Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
1998-05-02
2001-06-05
Kwok, Helen (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S579000
Reexamination Certificate
active
06240783
ABSTRACT:
TECHNICAL FIELD
This invention relates to monitoring systems, and more particularly to systems which monitor the structural integrity of bridges.
BACKGROUND ART
As known, bridges provide a structural pathway over depressions or obstacles to facilitate pedestrian and or vehicular and or railway travel over the obstacle. By far the bridges with the highest to frequency of use are the roadway bridges of the interstate and state highway systems, which have tens of thousands of vehicles crossing them each day. They are a critical part of the country's transportation infrastructure, and structural failure of a bridge creates a serious public safety risk as well as disruption of commerce.
State and Federal Departments of Transportation regulations require bridge safety inspections. These inspections have principally involved visual inspection of the bridge structure for signs of deterioration, such as corrosion, erosion, and signs of stress fatigue. However, the increased number of catastrophic bridge failures in the recent decade has prompted many state DOTs to begin conducting dynamic load testing of bridges to better evaluate their structural integrity.
The State of Alabama, for one, performs load tests on bridges which have been designated “load limited” due to actual traffic volume being larger than the theoretically allowable volume for the particular bridge or the bridge design. This testing program consists of instrumenting the bridge structure with strain gauges and/or linear variable displacement transformers, which are connected to a data acquisition system. Known static and dynamic loads are applied to the bridge using weighted DOT trucks which perform a sequence of maneuvers simulating actual traffic loads. The measured stresses (e.g. flexure of key structural members) are compared to the bridge's de sign values and a determination of structural integrity is made. If the bridge is unsafe it is closed, but in most instances the actual stress are far below design criteria due to conservative design calculations and the fact that materials used in the construction of the bridge often exceed specification. If, however, the bridge conditions change, such as an increase in traffic frequency, a higher proportion of heavy load vehicles, or more sever environmental changes, the operational life expectancy of the bridge changes accordingly.
Present methods of measuring bridge load capacity require extensive contact sensor or gage placement, long wire/cable routing, scaffolding, or cherry picker use to gain accessibility. These are labor intensive tasks which require special safety precautions to prevent worker injury. As a result these inspections can only be performed periodically. However, post analysis of structural failures indicates that bridges deteriorate in an unpredictable manner. In some cases structural deterioration may go unnoticed despite load tests due in part to the limited dynamics which may be observed with the staged loads. New methods and equipment are required to quantitatively measure the overall use (or real time) conditions of a bridge, and to correlate measured data to the bridge's condition of deterioration or change in load capacity with time.
DISCLOSURE OF INVENTION
One object of the present invention is to provide a bridge monitoring system for performing in situ monitoring of real time load dynamics of a bridge structure to quantify acceptable structural integrity characteristics which may be compared over time with actual bridge loads to determine structural degradation and thereby alert repair. Another object of the present invention is to provide a bridge monitoring system capable of providing non-contact measurement of structural loading of a bridge
According to the present invention a bridge monitoring system uses laser light reflected from structural members of a bridge being monitored to create velocity and displacement time signals of the bridge's vibratory response to quiescent conditions, and converts the sensed velocity time data to frequency domain data to provide a “signature” waveform for the bridge indicative of acceptable structural integrity characteristics of the bridge. In further accord with the present invention the bridge monitoring system obtains the velocity and displacement time signals without the need for sensor contact mounting with the bridge structure.
In still further accord with the present invention, the bridge monitoring system includes a site installed motion monitoring system for obtaining velocity time signal data of an on site bridge structure and for reporting the sensed data to a remote central analysis center which is responsive to sensed data from a plurality of site installed motion monitoring systems, thereby creating a centralized time history for a plurality of bridges in a jurisdiction.
The present invention provides a comprehensive bridge management system using noncontact techniques in obtaining real time data on the condition of a bridge, such as to allow the impacts and repercussions of bridge deterioration or failure to be dealt with in the most efficient, safe and cost effective manner.
These and other objects, features, and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying Drawing.
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“Technical Proposal for Global Bridge Evaluation and Monitoring”; Wiggs et al.; Jun. 6, 1994; pp. I-i to I-30.
Bishop Stephen Keith
McGugin Terry Curtis
Wiggs George Ashley
Friedland Norman
Kwok Helen
USBI, Co
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