What is Imperfection Load?

June 7, 2022
BLOG PROJECT TUTORIAL

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.

 
 
Imper load 1
           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

Imper load 2Figure 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
 

Imper load 3Figure 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.
 
Imper load 15
Imper load 14png
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


 
Imper load 5
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.


 

 
Imper load 6Figure 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.

 

Imper load 16Imper load 17 

Imper load 9
 
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

 

Imper load 10
 
Figure 9. The procedure of calculating the coefficient  in Midas nGen (Step 1)
 
 
Imper load 18

 

Figure 10. The procedure of calculating imperfection load  in Midas nGen (Step 2)
 
Imper load 12
Figure 11. The procedure of creating load combination  in Midas nGen (Step 3)
 
 

 

 

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Author Information
Yeong-il Seo | Principal Structural Engineer

Young-il has over 13+ years of experience in building design, especially high-rise buildings with column reduction analysis, plant structures, pushover analysis, health monitoring, and vibration control projects. Since 2016, he is planning and providing technical supports for midas building products such as midas Gen, nGen, and Design+.

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