Dynamic measurements are often difficult to take', says Brian Ellis of BRE's structural performance division. 'Our laser test system has proved useful for structures which are not easily instrumented.' Ellis has tackled structures including bridges and the Houses of Parliament. BRE claims the laser system is particularly useful for testing structures where access is difficult or hazardous, and offers comparable results with more traditional dynamic instrumentation such as accelerometers.
The laser system works in two ways to measure both the velocity and displacement of structural vibration. For velocity, the laser is aimed at the structure and interferometry (analysis of the interference of the incident and reflected beams) determines vibration velocity. A fast Fourier transform converts this signal into a spectrum in the frequency domain, allowing natural frequencies of vibration to be identified.
For displacement, the system works like a standard laser electronic distance measurement instrument in which displacement is calculated from the phase difference between the incident and reflected beams.
Ellis says reliable reflections can be obtained from unprepared concrete surfaces at distances up to 150m. Fixing reflectors on the structure means the range can be increased to 500m. The system is eye safe and uses a tracking system to reduce the effects of atmospheric turbulence.
Measurements can be made on targets with peak velocities of up to 8mm/s at frequencies from 1Hz to 1kHz. Displacement measurements can made with sub micron accuracy, allowing measurement of the response of structures to relatively low excitation loads, such as a single pedestrian on a footbridge.
The system has recently been used to survey 534 hard to reach stone pinnacles on top of the Palace of Westminster. The laser measured the pinnacles' natural frequencies due to wind excitation. Based on the assumption that a loss of stiffness would occur as the stonework on the steel reinforced pinnacles deteriorated - then low frequencies identified pinnacles that were damaged and required repair. To get a clear line of sight, the survey was conducted from across the River Thames, on the South Bank, and took only eight days.
The tests will be repeated at five yearly intervals and are expected to provide a key role in identifying deterioration.
Although commercial versions of the system are yet to be developed, Ellis sees many site applications, for example dynamic testing of inaccessible structures such as cooling towers, smoke stacks and arch dams. The laser test may also prove useful for monitoring low value structures such as lamp posts and construction joints as a basis for preventative maintenance.