 # Determining and Using Effective Lengths

### Tips & Tricks

The RF‑STABILITY and RSBUCK add‑on modules for RFEM and RSTAB allows you to perform eigenvalue analysis for frame structures in order to determine critical load factors including the buckling modes. It is possible to determine several buckling modes. They provide information about the model areas bearing stability risks.

The critical load factor is always determined for the entire structure. Based on this, the corresponding critical loads of the individual members, effective lengths, and effective length factors are calculated.

A simple example of a rigid column with load introduction on the head (for example roof loads) and on the top third (for example crane loads) exemplifies the procedure in detail.

For the structural system, the critical load factor of 8.591 has been determined. This is to be multiplied by the normal forces of members to obtain the critical loads.
Member 1: Ncr,1 = 8.591 × 550 kN = 4,725 kN
Member 2: Ncr,2 = 8.591 × 200 kN = 1,718 kN

The corresponding effective lengths for buckling about both axes are calculated by using the adjusted formula Ncr = E × I × π² / Lcr. Finally, the effective load factors are determined from the relation kcr = Lcr / L.

Now, it is possible to perform equivalent member design according to EN 1993‑1‑1. The critical load factors calculated in RF‑STABILITY or RSBUCK can be transferred directly into the RF‑/STEEL EC3 add‑on module. Depending on the model complexity and loading, you should always check whether the stability design using imperfections and internal forces according to the second‑order analysis would be a better alternative. 