January_1971 — Page 23

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SECTION "H-H'

1st DAY 2nd DAY

3rd DAY

4th DAY

5th DAY

6th DAY

3

PREVIOUS CYCLE

5

6

7 8

CYCLE REPEATS

TYPICAL WORKING CYCLE OF MANUFACTURING PRECAST

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1st DAY

2nd DAY 3rd DAY 4th DAY 5th DAY

BEAMS

bed and some delays in the production of the beams. The consulting engineer. was called in to assist in re-designing the device and strengthening the struc- ture.

The sequence of operations for making the precast beams was as fol- lows:

1) Fixing strands and debonding

tubes

2) Stressing

3) Fixing reinforcement

4) Fixing mould

5) Concreting

6) Curing and demoulding

7) Detensioning

8) Cutting strands

PREVIOUS CYCLE

CYCLE REPEATS ➜

570

9

10

11

12

TYPICAL CYCLE OF

Fig. 6. Detail of in-situ cantilever slab

The sequence of operations for the whole building was as follows:-

1) Fixing columnn reinforcing steels 2) Placing precast column casings 3) Fixing side formwork to the

9) Taking off the bed for stock column piling

Each cycle took six days. There were six beds altogether, and each bed could accommodate five beams.

Specially made steel moulds were used to cast the column casings and floor planks. There was some damage to the planks in the early casting, basi- cally due to mishandling during the de- moulding. This problem was solved simply by hammering the formwork in order to break up the adhesion be- tween the unit and face mould before lifting up from the base moulds.

Transporting beams

The weight of the precast beams was approximately 13 tons. To tran- sport them, the contractor designed a special cradle which was fixed to a 15-ton track chassis. The beams were lifted on the cradle by means of a P. & H. mobile crane or two fully-extended gantry cranes.

Erection of precast components

The method of erection of the pre- cast components was developed after discussion with the contractor. The consulting engineer suggested the use of four climbing erection booms plus four derricks on the top of the two

cores.

Far East BUILDER, January 1971

4) Concreting the column up to a level of 6in. below the soffit of the future precast beams

5) Placing precast beams on to the brackets

of the precast

precast column

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casings

6) Fixing reinforcing steel to the in-situ bracket as well as to the upper column

7) Fixing formwork to the column head and tie beams and fixing reinforce- ment to tie beam

8) Placing upper floor column

ERECTING THE PRECAST STRUCTURAL FRAMES

ing foundations, lifts and sprinkler system, is about HK$9 million. Con- struction of the high block started in April 1970 and is due to be completed in March, 1971. Conclusion

With this type of construction the prestressed precast units are used to carry the 'wet' in-situ concrete. After hardening, the in-situ concrete and the precast concrete together form a larger structural unit which is capable of sustaining the imposed loads while not overstressing either form of concrete in the new member.

In-situ concrete has no stress in it due to its own weight and, since it will conform on hardening to the profile of the supporting prestressed unit, no casings

propping during concreting of the top- 9) Concreting column head, in-situ ping is necessary. Thus, the in-situ con- bracket and tie beam

crete need not be of such high strength as the precast unit.

10) Fixing side formwork to upper column

11) Concreting the upper column up to 6in. below the beam soffit.

12) Raising the erection booms one floor, using one boom to lift the other.

13) Repeating the cycle 5-12 up to the top most floor

14) Placing floor planks in vertical sequence

15) Fixing topping reinforcement 16) Casting topping

Each cycle of 5-12 took five work- ing days.

The total cost of the high block, excluding professional fees but includ-

A further future advantage will be from the reduced cost of construction if the contractor concerned is able to organize his operations properly. Factors which contrubute to reduced

costs are:

1) Little or no formwork. 2) The elimination of the need to stress the floor transversely

3) The reduction in weight of the precast unit for a set span makes for greater ease in handling, transpor- tation and erection.

4) Repetition work in precasting lends itself to the long-line pretension system.

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