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D5.D.8.3 Clause 6.3.2.2 –Elastic critical moment and imperfection factors for LTB checks

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

The calculation of the LTB reduction factor χLT, requires the calculation of the "Elastic Critical Buckling Moment", Mcr. The Polish National Annex does not specify a particular method to calculate Mcr. Hence the calculation of Mcr has been based on the following NCCI documents:

SN003a-EN-EU – Elastic critical moment for Lateral torsional Buckling:

This document provides a method to calculate Mcr specifically for doubly symmetric sections only. Hence only doubly symmetric sections will be considered for this method. The equation to evaluate Mcr is given in the NCCI as:

M c r = C 1 π 2 E I s ( k L ) 2 [ ( k k w ) 2 I w I s + ( k L ) 2 G I t π 2 E I s + ( C 2 z s ) 2 C 2 z s ]

C1 and C2 are factors that depend on the end conditions and the loading conditions of the member. The NCCI provides values for C1 and C2 for the different cases as given in the tables below:

Table 1. Values of C1 for end moment loading (for k=1)
ψ C1
+1,00 1,00
+0,75 1,14
+0,50 1,31
+0,25 1,52
0,00 1,77
-0,25 2,05
-0,50 2,33
-0,75 2,57

This NCCI 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.

SN030a-EN-EU – Mono-symmetrical uniform members under bending and axial compression:

This document provides a method to evaluate the elastic critical moment (Mcr) for uniform mono symmetric sections that are symmetric about the weak axis. Hence, the elastic critical moment for "Tee-Sections" will be worked out using the method in this NCCI.

Note: Though this method could also be applicable to mono-symmetric built-up sections, STAAD.Pro currently does not have a means to specify/identify a mono-symmetric built-up section. Hence this implementation will use this method only for Tee-Sections.

The equation to evaluate Mcr for mono symmetric sections is given as :

M c r = C 1 π 2 E I s ( k x L ) 2 [ ( k x k w ) 2 I w I s + ( k x L ) 2 G I T π 2 E I x + ( C 2 z e C 3 z 1 ) 2 ( C 2 z e C 3 z 1 ) ]

The factors C1, C2, and C3 are dependent on the end conditions and loading criteria. This implementation will consider C1, C2, and C3 as given in the tables below:

The CMM parameter specified during design input will determine the values of C1, C2, and C3. The default value of CMM is 0, which considers the member as a pin ended member with UDL along its span. This NCCI does not however consider the “end moments and transverse loading” condition. You can use the C1, C2, and C3 parameters to input the required values for C1, C2, and C3 to be used in calculating Mcr.

Note: If MU as well as C1, C2, and C3 have been specified, the program will ignore MU and use the user input values of C1, C2, and C3. STAAD.Pro obtains these values from Annex F of DD ENV version of 1993-1-1:1992.

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 = kw = 1.0). STAAD.Pro takes into account of the end conditions using the CMN parameter for EC3. A value of K = kw =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 zj, please refer to Annex E of NBN NA 2018.

The term zg in the equation to calculate Mcr 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 zg 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 zg in the equation will have a value of zero.

Note: The program does not consider the case of cantilevers.