The stability of built-up axial loaded column in light of STR and EC3/Centriskai gniuzdomu spragotuju strypu pastovumas pagal STR ir EC3

The paper presents the analysis of built-up laced axially loaded steel columns in light of Eurocode 3 and Lithuanian design code STR 2.05.08:2005. The theoretical part analyzes two design methods. Some cases indicate principal differences. According to STR, axial forces are equally divided into two parts for both chords. However, in Eurocode 3, axial force (formula 8) for one chord increases due to the additional bending moment (Formula 6) that depends on the shear stiffness of lacings (Formula 5). For very slender columns, the axial force of one chord, considering Eurocode 3, is 2.7 times bigger than that taking into account the STR method. Another big difference between the methods is that according to Eurocode 3 it is not necessary to check the overall stability of the built-up member round the z-z axis (only checking the stability of one chord round the z1-z1 axis is obligatory). Both methods require checking the stability of one chord round the y-y axis. In two cases, calculations referred to the same initial data (Table 1, 2) applying different design codes. The obtained results are presented in the diagrams. The first case shows that column slenderness in both planes equals [λ.sub.y] = [λ.sub.z]. The axially loaded column calculated with reference to the STR method has bigger bearing capacity reserve than that calculated considering the Eurocode 3 method. In this case, the stability of one chord round the y-y axis (Fig. 3) is the most dangerous. This example illustrates that the stability condition of the axially loaded column according to Eurocode 3 is not satisfied; thus, a necessity of increasing the column cross-section arises. The main reason for the latter situation is a different method used for calculating the axial force of one chord. This difference is greater for more slender columns. In the second case--column slenderness makes [λ.sub.y] = [λ.sub.z]/2. When slenderness is [λ.sub.z] < 100, the axially loaded column calculated according to the STR method has similar results compared to the Eurocode 3 method (Fig. 10). The most dangerous according to STR is the stability of the entire column round the z-z axis (Fig. 8), whereas in accordance with Eurocode 3 it appears to be the stability of one chord round the y-y axis (Fig. 9). In such a case, the stability condition of the axially loaded column according to Eurocode 3 has more reserve only when slenderness is [λ.sub.z] > 100 (Fig. 10). Therefore, calculation according to Eurocode 3 is les