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Measured moves

A research team at Sheffield University has developed a system to monitor vibration in long span concrete floor structures.

A team from the department of civil and structural engineering at Sheffield University, led by lecturer Aleksander Pavic, is conducting research into the dynamic testing and monitoring of full scale structures in open environments.

A typical problem on, for instance, long span concrete floors, is that these structures may have a tendency to vibrate excessively, and under certain circumstances this can cause discomfort to users. However, interest in the vibration serviceability of massive concrete floor structures, as in multistorey car parks, is a fairly recent phenomenon. Until now most studies have focused on strength.

Pavic says: 'Civil engineers are constantly endeavouring to use improved and stronger materials, for example to make long span concrete floors lighter and thinner. However, although perfectly safe, too light and too thin a floor can result in a structure that vibrates excessively, impairing the comfort and efficiency of people who use the walkway or work on that floor.'

The group has developed a portable system for field testing, costing around £70,000. The system is based around Endevco's 7754-1000 Isotron accelerometers, which Pavic claims are ideal for the job, being capable of measuring very small vibrations while being exceptionally rugged.

In a typical application a medium-sized civil structure can be fully dynamically tested within 12 hours. This means that an office floor could be tested overnight or over a weekend. 'Previously,' Pavic claims, 'researchers in the field have had to close an office floor for anything up to a week because the equipment was so fragile, cumbersome to install and allowed only painfully slow testing'.

Current guidelines for vibration serviceability testing are based on measuring the vibrations generated by one person walking on a floor or footbridge surface. The new system allows a complete field test session to be done by just two or three people with data analysis carried out on site as part of the process.

For testing structures such as multistorey car parks the group uses an APS113 shaker, also supplied by Endevco. This supplies a known vibration to the floor surface which is measured at various points by the accelerometers. Pavic claims that previous generation transducers would have required a substantially bigger excitation on the massive floor to produce a measurable vibration.

A further reason for monitoring structures is to identify structural system characteristics. The dynamic behaviour of civil structures is affected by stiffness, damping and mass. The development of mathematical models in civil engineering is in principle, more complex than for say, aerospace or mechanical structures.

For example, identical buildings on different sites may have different characteristics because of differences in the ground on which they are built and the construction tolerances.

Modal testing is used on real structures to determine the accuracy of their mathematical models. It is difficult to dynamically excite large structures, but equipment developed by the group has proved useful in modal testing of civil structures.

Using the Endevco shaker and a 5.4kg instrumented sledgehammer fitted with a piezoelectric force transducer as excitation methods, the group is gathering a wealth of data which will highlight the differences between real life structures and mathematical models.

Ultimately mathematical model updating based on measured behaviour could be used to improve design guidelines for similar structures.

Another area of research is the effect of false floors on the vibration performance of long span floors. Pavic claims that current analysis methods neglect false floors as a structural component. The team is endeavouring to show that the false floor increases damping and stiffness, which in turn improves vibration performance.

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