An experimental investigation has been made of two identical beams of length 750mm bolted together (a composite beam) which have contact over the whole of the neutral plane. The objective is to determine how friction between the two beams influences the damping. In one set of experiments the beams were held apart using washers while in the second set the beams could contact each other over their whole length. The number of bolts used to hold the beams together was varied between three and 17. The damping of the first two or more modes of the beams was measured. The material damping of the beams when not bolted together was also measured. It was found that the material damping and the damping of the fully bolted composite beam were very similar (damping ratio 0.0005). However, when the beams were in contact and had few bolts (just 3) and the amplitude of vibration was large the damping ratio changed to a much larger value of 0.01.
The experiments suggest that the damping due to the bolts is similar to that of material damping. It is only when other friction sites, not involving bolts, are slipping that damping becomes large.
The consequences for the designer of a built-up structure who wishes to increase damping is thus to use the smallest number of bolts and to arrange for other surfaces to be in sliding contact.