Numerical hybrid simulation assessment of E‐BRBF and E‐FBF systems for mitigating P‐delta effects and enhancing post‐earthquake performance
Authors: Hariri Bashar , Christopoulos Constantin 
Published: July 2024

This article investigates the seismic stability of E‐BRBF and E‐FBF systems previously proposed as stability‐enhanced alternatives to conventional BRBF and FBF systems. The E‐BRBF system is a buckling restrained braced frame with an inverted‐V brace configuration in which one of the two braces is a conventional brace designed to remain elastic during a strong earthquake. When the buckling restrained brace (BRB) yields, an unbalanced vertical force develops at the brace‐to‐beam connection, which imposes flexural demand on the beam and creates a positive post‐elastic storey shear stiffness. The E‐FBF system is identical except that the buckling restrained braces are replaced by conventional bracing members constructed with an end friction connection designed and detailed to slip at a predetermined load. At every storey, the beam and the elastic braces are designed such that this post‐elastic stiffness is sufficient to cancel the negative storey shear stiffness resulting from P‐delta effects and introduce a re‐centring mechanism to achieve an enhanced performance in steel buildings subjected to interface subduction earthquakes. Using distributed plasticity numerical models, past studies demonstrated the effectiveness of E‐BRBF and E‐FBF systems in mitigating P‐delta effects, emphasizing the importance of a stable and predictable response of the beam, which is a critical element in the scheme that provides the post‐elastic stiffness of the system. This beam is subjected to axial and flexural demands and must remain near elastic to ensure the global seismic stability of the structure. The response of the beam can be affected by localized yielding and local instability effects resulting from residual stresses, stress concentration near gusset plate connections and geometric imperfections. Employing a refined nonlinear finite element model of the beam element, this article assesses the performance of 10‐storey E‐BRBF and E‐FBF systems utilizing multi‐platform numerical hybrid simulations. The evaluation involves static‐monotonic and dynamic response history analyses, incorporating seismic excitations representing Vancouver, BC's seismic hazard (Crustal, In Slab, and Subduction Interface). In the hybrid simulation model, the most critical member for the proposed system (i.e., first‐storey beam) is sub‐structured in ABAQUS using solid elements, while the remaining frames are integrated using OpenSees using fiber‐based sections. Numerical hybrid simulation dynamic nonlinear response history analyses demonstrate the adequacy of the E‐BRBF and E‐FBF systems in mitigating P‐delta effects with residual drifts within repair limits, unlike conventional systems where instability or excessive drifting occurred. Monotonic Pushover hybrid simulations reveal a more ductile beam behavior compared to standalone models, in which frame members are entirely modeled in OpenSees. The analysis also indicates a ductile plastic hinging beam failure mechanism with no instability failure modes at extreme drifts.