Steel Section Table
b
c
d
Imperfection factor a
0,13
0,21
0,34
0,49
0,76
Table 6.5. Selection of buckling curve for a crosssection (Table 6.2 of EN 19931I)
Table 6.5. Selection of buckling curve for a crosssection (Table 6.2 of EN 19931I)
The choice as lo which buckling curve (imperfection factor) to adopt is dependent upon the geometry and material properties of the crosssection and upon the axis of buckling. The appropriate buckling curvc should be determined from Table 6.5 (Table 6.2 of EN 199311), which is equivalent to the 'allocation of strut curve' tabic (Table 23) of BS 5950: Part 1.
Nondimensional slenderness for various buckling modes Clause 6.3.1.3 EN 199311 provides guidance for flcxural (clause 6.3.1.3), torsional (clause 6.3.1.4) and Clause 6.3.1.4 flexuraltorsional (clause 6.3.1.4) buckling modes. For standard hotrolled and welded structural crosssections, flcxural buckling is the predominant buckling mode, and hencc governs design in the vast majority of eases.
Buckling modes with torsional components are generally limited to coldformed members for two principal reasons:
♦ coldformed crosssections contain relatively thin material, and torsional stiffness is associated with the material thickness cubed
• the coldforming process gives a predominance of open sections because these can be easily produced from flat sheet. Open sections have inherently low torsional stiffness.
Flcxural buckling of a compression member is characterized by excessive lateral deflections in the plane of the weaker principal axis of the member. As the slenderness of the column increases, the load at which failure occurs reduces. Calculation of the nondimensional Clause 6.3.1.3 slenderness for flexural buckling is covered in clause 6.3.1.3.
The nondimensional slenderness A is given by
A= I—— = —^—• for Class 1, 2 and 3 crosssections VN„ i A,
where
for Class 4 crosssections
is the buckling length of the compression member in the plane under consideration, and is equivalent to the effective length Lz in BS 5950 (buckling lengths are discussed in the next scction)
is the radius of gyration about the relevant axis, determined using the gross properties of the crosssection (assigned the symbols rx and in BS 5950 for the radius of gyration about the major and minor axes, respectively)
Clearly, the BS 5950 definition of slenderness (A = /E/r.) is already 'nondimensional', but the advantage of the Eurocode 3 definition of 'nondimensional slenderness' A, which includes the material properties of the compression member through A„ is that all variables affecting the theoretical buckling load of a perfect pinended (Euler) column arc now present. This allows a more direct comparison of susceptibility to flexural buckling lo be made for columns with varying material strength. Further, A is useful for relating the column slenderness to the theoretical point at which the squash load and the Euler critical buckling load coincide, which always occurs at the value of nondimensional slenderness A equal to 1.0.
As stated earlier, flcxural buckling is by far the most common buckling mode for conventional hotrolled structural members. However, particularly for thinwalled and open sections, the designer should also check for the possibility that the torsional or torsionalflexural buckling resistance of a member may be less than the flexural buckling resistance. Torsional and torsionalflexural buckling are discussed further in Section 13.7 of this guide.
Table 6.6. Nominal buckling lengths Lcr for compression members  
End restraint (in the plane under consideration) 
Buckling length, Lcr  
Effectively held in position at both ends 
Effectively restrained in direction at both ends 

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