Expanded cellular beams (cutting an 'I'
beam in two and reassembling it) can
result in a beam up to 60% deeper than
the parent section, without increasing
the overall weight, allowing greater spans
to be created from smaller section solid
'I' beam stock. Welding two profiled tees
together also allows for a lighter top tee to
be combined with a heavier lower section
to provide the required load capacity,
which minimises the overall weight of the
section.
Disadvantages of this process can include
waste generated by the offset of a cell
radius; the difficulties of integrating
haunch and shear points and the time
taken to weld the tees into a complete
section. Due to the design of the sections,
omitting cells in a beam is particularly
difficult and requires forethought to
integrate haunches and shears. In addition,
the centre-line weld must be tested before
the beam is installed, which increases the
overall manufacturing time.
Introducing an aesthetic and
functional curve
The process to curve a cellular beam starts
with the creation of the two profiled teesections. The profiles must take account
of the subsequent bending process; the
radius of the holes in the lower tee-section
will increase during bending while they
will decrease in the upper section, so have
to be compensated for when cutting the
holes before being bent.
This slightly more complex cutting profile
depends on the specified curvature of the
beam and the radius of the holes. Usually
two top tee-sections are produced from
one original universal beam and each
is partnered with a similar bottom tee
after they have been bent to the required
specification.
The matched pairs are then aligned in a jig
and welded together.
The process becomes slightly
more complicated when the
beam needs to be curved and
tapered in order to
accommodate the growing
trend for more aesthetically
pleasing and even lighter
structures, particularly in roof
sections.
Essentially, the choice of production
method then includes beam
construction from plate sections
which can be cut and assembled, then
bent, or, pre-bent in the case of the upper
and lower sections, the centre is cut to
profile and perforated and then the three
elements are welded together to create
the finished fabricated beam.
This final option is the most expensive,
but essential when a tapered beam is
required, however it can be cost effective
when a curved beam with uniform depth
is required, this tends to be dictated by the
relative market cost of plate versus 'I' beam
section. When the depth requirement and
overall weight target of the final beam fall
within a popular 'I' beam section size, then
the economics tend to favour Porthole
Beam™ construction over fabrication.
A number of production
methods can be employed to
create both straight and curved
sections however, each has its
own advantages depending on
the demands of the application.
the correct section for their demands.
Fundamentally, the simplification of all
production processes of cellular beams
signifies their relevancy to the continued
progress of the industry.
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data required for specifying Porthole
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Making your choice
Porthole Beam™ construction has
simplicity and speed on its side, with
the ability to tailor the profile of every
hole and make allowances for tapers and
haunches. The production time is reduced
without the need for welding and there are
no testing requirements or potential issues
with misalignment.
However, time saved via less fabrication
can be lost due to CNC machine capacity
and calibration. As holes are cut into the
section after it has been curved, the bent
beam covers more of the machine tool
bed, restricting capacity compared to
cellular beams that are cut when straight.
In addition, each differing section will
require new CNC programming to achieve
the geometrically perfect finish.
Fabrication is a case of design demands, or
availability versus pricing conditions in the
steel market.
Through promoting strength and
maximising the height factor of steel
structures, straight and curved cellular
beams in all their forms typify the strides in
modern construction efficiency. Designers
can take full advantage of choice with
regards to structural beams, specifying
Issue 22 PECM
113