Simplified Methods for Predicting the Pushover Response of a Hybrid Steel Braced Frame with Concentric and Eccentric Connections

Abstract

Two methods to predict the pushover response of a hybrid steel braced frame that features concentric and eccentric connections are presented. The proposed hybrid frame configuration is intended for cold-formed steel (CFS) building structures and plays a dual role by providing gravity and lateral load resistance. The hybrid system features CFS columns for resisting gravity loads, tension only CFS straps connected concentrically to hollow structural steel (HSS) chords for resisting lateral loads, and prestressing strands connected eccentrically to the same HSS chords for further enhancing lateral load capacity. The first pushover response prediction method employs a nonlinear finite element modeling protocol that features only beam and truss elements. This method is presented as an alternative to three-dimensional (3D) nonlinear finite element models, which require considerable modeling effort due to the multitude of the components included in the hybrid system, and the variety of interactions that would need to be defined. The second method includes closed form equations that allow the definition of the pushover curves using manual calculations and is appropriate for preliminary analysis. Formulations for predicting the initial in-plane system stiffness, and secondary stiffness are presented. Similarly, formulations for predicting the load to cause the first and second yield as well as ultimate lateral load capacity are provided. A closed form equation is presented to obtain the ultimate lateral displacement, which completes the definition of the pushover curve. The validation of the proposed pushover response prediction methods is conducted by first comparing responses from 3D nonlinear FEA to those obtained from the simplified FEA model. Similarly, predictions from the simplified FEA model are compared to those obtained from the closed form equations. Good agreements are noted in both cases. The influence of hybrid steel braced frame aspect ratio on system push over response is quantified. The contribution of each lateral load resisting element to lateral load capacity is presented as a function of aspect ratio. Finally, improvements in behavior and enhancements in lateral load capacity offered by the hybrid frame-wall compared to the traditional strapped wall system are demonstrated through monotonic pushover, cyclic pushover, and response history analysis.