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