11.44: An integrated experimental and numerical assessment of force‐based design for concentrically braced steel frames

Abstract

ABSTRACTThis study uses an integrated experimental and numerical approach to assess the use of the force‐based design (FBD) methodology presented in Eurocode 8 (EC8) for multi‐storey steel concentrically‐braced frames (CBFs).In the EC8 approach, the design seismic action is defined by first generating an elastic response spectrum. This spectrum is reduced to create the design spectrum by applying an appropriate behavioural factor (q) to account for energy dissipation. The value of q chosen depends on the form of the lateral‐load resisting structure, the structural material(s) to be used, the ductility class and the degree of irregularity in the structural layout. The design base shear force calculated from the design spectrum is used to estimate the seismic forces on the structure and hence determine the final member cross‐section sizes.In this study, the FBD approach to the design of structures presented in Eurocode 8 is described and applied to 4‐ and 8‐storey CBF structures. Both dissipative (Ductility Class High – DCH) and non‐dissipative (Ductility Class Low – DCL) design is considered for each structure.A numerical modelling approach developed using the OpenSees seismic analysis software and validated from a series of full‐scale experimental tests of single‐storey CBFs is outlined. The two‐dimensional reference model assigns appropriate out‐of‐plane rotational stiffnesses to the ends of the bracing members based on the strength and geometry of gusset plates specified.The experimental work used to validate the model, carried out as part of the BRACED transnational research project funded by the European Commission's Seventh Framework Programme (FP7) is briefly described. The experimental observations support model validation at different levels of earthquake ground motion including elastic, inelastic and ultimate response conditions.The validated numerical modelling approach is used to perform a series of non‐linear time history analyses (NLTHA) of the designed 4‐ and 8‐storey frames by subjecting them to appropriately scaled simulated ground motion accelerations. The key recorded responses of the NLTHA of the modelled frames (displacements, drift, brace forces and ductility demands) are compared, and the implications of the assumed energy dissipations are discussed.