Experimental Results of a 3-dof Parallel Manipulator as an Earthquake Motion Simulator

Parallel manipulators are increasingly used in new applications by exploiting their better characteristics with respect to those of serial manipulators, such as higher stiffness, velocity and acceleration, payload. In this paper, experimental results are presented of a novel application of a 3-DOF CaPaMan (Cassino Parallel Manipulator) prototype to simulate point seismograms and 3D earthquake motion. The rigid body acceleration (linear acceleration, angular velocity and acceleration) has been experimentally analyzed to simulate real 3D earthquakes by using the parallel manipulator. Furthermore, first experimental results are reported to analyze earthquake effects on scaled civil structures. INTRODUCTION Earthquake simulators are widely used in the field of Civil Engineering to investigate both earthquake characteristics and resistant constructions. Therefore, it is of great interest to have earthquake simulators that can reproduce earthquakes to test nature-scaled or scaled civil structures. Several types of earthquake simulators are available in laboratories of Civil Engineering Departments, like for example those referred in [17]. But those simulators do not take into account and do not operate for a 3D motion of the terrain due to earthquake waves. In fact, usually seismograms that are available from seismic sites concern with the characteristics of point motion in terms of accelerometers. Nevertheless, the 3D motion of the terrain can strongly affect the dynamic excitation of the buildings. Therefore, recently during the 1997 earthquakes in center Italy, novel attention has been addressed to the 3D aspects of earthquake motion by installing proper systems to acquire and elaborate data, as explained in [8-10]. The problem of a proper analysis and consequent reproduction of earthquake motion has been attached in [11] by discussing preliminary experimental results of tests that have been carried out with a CaPaMan (Cassino Parallel Manipulator) prototype by using monitored data of earthquake events in central Italy in 1997, [8-10]. In particular, the 3D motion capability of CaPaMan parallel architecture has been used to simulate 3D earthquake motion through an extension of previous experiences, [12; 13]. A 3D motion for the terrain body can be considered as a novelty for earthquake simulators. In this paper, satisfactory results are reported for a study based on experimental activity for using CaPaMan architecture with 3-DOF as an earthquake simulator of general seismic motion. Furthermore, experimental results of the earthquake effects on scaled civil structures are reported. EARTHQUAKE CHARACTERISTICS A seismic motion is a function of chemical and physical characteristics of the terrain, which can be considered as a composite body with several stratified masses, [14]. One can identify the period of a seismic cycle and characteristic length for each seismic wave. Main types of seismic waves can be considered as: the compression expansion waves P, transversal waves S, and superficial waves M. They can be classified by referring to the propagation speed and terrain movements, as shown in Fig. 1. The P waves propagate through a spring-likemotion with a typical period between 0.1 and 0.2 sec. S waves refer to transversal waves with a typical period between 0.5 and 1 sec. Both P and S waves occur close to the epicenter. M waves involve only the terrain surface with a typical period between 20 sec and 1 min, they occur at a considerable distance from the epicenter. The intensity, shape and duration of a seismic motion can depend also on the characteristics of the terrain through which the seismic waves propagate. In Fig. 1c) the main difference among the seismic waves is represented in terms of acceleration magnitude and characteristic period of 2 Copyright © 2004 by ASME a) b)