CORNISH CONCRETE PRODUCTS HAVE DESIGNED, SUPPLIED AND
INSTALLED TWO FEATURE STAIRCASES FOR A NEW FOUR-STOREY OFFICE
BUILDING AT BP’S INTERNATIONAL CENTRE FOR BUSINESS AND TECHNOLOGY.
Designed by Scott Brownrigg,
the building was conceived from the inside out, following a
comprehensive workplace strategy to provide a modern, vibrant and
energising space to enhance collaborative working and effectiveness.
It is also adaptable to the future needs of the global corporation.
Two feature staircases are formed in high-strength
precast concrete elements connected by in-situ concrete stitches.
Large, curved, tapered spine sections are joined by prestressed
Macalloy bars and on to curved flights and half-landing units.
Working from an outline design of the staircase by
Buro Happold, Cornish Concrete Products’ engineers carried out the
detailed design of the individual precast elements and connections.
During this process, the proposed in-situ concrete solution was
switched to precast to achieve the required finish and quality. All
precast elements cantilever from the curved central spine beam which
is partly reinforced with 25mm solid steel plate.
To ensure all precast elements would fit together on
site, the curved and tapered sections were modelled in 3D using both
Solidworks and AutoCad as independent checks on compatibility.
Unlike most precast staircases, the main visual
feature was to be the curved and multi-faceted soffit and so bespoke
moulds were produced to ensure units could be cast the ‘right’
way up. Treads were rebated to ensure continuity, with in-situ
stitching of the precast elements.
The original design intent required the load to be
shared between the spine elements, which are supported on independent
foundations, and the tread units, supported from the intermediate
floors. Positive connections were therefore provided between every
tread and the corresponding spine unit.
However, as both vertical and horizontal incident
angles varied for each tread, couplers were incorporated within the
spine, welded to an embedded plate and angled to suit each tread –
a complex but necessary exercise.
All precast units included rebates, recesses and
cast-in conduit for lighting and balustrading, co-ordinated with the
reinforcement and embedded steel sections to provide the necessary
cover. The extensive liaison required during the detailed design
called for some re-analysis of the stairs to check suggested changes
to the moulds to ease fabrication and improve the final visual
appearance.
Installing the staircases was also challenging. As
the only connections to the in-situ concrete frame were the
cantilevered full landings at each storey and with all precast
components landed on props, lined and levelled, and remaining propped
until installation was complete, a detailed and comprehensive
temporary works solution was needed.
All units were stitched together in-situ and only
when the in-situ concrete had achieved the necessary strength could
the propping be sequentially dismantled.
All units were cast in a high-strength mix with white
Portland cement and white dolomite coarse and fine aggregates. Units
were then fair-faced and finally acid-etched to bring out the
attractive sparkle from the mica content of the aggregate.
The staircase was completed by adding a contrasting
inset natural stone tread and riser overlaid to conceal the
complexity of the construction underneath.
