Compression buckling (Columns: EC3 Eurocode)
General columns 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 column 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).
Restraints
Restraints to strut buckling are determined from the incoming members described within Tekla Structural Designer. The buckling checks are based on these.
Restraining members framing into either Face A or C will provide restraint to major axis strut buckling. Members framing into either Face B or D will provide restraint to minor axis strut buckling. Tekla Structural Designer determines the strut buckling restraints but you can override these.
To identify Faces A, B, C & D for an individual column, see Steel member orientation.
Note: The program assumes that any member framing into the major or minor axis of
the column provides restraint against strut buckling in the appropriate plane.
If you believe that a certain restraint in a particular direction is not
effective then you can either override the restraint or adjust the effective
length to suit – to 2.0L for example.
Torsional and torsional flexural buckling restraint is only provided at points restrained coincidentally against major and minor axis strut buckling.
Note: Provided a level is restrained coincidentally against major and minor axis
strut buckling, the program assumes that any member framing into the appropriate
faces provides restraint against torsional and torsional flexural buckling at
that level. There are a number of practical conditions that could result in
torsional restraint not being provided at floor levels. At construction levels
this is even more possible given the likely type of incoming member and its
associated type of connection. You must consider the type of connection between
the incoming members and the column since these can have a significant influence
on the ability of the member to provide restraint to one, none or both column
flanges. For example, consider a long fin plate connection for beams framing
into the column web where the beam stops outside the column flange tips to ease
detailing. The fin plate is very slender and the beam end is remote from the
column flanges such that it may not be able to provide any restraint to
torsional or torsional flexural buckling. The fact that a slab is usually
present may mitigate this. You are expected to override the ineffective
restraint.
Tekla Structural Designer always assumes full restraint at the base and at the roof level when carrying out buckling design checks – you are warned on validation if your restraint settings do not reflect this. Restraints are considered effective on a particular plane providing they are within ±45° to the local coordinate axis system.
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. You are expected to adjust the strut buckling effective length factor (up or down) as necessary. Different values can apply in the major and minor axis.
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.
The torsional and torsional flexural buckling effective length factor (1.0L) can not be changed.
Any strut buckling effective 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 effective lengths as “Continuous”.
There is no guidance in EC3 on the values to be used for effective length factors for beam-columns.
For general columns - The minimum theoretical value of effective length factor is 0.5 and the maximum is infinity for columns in rigid moment resisting (RMR) frames. Practical values for simple columns are in the range 0.7 to 2.0 (see For simple columns below). In theory, values less than 1.0 can be chosen for non-sway frames or for sway frames in which the effects of sway are taken into account using either the amplified forces method or P-Delta analysis. However, EC3 states that when second-order effects are included in this way then the design “may be based on a buckling length equal to the system length” i.e. an effective length factor of 1.0. The program default of 1.0 matches this requirement but allows you flexibility for special situations.
One such situation might be in RMR frames where the principal moments due to frame action preventing sway are in one plane of the frame. There will often be little or no moment out-of-plane and so, if using the amplified forces method, the amplification of these moments has little effect on the overall design. Nevertheless the stability out-of-plane can still be compromised by the lack of restraint due to sway sensitivity in that direction. In such cases a value of greater then 1.0 (or substantially greater) may be required. Similarly, in simple construction where only eccentricity moments exist, it is only the brace forces that 'attract' any amplification. Thus for the column themselves the reduced restraining effect of a sway sensitive structure may require effective length factors greater than 1.0.
For Simple columns - There is no concept of simple columns in EC3 and hence no information on effective lengths either. However, reference can be made to the “NCCI” on the subject of simple construction but none of this includes the clear guidance on effective lengths of simple columns that was included as Table 22 in BS 5950-1: 2000. Again the program defaults the effective length factor to 1.0
Compression resistance
The relevant buckling resistances are all calculated from Equation 6.47.
These consist of the flexural buckling resistance about both the major and minor axis i.e. Nb,y,Rd and Nb,z,Rd over the buckling lengths Lyy and Lzz and where required the buckling resistance in the torsional or flexural-torsional modes, Nb,x,Rd.
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.