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Nov/Dec 19
treatment water tank and thereby lessen the
impact on the landscape.”
The largest structure at Williamsgate,
housing the main water treatment facilities,
is the Rapid Gravity Filtration Building
(RGFB), which is a large steel frame.
Measuring approximately 90m × 90m,
the structure has dual-curving roof, to be
topped with sedum, a design specifically
chosen as it will help it blend into the
countryside.
“The framework to the building was
designed to BS EN 1993-1-1. As the
structure sits on a substructure of concrete
walls and slabs, of varying levels, and has a
dual wave-form roof, the design considered
to be the best solution for the superstructure
was a combination of braced steel frames
and portalised bays where a clear span
was required,” explains Border Steelwork’s
Technical Manager Ian Elliott.
As well as fabricating, supplying and
erecting the steelwork, Border is also
responsible for the RGFB’s roof and wall
cladding, including the installation of the
sedum. The steel erection programme
started once the groundworks and the
extensive concreting had been completed.
Internally, the RGFB is divided into
three main areas, an initial filtration zone, a
chemical dosing area – where chemicals are
added to further enhance the water quality,
and a second stage filter area. All of these
areas are of a different size and their internal
configuration also varies due to the differing
sizes of the water tanks and equipment.
Concrete walls, that compartmentalise
the entire building, presented the steel
erection team with their biggest challenge.
“Because of the walls, it was impossible to
run MEWPs into many areas of the building,
so we had to lift them into the structure
by crane so our erectors could
bolt the splice connections,”
says Border’s Contracts Director
Stuart Airey.
“We then removed
the MEWPs via gaps we
left in the roof steelwork.”
The initial filtration zone is five
bays wide, with each one measuring
up to 16m-wide. The widest central zone
also features an overhead gantry crane, with
its rail beams supported by the internal
columns.
The fifth bay, is an open canopy which
runs the length of the building, covering an
access road. For design purposes, this area
was treated as a lean-to in the structural
analysis.
Internal partition walls for the central
flocculation zone (main gallery) run at
90-degrees to the adjacent zones. This
again has five bays with the largest having a
21m-wide clear span.
Lastly, the final stage filtration zone also
has a longest span of 21m, in a zone that also
incorporates another overhead gantry crane.
The dual-curving roof that spans the
entire RGFB is formed with curved rafters
in one direction, that have stepped faceted
purlins between them to form the curve in
the other direction.
As the roof will be entirely covered with
sedum, which will exert heavier loads than a
traditional standing seam roof, lateral sway/
deflection had to be considered, using a 1 in
50 year return wind loading calculation.
The building’s roof also features a
2m-wide overhang around its full perimeter,
which follows the curve of the roof, and
contains the gutter. The design for this part
of the steel frame had to consider general
imposed loads, as well as the loading of a full
capacity gutter.
Meanwhile, the upstand of the perimeter
along the overhang was designed to consider
lateral loads from the bespoke stainless steel
edge protection.
Once the Williamsgate Water Treatment
Works is completed in 2022 and the wider
network and infrastructure is ready, the site
will be capable of treating 80 million litres of
water every day.
In summary, Ian Tomlinson, United
Utilities Project Manager, says: “The project
has been a great example of partnership
working - and not just with our contractors,
but also with our process colleagues who
will eventually operate the treatment works.
“We’ve used the latest 4D modelling,
Building Information Management (BIM)
tools and virtual reality tours to specify the
exact design detail, before a spade even hit
the ground. That all adds up to a smooth
construction process and a great final result.”
“The project has been a great
example of partnership
working…”
Utilities
A large earthmoving
programme was
needed before steel
erection could begin
The roof will be covered
with sedum to blend
into the surroundings
Much of the steelwork is founded
on a concrete substructure
/Construction#Mobile_cranes
/Braced_frames
/Portal_frames
/Fabrication
/Building_envelopes
/Construction#Steel_erection
/Design_codes_and_standards#General_rules_and_rules_for_buildings
/Design_codes_and_standards#Wind_actions
/Design
/Steel_construction_products#Stainless_steel_products
/Modelling_and_analysis#Modelling
/Construction