How do the code Directional Factors (C_dir), and wind directions correlate with those of Tekla Structural Designer
Environment
Eurocode
Question
When using the Wind Wizard, each direction is given a Directional Factor (Cdir) value. Where do these values come from and how are they assigned?
Answer
This article looks at the situation where we are looking at orthogonal load cases i.e. directions normal to the building faces such that they are at 90 degrees to each other.
Wind Wizard
The Directional Factor (Cdir), option appears within the Wind Wizard for Eurocode models which have utilised the Loading ribbon tab > Wind Wizard command.
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Direction Factors, Cdir
The Directional Factor (Cdir) values are determined from the United Kingdom Eurocode National Annex Clause NA.2.6 when working with Eurocodes as the head code and from Table 3 for BS 6399-2:1997 when working with British Standards.
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How then do these relate to the Tekla Structural Designer axis system
The below example uses the Eurocode values but are equally applicable to British Standards.
The circle below reports the angle and the Directional Factor (Cdir) factors from the United Kingdom National Annex. i.e. 270 deg (C_dir = 0.99)
Note that the values are reported from 0, at the top in a clockwise direction as denoted in Note 2 of the above table.
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If we consider a circle with the Tekla Structural Designer axis system referenced on it, 0 direction runs from left to right and the other angles are reported in an anti-clockwise direction.
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If we then overlay the Tekla Structural Designer Directions onto the United Kingdom NA Cdir values we can then align the Tekla Structural Designer model with the Cdir values using the Wind Wizard Orientation Known option by specifying the Orientation of North.
Example
For example if we know that the building is to be located so that the Tekla Structural Designer axis "270" aligns with North (0 on the C-dir axis) then;
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For each direction of wind normal to the face of the building, it uses the worst Cdir result in the +/- 45 degree quadrant (within the red line for the direction of Wind). Note 3 of the NA.2.6 table.
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So as an example Wind Direction 0 taking the Directional Factor values +/- 45 degrees to this direction provides Direction Factors Cdir values of 0.91, 0.99 or 1.0 . The worst value in accordance with Note 3 = 1.0
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For example if we assume that the Tekla Structural Designer axis 0 aligns with North (0 on the Cdir axis) then;
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For example if we assume that the Tekla Structural Designer axis 120 degrees, aligns with North (0 on the Cdir axis) then;
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