A truss element
arriving from Bourne’s
Poole facility
NSC 17
January 19
adjacent 19th Century-built Waterloo roof,
but never touches either.
A steel-framed solution was the
only viable design for the project as a
lightweight structure was needed because
no foundations could be installed to
support it. Piling the site was impossible
as a number of tunnels as well as Waterloo
station’s underground ticket hall sit beneath
the site.
“The entire roof structure including
glazing is only 400t,” says Mr Kitching. “But
however light this may be, we still needed
to work out where the loads could be
transferred to and if we could free-up any
capacity from the existing structures.”
The solution involved some nifty juggling
of loads and required the buffer stops in the
WIT structure to be moved 50m down line.
This was feasible as domestic trains are not
as long as Eurostar trains. So, by moving
any potential train impact away from the
new roof, nearly enough load capacity was
found for the new structure.
Nearly being the operative word, as
the roof still needs two 508mm-diameter
circular hollow section (CHS) columns to
support it in the middle.
“Using circular columns means the
steelwork is less harsh on the eye and
importantly they have been located so they
do not hinder the important views in the
station concourse,” adds Mr Kitching.
The vistas which Network Rail is keen
to preserve are the views of the Grimshawdesigned
WIT arches seen when entering
the former Eurostar terminal, and the listed
Waterloo station Victory Arch and Stairs
viewed when leaving the same platforms.
As well as providing additional support
to the roof, the CHS columns allow the
structure to have a central area with a 26m
clear span. However, as no foundations can
be installed, the CHS members are founded
directly on top of the WIT platform slab.
Because of the WIT’s propped cantilever
design the slab is subject to movement
of up to 100mm +/-, and so the columns
are placed on bearings to allow for any
potential shift, which would otherwise
crack the new roof ’s glazing.
Forming the main span of the roof is a
52m-long spine truss, which is 4.2m-deep
and weighs 27t. It was brought to site in
three sections, with the longest element,
which spans between the CHS columns,
weighing 13.5t.
The central spine truss supports a series
of eight pairs of gullwing trusses that sit
perpendicular to the main structure and
form overhangs on either side. Each wing
measures approximately 8.3m-long x
4.0m-deep.
Steelwork for the project has all been
erected by a 300t-capacity mobile crane
positioned towards the northern end of
the site. The working area for the project
is extremely tight, squeezed in between a
‘live’ functioning railway station and office
buildings. Finding a suitable location for
the crane was not only determined by space
constraints, but also by what is underneath
the site.
It had to be positioned to one side of the
new structure, away from any subterranean
facilities. This position meant a larger
capacity unit with a longer reach was
required; one much bigger than would
ordinarily be needed.
Keeping the frame stable during
erection was one of the main challenges for
steelwork contractor Bourne Steel. Due to
the confined nature of the site, the structure
was erected on substantial temporary
works comprising two towers over 20m in
height, a ‘planar’ trestle and a temporary
lattice girder spanning approximately 25m
between the towers.
The temporary works enabled sequential
erection from east to west and the
installation of the gullwing trusses and roof
steelwork without the spine truss being
fully complete. The temporary towers and
truss also allowed the wing trusses to be
set at the correct level before de-propping,
when the building load is then transferred
to the feature columns.
This was exacting work as Mr Davies
explains: “Each gullwing truss had to be
in an exact pre-set position before the
frame was de-propped and the glazing was
installed, so they needed to be individually
surveyed, checked and then released by
our designers before the erection sequence
could continue.”
The new roof is due to complete by early
2019, while the ongoing works on the
former Eurostar terminal will continue into
2019.
Transport
The lifting operation
was challenging
because of adjacent
buildings
A truss is made ready
for erection
A temporary support
tower is installed
/The_case_for_steel#Take_a_load_off_your_foundations
/Steel_construction_products#Structural_hollow_sections
/Bridge_articulation_and_bearing_specification#Bearings
/Steel-supported_glazed_facades_and_roofs
/Trusses
/Construction#Mobile_cranes
/Construction#Steel_erection
/Construction#Temporary_works