Compression buckling (Beams: EC3)

Beams must be checked to ensure adequate resistance to buckling about both the major and minor axes and they must also be checked in the torsional mode over an associated buckling length. Since the axial force can vary throughout the beam and the buckling lengths in the two planes do not necessarily coincide, all buckling modes must be checked. There may be circumstances where it would not be safe to assume that the combined buckling check will always govern (see below).

Note: A warning message will be given in the compression buckling check results whenever (major or minor axis) high shear is present in a load combination. “HIgh shear is assumed not to affect buckling design. This assumption should be verified by the Engineer.”

Effective lengths

In all cases Tekla Structural Designer sets the default effective length to 1.0L, it does not attempt to adjust the effective length in any way. Different values can apply in the major and minor axis. It is your responsibility to adjust the value from 1.0 where you believe it to be justified.

Note: It is assumed that you will make a rational and “correct” choice for the effective lengths between restraints. The default value for the effective length factor of 1.0L may be neither correct nor safe.

Tip: As an alternative to using each beam's Properties dialog box to set restraints and effective lengths, you can review and set more effectively via Review View > Show/ Alter State. For more details see: Review and modify restraints and Effective Lengths and Factors - review and copy.

Coincident restraint points in the major and minor axis define the 'check length' for torsional and torsional flexural buckling (which also has an effective length factor but is assumed to be 1.0L and cannot be changed).

All intermediate major and minor restraints in a cantilever span are ignored.

Any major or minor strut buckling 'check length' can take the type 'Continuous' to indicate that it is continuously restrained over that length. There is no facility for specifying torsional or torsional flexural buckling 'check lengths' as 'Continuous'.

There is no guidance in EC3 on the values to be used for effective length factors for beam-columns.

Compression resistance

The relevant buckling resistances are calculated from Equation 6.47.

These consist of the flexural buckling resistance about both the major and minor axis i.e. N_b,y,Rd and N_b,z,Rd over the buckling lengths L_yy and L_zz and where required the buckling resistance in the torsional or flexural-torsional modes, N_b,x,Rd.

The elastic critical buckling load, N_cr for flexural buckling about major and minor axes is taken from standard texts. The elastic critical buckling loads for torsional, N_cr.T and for torsional flexural buckling, N_cr.TF are taken from the NCCI “Critical axial load for torsional and torsional flexural buckling modes” available free to download at www.steel-ncci.co.uk.

All section types are checked for flexural buckling. It is only hollow sections that do not need to be checked for torsional and torsional-flexural buckling.