Vibration tests were applied to ascertain the flexural rigidity of Howe-type wooden deck-trusses in two kinds of forest-railway bridges, one of which was relatively new (constructed 1 1/2 years ago) and the other comparatively old (in use for 11 years).
The truss was vertically vibrated by means of a rotary eccentric mass vibrator operated by hand. The resonance of the fundamental vibration of the truss to this damping forced vibration was recorded on a displacement vibrograph. The measured value (T) of the natural period of vibration of each truss was compared with the ideal theoretical period (T₀) of the same truss. The rate (φ) of decrease in the flexural rigidity of each truss was calculated by the following formula:
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It was found that the values of φshowed 30-35% in the case of the older bridge, on which locomotive traffic was prohibited because of danger, while in the case of the new bridge, φshowed a slight difference from the theoretical value. Moreover, the value of φ indicated the looseness of of the bolts and nuts in the vertical members and whatever slight irregularity existed in the structure. Thus, the vibration test showed the practical possibility of some exact and quantative judgement of the flexural rigidity, and finding of the structural defects of a bridge.