The Application of Fiber Optic Gyroscopes for the Measurement of Rotations in Structural Engineering

Abstract

The effects of rotations have been neglected in studies on the seismic properties of civil engineering structures in the past. This was mainly because their influence was thought to be small and there were no suitable sensors available to measure the system response of buildings to rotations properly. Only the effects of torsions caused by asymmetries in buildings, where the center of stiffness differs from the center of mass, are known from differential measurements of accelerometers. Different types of inertial rotation sensors exploiting the Sagnac effect have now reached the necessary sensitivity to be used for the investigation of rotational excitations in buildings. Because large ring lasers (Schreiber et al. , 2003, 2004) have successfully recorded signals of earthquake induced rotations from teleseismic events (Igel et al. , 2005), it is now time to study the behavior of buildings with respect to rotations. Fiber optic gyroscopes (FOGs) are commonly used for applications in inertial navigation. They are exploiting the Sagnac effect in a passive optical interferometer design in order to measure rotations with high precision. For that reason, these gyros can measure absolute rotations and do not require a specific frame of reference. Because the concept of operation is entirely based on optical signals, there are no mechanical moving parts inside the sensor, so the transfer function is constant and the system works over a very wide range of excitation frequencies (10-3 Hz< f FOG<2 kH z ). Furthermore, one can obtain a well-defined reference to north from an FOG, which provides the additional advantage of using these sensors for the long term monitoring of structural stability. In this article we report initial measurements with an FOG on a shake table as well as results from in situ applications in very tall structures.