Seismic Demand and Capacity Evaluation of Rectangular Concrete- Filled Steel Tube (RCFT) Members and Frames

This paper presents analysis tools to be used in quantifying the demand and capacity of composite frames composed of rectangular concrete-filled steel tube (RCFT) columns and steel girders. Worldwide experimental research on RCFT members were consolidated into a database. The experimental results stored in the database were used to quantify the capacity of RCFT members at multiple levels of loading, which in turn were identified based on the local damage levels reported during the tests. A three-dimensional (3D) fully nonlinear mixed fiber finite element formulation was developed to simulate the cyclic loaddeformation response of RCFT members as part of composite frames. The formulation has the capability to account for slip displacement between the steel tube and concrete core, as well as all key cyclic phenomena in the steel and concrete critical to capturing the composite characteristics of RCFT members. dition, detailed information related to the local damage states of the specimens were recorded in the database with their load and deformation values from the applied loading history. The damage experienced by RCFT members was quantified through the use of displacement-based and energy-based damage functions that documented the damage relative to the peak strength and ultimate deformation attained in the specimen. These damage functions were evaluated for each specimen in the database at each available local damage level (e.g., concrete cracking, yielding of steel tube, local buckling etc.). The damage function values were then correlated to the structural parameters of the specimens (e.g., compressive strength of the concrete, yield strength of the steel tube, depth–to-thickness ratio, etc.). For each category of RCFT specimens, this process was repeated to yield different sets of damage function equations to quantify damage at all the available local damage levels. As an example, Equation (1) shows the deformation-based damage function ( $ D ) of monotonically loaded beam-columns in double curvature for the local damage state of yielding of the tension flange, correlated to the parameters of the axial force ratio ( P Po / ) and the steel tube strength ratio ( P P s o / ).