News & Events - Institution of Engineers Singapore

Cover Story
Design and installation of sky bridges
The 12 sky bridges, connecting the seven
blocks, at the 26 th and 50 th storeys, form
part of outdoor sky gardens which are
equipped with amenities for recreational
purposes. The bridges are made of steel
with concrete slabs on top. The lengths
of the bridges vary, with the longest
spanning 48 m and weighing 327 t. The
widths and heights of the bridges are 20
m and 3.9 m, respectively.
The design of the sky bridges was
carried out by T.Y.Lin International
Pte Ltd, HDB’s consultant. The design
adopted a 3-dimensional triangular
truss layout which is stable without
lateral support and could be erected
independently. The bridges are designed
to withstand wind forces in all directions.
The side faces are tapered to reduce
the obstruction to wind flow, thus
minimising the wind pressure on the face.
One end of each bridge was designed
to be fixed to the building, to improve
the natural frequency of the bridge and
reduce vibrations from walking and
jogging, thus enhancing comfort levels
for people.
Owing to the tight site conditions,
there were many challenges in the
erection of the bridges, relating to the
installation method and procedures,
availability of space at site, and duration
of the installation. Owing to the sizes,
the bridges were fabricated in segments
off-site, at a factory (Fig 7), transported
to the site, and assembled onto the
complete structure. To overcome the
space constraints, the 50 th and 26 th storey
bridges were stacked on top of each
other (Fig 8). This also facilitated the
subsequent lifting operation.
In the factory, as well as during the onsite
assembly of the sky bridge members,
the fitting up, welding, testing, and trial
assembly of the sky bridge trusses were
supervised by an independent checker.
The progress of the work was closely
supervised by the Resident Engineers and
Resident Technical Officers who were
stationed on-site and at the fabrication
yard.
After fabrication, the sky bridges
were lifted to their respective heights
using the strand jack system which was
used instead of cranes, due to the height
of the buildings. In addition, this system
Fig. 7: Welding, testing, and trial assembly, of sky bridge steel trusses at the factory.
Fig 8: Assembly of the sky bridge steel trusses on site. The 50 th and 26 th sky bridges are stacked on top
of each other.
allowed the bridges to be assembled at
a lower level before hoisting the whole
assembly, thus enhancing safety on site.
Four strand jacks were required for
each lifting. The jacks were installed
and positioned at four corners between
the two buildings at the 50 th storey roof
top position (Fig 9). Prior to the lifting,
a trial jacking was done, that raised the
skybridge 300 mm off the ground, to
ensure that the jacks were functioning
properly.
The bridges are fixed to the core
walls of the residential buildings. To fix
the bridges safely and securely to the core
walls, base plates and sleeves for tension
bars were cast together with the core
walls. Great care was taken to ensure that
the cast-in items aligned accurately with
the tower blocks. When the bridges were
lifted to their final positions at the 50 th
and 26 th storeys, and adjusted, the main
trusses were connected to the building by
high strength bars and locked in-place by
casting the concrete slab which forms the
floor of the bridge.
14 · THE SINGAPORE ENGINEER Jun 2010