Early Warning of Structural Collapse After a Disaster

Even though a building remains standing after a natural disaster and, to the untrained eye looks safe, it could be on the brink of a structural failure. Thanks to new technology, it is now possible to determine whether or not structures are about to collapse. The way structures vibrate indicates their state of health in much the same way as a heartbeat will give information about the state of health of a person. A system has been developed in which a portable device is taken to a structure and a vibration measurement is made over a period of about three hours. The vibration signature is then returned to the laboratory for in-depth analysis.
 

The building (Fig.1- showing damage from earthquake) was affected by an earthquake that caused extensive damage. It was necessary to determine whether the building was safe, and whether it would be possible to repair it. In this case it was possible to give assurances about the condition of the building by collecting and analyzing minute vibration signals from the structure.

A structure that is about to collapse displays a distinctive vibration signature. This comprises a series of very low frequency resonances, and are an indication that the structure is breaking up. These frequencies are typically at less than one cycle every 10 seconds.

It is possible to conduct an in-depth analysis of the vibration signatures and use these to compare what the structure is actually doing with how it should have behaved when it was newly built. Equipment has been developed for quick and affordable assessment of vibration signatures for entire buildings.

This equipment (Fig 2: Portable Integrity Assessment System) is portable, battery operated, and can record vibrations caused by natural phenomena such as wind, passing traffic or microtremors. The equipment is so sensitive that it can resolve motion to better than the wavelength of visible light. With such a sensitive instrument it is possible to obtain usable vibration signatures even when there is no perceptible motion of the structure. Even when the vibration is very small the structure behaves in a similar way, and it is possible to determine what will happen to the structure if the vibration increases.

In this way it is possible to conduct a safe investigation even if the structure is in danger of collapse and to get a good measure of the state of health of the structure.The system has been used in several parts of the world so far including assessments of earthquake-damaged buildings in Asia, and ageing bridges and offshore structures in South America. In each investigation, the data are sent to laboratories in Hong Kong and Australia for analysis.

Prof Alan Jeary is with the University of Western Sydney, School of Construction, Property and Planning. He can be contacted at a.jeary@uws.edu.au