NSC 19
March 18
A number of factors came into play when
the design team chose a steel core instead of
a concrete one. The site’s basement and raft
foundations have both been reused and a
lighter steel option helped avoid the need for
new piles.
“As well as the steel option being lighter,
it was also deemed to be in keeping with the
desired overall design aesthetic of exposed
steelwork throughout the building,” explains
Arup Project Engineer Tim Bennett.
“The former post office underground
railway runs directly beneath the site and so
it was also important not to add unnecessary
loads.”
Having stabilised the retained steelwork
BHC then set about reconfiguring the large
steel beams in readiness for the insertion of
new steel mezzanine levels.
The original grid pattern for the Post
Building’s ground floors was 12m × 20m
to suit post office vehicle movements.
Consequently, a series of deep transfer beams
was originally installed to support these
spans. These transfer beams had the effect of
concentrating the original building loads into
heavily-loaded, widely-spaced points on the
raft foundation.
As the ground floor is no longer subject to
vehicle movements, the very long spans over
this area are no longer necessary. As such, by
providing new columns to cut down the long
spans, the increased overall building mass
can be spread more evenly on the existing
foundations.
By removing the transfer beams’
concentrating effects, the widely-spaced high
point loads are replaced by more frequent,
lower point loads. This helps limit punching
shear and bending forces within the raft,
such that the increased building size can be
carried.
The now redundant transfer beams have
been slimmed down from 1.8m-deep to
500mm-deep members. This involved a large
amount of site modifications to the existing
plate girders, with a large team of welders
on-site.
Where mezzanine floors have been
inserted, the existing steel beams have been
trimmed to maximize available headroom
within the existing floor-to-floor heights.
An entirely new steel frame has been
erected around the retained portion
completing the lower three floors and filling
up the entire site’s footprint. At the south east
corner of the site a new residential zone has
been created and integrated within the overall
structural footprint.
Elements of the retained building were
integrated at levels one, two and three, and
this dictates the floor-to-floor height of these
storeys. Above this, new construction is
provided following the original grid at levels
four and five.
These floors step-in at levels five and eight,
where transfer beams have been installed
to support the column location changes.
Substantial transfer structures were included
at level five to permit the upper floors to
follow a more attractive spatial planning grid
Level eight also has a slightly higher floorto
ceiling height than the other new floors
and includes another mezzanine. The roof
profile has been stepped back to minimise
visual intrusion, and trusses spanning along
the step lines have been used to reduce the
number of internal columns, maximising
clear internal floor space.
“This is a very challenging project and site,
with lots of trades working simultaneously,
all of which has required a good deal of
coordination,” sums up Mr Veness.
“Retaining and altering the old steelwork
was an engineering feat requiring the frame
to be surveyed throughout the works. We also
found the old steel to be in good condition,
in some ways due to it being coated in lead
Mixed-use
20
How the steel erection
looked last May
The completed
building’s New Oxford
Street façade
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