Steel column design
Tekla Structural Designer allows you to analyse and design a structural steel column which can have moment or simple connections with incoming members, and which can have fixity applied at the base. The column can have incoming beams which may also be capable of providing restraint, and may have splices along its length at which the section size may vary. You are responsible for designing the splices appropriately.
In its simplest form a steel column can be a single pin ended member between construction levels that are designated as floors.
More typically it will be continuous past one or more floor levels, the whole forming one single entity typically from base to roof.
Steel columns that share moments with steel beams form part of a rigid moment resisting frame.
In all cases you are responsible for setting the effective lengths to be used appropriate to the provided restraint conditions. All defaults are set to 1.0L.
Design is performed using a set of design forces obtained from 3D Analysis. (Grillage Chasedown Analysis and FE Chasedown Analysis results are not considered.)
If working to either AISC or Eurocodes, the design will take into consideration additional moments resulting from the eccentricity of pinned beam end connections. If working to other head codes, while the eccentricity moments are calculated, they are not used in the design.
A steel column can be designated as a Simple column in the column properties - in which case specific design rules are required.
A simple column should not have any applied loading in its length.
Simple columns are subject to axial forces and moments due to eccentricity of beam reactions.
The Fabrication type is specified in the column properties.
The steel column fabrication types that can currently be designed in Tekla Structural Designer are dependent on the design code, and also the construction type specified in the column properties - refer to the following table for details.
|Head code||Rolled||Plated||Concrete filled||Concrete encased|
Composite, concrete filled, and concrete encased steel columns
Whilst composite columns, concrete filled hollow sections and concrete encased sections can be specified in Tekla Structural Designer they are not designed.
For composite columns and concrete filled hollow sections, the analysis uses the bare steel inertia and not the ‘effective composite inertia’. This is conservative as the lower stiffness of the bare section will promote more second-order effects. On the other hand for concrete encased columns Tekla Structural Designer takes into account the encasement based on the size specified by the user.
When working to the US head code: AISC 360-16, sub-section I1.5 deals with the calculation of stiffness for concrete filled hollow sections and concrete encased sections used in the Direct Analysis Method (DAM). In accordance with C2.3, Tekla Structural Designer takes account of the standard stiffness adjustment factor of 0.8 with τb set to 1.0 given the additional notional load of 0.1% is also applied. I1.5 requires these types of member to take τb as 0.8 when considering the flexural stiffness. The user can achieve this for concrete encased columns by adjusting the Modification Factors in the Analysis Settings. (For composite columns and concrete filled hollow sections which both use the bare steel stiffness, no adjustment is required).
Plated columns - Indian head code
The section shapes supported are:
- Rolled Channels back to back
- Rolled Channels toe to toe
- Doubled rolled I sections
- Doubled rolled I sections with plates
The scope of design of these compound sections includes both beams and columns and autodesign.
In general, the scope of strength checks undertaken is the same as for rolled sections with the following current limitations:
- The design is for non-composite sections only.
- Slender Sections are beyond scope.
- High Shear case with minor axis moment is beyond scope.
- Design of the lacing or battening system is beyond scope.
- Only parallel flange sections can be used.
Plated columns - Australian head code
When the Fabrication type is set to Plated, only welded doubly symmetric I-section columns can be designed to the Australian code.
A pre-defined range of these is available from the Welded Columns page of the Select Section dialog - these can be both checked and autodesigned.
While you can add to the range of welded columns in the Select Section dialog by clicking the Add... button, any such user-defined columns can only be analyzed, but are not designed.
Restraints to flexural and torsional buckling are determined from the incoming members that connect to the column. The buckling checks are based on these restraints.
Restraints are considered effective on a particular plane providing they are within ±45° to the local coordinate axis system.
In all cases Tekla Structural Designer sets the default unrestrained length factor between restraints to 1.0.
You have the control to set any unrestrained length to be continuously restrained over that length - when set in this way the relevant buckling check is not performed during the design process.
Lateral torsional buckling (LTB):
- Members framing into either Face B or D are by default assumed to provide full LTB restraint (i.e. Face A and Face C are both set as restrained). You therefore need to consider whether or not your particular configuration of incoming members is capable of providing this level of LTB restraint. If necessary you can edit the default restraint provision by selecting the Face A override and/or Face C override checkboxes as appropriate.
Note: Face A can be identified graphically, from which the other faces follow, see: Steel member orientation
- Members framing into either Face A or C will by default provide restraint against major axis compression buckling. Members framing into either Face B or D will by default provide restraint against minor axis compression buckling. You can remove these default restraints if required by selecting the Major override and/or Minor override checkboxes.
Splice and splice offset
Splices can be specified by checking Splice under the appropriate Stack heading (apart from Stack 1) in the column properties and then specifying the Splice offset from the lower end of the stack.
Splices are allowed at floor levels only and must be placed at changes of angle between two adjacent stacks and at changes of section size or type. A validation error will result if this is not the case. The splice offset from the floor level defaults to 500mm and is considered not to be structurally significant.
Each lift (length between splices) of a general column can be of different section size and grade. Different section types within the same column are not allowed due to the particularly complex design routines that general columns require. You are responsible for guaranteeing that the splice detail ensures that the assumptions in the analysis model are achieved and that any difference in the size of section between lifts can be accommodated practically.
You can define rectangular or circular openings and these can be stiffened on one, or on both sides.
Openings cannot be defined from the Properties window, they can only be defined from the Properties dialog box, (by right-clicking on the member and selecting Edit...)
Web openings can be added to a column by a 'Quick-layout' process or manually.
The 'Quick-layout' process, which is activated using the checkbox on the Web openings dialog page, adds web openings which meet certain geometric and proximity recommendations (taken from Table 2.1 of SCI Publication P355 if the Head Code is set to Eurocode, or taken from SCI Publication P068 if the Head Code is set to BS). The openings so created are the maximum depth spaced at the minimum centers recommended for the section size.
Web openings can also be defined manually. With Quick-layout cleared, the `Add' button adds a new line to the web openings grid to allow the geometric properties of the web opening to be defined.
Long members in a model that have axial force in them can be unstable during second-order analysis because their individual elastic critical buckling load factor is lower than the elastic critical buckling load factor of the building as a whole and is less than 1.0.
However, often such members, for example the rafters in a portal frame, are stable in design because, for example, there are many smaller members or sheeting, that restrain the member in reality. They fail in the analysis because it is too resource intensive to model all the individual restraining members in the model which would also add unwanted clutter.
To prevent or to reduce the incidence of such failures during the analysis a multiplier can be applied to the minor axis inertia of these members which caters for the effect of the restraining members.
This multiplier is defined by selecting Prevent out of plane instability in the column properties and then entering a suitable value in the Instability factor field.
When activated, the minor axis inertia of the member is multiplied by the factor, thus increasing its minor axis (theoretical elastic) buckling capacity. This only occurs in the 2nd order analysis solver model. The default value for the factor is 20. This is just a sensible initial value, not a 'magic' number or one that will work in all circumstances. This behaviour also applies to the pre-size sections assigned during steel auto-design using 2nd order analysis. Hence the setting can be used to assist when buckling is occurring in the 2nd order analysis that is part of steel auto-design.
Since the setting adds ‘artificial’ stiffness to the model, the engineer should take care to ensure that it does not inappropriately increase the lateral stiffness of the structure as a whole leading to inaccurate results.
This multiplier is applied to prevent unwanted behaviour in the analysis model. While the analysis results may be affected by this adjustment, the increased minor axis inertia is NOT used in design checks.