4.2 - VSL

Figure 68: The Orchard Towers shortly
before completion
shops the slabs cantilever out beyond the
outermost columns. In this region it was
possible to keep within the depth specified by
the architect for the load-bearing structure
Figure 69: View during construction
Figure 71: Plan and cable distribution in high-rise section of rear
thanks to the use of post-tensioning. In the
upper storeys the slabs are strengthened
with post-tensioned edge beams, which
assist in supporting the heavy facade
cladding.
The column spacing in the rear building in
the central part of the low structure and in the
storeys of the high-rise section is 8.25 m in
both directions. In the low structure the most
economical arrangement proved to be a
combined floor structure, namely low main
beams in the transverse direction and thin
flat slabs in the longitudinal direction. The
depth of the slabs is 150 mm and that of the
beams 380 mm. By the use of this shallow
structural depth it was possible, without
changing the overall height of the building, to
incorporate a complete additional storey for
car parking.
The slabs of the rear high-rise building are
flat. Their thickness ranges from 150 to 200
mm. They are post-tensioned in both
directions. Like the slabs of the low level
portion, some of them possess fairly large
cantilevers.
The post-tensioning ensures the necessary
limitation of deflections. As a result, problems
such as those associated with service pipes
etc. were largely eliminated. The advantages
of post-tensioning in respect of
watertightness of the concrete become
evident in the roof slabs.
Construction
The slabs of the low buildings were each
constructed in two sections, a system which
favoured the construction program and the
course of the other work. In the high-rise
slabs, the construction program provided for
the erection of one storey every fourteen
days. After an initial phase, it was possible to
reduce this cycle to 9 days. To permit early
removal of formwork and thus a rapid
resumption of work on the next slab,
stressing was carried out in two stages and
the formwork was transferred on the fourth or
fifth day after concreting, i.e. at a concrete
strength higher than 21 N/mm 2
(Fig. 69).
Post-tensioning
For all the slabs, bonded tendons were used.
Each cable consists of four strands ∅ 13 mm
(0.5"), lying in a flat duct and fitted with VSL
anchorages. The service load per cable after
deduction for all losses is 440 kN. The main
beams in the rear low level building, which
are 1.83 m wide, each contain 6 cables. In
the slab, the tendons are almost uniformly
Figure 70: Plan and cable distribution in low level portion of rear block
Figure 72: Plan and cable distribution in high-rise section of front
block
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