University of Cincinnati Infrastructure Institute

Introduction:

How it all began...

The University of Cincinnati Infrastructure Institute (UCII) research with instrumented bridge monitoring began in 1989 when displacement sensors were installed beneath a reinforced concrete slab bridge. The sensors measured the displacement of bridge for truck loads. UCII researchers further explored bridge instrumentation related issues in several subsequent research projects. In 1994, UCII researchers were given the opportunity to conduct a rigorous scientific investigation of issues associated with the design and implementation of a bridge monitoring system. A multi-disciplinary team of civil and electrical engineers were used for the project as it presented a need for a broad range of expertise.

A major thrust of this project was to conduct a comprehensive scientific study of the critical bridge responses and their corresponding causative effects for a typical steel-stringer bridge during fabrication and construction. A bridge's actual response and behavior during construction, combined with environmental effects is an essential component for understanding its overall life cycle behavior and performance. By examining a structure only after it is built, critical information about its stress state can only be estimated. Such information may prove critical to understanding the root causes of various defect, deterioration and damage mechanisms.

The need for this research...

This research is meant to complement and extend the work currently funded by the Ohio Department of Transportation, the Federal Highway Administration, the National Science Foundation and other agencies through the University of Cincinnati Infrastructure Institute (UCII). The UCII researchers have developed a rational, global condition assessment technique, which addresses the conceptualization and measurement of several unknowns for bridges, which include:

A lack of quantitative knowledge on the as-built state parameters (e.g., initial stresses, strains and displacements, local and global stiffness) and their variation over time;

A lack of clear and quantitative definitions for the performance parameters (e.g., functionality, serviceability, safety, lifecycle cost, etc.) and relationships between the state and performance parameters; and,

A lack of a clear and complete understanding of the phenomena which influence the state-of-force in a bridge; which lead to changes in state parameters; and/or which lead to a decrease in performance.

Fifteen steel-stringer, four steel truss, two reinforced concrete slab, and one fiber-reinforced polymer (FRP) deck on steel bridge have been tested nondestructively by a variety of techniques, while several of which were tested numerous times. Three decommissioned bridges (two trusses and one stringer bridge) were further tested to damage and failure. A global NDE method was developed based on the structural identification concept, and employing truckload testing, modal impact testing, and instrumented monitoring as its principal experimental tools. The test results are transformed to modal flexibility, which has been demonstrated to be a conceptual, quantitative, comprehensive, and damage-sensitive signature [Rubin, 1983]. Flexibility also provides a conceptual condition index, since it may be used to conveniently obtain the deflected shapes of a bridge under any loading pattern. Another such conceptual signature is the fundamental structural parameter of the unit influence line, the characteristic response at any instrumented bridge node due to the position of a unit load. Both testing approaches will provide best results under controlled loading conditions, which can be performed on a regular basis or in response to suspected or known damage. Ambient monitoring is conducted during the interim to track, document, and alert the bridge engineer to any gradual or sudden changes in these or component damage indices.

The scope and complexity of bridge condition assessment, however, does not permit a simplistic, reductional formulation. A holistic approach has been followed in the design and implementation of UCII research, and includes:

The strategies, concepts, and tools needed to confidently measure bridge state parameters;

The lessons learned from instrumented health-monitoring of actual bridge specimens;

The hardware, software, and analytical and experimental expertise required for linearized structural identification of complex bridges; and,

The concept of bridge-type-specific behavior mechanisms that have so far been identified for steel-stringer bridges.

Based on the principal attributes that influence state and performance, it is possible to rationally classify steel-stringer bridges into several groups of like behavior, each of which can then be represented through a statistical population of bridges. Once the statistical population is rigorously tested and studied, practical, type-specific global condition assessment procedures can then be developed for the whole group. The UCII researchers have extensively tested and monitored three steel-stringer bridges in Cincinnati and have just contracted with ODOT to test another thirty-four steel-stringer bridges throughout Ohio. Twenty-four of these bridges to be tested will be essentially in good condition but will cover a wide range of structural parameters, including number and length of spans, skew, non/composite design, integral/stub abutment, etc. in order for a more diverse database. The other ten bridges to be tested will be the lowest rated bridges of greatest concern in the state of Ohio.