Definition of Imperfection load & Design Procedure by program
What is Imperfection Load?
Vertical members like a column and wall are not actually straight due to construction imperfection.
This geometric instability may cause a problem in the safety of the column. Generally, geometric imperfections can be divided into two categories, global imperfection, and local imperfection.
Figure 1. Global Imperfection(Initial sway imperfection) Figure 2. Local Imperfection (Initial bow imperfection)
Imperfection Load as per EN1992-1-1 & EN1993-1-1
The method to apply to structural design for each imperfection type is as follows.
a) Global imperfections for frames and bracing systems
Figure 3. Global Imperfection
Global imperfection is reflected in the model so that it is considered in the analysis result. Therefore, global imperfection is realized by adding loads by multiplying the imperfection angle, Φ by the axial force in the lateral direction as in the above image.
b) Local imperfections for individual members
Figure 4. Local Imperfection
In general, local imperfection is not reflected in analysis but is reflected in the design.
The design is performed by adding the bending moment generated by e0 (=L/500) to the bending moment resulting from the first-order analysis.
IIn Eurocode, the lateral loads for Global Imperfection are calculated according to the formula below.
Figure 5. Calculation of lateral loads for global imperfection
Clause 5.3.2(4)B in EN1993-1-1 states that where the overall applied lateral load is more than 15% of the vertical load in a member then the notional horizontal load can be ignored. This is expressed as HEd ≥ 0.15 VEd.
Global initial sway imperfections are determined as a coefficient, Ф, which is multiplied by the vertical load of the structure.
How to create the imperfection loads in the analysis model
Figure 6. Example to create imperfection loads
The above image is an example of how to calculate nodal loads to finally implement imperfection.
The imperfection coefficient, Φ was entered by a user-defined method.
Figure 7. Example of how to calculate global initial sway imperfections (Φ)
The above image is an example of how to calculate global initial sway imperfections(Φ) and nodal loads to finally implement imperfection.
**** Note.
The error between the values by manual and by the program is due to the number of decimal points.
Tutorial to create the imperfection load in Gen / nGen
How to create the imperfection load in MidasGen
The procedure of creating imperfection loads in Midas Gen is as follows.
Step 1 . Calculate the coefficient, Ф
Step 2. Calculate Imperfection Load
Step 3 . Create Load Combination
Step 4. Check Imperfections in the table according to EC.
Figure 8. The procedure of creating imperfection loads in Midas Gen
How to create the imperfection load in Midas nGen
The procedure of creating imperfection loads in Midas nGen is as follows.
Step 1 . Calculate the coefficient, Ф
Step 2. Calculate Imperfection Load
Step 3 . Create Load Combination
Figure 9. The procedure of calculating the coefficient in Midas nGen (Step 1)
Figure 10. The procedure of calculating imperfection load in Midas nGen (Step 2)
Figure 11. The procedure of creating load combination in Midas nGen (Step 3)