Vibration of truss structures

Abstract

Recent trends in underwater vehicle design suggest the use of trusslike structures to support vibrating machinery. Experimental measurements are used to understand the dynamic behavior of a set of 1:15 model, three-dimensional truss structures over the full scale equivalent frequency range 10–1400 Hz. A cubic truss with all equal struts, a rectangular truss with two strut sizes, and a less practical truss composed of pyramid-shaped cells are considered. In conjunction with experiments, a direct global stiffness matrix numerical model is used to identify fundamental truss processes. The periodic nature of trusses causes them to act as mechanical comb filters in frequency with 3–5 dB per bay of attenuation at nonresonant frequencies. In trusses with long runs of axial struts, energy is carried down the truss axis primarily by compressional resonances. The spacing of the resonant peaks is controlled by strut length. A filtering strategy uses strut length to reduce the overall truss response via destructive interference of the frequency resonance structures of the individual struts. Multiple periodicities and impedance discontinuities at the truss joints cause the resonant peaks at low frequencies to widen. The consequence of this is that trusses with varied strut lengths tend to a low-pass frequency characteristic. This effect is compared to an equivalent applied damping and is shown to achieve up to 6 dB per bay of attenuation at the higher frequencies.