28 days.

The Shanghai plaster finishings, steel handrail and guard rail followed up immediately behind the concreting of the deck slab and edge beams. Temporary lighting has been fixed to the handrail pending arrival of the lanten-type balustrade lighting from UK.

The flyover was opened to traffic at 3.00 p.m. on April 4, 1966, just un- der 12 months from the commence- ment of work on stage II and four and a half months ahead of the ex- tended scheduled completion date. The total cost was HK$3,200,000.

Mr. Paul Lee, BSc.. of Paul Lee Engineering Co., Ltd. acted as re- sident supervisor on the project and Mr. K. T. Philcox, AMICE, MIStructE, MASCE., MACI, acted as consultant for the prestressed beams.

The design and supervision of the Princess Margaret Road /Waterloo Road flyover was carried out under the direction of Mr. J. J. Robson, MICE, AMI Struct E., Director of Engineering Development. PWD. and Mr. G.A.G. Sapstead, AMICE.. MASCE.. MIHE.. Chief Engineer. Roads & Drainage Kowloon Divi- sion, CEO, PWD.

Flyover seen above ground level complex

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Far East Architect & Builder May, 1966