Commercial
NSC 19
January 19
design,” explains Sir Robert McAlpine
Project Manager Peter Watts.
“The biggest constraint however, was
the fact that we couldn’t gain access to the
existing foundations because ground floor
retail units and the mall have to remain open
throughout our programme.”
Sustainability is also at the core of this
design, as only steel construction would have
allowed the project to reuse the foundations,
and then strengthen retained steelwork to
allow them to support new lightweight steel
floors.
The mall and the west entrance to the
railway station also played a significant role
in determining when the project kicked-off.
Initially a start date at the end beginning of
2017 was penciled in by the team, however
with a Christmas closure of the entrance and
its shops organised during the festive season,
the programme started two weeks early.
Taking advantage of the lull in passenger
numbers coming into the City, Sir Robert
McAlpine was able to partition the mall
and begin the minor demolition works
which would allow the steelwork erection
programme to begin. Although the mall
has remained open, access routes have been
moved to allow work to continue, with
most work involving the installation of large
beams being carried out overnight.
The existing steel-framed buildings
were built in the 1980s by main contractor
Bovis, with the steelwork contract being
undertaken by Redpath Dorman Long. The
records of this construction programme and
its structural design were readily available
and proved to be invaluable when AKT II
needed to carefully analyse the structure.
“Analysis of the structure allowed us to
identify and utilise redundancies in the
original design, and work out which areas
of the retained steel frames would need
strengthening,” explains AKT II Technical
Director David Watson.
“The lightweight nature of a new steel
composite design, using Fabsec cellular
beams means we have been able to reuse the
foundations and only had to strengthen 33%
of the existing columns to support the new
build elements.”
New steel columns are bolted to the
existing steel frame where possible, and
the building follows the original structural
grid, based around a 7.5m × 7.5m column
spacing.
Adding some architectural interest to
the new steelwork, fabricated steel transfer
beams have been installed to enable
set-backs on the new upper levels. These
set-backs occur at level eight and 12 on the
southern elevation and on level 12 along the
northern façade.
Tying in a new steel frame to an existing
1980s frame has been done as seamlessly
as possible. Floor slab thicknesses vary 20
New composite
flooring abutting
existing flooring
Some existing columns
have new steel beams
connected to them
Underground Infrastructure
Metropolitan Line
Disused tunnel
Crossrail
Central Line
Liverpool St
Underground
Ticket Office
/Sustainability
/Construction
/The_case_for_steel#Take_a_load_off_your_foundations
/Construction#Steel_erection
/Braced_frames
/Design
/Composite_construction
/Long-span_beams#Composite_beams_with_web_openings
/Long-span_beams#Composite_beams_with_web_openings
/Concept_design#Floor_grids
/Concept_design#Floor_grids
/Steel_construction_products#Plate_girders
/Steel_construction_products#Plate_girders
/Facades_and_interfaces