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Civil Modal Analysis

Written by Rick Tavares, Ph.D. on .

We've received several inquiries on how civil modal analysis is performed and what instruments are used. 

Shown in the photo is ISE's 'modal sledgehammer' capable of producing an impulsive force on an object with magnitudes up to 5,000 pounds. The force is measured through a pressure transducer in the head of the hammer (the silver part shown) while the tip of the hammer (the grey part) is adjustable with different tips of varying hardness allowing us to control the frequency content of the impulse (i.e., the harder the tip material, the higher the frequency roll-off).

Data from the impulse is fed into a spectrum analyzer as our input signal while similar data from a nearby seismometer measures the ground response due to the hammer blow. The output signal (the seismometer) divided by the input force (from the hammer) allows us to determine what is known as the 'transfer function' of the object being tested which can range from a civil concrete structure to something as simple as dirt.

Once we know the transfer function of the system, we can then determine the engineering parameters of interest to the project such as frequency response, relative damping, attenuation rates, etc. ISE is the only firm that we know of to apply aerospace modal analysis methods to conventional highly-damped civil engineering structures.