📄 Nominal Column Moments
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Nominal Column Moments

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Under Construction

Nominal column moments are used in Steel design primarily to account for geometric eccentricities that are often ignored or simplified in the initial structural analysis model.

Function and Necessity

The computation of nominal additional column design moments addresses the practical reality that structural members are often offset from the theoretical centreline used in the MasterFrame model analysis.

  • Analytical Model vs. Design: MasterFrame analysis typically relies on an analytical model where the centreline of a beam aligns precisely with the centreline of a column, meaning eccentricities are not automatically included in the analysis.
  • Design-Time Adjustment: Standard nominal beam to column eccentricity value are assumed at design time  to calculate additional nominal moments. These eccentricities are both parallel with the beam axis (ecc) and also horizontally perpendicular to the beam axis (offset).
    • The ecc value is taken by default as half the dimension of the connected depth + 100mm. For beam on I section flange this is D/2 + 100.
    • For beams in to I section column webs this is tw/2 + 100, however where  a toe plate thickness is specified this becomes B/2 + 100.  The perpendicular offset is taken as zero by default.
    • The nominal eccentricity values can also be controlled from the model using The Beam Eccentricities for Nominal Column Moments MasterFrame feature (available in the design menu). This allows these eccentricities to be defined.
    • In a Columns in Simple Construction check the value can also be controlled by design brief inputs, which supersede any Beam Eccentricities for Nominal Column Moments values specified. These eccentricities do not affect the analysis results, but they are factored into the calculation of the nominal additional column design moments during the design phase.
  • Threshold: It is generally not necessary to define offsets for all beams, only those where the offset is beyond typical tolerance (e.g., outside the normal 100 mm from the flange or web of the columns).

Implementation in Steel Column Design

For steel members, there is an option to include consideration of nominal moments within both the Axial with Moments design check and the Columns in Simple Construction check.

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A Columns in Simple Construction check assumes the column has only axial load from the analysis and all moment is derived at design time from the nominal moment calculation.

In an Axial with Moments design check both analysis axial forces and moments are taken. The purpose of the nominal moment is to ensure that the end moment are at least equal to the nominal moment. Where the analysis moment is greater than the nominal moment, the nominal moment is not applied. In other case the difference between the analysis moment and full nominal moment is applied to ensure the design moment is at least equal to the nominal moment.

Defining Beam Eccentricities

The Design> Beam Eccentricities for Nominal Column Moments MasterFrame feature allows you to define offsets between beams and columns for design purposes. See also 📄 Beam Eccentricities (Pro). You can specify the offset in detail:

  • The offset can be defined both parallel to the beam and laterally perpendicular to the beam.

    Offset Defined Perpendicular to the Beam


    Offset Defined Perpendicular to the Beam
  • The lateral alignment can be set relative to the centreline of the column or the edge of the flange.
  • An additional offset dimension can be added.
  • The offset dimensions can be specified differently for each end (End 2) of the beam if required.
  • The parallel 'Plus Eccentricity' value if set at 0 is taken as 1the default 100mm. This is a design convention and not a physical eccentricity.

Conceptual Analogy: Consider nominal column moments like a required margin of error in manufacturing. Even if your design drawing specifies a perfectly centred beam on a column (the analytical model), you know during construction there will be slight offsets (eccentricities). The nominal moment ensures that the column reinforcement is strong enough to handle the extra bending created by these small, expected misalignments, essentially protecting the structural integrity by designing against anticipated imperfections.

FAQs

1) What are “nominal moments” in MasterSeries?

  • A lower-bound design moment applied to columns in simple construction to reflect real connection eccentricities.

  • Calculated post-analysis in Steel Design (not in the frame analysis).

  • Basis: shear from incoming beams acting at a nominal lever arm from the face of the column flange/web (UK standard practice from BS era and SCI guidance; carried forward for EC3 design workflows).

Key consequences

  • They don’t appear in graphical analysis results or end-force diagrams; you’ll see them only in the Steel Design calculation as “Additional Nominal Moments”.

  • They are used only if larger than the analysed end moment at that level. If the analysed moment is higher, the nominal moment is ignored.

  • They do not get applied to column ends axis where a member ends release is present.


2) Where do they come from in the model?

  • Incoming beams: The vertical reaction from a supported beam is assumed to act 100 mm from the column face → produces a nominal moment about the column.

  • Bracing members: The vertical component of bracing forces framing into the column also contributes nominal moments.

  • Nodal loads on the column: Not included in the nominal-moment mechanism. If you want construction eccentricity on a direct nodal load, manually add a nominal moment at the node.

3) How are nominal end moment distributed at design time

  • Total nominal moment at column end: The beam shear forces x the eccentricities in both direction for column major and minor axis are calculated. 

  • Nominal moment distribution: The total nominal moment is then distributed to the system of columns that may exist above and below the column being designed, based on the proportions of the column stiffnesses. This calculation is given explicitly in the Columns in Simple Construction check, however in the Axial with Moment check only the final column distributed end moment are presented. The presence of member end released in the column above and below affect this distribution, for example, if the column above has an end release at the bottom, then all of the nominal moment from the top beams will be distributed to the top of the column being designed.

4) Controlling nominal moments in MasterSeries

  • Global switch: Steel Design → Options → Use Column Nominal Moments (commonly ON by default).

  • Per-member setting (Design brief):

    • Axial with Moments brief → dropdown “Nom. Column Moments”:

      • As Global Setting = follow the global setting.

      • Nominal Moments ON/OFF = override for this member.

  • Lever arm value: Design → Beam eccentricity for nominal column moments → set default 100 mm (value can be amended) → Select and apply to the whole frame or a selection. The Columns in Simple Construction check also has input over rides for the nominal eccentricity values.

5) End releases, torsion, and “where did this moment come from?”

  • If beams frame in pinned but the column shows end moments, it is commonly due to torsion from the beams.

    • Remedy: add torsional end releases to the beam ends (not the column head) to prevent torsion being passed into the column.

6) Choosing the right design brief

  • Columns in Simple Construction (EC3 workflow using BS5950 4.7.7 interaction logic as per NCCI SN048b-EN-GB ):

    • Use where all beams to the column are pinned.

    • Includes nominal moments and ignores local bow.

    • Has local input for the nominal eccentricities and overrides the default and any values specified in the Beam Eccentricities for Nominal Column Moments MasterFrame feature.

  • Axial with Moments (general case for EC3 columns):

    • Use for most column design scenarios.

    • Will consider nominal moments (if enabled) only where they exceed analysed moments.

    • If all framing beams are pinned, it effectively reduces to an axial check (plus nominal moments if larger).

    • The nominal moment can be turned off in this type of design using the “Nom. Column Moments” design brief setting

    • The nominal eccentricity values can only be controlled using the  Beam Eccentricities for Nominal Column Moments MasterFrame feature.

7) Beam offsets vs “eccentricities”

  • The “100 mm eccentricity” used for nominal moments is a design convention, not a geometric offset in the analysis.

  • Beam offsets you set in the model are largely graphical / documentation aids and only influence the nominal moment calculation—not the analysis centrelines.

  • For typical lattice or braced frames, avoid modelling large artificial offsets for braces; model on centrelines unless a genuinely large eccentricity (>100 mm) must be captured in the nominal-moment rule.

8) Storey-by-storey design vs super-members

  • Best practice: Design each storey column separately (not merged into one long physical member over multiple levels).

    • Then nominal moments are assessed at each level from the supported beams.

  • If you merge columns, nominal moments may only be assessed at the top of the physical member, missing intermediate floor levels.

9) When you won’t get nominal moments (even if enabled)

  1. Nodal loads placed at the column head (through the centroid) → no nominal moment; add MX manually if required.

  2. Column end release - the assumption made by MasterSeries is that the column has simple connection 'to' the beam, such as beam-over-column at top or beam-under-column at bottom, and not a simple connection for the beam into the continuous column. In this case no nominal moment will be present in the axis of the column end that is released.

  3. Applied end moment > In an 'Axial with Moment' check the purpose of the nominal moment is to ensure that the the ends moment are at least equal to the nominal moment. Where the analysis moment is greater than the nominal moment, the nominal moment is not applied. In other case the difference between the analysis moment and full nominal moment is applied to ensure the design moment is at least equal to the nominal moment.

  4. If a beam has the 'UT (Rigid Link Member)' attribute applied, the shear force from the beam is not considered in the nominal moment calculation.

10) Typical workflows & checks

A) Simple multi-storey frame with pinned beams

  1. Ensure beams have torsional end releases into the column.

  2. Use Axial with Moments or Columns in Simple Construction.

  3. In an Axial with Moments check keep global Use Column Nominal Moments = ON.

  4. Review Steel Design → The first lines show Nominal Moments and their combination with analysed moments.

  5. If local bow is being triggered purely by nominal moments and this is inappropriate for your scenario, switch to Columns in Simple Construction brief.

B) Temporary prop (nodal load at top, lateral restraints at floors)

  1. Keep Nominal Moments ON for beam/shear cases (if any).

  2. For the top nodal load, compute M = P × e manually and add as MX moment with the nodal load.

  3. Check buckling lengths per restraint level; choose Axial with Moments brief.

C) Reducing the nominal lever arm (e.g. 50 mm)

  1. Design → Beam eccentricity for nominal column moments → set 50 mm.

  2. Select the relevant members (or All) to apply.

  3. Re-design → check reduced “Additional Nominal Moments” values.

11) Interpreting results

  • Example line items you’ll see in Steel Design:

    • Major axis member bending (analysis) = e.g. 0.012 kNm

    • Additional Nominal Moments = e.g. 0.974 kNm

    • Design moment taken = max of the two (e.g. 0.986 kNm)

  • Remember: nominal moments aren’t drawn in diagrams; they’re reported in the calc sheet.

  • To get splice moments at a given distance from the CL intersection: Results → Internal Member Forces → Distinct Interval to read My/Mz at the exact location, or define the splice position in the Connections module and let it read from the model.

12) Troubleshooting checklist

  • Seeing unexpected end moments in the column with “pinned” beams?

    → Release torsion at the beam ends.

  • “Graphical” moments not matching design calc?

    → Nominal moments are design-only; check the calc sheet.

  • Want nominal moments but getting none?

    → Check Options → Use Column Nominal Moments; confirm not overridden OFF in the member brief; confirm the floor beams/braces actually frame in at that level. Check that you can not applied a member end release to the column end.

13) Quick references inside MasterSeries (where to click)

  • Global toggle: Steel Design → Options →  Global Design Options→ Apply Nominal Column Beam Eccentric Moment - 'Axial With Moments' brief.

  • Per-member: Steel Design → Axial with Moments brief → “Nom. Column Moments” (As Global / ON / OFF).

  • Lever arm value: In MasterFrame Design → Beam eccentricity for nominal column moments → set mm → Select members.

  • Eliminate beam torsion-induced column moments: Member → End Releases → Torsion at beam ends.

Notes on background sources (for your engineering file)

  • Legacy BS5950 §4.7.7 interaction for columns in simple construction permitted in EC3 through NCCI SN048b-EN-GB 'Verification of columns in simple construction – a simplified interaction criterion'.

  • SCI guidance (e.g., P365 Section 6.2; AD 404 commentary on simple construction).

  • UK practice re: 100 mm nominal lever arm.

See Also:

🎥 📺Steel design in complex design situations [WEBINAR RECORDING]

🎥 📺Concrete Design to EuroCode 2