NSC 17
One of the seven
internal columns
Sept 19 Senior Project Manager Rohit Gorasia.
From level 10 to 14, the building’s
floorplates decrease in size as each of these
levels has a roof terrace. Each outdoor
space is in a slightly different position to
the others, as they are arranged in a stepped
formation that ascends around two sides of
structure.
“If you look at the top of the building in
plan and divide it into nine squares, five of
the outer squares become terraces from level
10 to 14 and the floorplate progressively
decreases creating a ‘Giant’s Causeway-like
effect’,” says Pell Frischmann Associate
Director Kee Leung.
The building’s steel frame generally has
a 15m × 9m grid pattern, with one area
designed around a larger 15m × 15m column
spacing. Fabsec cellular beams – chosen to
integrate the services within their depth –
support metal decked composite floors.
The long span arrangement means there
are only seven internal columns, all of which
will remain exposed in the final scheme.
“As the columns will be feature elements
within the building, we will apply a
decorative final coat of paint once the
internal fit-out is nearing completion,” says
Severfield Senior Project Manager Michael
Bryars. “Doing this late in the construction
programme, means there is less chance of
them getting damaged.”
Aesthetics has played a further role in
the design of these seven exposed columns,
as 460 grade fabricated plate girder sections
have been used, as opposed to conventional
355 grade members, which have been used
around the building’s perimeter.
“These columns were chosen as they
offer the required stiffness, but are slimmer,
meaning they take up less of the valuable
office footprint,” says Mr Leung. “They may
cost more, but we ended up using less steel,
which evens out economically.”
The column splices are also concealed
in the structural floor zone, again adding to
the overall clean and neat aesthetic look the
architect and client desired.
Stability for the frame is predominantly
derived from an offset concrete core, which
is positioned along the eastern elevation.
Similar to the choice of column sizes, the
core’s position was chosen so it would not
intrude into the floor space, while also
giving the impression of a more open-plan
environment.
The opposite western elevation has been
designed as a sway frame, to restrict torsion
and provide some extra stability to the area
furthest away from the offset core. This is
particularly important as the only area to
feature a 15m × 15m grid is in the south west
corner of the building.
Commenting on the sway frames, Mr
Leung adds that although stability was an
important consideration, the main reason for
the design was the strict cladding deflection
limits along this elevation.
The sway frame is created by two
parallel ladder frames that extend up the
whole western façade. They are formed by
a series of 8m-long × 4m-high H-shaped
fully-welded assemblies each weighing 6.5t.
They are fabricated from 725mm × 300mm
plated beams and 600mm × 400mm plated
columns.
Possibly the most impressive feature of
80 Fenchurch Street is the full-height central
atrium, which will have a glazed roof.
Standing at ground floor and looking up
through the 9m by 9m void, one can observe
a spiralling effect as the floor beams along
two sides of the atrium are slightly cranked
at a different angle on each level to give the
impression of a twist.
Forming the roof of the atrium has been
one of the more significant design challenges
of the project. Taking into account the extra
loading that the landscaping of the terraces
will impose on the structure - as it will
include some large planters with small trees -
the atrium roof structure has been designed
as an independent Vierendeel frame.
It sits between floors 12 and 14, and this
standalone structure supports the glazing
and is completely detached vertically from
the slab edges at levels 13 and 14, which
isolates it from the post-construction
movements.
The box is approximately 8.6m tall
and works as a Vierendeel frame in two
directions, spanning 13m in one direction
and 7.5m in the other. It is supported
vertically at key discrete points of the main
frame.
According to Pell Frischmann Senior
Engineer Alexandru Gaftea, the frame was
designed so that the deflections are limited
during each of the construction stages,
starting from the erection of the frame and
finishing with the building being fully-
occupied and operational.
“By closely coordinating with the
architects and the façade subcontractor,
we managed to achieve the best balance
between the sizes of the steel frame elements
and the predicted movements that would
have an impact on the glazing panels.”
Due to the large size of this frame and the
transportation limitations, hidden splices
were introduced at specific locations in order
to minimise the visual impact of the exposed
steelwork.
80 Fenchurch Street is due to complete in
June 2020.
Commercial
The western elevation's
ladder frames have a
kink at second floor
level to accommodate
the changing profile of
the cladding
Model showing
the structurallyindependent
roof frame
Possibly the most impressive
feature of 80 Fenchurch Street
is the full-height central atrium.
/Concept_design#Floor_grids
/Service_integration
/Construction
/Design
/Steel_construction_products#Plate_girders
/Simple_connections#Column_splices
/Concept_design#Structural_options_for_stability
/Concept_design#Concrete_or_steel_cores
/Continuous_frames
/Facades_and_interfaces
/Fabrication
/Steel-supported_glazed_facades_and_roofs
/Trusses#Vierendeel_trusses
/Construction#Steel_erection
/Fabrication#Handling_and_transportation
/Visually_expressed_structural_forms
/Visually_expressed_structural_forms