For predefined circular section of column you can check the instability of the column with nominal curvature method (§5.8.8 EC2). This method is primarily suitable for isolated member with constant normal force and defined length l0 (pre-calculated according to § 5.8.3.2 EC2). For columns with rectangular section the same type of checks is performed by program in the pertinent typology "Rectangular section of columns". For the present circular section this type of calculation The "Section Data" Tab looks like:
The relevant input data in this window are:
- Diameter of concrete circular section
- Concrete Class and Steel grade
- Number and diameter of bars arranged along the circumference with a given concrete cover (measured from centroid of bars)
- Design constant axial force Nd
- Effective creep ratio according to eq. (5.19) EC2: φef = φ(∞,t0) MX0qp/MX0d where MX0qp is the first order moment in SLS quasi-permanent combination
- First order bending moment in ULS combination: MX0d
- Eccentricity eY: = l0Y/400 to take into account geometric imperfections according to simplified alternative in § 5.2(9) EC2.
- Effective length l0Y: to evaluate in advance according to § 5.8.3.2 EC2
The curvature 1/r0 in eq (5.34) can be assessed by the program (see in above figure the option "Mode of calculation of curvature 1/r") with the direct formula in § 5.8.8.3 EC2 or by means the moment-curvature diagram as in "model column" method. Below is an example of such a diagram for a circular section:
