Function
- Enter the load cases and related data for the buckling analysis of a structure.
- Gen NX follows the procedure noted below for buckling analysis.
Details...
1. A buckling analysis process requires static analysis results to formulate the initial geometric stiffness matrix. Accordingly, enter the load cases to be applicable for the buckling analysis following the procedure for static analysis.
2. Invoke the dialog box of the Analysis > Buckling Analysis Control menu and enter the required number of modes and data necessary for convergence and assign the load combination to be applied to the buckling analysis.
3. Select the Analysis > Perform Analysis menu or click Perform Analysis to carry out the analysis.
4. Once the analysis is successfully completed, use either the Results > Buckling Mode Shapes function or the Results > Result Tables > Bucking Mode Shape function to review the buckling mode shapes and critical load factors for each mode shape.
5. To calculate buckling loads, enter the self-weight (Constant) and additional loads (Variable) and then perform the analysis. The Eigenvalue resulting from the analysis is the critical load factor. Accordingly, the buckling load = critical load factor X additional load (Variable) + self-weight (Constant).
Call
From the Main Menu select [Analysis] tab > [Analysis Control] group > [Buckling]
Input
Buckling Modes
Number of Modes : Enter the number of buckling modes.
Positive Value Only : Produce the eigenvalue results only for the loading direction.
Note
Once buckling analysis is performed, buckling modes are computed for both the loading direction and the opposite direction. However, for static loads whose loading direction is fixed, it is not necessary to compute the buckling mode shape and factor for the opposite direction.
Search : Calculate the buckling load factor within the defined range.
From : Minimum value of the buckling load factor
To : Maximum value of the buckling load factor
Note
If the same values are entered in the 'From' and 'To' field, the buckling load factors closest to the entered value are calculated.
Sturm Sequence Check : Check to detect any missed buckling load factors. If there are any missed buckling load factors, an error message is displayed in the message window.
Frame Geometric Stiffness Option
Option for formulating the geometric stiffness matrix in a frame member
Consider Axial Force Only : Consider axial force only when formulating the geometric stiffness matrix.
Buckling Combination
Enter the load cases for the buckling analysis. Different Load Cases with the corresponding Scale Factors may be entered.
Load Type
Variable : Consider increase or decrease in loads (live loads, etc.)
Constant : Do not consider increase or decrease in loads (self-weight, dead loads)
Note
Strictly speaking, Constant should contain only the self-weight of the structure. However, it is common to apply dead load to slab, along with the self-weight. Accordingly, dead load can be entered as Constant Type.
- To enter a new or additional load case
Enter the Load Case and Scale Factor; then click the Add Button.
- To modify the scale factor for a previously entered load case
Select the load case, modify the Scale Factor and click the Modify Button.
- To delete a previously entered load case
Select the load case to be deleted from the load case list at the bottom of the dialog box and click the Delete Button.
Remove Buckling Analysis Data
To delete all the entered data, click the Remove Buckling Analysis Data Button.
Note
Buckling Analysis may not be carried out in conjunction with P-Delta Analysis or Eigenvalue Analysis. Buckling Analysis is limited to truss element, beam element (including tapered beam element) and plate elements.
Q&A
Q1. Can midas Gen consider second-order incorporating lateral torsional buckling directly?
A1. In midas Gen, both flexural buckling and lateral torsional buckling of the beam elements are considered in buckling analysis. After that, local second order effect and imperfection can be considered as per Cl.6.3 in EN1993-1-1 during Steel Code Checking in midas Gen. It is according to the Cl.5.2.2(3)b. In Steel Code Checking, LTB check is also verified for steel I-shape section.
However, if the user wants to consider the local second order effect and imperfection as per Cl.5.2.2(3)a, we can use buckling analysis. In midas Gen, both flexural buckling and lateral torsional buckling are considered in Buckling Analysis. For this verification, I had a test as below.
The report below is generated from "LTB.mgb" model file, which considers Cl.5.2.2(3)b.
By performing buckling analysis, we can see the 1st buckling factor, which is LTB, is determined as 1.3618. "My" value is 250kNm. Therefore lateral torsional buckling moment is 1.3618*250 = 340.45kNm. We can see this value is very similar to Mcr above.
After checking buckling load, we can update the geometry of model with the buckling mode 1 in order to consider local imperfection using the Results > Buckling Mode Shape function. This model can be used for the verification of buckling force by performing geometric nonlinear analysis. Here, maximum eccentricity was entered as 0.015m.
*Why 0.015m is determined?
e0/L for buckling curve a = 1/300
L = 10m
e = 0.03m
For LTB, k = 0.5
Therefore, eccentricity is 0.5*0.03 = 0.015m
In the updated model with 1st mode imperfection, if we check the value of My, it is shown as 362kNm which is also similar to Mcr.