Assessment of bridge natural frequency as an indicator of scour using centrifuge modelling.

One of the most prevalent causes of bridge failure around the world is "scour"-the gradual erosion of soil around a bridge foundation due to fast-flowing water. A reliable technique for monitoring scour would help bridge engineers take timely countermeasures to safeguard against failure. Although vibration-based techniques for monitoring structural damage have had limited success, primarily due to insufficient sensitivity, these have tended to focus on the detection of local damage. High natural frequency sensitivity has recently been reported for scour damage. Previous experiments to investigate this have been limited as a result of the cost of full-scale testing and the fact that scaled-down soil-structure models tested outside a centrifuge do not adequately simulate full-scale behaviour. This paper describes the development of what is believed to be the first-ever centrifuge-testing programme to establish the sensitivity of bridge natural frequency to scour. A 1/60 scale model of a two-span integral bridge with 15 m spans was tested at varying levels of scour. For the fundamental mode of vibration, these tests found up to a 40% variation in natural frequency for 30% loss of embedment. Models of three other types of foundation, which represent a shallow pad foundation, a deep pile bent and a deep monopile, were also tested in the centrifuge at different scour levels. The shallow foundation model showed lower frequency sensitivity to scour than the deep foundation models. Another important finding is that the frequency sensitivity to "global scour" is slightly higher than the sensitivity to "local scour", for all foundation types. The level of frequency sensitivity (3.1-44% per scour depth equivalent to 30% of embedment of scour) detected in this experiment demonstrates the potential for using natural frequency as an indicator of both local and global scour of bridges, particularly those with deep foundations.

The role of vibration and drainage in femoral impaction bone grafting.

Vibration is commonly used in civil engineering applications to efficiently compact aggregates. This study examined the effect of vibration and drainage on bone graft compaction and cement penetration in an in vitro femoral impaction bone grafting model with the use of 3-dimensional micro-computed tomographic imaging. Three regions were analyzed. In the middle and proximal femoral regions, there was a significant increase in the proportion of bone grafts with a reciprocal reduction in water and air in the vibration-assisted group (P < .01) as compared with the control group, suggesting tighter graft compaction. Cement volume was also significantly reduced in the middle region in the vibration-assisted group. No difference was observed in the distal region. This study demonstrates the value of vibration and drainage in bone graft compaction, with implications therein for clinical application and outcome.