Adaptive constitutive analytical model code for rubber bearings with load-dependent variable analytical model in seismically isolated steel structures

AbstractNear-fault earthquakes, characterized by high-amplitude vertical acceleration components, induce significant fluctuations in the vertical stresses of structural base isolators. This phenomenon causes isolators such as rubber bearings to experience alternating tensile and compressive forces during different phases of seismic excitation, which could lead to considerable changes in their lateral behavior. Therefore, this study introduces a novel development of adaptive modeling approach for rubber bearings, incorporating the effects of vertical load variations in three conditions of compression, neutral and tension to enable real-time adjustment of the isolator’s constitutive behavior. For this purpose, experimental tests were conducted on rubber bearings by applying cyclic loads with different amplitudes under compressive, negligible, and tensile preloads to accurately extract their force-displacement relationships (hysteresis responses) corresponding to different vertical stress conditions. Thereafter, a dedicated computational algorithm and macro program were developed and implemented within a finite-element software environment to enable real-time adjustment of the lateral stiffness and damping properties of rubber bearings at each time step based on instantaneous vertical reaction forces. The integration of these experimental results with the adaptive programming framework facilitated the development of a Variable Analytical Model (VAM) for the rubber bearings, which was compared against a conventional Constant Analytical Model (CAM) representing traditional modeling practices. The proposed VAM was subsequently applied to the analysis of a seven-story steel frame structure subjected to seven near-fault ground motions selected from FEMA P695 database. The results indicated that isolator displacements predicted by the VAM increased by approximately 30% compared with those obtained using the CAM. Furthermore, the evaluation of roof-level accelerations demonstrated a reduction of up to approximately 5% in the VAM analysis compared with the CAM results. These findings underscore that the VAM provides a more realistic representation of structural behavior, where increased displacement demands, if overlooked, could potentially result in significant damage or even failure in critical buildings.

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