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Note 16 Level 1
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TheStructuralEngineer
October 2012
Technical
Technical Guidance Note
Lateral torsional buckling
Introduction
Elements within a steel frame structure are at risk of buckling under load.
If measures are not taken when designing steel elements that recognise
this risk, then the likelihood of its failure is significantly increased. This
Technical Guidance Note explains how steel elements are restrained
against buckling and what the structural engineer should consider when
analysing steel structures with respect to buckling resistance.
ICON
LEGEND
W Design principles
W Applied practice
W Worked example
W Further reading
W Web resources
Design principles
Steel beams have a tendency to buckle
along their length. In the case of simply
supported beams, this is prevented by
restraining its compression flange, which
prevents it from rotating along its axis. This
phenomenon, known as lateral torsional
buckling, must be fully understood and
allowed for by the structural engineer when
designing structures that consist primarily of
steel elements.
Eurocode 3 (BS EN 1993-1-1), Clause 6.3.2
explains that in order for a steel beam
element to be classed as ‘restrained’, its
compression flange must have sufficient
restraint so as not to be susceptible to
lateral torsional buckling. Beams with certain
types of cross sections e.g. closed hollow
sections with a height/depth ratio of less
than or equal to 2, are not susceptible to
lateral torsional buckling (Figure 1).
Methods of restraint
As a general rule, a restraint to a top flange
of a beam must be capable of resisting
a force that is equivalent to 2.5% of the
ultimate compression load in the top flange
of the beam element it is restraining.
The methods of restraining steel beams are
dependent upon meeting this load resistant
requirement – and for most structures (such
as concrete) – can be easily met. Care must
be taken however when determining the
capacity of the floor structure to act as a
restraint, with regards to how it is supported
by the steel beam. If the top flange of the
beam is not directly supporting the floor
structure, then it is not restrained. Examples
of this are shown in Figures 2 and 3.
Note that the examples in Fig. 3 can be
analysed on the basis that they provide
a stabilising load, even if the top flange is
unrestrained. More detailed advice on
how to carry out such analysis and other
methods of continuous restraint can be
found in The Steel Construction Institute
publication Stability of Steel Beams
and Columns.
Intermediate restraint
It is not uncommon for steel beams to have
restraints at discrete locations along their
length. This typically occurs where openings
within the floor structure require additional
support, which tend not to have the floor
structure sitting on them. Figure 4 is an
example of this type of beam.
N Figure 2
Steel beams restrained by the floor structure they support
N Figure 1
Lateral torsional buckling of an open section steel beam
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N Figure 3
Steel beams unrestrained by the floor structure they support
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"It is important to
note that restraints
cannot be simply
fixed back to an
adjacent beam"
Primary
beam
When assessing the amount of restraint
provided, the engineer must consider how
the beam is supported and the level of
torsional restraint that is offered at its end.
Figure 6 shows an example of cantilever
beams with varying degrees of restraint at
their supports, while Figure 7 illustrates a
variety of methods of restraining the tip of
the cantilever.
These varying conditions have an impact
on the design of the cantilever beam and
must be considered in order to arrive at an
accurate result.
Secondary
beams
N Figure 4
Steel beam with intermediate
restraints via secondary beams
The integrity of the restraint must be that it is
fixed to a point of support that can withstand
the axial load applied to it. Examples of such
supports include walls and braced elements.
It is important to note that restraints
cannot be simply fixed back to an adjacent
beam, as the support is not stiff enough to
withstand the applied load. The presence of
intermediate restraints reduces the effective
length of the steel beam, which results in a
smaller section size of the element than if
there were no restraints at all.
N Figure 6
Cantilever beam restraint at support (shown in elevation)
Restraints to cantilevers
Another type of beam element whose design
is impacted on the level of lateral torsional
restraint present, is the cantilever beam. The
buckling mechanism is somewhat different
to simply supported beams in that the
bottom flange needs to be restrained more
than the top flange, as shown in Figure 5.
N Figure 5
Cantilever beam buckling
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N Figure 7
Cantilever beam restraint at tip (shown in section)
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Note 16 Level 1
30
Technical
Technical Guidance Note
TheStructuralEngineer
October 2012
Purlins
Restraint
Bottom
chord
Restraint to trusses
Like beams, trusses also require restraint,
but they are more often than not placed
within the roof structure of a building. This
can lead to them being subjected to uplift
loads due to negative air pressure from
prevailing wind. This is known as ‘wind
reversal’ and has an impact on the design
of the bottom chord of the truss, which
is normally subjected to tension loads.
Reversal causes the bottom chord of the
truss to withstand compression loads,
which can result in a buckling failure. To
overcome this, lateral restraints are
installed (Figure 8).
The restraints provided are designed for
the restraint force as a strut and tie only.
N Figure 8
Examples of restraint to bottom chord of truss
Eurocode 0.
Applied practice
The applicable codes of practice for lateral
torsional buckling of steel members are
as follows:
Glossary and
further reading
Lateral torsional buckling – The
buckling of the compression flange, which in
the case of simply supported beams is the
top flange when placed under load.
BS EN 1993-1-1 Eurocode 3: Design of Steel
Structures — Part 1-1: General rules and
rules for buildings
Restraint – Method by which lateral
BS EN 1993-1-1 UK National Annex to
Eurocode 3: Design of Steel Structures —
Part 1-1: General rules and rules for buildings
Top flange – The section of the steel
element that needs to be restrained in order
to prevent lateral torsional buckling.
Worked example
A 10m long, 686x254x140 UB is supporting a UDL of 10 kN/m ultimate load and
lateral restraints at 3rd points along its length. Calculate the axial force that needs
to be resisted by the restraints.
torsional bucking is prevented.
Wind reversal – The effect of the
direction of applied load being reversed,
which results in elements of the supporting
structure being subjected to inverted
compression and tension forces.
Further Reading
The Institution of Structural Engineers
(2010) Manual for the design of steelwork
building structures to Eurocode 3 London:
The Institution of Structural Engineers
The Steel Construction Institute (2011)
Stability of Steel Beams and Columns Ascot,
UK: SCI
Eurocode 0.
Web resources
The Institution of Structural Engineers library:
www.istructe.org/resources-centre/library
The Steel Construction Institute:
http://www.steel-sci.com/
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