Gianmario Benzoni

Friction Model for Sliding Bearings under Seismic Excitation

In this article, an experimentally validated model is proposed in order to take into account main sources of performance degradation that could be experienced by friction-based devices during a seismic event. Particular attention is dedicated to the degradation of friction characteristics due to repetition of cycles and consequent temperature rise. This effect can be responsible for overestimate of the dissipation capacity of the device. The proposed model of frictional behavior is suitable for immediate implementation in generalized structural analysis codes and provides an important design tool for realistic assessment of the seismic response of structures equipped with friction-based isolators.

Effects of Vertical Load, Strain Rate and Cycling on the Response of Lead-Rubber Seismic Isolators

Lead-rubber isolators represent a valid and economic solution for the seismic isolation of bridge structures and modern manufacturing techniques make available large devices. Velocity effects on small to medium-scale isolators have been discussed by several authors (e.g., Clark et al., 1997; Thompson et al., 2000) as well as included in reports of experimental programs (e.g., CERF, 1999). Only recently, however, the behavior of large devices was validated under full-scale displacements, loads, and velocities. In this article, results obtained from an experimental investigation on the effects of axial load and strain rate on the performance of a full-scale lead-core elastomeric bearing for bridge applications, are reported. The bearing response was analyzed with particular attention to the variation of critical performance characteristics in order to produce a set of information that could be implemented in a physically motivated numerical model. The results, in line with additional tests performed on similar full-scale bearings at the Caltrans SRMD Testing Facility at the University of California San Diego, indicate a moderate effect of the applied vertical load but a significant effect of the strain rate and cycling on all the significant response parameters. This information should be taken into account by designers, particularly when high component of velocities are associated with the expected seismic motion. A simplified numerical model is proposed for the assessment of lead-rubber bearing performance.

Seismic response of linked marginal wharf segments

The seismic response of marginal wharf segments is considered based on simple analytical relationships, and dynamic inelastic time-history analyses. Of particular interest was the amplification of transverse displacements caused by torsional response under longitudinal excitation, and the magnitude of shear key forces developed at movement joints between adjacent wharf segments. Different assumptions were made about pile stiffness, particularly recognising the differences resulting from unequal foundation stiffness and strength in the upslope and downslope directions. Simple design recommendations were verified or calibrated against the time-history results.

Mobile acoustic system for the detection of surface-breaking cracks in pavement

Monitoring the structural condition of road and airport pavement is an extremely critical task to ensure the safety and efficiency of teh transportation. The topic is relevant to both civil and military transportation infrastructure. The presence of damage in pavement, including surface cracking, depressions, swells, and wear, is inevitable due to the sever environmental and service loads that these structures must be subject to. Existing NDE techniques aimed at assessing the structural condition of pavement include Falling Weight Deflectometer, Ground Penetrating Radar, and acoustic methods based on surface waves. This paper presents improvements to the traditional surface-wave method for the detection of surface-breaking cracks in pavement. The advances include 1) the modeling of the problem as dipsersive waves propagating in a multilayer system, 2) the inclusion of post-processing algorithms based on the Wavelet Transform to improve the sensitivity and accuracy of the inspection, and 3) the use of non-contact, air-coupled acoustic detectors to enhance the mobility of the inspection unit. The crack detection procedure consists of first generating a dispersive wave with an impulse hammer, and then measuring the changes in velocity, amplitude and/or frequency content as the wave travels across the flaw with the aid of the Continuous Wavelet Transform. Multilayer wave propagation modeling provides a better understanding of the experimental results by predicting how the various frequencies interact with cracks of different depths. The results of field tests will be presented for both rigid (concrete-based) and flexible (bitumen-based) pavement with surface cracks.

Structural Health Monitoring of a Bridge with Energy Dissipators

After natural events like the 1994 Northridge (USA), the 1995 Kobe (Japan), the 1999 Chi‐Chi (Taiwan) and the 1999 Duzce (Turkey) earthquakes it became evident that the demand for bridge structures could greatly benefit from the application of isolation/energy dissipation techniques. Despite the level of maturity achieved in the field of seismic isolation, open questions still remain on the durability of seismic response modification devices (SRMD) under working conditions. The option of removal of sample devices from the bridge structure to verify their performance characteristics involves a significant economical effort, particularly if associated to disruption of the regular traffic. It provides also a device response verification difficult to correlate to the global structural performance. Health monitoring techniques offer a valuable alternative. The main objective of this research is the definition of an effective health monitoring approach to be applied to bridges protected with the most common sei...