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2017-07-31

Imperfections According to EN 1993-1-1, Section 5.3.2: Global Initial Sway Imperfection

According to EN 1993-1-1 [1], it is necessary to use the equivalent geometric imperfections with values that reflect the possible effects of all types of imperfections. EN 1993-1-1, Section 5.3, specifies basic imperfections for the global analysis of frames as well as member imperfections.

In the case of structures with a mode shape, which is characterized by a lateral displacement, the effect of imperfections can be considered for the structural calculation by a global initial sway imperfection (and individual bow imperfection, if necessary).

The global initial sway imperfection Φ is determined according to Equation (5.5):
Φ = Φ0 · αh · αm

The resulting basic value Φ0 is equal to 1/200, as specified in DIN 18800, and can be reduced by two factors. These factors take into account the favorable effects of very high columns as well as several supporting columns in a row. The background to this is that the imperfections are structurally distributed and thus very unlikely to be unfavorable on all components.

αh is the reduction factor for the height h applicable to columns

23 < αh = 2h < 1.0
Formula 1

Reduction Factor αh
Reduction Factor αh

αm is the reduction factor for the number of columns in a row
m is the number of columns in a row with a vertical load > 50% of the average value of the column

αm = 0.5 · (1  1m)
Formula 2

Reduction Factor αm
Reduction Factor αm

Compared to DIN 18800, there are greater sway imperfections in the case of a larger number of columns m and higher columns resulting from the calculation according to EN 1993-1-1.

References

[1]  EC 3. (2009). Eurocode 3: Design of Steel Structures – Part 1-1: General Rules and Rules for Buildings, EN 1993-1-1:2010-12.
[2]  Dlubal Software. (20017). Training Manual EC3. Leipzig, Dlubal Software, March 2017.
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