Mixed-use
The trusses will form an
architectural feature in
the completed scheme
NSC 17
March 19
the smaller RC column grid of the towers to
the larger steel structural grid below. This
was resolved by using the trusses to support
the change in the column grid pattern.
This is the transition zone between the
steel and concrete parts of the scheme and
it required a lot of coordination during the
design phase between steelwork contractor
Severfleld and the reinforced concrete
contractor.
“There are a lot of steel to concrete
interfaces, particularly along the top of the
trusses where the tower’s columns will be
sat. Here we’ve had to install stiffeners for
the rebar, while on the underside of the
trusses, at these column locations, we’ve
added stiffened nodes, some of which are
up to 150mm thick,” says Severfield Project
Manager Richard Grey. “
There are 15 trusses in total of varying
length, with the longest being 25m. There
are eight double-height trusses, that
accommodate two floor levels, and seven
single level trusses.
Some of the single trusses are located
where there is a set-back in the building
accommodating a terrace, or in areas that
will not need to absorb such heavy loadings
as other zones. Above these slimmer trusses,
a series of stub columns bring the steelwork
up to level nine.
A variety of UC section sizes are used for
the trusses with a number of them being
356 × 406 × 634 UC’s. In some areas, the
diagonal bracing elements have been pulled
apart (un-noded) to provide sufficient space
for doorways.
“The result of this un-noding has resulted
in a huge local shear load - V=10,620kN,
equivalent to 84 double decker busses - in
the top boom,” explains Mace Senior Project
Manager Thomas Kercel.
“In these locations, in order to provide
a member with adequate capacity the
standard rolled UC has been replaced with
a fabricated plate section. The webs of these
fabricated sections are up to 150mm in
thickness in order to provide the required
shear capacity,” he adds.
The trusses are supported at either end
by a series of 600mm squared and doublewebbed
mega-columns, each fabricated from
four plate sections.
All of the podium’s columns are founded
at ground level above a three-level deep
basement. The subterranean level is
predominantly formed with concrete,
although some columns are steel members
encased in reinforced concrete.
Based around a centrally positioned core,
Fabsec cellular beams, accommodating all of
the services, span outwards to the perimeter
to form the desired column-free office spans.
The mega-sections account for
approximately 50% of the podium’s columns
and are spliced at every second floor. Like
all of the project’s steelwork, the megacolumns
had to be restricted to 9t sections so
that each piece would fall within the lifting
capacity of the site’s tower cranes.
“It’s a very confined project with no room
for a mobile crane on site or on any of the
surrounding streets, so everything has to
be lifted by one of our three tower cranes,”
explains Mr Kercel. 18
FACT FILE
One Crown Place,
London
Main client:
AlloyMtd
Architect:
Kohn Pedersen Fox
Main contractor: Mace
Structural engineer:
AKT II
Steelwork contractor:
Severfield
Steel tonnage: 2,600t
A six-storey steel
podium is topped
by the structurallyimportant
trusses
/Concept_design#Floor_grids
/Design
/Steel_section_sizes
/Braced_frames#Vertical_bracing
/Steel_construction_products#Plate_girders
/Fabrication
/Concept_design#Concrete_or_steel_cores
/Service_integration#Composite_beams_with_web_openings
/Multi-storey_office_buildings#Long_span_systems
/Construction#Tower_cranes