Steel frame structures are often analyzed for stability in the elastic range, while most of the columns exhibit inelastic behavior at the buckling stage. Most column design provisions allow for inelastic behavior, but overall inelastic stability analysis is rarely performed.
In the presented stability analysis, the method for calculating the critical load and the effective buckling length of the frame structures based on the global stability analysis has been formulated accounting for inelastic behavior at the buckling stage.
The theoretical and numerical analysis is performed using the tangent modulus approach. For structural member where the proportionality limit is exceeded, for each new load increment, the member stiffness has to be changed and the corresponding tangent modulus should be used for that member. In this investigation are applied some of the most used expressions that describe the relationship between the modulus of elasticity and tangent modulus.
For the purpose of this investigation a computer code for stability analysis of frame structures in the elastic and inelastic range has been developed. The calculation algorithm implemented in the code is based on the application of stiffness matrices derived using the tangent modulus approach. This algorithm introduces more accurate calculation of buckling not only in elastic, but also in the plastic domain. It allows monitoring of the phenomena of stability loss of the frame structure in the plastic domain and direct calculation of the critical force.