For design purposes the engineer can select to apply the column effective length factors in the Criteria-Effective Length dialog (See BS5950-1:1990 discussion above), or specify a specific value to be used. The engineer may specify that the effective length ratio L_E/L either be calculated per Annex E or that it be set to a specific value. Per Annex E the L_E/L ratio will be calculated based on whether the column is non-sway or sway-sensitive. This is true in all cases except as described in the section on moment amplification below. Refer to the BS5950-1:1990 discussion of Effective length for more detailed description of the calculation of the effective length factor. The method used by the program to calculate the columns effective length factor is as depicted in the following figure.

The following discussion regarding effective length for columns assumes that the columns axis sway state is known. That is, the program knows if a column is sway-sensitive or non-sway in a particular axis.

For a non-sway axis the program will consider if the user has designated a specific effective length value or indicated to use Annex E to calculate the effective length. In the case of using Annex E the program will use the No-Sway Annex E formulations.

For a Sway-Sensitive column axis the program first determines if P-Delta has been performed.

• If P-Delta is performed the program will either use the user specified effective length factor (if assigned) or will use the No-Sway Annex E effective length factors per 5.6.4(b). The No-Sway effective length factors are used as in this case it is assumed that the amplified sway approach (2.4.2.7b) is being applied and the " kAMP x Ms " values will be replaced by the sway moments from the P-Delta Analysis (refer to Moment Amplification (BS5950-1:2000) ).
• If P-Delta is not performed the program will respond based on one additional criteria the user can assign. The user can designate to either use the effective length factor per 2.4.2.7a or the amplified sway method per 2.4.2.7b (refer to 2.6.6 Sway-Sensitive Design Method (BS5950:2000 Only) ). If the effective length approach is selected the program will use the user specified effective length factor or the Sway-Sensitive Annex E Formulations per 5.6.4(a)/2.4.2.7(a). If the amplified sway approach is selected the program will use the user specified effective length factor or the Non-Sway Annex E Formulations per 5.6.4(b)/2.4.2.7(b).

The logic used to calculate the effective length factor for columns is illustrated below.