Mixed-use
20 NSC
July/Aug 19
divided into four blocks with two portions
on the northern elevation topping out at
levels 15 and 17 respectively. Meanwhile,
on the southern façade, there is a further
rooftop step as one portion accommodates a
two-level plant deck.
To help create the 15th floor step in the
building, two large transfer girders with
depths of 590mm and each weighing 11.5t
have been installed to support the set-back
columns above. In order to accommodate
the girders into the overall shallow floor
design, the beam’s top flanges are exposed
to the structural slab level with no concrete
topping.
The step at floor 17 is close to primary
columns so the floor edge is cantilevered
and does not require transfer beams. The
uppermost step at roof level supports two
plant decks, which is off the main column
line. A transfer beam is included here as
there is greater structural depth available.
Adding some more interest into the
building’s design, One Braham also features
a three-storey atrium/viewing gallery located
on level 14’s north western corner, offering
views to the City.
Meanwhile, the ground floor is a doubleheight
space, accommodating the entrance
lobby. The first floor, which is set back to
create this large space, accommodates a
cantilevering walkway overlooking the
entrance. Large portions of the floor are
suspended from the underside of the second
floor via a series of Macalloy hangers and
steel plate hangars to form recessed low level
façades.
One Braham is due to complete in May
2020.
19
Riveted plate girders dating from Victorian
times in use for long span railway bridges
can still be seen. Flanges are laminated
from plates and riveted angles connect flanges
to webs and webs to stiffeners. Material was
expensive and many such structures clearly show
the distribution of internal forces.
Plate girders continue to be used for long
spans and high loads (such as transfer beams
in buildings) and also for elements required to
meet particular geometric constraints that mean
rolled sections are not suitable. Material cost
is relatively less significant now and plates are
more often of uniform thickness. For design, the
relevant part of Eurocode 3 is part 1-5 Plated
structural elements.
The possible combinations of plate size and
thickness mean that phenomena not usually
encountered with rolled sections may need
to be considered. Wide flanges result in shear
lag – where the shear flexibility of the flange
outstands leads to corresponding non-uniform
axial stresses across the flange. This behaviour is
more relevant to box girders (eg in bridges) than
in I section girders. Slender flange outstands may
also lead to local buckling of the flange edges
and require a design based on effective area,
although use of material in such a design may
not be considered efficient.
Designers of plate girders not susceptible
to fatigue could use P4191 when considering
the thickness and toughness requirements of
flanges. A “hybrid” approach is possible with high
strength steel for flanges and a lower strength
steel for the web2. Plate girders with a web depth
selected to reduce flanges to manageable sizes
may mean the section classification is 4 (slender)
and the webs are susceptible to shear buckling.
Selecting webs thick enough to avoid this may
not be an efficient use of material and stiffeners
have to be provided. Thin webs may also mean
that bearing stiffeners are required to transfer
local point loads into the girder web. Plate girders
with stiffened webs may therefore be an effective
choice3. Tension field behaviour where the webs
act like diagonal tension members and the
stiffeners the verticals in a Pratt truss may result
and require the corresponding design approach.
In buildings, deep girders designed as pin
ended will require careful consideration of the
end connections. Finally, in addition to sizing
the plates, the web to flange welds and welds to
stiffeners are the designer’s responsibility.
1. Brown D G, Cosgrove T C, Brittle Fracture:
Selection of steel sub-grade to BS EN 1993-1-10,
SCI publication P419, 2017
2. Brown D G, The design of hybrid fabricated
girders, NSC, June 2017
3. Brown D G, The design of hybrid fabricated
girders – part 2, NSC, July/Aug, 2017
Plate girders Richard Henderson of the SCI
discusses some of the design issues
Model showing the
northern elevation,
which contains the core
and the parts of the block
that reach full height
/Facades_and_interfaces
/Steel-supported_glazed_facades_and_roofs#Atrium_Roofs_and_Sky_lights
/Steel_construction_products#Plate_girders
/Steel_construction_products#Standard_open_sections
/Design
/Design_codes_and_standards#Eurocode_3_-_Steel_structures
/Bridges
/Fatigue_design_of_bridges#The_mechanism_of_fatigue
/Stiffeners
/Stiffeners#Bearing_stiffeners
/Trusses#Pratt_truss_.28.27N.27_truss.29
/Welding