Dynamic impact of Diamond Light Source foot bridge

Abstract

The main foot bridge provides access to linac, booster and storage ring facilities in the synchrotron of Diamond Light Source. The impact of the passage of pedestrian traffic and equipment across the bridge structure was noticeable to the site of beamlines below. One of them, I20, is the most sensitive beamline to such impact. The bridge obviously oscillated with even light traffic, and it was also assumed that this would couple to the storage ring structure where the bridge is mounted. The optics for beamline I20, for stability, stands directly on the slab within the I20 experimental area; this was however subject to excessive vibration transmitted by foot traffic from the overhead footbridge producing a vibration on the experimental floor of 86 nm whereas elsewhere in the experimental hall experiences only about 20 nm, demonstrating a four times increase in vibration caused by the pedestrian bridge. Vibration measurements on the ground underneath the bridge and finite element analyses clearly show that frequencies of 2 and 5 Hz were caused by the bridge and traffic on it. Several remedies were proposed. However, dampers will only damp out vibrations of around 5–6 Hz but not to damp out 2 Hz, which is caused directly by human foot steps. After investigation of cost and effectiveness and several vibration tests conducted, a compromise with extra propping at the mid-span of the bridge was eventually selected. Such reinforcement has been now implemented. The 5 Hz frequency has been successfully removed and a amplitude of 2 Hz also considerably reduced.