DAMPING MEASUREMENTS ON STEEL AND COMPOSITE BRIDGES

Measurements were made on twenty three bridges having spans 17m to 213m long. The main types were steel box girder with steel deck, steel box girder with concrete deck and steel plate girder with concrete deck. From the measurements made it was found that damping increases with amplitude of vibration and stabilitises at an upper level which can be up to four times higher than the level at small amplitudes. The lower damping is associated with behaviour of the superstructure. The upper damping includes contributions from joints and substructure. It was also found that there is a tendency for damping to increase with frequency. Measurements made at different parts of the structure indicated that there can be relatively large movements above the abutments so that a significant amount of damping may be produced particularly for short single-span bridges. In general it was found that the lowest values of damping, expressed as logarithmic decrement, were exhibited by multispan wholly steel structures, 0.02 to 0.06. Composite bridges exhibited higher damping, 0.05 to 0.07, than comparable wholly steel bridges. Single-span bridges, 0.05 to 0.13, tend to have higher damping than multi-span bridges, 0.02 to 0.08. /Author/