Vertical Seismic Isolation Using Transversely-Loaded Buckled Beams to Achieve Quasi-Zero Stiffness

Abstract

In seismically active regions, earthquakes pose a significant threat to the structural integrity of buildings and their contents, often leading to substantial economic losses and operational disruptions. Seismic base isolation devices are commonly implemented to mitigate these accelerations. In near-fault seismic zones, strong vertical ground motions can amplify the response of nonstructural components, leading to significant loads and damage. Smaller-scale isolation systems can be employed to protect sensitive equipment from these vertical dynamic vibrations. This research proposes a vertical base isolation system that uses the nonlinear behavior of a transversely-loaded elastic buckled beam to generate a negative stiffness. By combining the beam’s negative stiffness with linear positive stiffness springs, it creates a quasi-zero stiffness (QZS). A QZS system can provide a high initial static stiffness to support gravity loads while reducing the dynamic stiffness for improved vertical vibration isolation.An experimental model of the vertical isolator was developed to evaluate its isolation performance. The system consists of a carbon fiber beam with fixed-end constraints, elastically buckled into an initial configuration that, when transversely loaded, produces a negative stiffness. Compression springs are incorporated to offset this negative stiffness and bring the system toward a low effective stiffness. Theoretical predictions of the beam’s negative stiffness were formulated based on the beam’s geometry and material properties and were used to establish a target stiffness for the experimental isolator. Static and dynamic tests were conducted on the experimental system. Static tests were used to characterize the negative stiffness in the beam and measure the QZS of the combined system. Dynamic tests were performed using a vertical shake table under varying ground excitation intensities to evaluate the isolation performance. The acceleration response of the isolator was measured and compared with the ground input, representing the accelerations transmitted to a payload. The results from the dynamic tests identify the critical frequency range for the system to achieve effective isolation. A numerical model of the isolator was developed by idealizing it as a single-degree-of-freedom system. A Bouc-Wen model was used to capture the isolator’s nonlinear frictional behavior. The model parameters were selected based on the experimental results. The governing equations were solved using a numerical ordinary differential equation solver in MATLAB, and the simulated responses were compared with experimental data to validate the model’s nonlinear dynamic behavior.