Mixed-use
The increasing use of S460 steel in buildings David Brown
20 NSC
April 19
19
however the main five-star W Hotel is a
different proposition.
To be erected around its own centrallypositioned
concrete core, the W Hotel will
be 13-storeys high and sits on a separate
grillage of transfer beams positioned at level
one.
The building is an egg-shaped structure,
formed with radial floor beams creating
a column spacing which is 4.8m at its
widest. Adding to its complex design, the
lowest three floors are inclined with raking
columns, while the upper three are set-back,
forming large terraces.
To create this complex-shaped structure,
BHC will use a combination of facetted and
curved steel members to form the balconies
and perimeter walls.
As of March 2019, BHC had erected
approximately one-third of the overall
15,000t of steel, a total that amounts to
the company’s largest ever project. The
steelwork package is due to complete in June
2020, with the project opening in phases,
beginning with the retail and leisure element
anticipated to complete in 2020, and the
remainder of the development in late 2021
when the W Hotel and residential parts will
be ready.
Twenty five years ago, designers were
being encouraged to use S355 (known as
Grade 50 then) rather than S275 (Grade
43). In a similar way today, the use of S460
offers benefits as demonstrated by its use in the
Edinburgh St James project.
The major structural advantage is seen in
the design of compression members, where
a higher column buckling curve is specified
for S460. In the minor axis, which is generally
critical, the buckling curve improves from curve
‘c’ to curve ‘a’ for a typical column section. There
is a minor disadvantage because the nondimensional
slenderness increases with design
strength, but this is more than offset by the
improvement in buckling curve and the 30%
increase in design strength. For typical column
lengths in multi-storey buildings, the increase
in resistance by using S460 can be in the order
of 35 – 40% compared to S355. Smaller, lighter
column sections can therefore be used.
In bending, the increase in resistance is
solely due to the increased strength, offset
very slightly by the increased slenderness if the
member is unrestrained.
The Edinburgh St James project used S460
HISTAR. This steel is not covered by BS EN
10025, but by a European Technical Assessment
(ETA), number 10/0156. The ETA can be found
on the EOTA website by searching for the ETA
number.
An ETA is an alternative to a harmonised
of the SCI comments
Standard, and in this case tabulates the same
mechanical, chemical and toughness properties
as BS EN 10025, but specifically for HISTAR steel.
The special feature of HISTAR steel is that the
design strength for S460 does not reduce until
the thickness is greater than 100 mm. There are
no reductions at 16, 20, 63, 80 mm that would
normally be anticipated – the design strength
to be used in calculation is 460 N/mm2 until the
thickness exceeds 100 mm. Above 100 mm, the
design strength drops slightly to 450 N/mm2.
For relatively thick sections, the use of
HISTAR would lead to an increase in resistance
of around 15%, simply because there is no
reduction in the design strength.
Designers should not be concerned about
welding S460. The key measure of weldability
is the carbon equivalent value (CEV). Table 4
of BS EN 10025-4 shows that the CEV for S355
up to 120 mm is 0.45, and for S460 is 0.48 (so
not a significant increase). From Table 5 of ETA
10/0156, S460 HISTAR has a maximum CEV of
0.43 indicating that welding should not be
more difficult with the higher grade.
The smaller residential
grid sits above the
retail elements of the
project
/Concept_design#Concrete_or_steel_cores
/Design
/Fabrication#Bending
/Residential_and_mixed-use_buildings#Balcony_systems
/Construction#Steel_erection
/Retail_buildings
/Leisure_buildings
/Multi-storey_office_buildings
/Steel_material_properties#Toughness
/Steel_material_properties#Yield_strength
/Welding