Eurocode Steel Design - When can an effective length factor of 1.0 be used?

Tekla Structural Designer
Not version-specific
Tekla Structural Designer
Environment
Eurocode

Background

For the design of steel members to all design codes, an effective length factor is generally required to define the design length to be used in member stability checks.  There is no guidance on effective lengths given in EC3 so the Eurocode itself does not state what value should be used for any case.  Non-conflicting complementary information (NCCI) documents must be referred to for guidance, which for the UK are generally produced by the Steel Construction Institute (SCI).  See for example NCCI: Elastic critical moment for lateral torsional buckling (SN003b).  Much of this NCCI guidance is adopted from the BS 5950:2000 design standard and common practice for this in use for decades in the UK.

The effective length factor is applied to the unrestrained member length or “system length” to determine the effective or ‘buckling’ length (termed Lcr in EC3 for strut buckling) to be used in design checks.  In Structural Designer, the system length depends on; the member Characteristic (Beam, Column, Brace...etc), its Restraint Settings and its location and connectivity in the model.  It can variously be; the entire physical member length, the individual stack length of a continuous column or span length of a continuous beam, a length of member between connected/ supported member locations - i.e Restraint locations.

Member stability relates to buckling failure of which there are principally two types:

  • Lateral Torsional Buckling (LTB) - lateral twisting buckling of a member subject to bending about its major axis (with or without axial compression) in the minor axis direction.

  • Flexural, Torsional and Torsional-flexural Buckling (also termed ‘Strut’ or ‘Compression’ buckling) - buckling of a member subject to axial compression (with or without bending) about any axis.

For more information on buckling, both in general and specific to the Eurocode, see the excellent online guide at SteelConstruction.info  Member Design - Buckling Resistance of Members and SCI P360 Stability of Steel Beams and Columns, 2011 and the excellent New Steel Construction article by Mary Brettle of the SCI Lateral torsional buckling and slenderness

Since LTB and Strut (Flexural, Torsional and Torsional-flexural) buckling are separate behaviours and checks, there are separate restraint settings and effective length factors for each in Structural Designer.  These can be reviewed and edited in the member properties dialog on the “Lateral restraints” and “Strut restraints” pages, as illustrated in the picture below (this shows these settings for a single span beam with no connecting/ supported members).  They can also be reviewed and edited in Review View > Show/Alter Statue using the Restraints Attribute.

Default restraints are set for each flange at the locations where other members connect to a beam or column, and for the Sub-beam/ Sub-stack lengths between these.  The defaults settings are not necessarily appropriate for all situations, and so we recommend should always be reviewed and set as appropriate by the engineer.  For more on this see the Help Topics Steel column restraints and Review and modify restraints and TUA Video Show/Alter State - Restraints.  The default value of the effective length factor for each Sub-beam/ Sub-stack in all cases will be 1.0.  While this value will be appropriate in many circumstances, there are times when it could be un-conservative, hence we advise these settings should always be reviewed by the engineer and set as they see fit (see below regarding cantilevers).

What method of analysis and design is implemented for steel members in Structural Designer for the Eurocode, and what should these factors be set to?

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TSD Member effective length factors 1.png


Answer

For the design of steel frames as shown in the extract below, BS EN 1993-1-1 Cl. 5.2.2 (7) b) states that, providing the provisions of the clause are satisfied:

         “...verification... may be based on a buckling length equal to the system length”  (our emphasis)

In other words an effective length factor = 1.0 may be used (or more properly an effective length ≯ 1.0 since the clause does not preclude the use of a factor < 1.0).  This is providing that the system length - i.e. the unrestrained length - is set appropriately.

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EC3 5.2.2(7).png

The provisions of this clause are catered for in Structural Designer as follows:


Implementation

The engineer must take deliberate action and make specific settings in Structural Designer in order to comply with Cl. 5.2.2 (7) b) which requires that account be taken [in the analysis] of end moments and forces from the global analysis of the structure, including global second order effects and global imperfections

For more information on second order effects and imperfections see for example the excellent online guide SteelConstruction.info - Allowing for the effects of deformed frame geometry.  See also the Structural Designer Help Topic Choice of analysis type (Eurocode).

Global Imperfections

These are catered for in Structural Designer by the application of Equivalent Horizontal Forces (EHFs) per BS EN 1993-1-1 Cl. 5.3.2 (7).  EHFs are automatically calculated and applied to the analysis model in Structural Designer.  Note however that to activate their application the engineer must deliberately check on the option to apply EHFs in specific direction(s) for appropriate combinations in the Combination Generator and manually include EHF direction load cases in any manually created combinations.

The engineer is also advised to review and edit as they consider appropriate the EHF settings via Home > Model Settings > EHF as shown in the picture below.  For more on this topic see the following Help Topics Equivalent horizontal forces (EHF) (Eurocode) and EHF settings and the TUA article How are EHFs used in Tekla Structural Designer and what for? .

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TSD EHF settibngs.png

Global Second Order Effects

For more information about second order effects see for example the excellent SteelConstruction.info online guide and section Modelling and analysis > Second-order analysis.

Second Order Effects are catered for in Structural Designer by using either of two “Second-order…” analysis options which are activated via Design > Settings > Analysis as shown in the picture below.  The default analysis setting is First-order, which is not necessarily safe nor conservative.  The analysis type used is a matter of engineering judgement and practicality so is not automated and must be deliberately set by the engineer.  For more information and guidance on this see the Structural Designer Help Topic Choice of analysis type (Eurocode) especially the sections; Amplified forces (k amp ) method, Validity of the amplified forces method and Second-order analysis.

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TSD design analysis settings.png

Both “Second-order…” analysis options (in conjunction with the application of EHFs) will satisfy Cl. 5.2.2 (7) b) by taking “...account of end moments and forces from the global analysis of the structure, including global second order effects...”.  The second (lower) of the two options - which is a rigorous second order computer analysis - is the recommended option, since it is widely acknowledged to be the more accurate and is applicable to any structure and geometry, while the amplified forces method has a number of limitations and may only be used for multi-storey frames provided the requirements of Cl. 5.2.2 (6)B are met.

Per BS EN 1993-1-1 Cl. 5.2.1 (3), it is only necessary to account for 2nd order effects when the elastic crtical buckling factor αcrit is < 10 (for elastic analysis, which the analysis in Structural Designer is).  See BS EN 1993-1-1 and SteelConstruction.info for more on this aspect.  A Sway Check which calculates αcrit (for all columns and walls by default) is automatically performed by Structural Designer when analysis or a design process is run.  The results of this should be checked by the engineer to assess the sway-sensitivity of their frame and hence decide on the appropriate method of analysis.  For more on the Sway Check see this Structural Designer Help Topic > The sway check and this TUA article Why does the Status tree Report a Design > Sway Warning?.


Summary

In summary then, an effective length factor = 1.0 (i.e. accepting the default value) can in general be used in Structural Designer for steel frame members as per BS EN 1993-1-1 Clause 5.2.2(7) b) providing and considering:

  • EHFs are included in combinations as appropriate.

  • One of the Second-order analyses options is activated where second order effects are too large to be ignored i.e. αcrit < 10.

    • For multi-storey frames and when using the amplified forces method that the requirements of Cl. 5.2.2 (6)B are satisfied.
  • ​Restraint settings are reviewed and edited as necessary to ensure appropriate unrestrained lengths are considered.
 
  • Proviso For Cantilevers - note that special consideration still needs to be made for cantilevers, the effective length factor for which may still need to be > 1.0 depending on the nature of restraint at both root and tip.  For more on this see for example SCI AD-408.

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