posal put forward by the structural consultant.
The most difficult part of the struc- tural design was the rectangular plates at each floor level on top of the core walls which housed two toilets. These plates were subjected to a very com- plex system of forces. Assumptions had to be made in order to solve the problems rationally by using the theory of plates and shells. Some of the so- called secondary stresses were no long- er secondary.
In designing the plates and core walls, particular attention was given to the problems of end rigidity, two-way bending and torsion, stress concentra- tion at the outer front corners of the core walls, principal compressive stress in the concrete, yield lines of the plates, the magnitude of various de- formations and the vibration of each individual floor.
In the cantilever plates the end rigidity was not 100 per cent, so that the redistribution of the end bending moment took place between the plates FP-1, FCP-1 and FCP-2 in Fig. 1F. The yield lines within the plates FP-1, FCP- 1 and FCP-2, due to two-way bending and torsion, were investigated and the results were used in the reinforced con- crete design where principal compres- sive stress was taken as a governing factor
Special reinforcements were pro- vided to resist this stress where neces- sary. The working stress for some of the main reinforcements had to be reduced in order to keep the stress, due to two-way bending and torsion, within the concrete strength.
The stress concentration problem could be solved either by providing a much stronger material to resist the stress or by using a deformable material
CORE WALLS
CANTILEVER
PLATES
FLOOR PANELS-
ECAST CONCRETE
EDGE
Perspective view of structural frame
BEAM
WITH FLOOR
SLAB OMITTED
BASEMENT
WITH
PILE
FOUNDATION
around the stress concentration zone to dissipate the stress. The latter method was adopted.
The deformation of the cantilever core walls, cantilever plates and canti- lever edge beams, due only to dead load, were carefully computed. In cal- culating these deformations various de- grees of rigidity of the structural mem- bers were assumed and the actual build- ing movements due to dead load, after taking off the formwork, indicated that the assumptions made were very near- ly correct.
The vibration problem was studied to see whether there would be a pos- sible "spring board" effect to the in- dividual cantilever floors. The results once again indicated that the lowering of the floor slabs aided the elimination of the spring board.
Construction
During sheet piling on the site it was found that the adjacent building had started to settle. Investigation re- vealed that the sub-soil condition un- derneath the adjacent building was very bad due to the "migration of fine materials" caused by pumping water from wells during dry years. Any small vibration from the piling caused rearrangement of the soil particles.
Following negotiations with the architect of the adjacent building, its foundation was stabilized by cement grouting, which proved very effective. No further settlement was observed during the major piling works which involved the sinking of 82 prestressed concrete piles.
The whole sub-structure work, in- cluding the remedial work to the ad- jacent building and a 13 ft. deep base- ment, was completed within six months.
Being an unusual structure, the superstructure contractor commission- ed the structural engineer to design special formwork which could be re- used, except directly beneath the edge beams where the supports had to re- main in position until the top floor could support itself.
The calculated deflections of the cantilever plates and edge beams due to dead load were incorporated in the formwork in order to keep the soffit of the floor as level as possible after taking off the formwork.
Costs
The total building cost, excluding professional fees but including founda- tions and sub-contracts, was about HK$3,400,000. An approximate break-
Far East BUILDER, October 1968.
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