Structural Engineering

Post Seismic Structural Health Monitoring of Bridges

Seyed. A. Bassam, PhD Candidate, Advisor: Dr. Farhad Ansari

CME StudentsThe focus of this study is on a quarter-scale test model of a four-span bridge frame supported on three shake tables. The experiments were performed at the University of Nevada, Reno shaketable test facility (Fig. 1). The bridge was subjected to the Northridge ground motion. The study consists of preliminary analytical studies of the system, construction of the model, instrumentation, data collection, and data processing. The design and installation of this system was prompted by need to design an automatic structural health monitoring system in a vast number of highway bridges and the need to present a quantitative assessment of structural damage.

The bridge was instrumented with FBG sensors specially designed to be mounted on the concrete surface (Fig. 2). Information regarding the change of structural characteristics was then obtained through visual observations, moment curvature analysis, spectral and also modal analysis from the acquired data. Depending on the location and severity of damage, modal parameters, including natural frequencies, damping ratios and mode shapes change in the structure.

The results provide a tool to predict the time and location of the formation of plastic hinge in the column.

Following general conclusions were drawn from the results:

1- The fiber optic sensors used in this study could survive even following the full scale seismic loads experienced by the bridge. Considering their high resolutions and high resistance in harsh environmental conditions, using these types of sensors is recommended for the health monitoring applications where high reliability of the measuring sensor is required.

2- Moment curvature diagrams obtained from the surface mounted FBG sensors could precisely detect the cracking of concrete, yielding of the steel and the formation of the plastic hinge.

3- Fiber optic sensors could obtain natural frequency of the column with great precision for use as a tool to measure the extent of the damage in the structure.

4- Strain Modal analysis provides a powerful tool to automatically determine the damage localization and its severity. Also, fewer sensors are required with respect to more traditional modal analysis methods.

Current Bridge Monitoring Research at the UIC

Iman Talebinejad and Chad Fisher, PhD Candidates, Advisor: Dr. Farhad Ansari

Sensors for monitoring bridges have received a great deal of media coverage following the recent bridge collapse in Minnesota. Such sensors are being researched and designed to monitor the performance of bridges and detect damaged locations. Sensors are a tremendous supplement to visual bridge inspections as they can detect damage in areas difficult to access by inspectors and can continuously monitor a bridge between periodic visual inspections. The Smart Sensors and NDT Laboratory, headed by Dr. Farhad Ansari, has developed fiber optic sensors that have been used to monitor the “health” of bridges and is continuing to research practical applications for their use. One such recent project was the monitoring of a cable supported pedestrian bridge.

CME StudentsA long span pedestrian bridge constructed to link the athlete village with the rest of the city was constructed in Torino, Italy for the 2006 Olympic Winter Games. The bridge design was quite complex and included a number of different structural elements and materials. For example, parts of bridge deck were supported by cables that were attached to an elliptical arch pylon that was set on a diagonal with cables at various levels. The bridge construction consisted of a steel framed superstructure, concrete deck, high tension steel cables and an arched pylon.

As a result of the complex bridge geometry along with performance and construction issues following its erection, access to the bridge was limited during the Olympic Games. Following the Games, it was decided to monitor the performance of the bridge cables and monitor the health of the bridge. The bridge cables were outfitted with long gauge fiber optic deformation sensors. The fiber optic sensors possess the capability for making dynamic real-time measurements. Aspects of the structural health monitoring involving dynamic modeling of the entire structure, the design and placement of the sensors and the analysis schemes for assessment of damage were researched. The bridge has since been re-opened for full use.

Mr. Talebinejad and Mr. Fischer are continuing their PhD research on upcoming bridge monitoring projects which include building a small scale bridge for monitoring and fatigue crack monitoring of a bridge in New York. They are working under the direction of Dr. Farhad Ansari. Mr. Talebinejad graduated from University of Tehran, Iran with his masters and bachelors degrees. Mr. Fischer attended the University of Illinois at Urbana-Champaign for his undergraduate and masters degrees and is also currently employed as a structural engineer with Engineering Systems Inc. in Aurora, Illinois.

Performance-Based Aspects and Structural Behavior of High Performance

Mohammad Abedalkareem Alhassan, Ph.D. Candidate, Advisor:Dr. Mohsen A. Issa

Bridge decks are deteriorating under repeated mechanical loading, shrinkage-induced stresses, thermal cycles, and environmental attack. Problems with corrosion of reinforcing steel and consequent spalling and delamination in reinforced concrete bridge decks are considerably intensified by the use of deicing salts. As a result, protective concrete overlays including latex-modified concrete (LMC) and microsilica concrete (MSC) overlays are being employed on bridge decks as part of rehabilitation and corrosion protection strategies. The overlay also provides an aesthetic product and a good riding quality. In addition to the typical advantages of the overlay; this study addresses the effect of bonded LMC and MSC overlays on the overall structural behavior of the bridge system and the advantages that can be gained from adding discontinuous synthetic and steel fibers to the LMC and MSC overlays.

This study encompassed extensive laboratory investigations and field application on a full scale prototype bridge system, 82 ft long and 18 ft wide with equal span lengths of 40 ft. As a result plain and fibrous LMC and MSC overlay mixtures were developed (plain LMC, LMC with synthetic fibers, LMC with steel fibers, plain MSC, MSC with synthetic fibers, and MSC with steel fibers) to meet target performance characteristics in terms of strength, permeability, hardened air-void system, bond strength, shrinkage, and crack arresting mechanism (toughness). Typically, the mixtures with synthetic fibers showed favorable performance over the mixtures with steel fibers due to the uniform distribution of synthetic fibers throughout the concrete, and LMC overlay mixtures showed superior performance over MSC overlay mixtures mainly in terms of permeability and shrinkage. The bond strength and the composite action between the overlay and the bridge deck were also evaluated under actual environmental exposures and full-scale low cycle fatigue load tests simulating AASHTO truck service load, overload, and ultimate load conditions. Following the laboratory and field studies, an innovative technique for casting LMC with synthetic fibers using mobile mixers is tried and proved to produce LMC with uniform distribution of the synthetic fibers and with high performance characteristics.

Experimental and Theoretical Behavior of Reinforced Concrete Beams and Columns Wrapped with CFRP-Composites

Rajai Alrousan, PhD Candidate, Advisor: Dr. Mohsen A. Issa

Many of the nation’s constructed facilities are in need of rehabilitation or strengthening due to their deterioration, aging, and underestimated design loads. To produce effective and durable repairs/strengthening projects, innovative construction materials with substantial bond characteristics, strength, and resistant to environmental exposures are required. This study deals with two major subjects: the application of carbon fiber reinforced polymer (CFRP)-composites for enhancing the shear strength capacities of reinforced concrete beams that are deficient in the shear behavior; and the use of the CFRP-composites for enhancing the ultimate axial strength capacity of the columns. The decrease in the costs of the composite materials as a result of the technology improvement has made the CFRP-composites an alternative to conventional materials with superior performance for many applications.

CME StudentsThe major parameters that were evaluated are the effect of the number of applied layers of CFRP-composites and the fiber alignment on the behavior of the test specimens. In the beams, the CFRP-composites were applied in different number of layers and in different configurations that include U-wrapping and strips at varied spacing and orientation. In the columns, the number of layers of CFRP-composites was the major varied parameter. The initial phase of the study included fabrication and testing of large number of beams and columns; control and strengthened specimens. A total of 28 reinforced concrete beams; 5 ft-long with a cross section of 6 in. x 9 in., and 18 circular reinforced concrete columns; 24 in.-long with 6 in.-diameter, were tested to failure under monotonic loading. The experimental test results were then used to create and validate three-dimensional nonlinear finite element models identical to the tested specimens. The exact geometries, reinforcement details, and materials properties were employed in the nonlinear finite element analysis (FEA). After reasonable validation, the FEA were expanded to study the effect of various critical parameters on the behavior of the specimens; such as the effect of the thickness of CFRP-composites and orientation as well as the effect of the concrete strength. The current stage of research includes providing rational models that are capable of predicting the shear capacity of CFRP-strengthened beams and ultimate axial strength capacity of CFRP-strengthened circular columns based on the number of layers of CFRP-composites, concrete strength, and composites orientation. In addition, guidelines will be established to allow for optimum selection of the CFRP-strengthening scheme. At this stage, preliminary models were proposed, and their effectiveness and accuracy were evaluated based on the available experimental and FEA results. The preliminary models showed encouraging results and good agreement with the results available in literature. With further validation and calibration, the objective is to generalize the models to cover any CFRP-strengthening scheme and any member geometry and materials properties. The results of the ongoing research on CFRP for the last eighteen years at UIC were used to perform actual rehabilitation on many projects and mainly the rehabilitation of three damaged precast/prestressed concrete bridges in the State of Illinois.

Topic revision: r5 - 2010-08-31 - 16:45:43 - Main.tmatthes
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