designed as post-tensioned concrete
structures, but they were changed to steel
frames primarily because of cost and
building weight.
On S1, structural steelwork’s flexibility
and the ease with which last minute
changes can be made during the fabrication
process has come to the fore.
Working on behalf of BAM
Construction, Elland Steel Structures
completed the main steel frame for S1
in December 2019 on schedule, despite
having to incorporate numerous design
changes that totalled nearly 1,000t.
“The completed building has been let
to two tenants, both of which requested
changes to be made to the steel frame
design,” explains BAM’s Construction
Manager Justin Brown.
“They both wanted numerous staircase
openings to be included to allow their
floors to be linked, while the occupier of
the lower six levels also wanted a fourthfloor
atrium.”
All of the new openings, including
the atrium have been formed with a
reinstatement method, whereby design
capacity, fixings and detailing have been
provided ready for extra steel beams to be
brought into the building, to simply infill
the voids if the current tenants or new
tenants in the future want an uninterrupted
floorplate.
Prior to the steel frame being erected,
the S1’s most challenging aspect, namely
two Thameslink tunnels that run beneath
the building, had to be bridged over with a
large ground floor transfer structure.
This concrete structure not only
transfers the building’s loads away from
the tunnels, it also forms the structure’s
basement level.
“Because of the tunnels, the project
has had to be conducted with full
agreement from Network Rail, while
steelwork’s relative lightness compared
to other framing materials was an
important design choice on account of
the underground rail assets,” says Mr
Brown.
The steel frame begins at ground floor
and includes a double-height space for
this lowest level. This floor accommodates
the building’s main entrance alongside a
number of retail outlets.
From first floor upwards the steel design
is fairly regimented, based around a 9m
column grid pattern, with internal spans
up to 12m long.
One concrete core supplies the majority
of structural stability to the steel frame,
although as Mr Banfield explains, it was
designed to be as small as possible in order
to minimise the weight of the building over
the railway tunnels.
“The core is also positioned slightly off
centre and so structurally the building
wants to twist. To prevent this and to
supply extra stability, there are two
3m-wide braced bays that extend up the
entire building.”
One of the exceptions to the regimented
design is the fourth-floor atrium, which is
positioned within one half of the building
and extends upwards to the underside of
level seven. A fifth-floor area wraps around
the perimeter of the atrium and, because
this floor area does not line up with the
rest of the building’s column lines, it is
hung from the floor above via a series of
steel hangars.
In order to install this hanging floor,
Elland Steel had to erect the level above
and then wait until the composite flooring
had been installed. Once this was done, the
hanging fifth-floor level was installed with
the use of temporary supports.
Aesthetics have played a key role in
the steelwork design of S1 as the interior
steelwork, as well as the underside of the
soffit, will be left fully-exposed within the
completed scheme. Cellular beams have
been used throughout to accommodate the
Commercial
Model showing the main
frame and the Thameslink
tunnels that run beneath
the site
20
NSC 19
Mar 20
S1 and its neighbour
S2 (on left) have been
designed to look like
one single entity
“Steelwork's
relative
lightness
compared to
other framing
materials was
an important
design
choice on
account of the
underground
rail assets.”
/Cost_of_structural_steelwork
/Multi-storey_office_buildings#Flexibility_and_adaptability
/Fabrication
/Design
/Steel-supported_glazed_facades_and_roofs#Atrium_Roofs_and_Sky_lights
/Construction#Steel_erection
/The_case_for_steel#Take_a_load_off_your_foundations
/Concept_design#Floor_grids
/Concept_design#Concrete_or_steel_cores
/Concept_design#Structural_options_for_stability
/Floor_systems#Composite_slabs
/Construction#Temporary_works
/Steel_construction_products#Cellular_beams