# D5.D.10.2 Clause 6.3.2.2 –Elastic critical moment and imperfection factors for LTB checks

The NBN-NA recommends the use of Table 6.3 and 6.4 of EN 1993-1-1:2005 to calculate the imperfection factors for Lateral Torsional Buckling (LTB) checks.

The calculation of the LTB reduction factor χ_{LT},
requires the calculation of the Elastic Critical Buckling Moment,
M_{cr}. The NBN-NA gives a method to calculate
M_{cr} in Annex E, which is used by STAAD.Pro. Annex E, however, only deals with the calculation of
M_{cr} for doubly symmetric sections and mono symmetric
sections that are symmetric about the minor axis (i.e, Tee sections). For any other type of
section that is not dealt with by Annex E, STAAD.Pro uses the
method and tables given in Annex F of DD ENV 1993-1-1:1992:

## Doubly symmetric sections

Annex D of NBN-NA provides equation used to calculate M_{cr}
specifically for doubly symmetric sections:

C_{1} & C_{2} are factors that depend on the end
conditions and the loading conditions. The Annex provides values for C_{1} &
C_{2} for the different cases as given in Table1 and Table 2 of the Annex.
Table 1 deals with the condition of a simply supported member with end moments and the
value of C_{1} is determined by the end moment ratio (Refer to the NA for
details). Clause 3.2 of the National Annex however gives a formula to calculate
C_{1} as:

C_{1} = 1.77 - 1.04ψ
+ 0.27ψ^{2} ≤ 2.60

The value of C_{2} is determined based on the Table 2 of the Annex,
based on the loading and end conditions as specified using the `CMM`
parameter.

This NBN-NA considers three separate loading conditions:

- Members with end moments
- Members with transverse loading
- Members with end moments and transverse loading.

STAAD.Pro accounts for the loading condition
and the bending moment diagram through the `CMM` parameter.

## Mono-symmetric sections with symmetry about their weak axis

Annex D of NBN-NA also provides a method to evaluate the elastic critical
moment, M_{cr}, for uniform mono symmetric sections
that are symmetric about the weak axis. Hence for this implementation the elastic
critical moment for Tee-Sections is evaluated using the method in this Annex.

The equation to evaluate M_{cr} for
mono symmetric sections is given as:

The factors C_{1}, C_{2}, and C_{3} are dependent
on the end conditions and loading criteria. This implementation will consider
C_{1}, C_{2} and C_{3} as given in the tables below:

The `CMM` parameter specified during design input will
determine the values of C_{1}, C_{2}, and C_{3}. The default
value of `CMM` is 0, which considers the member as a pin ended member
with uniformly distributed load (UDL) along its span. This NCCI does not however
consider the "end moments and transverse loading" condition. You can
use the parameter `MU` to describe the moment distribution for cases
where the end moments vary (i.e., `CMM 7` or `CMM 8`).
Alternatively, you can use the `C1`, `C2`, and
`C3` parameters to input the required values for C_{1},
C_{2}, and C_{3} to be used in calculating M_{cr}.

Both the NCCI documents mentioned above assume that the member under
consideration is free to rotate on plan and that there are no warping restraints for the
member ( k = k_{w} = 1.0). STAAD.Pro takes into
account of the end conditions using the `CMN` parameter for EC3. A value
of K = k_{w} =1 is indicated by a value of
`CMN` = 1.0 in the design input. Hence the above methods will be used
only for members which are free to rotate on plan and which have no warping restraints
(i.e., `CMN` = 1.0). For members with partial or end fixities (i.e.,
`CMN` = 0.5 or `CMN` = 0.7), this implementation will
fall back on to the method and coefficients in DD ENV 1993-1-1:1992 – Annex F.

For all cases that are not dealt with by the National Annex (or the NCCI documents) this implementation will use the method as per the DD ENV 1993-1-1:1992 code.

For the term z_{j}, please refer to
Annex E of NBN NA 2018.

The term z_{g} in the equation to
calculate M_{cr} refers to the distance between the point of
application of load on the cross section in relation to the shear center of the cross
section. The value of z_{g} is considered positive,
if the load acts towards the shear center and is negative if it acts away from the shear
center. By default, the program will assume that the load acts towards the shear center
at a distance equal to (Depth of section/2) from the shear center. The use will be
allowed to modify this value by using the `ZG` parameter. Specifying a
value of `ZG` = 0 in the design input would indicate that the load acts
exactly at the shear center of the section so that the term z_{g} in the equation will have a value of zero.