Commercial
NSC 19
Nov/Dec 18
plate girder weighs in excess of 100t and is
14m-long.
An existing access route to the basement
loading bay necessitated steelwork contractor
Severfield to design, fabricate and erect a
structure dubbed the ‘Rhino’ truss because of
its shape.
“This is circa 150t in weight and was
erected using two tower cranes. We had to get
a special dispensation from the manufacturer
to uprate the cranes for the erection of this
truss,” explains Severfield Project Manager
Kyle Fletcher.
“It is made up of site bolted booms,
nodes and diagonals, with the heaviest node
weighing approximately 20t.”
In order to maximise the building’s
floor space and help the remainder of the
perimeter columns locate on existing pile
positions, the structure’s lower columns, up to
level 7, are mainly inclined.
As the columns are raking outwards
there are some large structural reactions and
transfer forces. To counteract this, the floor
beams are slightly larger on these levels and
installed in a diagonal orientation, as opposed
to being perpendicular to the core.
Like most UK high-rise commercial
buildings, the superstructure comprises
a composite design of cellular beams
supporting metal decking and a concrete slab.
This provides a diaphragm action restraint
to the perimeter columns.
A centrally-positioned core contains
35 lifts, providing direct access from the
entrance lobby to all levels. Squeezing so
many lifts into one core, without intruding on
any of the valuable floor space, meant a long
slender core design was chosen.
Consequently, the core cannot provide
the necessary structural stability to the
superstructure’s steel frame during high
winds. This has required the installation
of two giant outrigger stability systems
positioned on two intermediate floors, adding
some stiffness to the building and controlling
sway and acceleration.
Mr Padilla Philipps uses a skiing analogy
to describe the work the outriggers perform.
“If a skier has his arms straight down by his
sides, there is very little stability. However, by
introducing poles with arms outstretched one
can maximise stability.”
Positioned at levels 25 and 41, both of
which are double-height floors containing
plant equipment, the outriggers are large
floor-to-ceiling V-shaped trusses that link the
core to the perimeter columns and provide
bracing.
There are three sets of outrigger trusses
on both floors, all of which extend from the
building’s east elevation, through the core and
then connect to the west perimeter columns.
They were fabricated and erected in
sections to form a truss. These were typically
made up of a top and bottom boom, plus
Managing windy conditions
Working on a building that will ultimately
reach a height of 278m and with three
tower cranes looking down on the
structure from an even loftier position, it is not
surprising that Severfield has had to manage the
effect of high winds and gusty conditions on steel
erection.
Wind is the one weather element that can
occasionally delay a steel erection programme
that relies on craneage, and wind speeds generally
increase with height so it has more of an effect on
tall buildings. However, what is surprising about
this scheme is that the high winds that have
occasionally suspended crane duties have mostly
been encountered at low levels along the main
Bishopsgate thoroughfare.
“The street is like a wind tunnel at times as gusts
speed along the road between the numerous highrise
buildings,” says Severfield Senior Site Manager
Micky Reilly.
“This stops us picking up steel from our delivery
point and lifting it up to the erectors who are
working further up the building. Strangely at these
times the conditions have been mostly alright for
working and lifting steel on the top parts of the
structure.”
To guard against such wind stoppages and
mitigate any potential delays to the programme,
Severfield’s erection team is working on a 24-hour
basis, with a night-time shift – wind permitting
– delivering and placing as much steelwork as
possible in readiness for the daytime shift to erect.
20
Outrigger systems have
been installed on two
levels
/Fabrication
/Construction#Tower_cranes
/Concept_design#Concrete_or_steel_cores
/Composite_construction
/Steel_construction_products#Cellular_beams
/Steel_construction_products#Decking_for_floors
/Braced_frames#Horizontal_diaphragms
/Concept_design#Structural_options_for_stability
/Trusses
/Construction#Steel_erection