Mixed-use
The four plate girders described in the main
seen in the photographs. In terms of structural
resistance, a fabricated box section has some
significant advantages. Firstly, a box section with
a rectangular cross-section of this geometry will
not suffer from lateral torsional buckling, so the
moment resistance will be the resistance of the
cross section. Very tall, very narrow rectangular
cross sections may experience lateral torsional
buckling – but not the form of the cross section
used at the Ilona Rose development.
Double webs obviously increase the shear
resistance of the member. Normally, shear
resistance is not critical for beams, but with high
loads close to supports, shear will be much more
important.
A further beneficial contribution to the
resistance is that the classification of the allimportant
22 NSC
article are not ordinary I-shaped beams,
but are fabricated box sections, as can be
Jun 20
compression flange changes from
the consideration of an outstand element to
classification based on an internal element –
between the two webs. Outstand flanges are
of course still possible, but the outstands of the
fabricated sections at the Ilona Rose development
can be seen to be very small.
From Table 5.2 (sheet 2) of BS EN 1993-1-1,
the Class 3 limitation on an outstand flange is a
c/t ratio of 14ε. The Class 3 limit for an internal
element from sheet 1 of the same table is 42ε.
Based on the outstand classification one might
have expected 28ε for internal elements, so the
50% increase is valuable – the compression flange
of a box member with a double web is more
effective than that of an I section. The classification
of the compression flange is unlikely to be a
concern in this instance – the flanges are so thick
that they will be Class 1. The significant thickness
means that the design strength – taken from
BS EN 10025 – will be significantly lower than the
nominal grade. A 100 mm thick flange in S355 will
have a design strength of 315 N/mm², for example,
which affects the resistance of the cross section
and the value of ε when classifying the element.
Fabricated box sections do have some
disadvantages. The section must be completed by
welding from the outside only and connections
to the webs may be more involved since access
to the inside is not possible. Fin plates may be
appropriate, or end plates offset from the web
and welded to the flanges, as can be seen in the
photograph.
Fabricated plate girders
David Brown of the SCI comments
on the fabricated plate girders used
at the Ilona Rose Development
restrained by the primary beam line
on the east side of the core.
The girders are between 15m-long and
18m-long, and weigh in the region of 40t
each. They are tapered and have a depth
that varies from 730mm to a maximum of
1.5m depth.
As these sections would have also
exceeded the lifting capacity of the onsite
tower crane, each girder had to be delivered
to site in three pieces.
Summing up, Sir Robert McAlpine
Project Director Allan Cameron says:
“From the outset, William Hare has worked
hand-in-hand with Tier Consult to provide
the most efficient solution with a key
consideration given to buildability.
“With a fully coordinated and integrated
3D model in place prior to any site works
commencing, all key interfaces have been
resolved with risk reduced of any onsite
clashes.
“On the large truss, they designed both
welded and bolted connections to enable
erection during the winter ‘windy’ period
with minimal fuss and overcame what was a
high-risk element of the build.”
Ilona Rose House development is due to
complete in 2021.
A fabricated plate girder
is lifted into place
Steelwork is erected
around a centrallypositioned
concrete
core
Each girder was supplied in three sections, red,
orange and blue, and welded together onsite,
while supported on temporary props - yellow
21
/Steel_construction_products#Plate_girders
/Member_design#Lateral_torsional_buckling_resistance
/Member_design#Classification_of_cross_sections
/Fabrication
/Steel_material_properties#Yield_strength
/Member_design#Resistance_of_cross_sections
/Welding
/Simple_connections#Fin_plates
/Simple_connections#Flexible_end_plate_connections
/Construction#Tower_cranes
/Fabrication#Handling_and_transportation
/Trusses
/Construction#Steel_erection