“Computational Civil Engineering - "Intersections" International Journal

“Computational Civil Engineering 2005”, International Symposium 2293. SINGLE EXCITER TECHNIQUES3.1. Peak Amplitude MethodThe simplest and most commonly used technique is the peak amplitudemethod, in which the structure is excited by a sinusoidal force from a single exciterand the response curves of total amplitude, obtained at several points on thesuperstructure of bridge, are recorded as a function of frequency. Measurement ofthe exciting force and the resulting motion at a number of points over a range offrequencies would be sufficient to describe the vibrator behavior of the structure.This information can be presented by plotting the ratio of motion to force as afunction of frequency. This is the most commonly used method of carrying out aresonance test [4].In the instrumentation set-up used for obtaining a response curve thevibration exciter is fed with a certain amount of power at a slowly changingfrequency. Due to resonances in the structure and the vibration exciter, the powernecessary to subject the structure to a constant force level will not remain constantduring the test, but will be a function of frequency. To keep the force levelconstant, a servo-loop is used in which the output from the force transducer,mounted between the exciter and the superstructure of bridge, is fed back to theexciter control via a preamplifier. The output from the accelerometer mounted onthe specimen is fed to an X-Y recorder, which traces out a response curve as thefrequency is scanned (fig. 3).CHARGEPREAMPLIFIERACCELEROMETERVOLTMETERFORCE TRANSDUCERCHARGEAMPLIFIERVIBRATION EXCITERX - Y RECORDERPOWER AMPLIFIEREXCITER CONTROLFig. 3. Instrumentation set-up for the peak amplitude method