Fixed Base FRF Using Boundary Measurements as References: Experimental Results

Abstract

Modal parameters extracted from test articles with known boundary conditions are useful for model correlation and updating but are often not obtained because of the time and cost associated with moving and installing the test article on a rigid or isolated seismic mass and performing a separate modal test. It would be advantageous if frequency response functions from known boundary conditions, such as fixed base frequency response functions, could be obtained while the structure undergoes testing on fixtures that contain compliance or dynamics in the frequency range of interest, such as base shake tables.

A method that uses response measurements at the structure’s boundary to mathematically constrain the test article and produce equivalent fixed base frequency response functions is proposed. Fixed base frequency response functions can be obtained by using the boundary measurements as additional references to mathematically constrain these degrees of freedom in the frequency range of interest.

This paper presents the experimental results of applying the developed methodology to two test articles: a cantilever beam and a mass simulator mounted on a flexible bracket. Each test article was mounted to an aluminum block that was supported in two ways either resting on flexible foam supports or rigidly attached to ground. The mathematically constrained frequency response functions from the flexible boundary conditions compared favorably to those obtained when the structure was rigidly attached to ground, which demonstrates the potential of the developed method.