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# Instability check

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# Instability check

In the present typology "rectangular section of column" (selected in General data Tab) 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 (to be pre-calculated by the user according to § 5.8.3.2 EC2). The input windows for this calculation is as follows:

The relevant input data in this window are:

- Dimension of side X and Y of the concrete rectangular section

- Concrete Class and Steel grade

- Number and diameter and cover of corner bars and web 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 of interest:  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. In biaxial bending it is difficult to use the direct formula (5.34) of § 5.8.8.3 EC2 as it is hard to meet the conditions of applicability (5.38a) and (5.38b) EC2. The biaxial moment-curvature diagram provided without problems by the program make always possible to check biaxial instability. In the below results such a diagram is shown together with the windows to print numerical results. The graphic of diagram can be save as dxf file or printed directly on paper.