with the adjacent structures. However, two
of the most noticeable elements formed with
steelwork are two tree-like structures, that
help to create open-plan areas and evenly
transfer substantial loads to the foundations.
Both of the trees have four steel raking
branches, are similar in size and provide
similar solutions. Tree one is supported by
a column, stemming from the first floor
through a second-floor void, and supports
the third floor. Tree two supports the slab at
level four and is founded on a steel column
that is sat on top of a transfer beam at level
one.
“Steel was used for the trees as the four
branches collectively support approximately
3,500t, which was not feasible in concrete
given the structural zones available,’ explains
AKT II Associate Wai Pang.
“Steel was also chosen as the most
suitable form of construction due to the
concentration of load effect from raking
columns.”
Tree one was delivered to site in six pieces,
which consisted of a base plate, central node
and four branches. Each of the branches
weighed 8t and were fabricated from 700mm
× 700mm × 50mm rectangular hollow
sections (RHS).
The second tree supports the level four
slab and is supported on an 8m-high, 10t
column. It helps to form a central atrium
within the Marshall Building.
Tree two came in ten pieces, which
consisted of a column, central node, four
branches and a ring beam. The branches
weighed 6t each and were fabricated from
slightly smaller 500mm × 500mm × 40mm
RHS.
The architectural intent for this tree
was to have a void in the slab at level four,
and therefore it was not possible to have
connecting Macalloy bars between the
opposite raking branches, like the other
tree. As the forces are lower, the design team
decided to redirect the horizontal forces
around the perimeter of the void with a steel
ring beam encased in concrete.
“Coordination between the individual
trades was key to installing the trees
successfully,” explains Mace Senior Design
Manager Richard O’Shea. “Our steelwork
contractor, Bourne Steel, had to work in
tandem with the concrete contractor, as
the trees were erected within the concrete
programme. Once the trees were installed
the encasement was begun and the slabs
were cast on top of them.”
The installation process for the first tree,
sat at level two, had to wait until the slab
was cast around the 2.5m2 cast-in base
plate. This then allowed Bourne to install
the prefabricated node, which weighed
approximately 5.5t. Once the node was
bolted into position, using grout between
the base plate and node for positional
tolerance, each of the 7m-long branches were
individually lifted into place and held in
place with temporary bracings.
In a cross formation, a series of hightensile
Macalloy bars connect the tops of
diagonally opposite branches. The bars were
installed as each branch supports a column
and, due to the large axial compression loads
in the raking steel elements, a significant
component of tension is introduced at the
Education
NSC 23
May 20
“Steel was used for the trees as
the four branches collectively
support approximately 3,500t,
which was not feasible in
concrete given the structural
zones available.”
Both trees are similar
in design and provide
comparable structural
solutions.
FACT FILE
Marshall Building, London School of Economics
Main client: London School of Economics
Architect: Grafton Architects
Main contractor: Mace
Structural engineer: AKT II
Steelwork contractor: Bourne Steel
Steel tonnage: 126t
The trees form open
plan areas within the
building
Visualisation of the
completed interior
Once the steelwork was
installed, the entire
tree was encased in
concrete
Tree two has a steel
ring beam in order to
create a void in the
level four slab
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/Construction
/Steel_construction_products#Structural_hollow_sections
/Steel_construction_products#Structural_hollow_sections
/Steel-supported_glazed_facades_and_roofs#Atrium_Roofs_and_Sky_lights
/Fabrication
/Design
/Construction#Steel_erection
/Construction#Temporary_works