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Technical Guidance Note

Technical

October 2012

Note 16 Level 1

›

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 fl ange, 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 fl ange must have suffi cient 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).

Lateral torsional bucklingIntroduction

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 signifi cantly 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.

W Design principles

W Applied practice

W Worked example

W Further reading

W Web resources

ICON LEGEND

N Figure 1Lateral torsional buckling of an open section steel beam

N Figure 2Steel beams restrained by the fl oor structure they support

N Figure 3Steel beams unrestrained by the fl oor structure they support

Methods of restraintAs a general rule, a restraint to a top fl ange of a beam must be capable of resisting a force that is equivalent to 2.5% of the ultimate compression load in the top fl ange 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 fl oor structure to act as a restraint, with regards to how it is supported by the steel beam. If the top fl ange of the beam is not directly supporting the fl oor 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 fl ange 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 fl oor structure require additional support, which tend not to have the fl oor structure sitting on them. Figure 4 is an example of this type of beam.

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N Figure 5Cantilever beam buckling

N Figure 6Cantilever beam restraint at support (shown in elevation)

N Figure 7Cantilever beam restraint at tip (shown in section)

N Figure 4Steel beam with intermediate restraints via secondary beams

Secondary beams

Primary beam

The integrity of the restraint must be that it is fi xed 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 fi xed back to an adjacent beam, as the support is not stiff enough to withstand the applied load. The presence of intermediate restraints reduces the eff ective length of the steel beam, which results in a smaller section size of the element than if there were no restraints at all.

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 diff erent to simply supported beams in that the bottom fl ange needs to be restrained more than the top fl ange, as shown in Figure 5.

When assessing the amount of restraint provided, the engineer must consider how the beam is supported and the level of torsional restraint that is off ered 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.

"It is important to note that restraints cannot be simply fixed back to an adjacent beam"

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.

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TheStructuralEngineer30

Technical Guidance Note

Technical

October 2012

Note 16 Level 1

›

Glossary and further reading

Lateral torsional buckling – The buckling of the compression fl ange, which in the case of simply supported beams is the top fl ange when placed under load.

Restraint – Method by which lateral torsional bucking is prevented.

Top fl ange – The section of the steel element that needs to be restrained in order to prevent lateral torsional buckling.

Wind reversal – The eff ect 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/

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.

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 8Examples of restraint to bottom chord of truss

Bottom chord

Purlins

Restraint

Eurocode 0.Applied practice

The applicable codes of practice for lateral torsional buckling of steel members are as follows:

BS EN 1993-1-1 Eurocode 3: Design of Steel Structures — Part 1-1: General rules and rules for buildings

BS EN 1993-1-1 UK National Annex to

Eurocode 3: Design of Steel Structures — Part 1-1: General rules and rules for buildings

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