| A total of four test boreholes
were conducted, each to a depth of about 64m for
the sub-structure design. Generally, these borelogs
indicate the site to be underlain by successive
layers of very soft to soft clay, followed by medium
stiff to very stiff silt and silty sand. Ground
water table is generally between 2.7m to 3.5m below
Ground level. Column loadings vary from 3,200
Tonnes to 1,800 Tonnes for the widely -spaced
columns. For the more closely spaced columns,
the loading is about 740 Tonnes.
In the selection of foundation for the structure,
shallow foundation like pad footing and raft was
considered to be obviously not suitable in view
of very poor soil (N - value of 3) to a depth
of 9m below Ground level. Bored piling was not
adopted in consideration of high water table with
silty sand and low N-values at the upper layers.
The requirements of long length of steel casings
associated with boring in such soil to prevent
collapse of bore holes would not merit value engineering
decision.
For such soil condition and medium range column
loadings, it was considered most appropriate to
adopt driven reinforced concrete piles. Further
reasons to justify the use of driven r.c. piles
are that they are economical (especially, when compared
to steel piles) and could be installed relatively
quickly. Information on the piles used are as follows:
| Size = |
400mm x 400mm, with welded joint |
| Grade of Concrete = |
G45 |
| Driven Length = |
Average 55m |
| Working load = |
185 Tonnes |
Maximum No. of piles/column =
|
18 |
Essentially, these piles are skin friction piles
which mobilise the good soil resistance properties
at depth of 30 to 55m. The idealised structure
consists of moment resisting frames coupled to
a shear wall. Horizontal and vertical r.c. members
are rigidly connected together in a planar grid
form which resists lateral wind loads primarily
through the flexural stiffness of the members.
This type of structural system is efficient to
enhance the sway serviceability performance of
the building. The structural analysis was carried
out using the computer software STAAD-III, with
the appropriate gravity loads and wind loads,
derived from a basic wind speed of 35.8m/s (80mph).
The maximum computed horizontal deflection of
98mm, is well within the deflection limit of H/500
(85m/500 = 170mm).
The building was designed for conventional r.c.
beam and slab construction which is most economical
for such medium height range. Furthermore, many
local builders were able to tender for this job.
The quantity of concrete (G30) and steel reinforcement
(Fy = 460 Mpa) used for the superstructure are
as follows:
Concrete : 5,696 m3
Steel: 1,195 Tonnes
In order to achieve an early hand over of the
lift r.c. wall for lift installation, the contractor
adopted the “Jump Form” construction
with a construction cycle time of 8 days for 3.9m
height of wall. With this method, the contractor
was able to complete the r.c. wall construction
3 months ahead of the other areas which was constructed
using normal steel and timber framework.
The entire project, including piling works,
was completed in 22 months.
|
