the effect of lateral force on stability margin of flexible frame systems

Investigation on the stability margin of typical structures subjected to gravity loads is usually based on eigen analysis for the first buckling mode of the system using stiffness characteristics of the structure (including its geometrical stiffness) (bazant & cedolin, 2010; timoshenko & gere, 1985). however, introducing lateral force on a structural system makes its stability assessment difficult. in the presence of lateral loads in a structural system, material nonlinearity (in theform of plastic hinges in structural members) and geometrical nonlinearity (due to p - d effect) are the main concerns in determination of stability margin for the system (bernal, 1992, 1998). in these cases, eigen analysis cannot be directly used to address stability problems of the structural system. on the other hand, in tall structures and braced frame systemssubjected to lateral loads there is another source of nonlinearity in the system that makes identification of the first bucking mode of the structure difficult. the fact is, introduction of lateral force on a structural system makes the axial force distribution in the structure dependent on the magnitude of lateral force. this in turn modifies the geometrical stiffness ofthe system and results in changing the first buckling mode of the system and its stability margin. such tendency in shifting the stability margin of structural systems is considered important in the design procedure of tall flexible frame systems (ziyaeifar, 2002).