A variable limit for the instability parameter of wall-frame or core-frame bracing structures

This work aims to investigate the viability and convenience of adopting a variable limit α1 for the instability parameter of buildings with reinforced concrete wall-frame or core-frame structures. Initially, the evolution of tall buildings global stability theory is summarized, giving emphasis to define when a second order analysis is needed. The treatment given to this subject by the present Brazilian code for concrete structures design (NBR 6118:2007) is also showed. It follows a detailed analytical study that led to the derivation of an equation for the variable limit α1; a series of examples is presented to check its accuracy. Results are analyzed, showing the validity bounds of the equation and research directions are suggested, in order to improve it.