Local Bow Imperfection (2025)
When EN 1993-1-1:2005 local bow imperfection loading is required (NEd > 0.25*Ncr), as determined at the MasterKey Steel Design stage, the application of this member attribute signals to the member that local bow imperfection loads will be applied in accordance with EN 1993-1-1:2005 5.3.2 (6) & (7).
This check is done for both the major and minor axis. This is part of an approach that accounts for in-member geometric nonlinearity (small P-delta). The loads generated by the Local-Bow member attribute (see EN 1993-1-1:2005 figure 5.4) are automatically applied during the second order P-Delta analysis. Local-Bow loading will not be applied in loading cases where P-Delta is not used. Local-Bow loading will not be applied where Ned< 0.25*Ncr.
UT Local-Bow 0 00.00 (Axis, L)
The Local-bow loading has two inputs. The first item is either '0' for major axis loading, or '1' for minor axis loading. If Local-Bow loads are required in both axis, then two Local-Bow member attributes my be applied. The second input is a member length override for the calculation of the Ncr figured used in the local-bow 0.25*Ncr limit and e0 equation. If the default 00.000 is used, the physical length of the member is employed in the Ncr and e0 evaluation.
The direction of the local bow loading is automatically determined based on the direction of any natural bending of the member, hence the local bow loads always attempt to augment (increase) the bending.
The Local-Bow load does not observe the load factor applied to the load group, other than any load factor greater than 0.00 will activate it. When included in the UT load group it will be present in all loading cases, which is recommended.
Prerequisites for a Successful Analysis
For the local bow loading and the associated small P-delta effects to be included correctly, the physical member needs to be subdivided into smaller analytical members.
- It is recommended to have three evenly distributed internal nodes, which creates four analytical parts.
- This subdivision can be generated automatically if you use the 'Modify Geometry > Modal Auto-Connect and Clean Up' feature with the correct settings enabled for automatic subdivision.
- This ensures that the small P-delta effects from the imperfection are properly captured in the analysis. The software will still treat it as a single physical member for design purposes.
- If manual internal nodes need to be generated use the 'Modify Geometry> Split Member' geometry function in MasterFrame, ensuring to use the 'Insert analytical node only' option.
Final Steps and Considerations
After applying the attribute and ensuring member subdivision, you must activate a P-Delta analysis for the relevant load cases.
Note when local bow imperfection is handled in this way the steel design combined Axial and Moment Checks are modified in that equation EN 1993-1-1:2005 6.61 is omitted (as per 2022 equation 8.88). EN 1993-1-1:2022 is more explicit on this (7.2.2 (7) b) Method M4), but 2005 alludes to it in 5.2.2. (3) & (7).
Opting Out
There is an option in the 'Axial with Moment' design brief to not check local bow, which must be used at the engineers discretion.
You read more on this check here