A. E. Aktan

Bridge-condition assessment by modal flexibility

A bridge-condition assessment method for evaluating the global state of health is formulated based on modal flexibility directly obtained by processing modal test data, complemented by structural identification. The method is proven and the experimental and analytical tools that have been developed for implementation are demonstrated on a three-span reinforced-concrete high-way bridge.

Experimental Vibration Analysis for Structural Identification of a Long-Span Suspension Bridge

Structural identification (St-Id) of long-span bridges by ambient vibration testing provides a starting point for quantitatively characterizing the actual in-service mechanical characteristics and behaviors of these complex constructed systems. However, various uncertainties involved in the experimental and identification processes impact the reliability of St-Id, especially if vibration testing is the sole experiment. Such uncertainties represent perhaps the most fundamental barrier to more widespread applications of measurements in civil engineering practice. The goal of this paper is to leverage a vibration test of a long-span suspension bridge to illustrate a number of possible strategies for coping with the uncertainties confronted when identifying the dynamic characteristics of large-scale constructed systems. The design and implementation of a field test in the context of St-Id are first presented to illustrate how uncertainties can be mitigated by following a disciplined approach to designing the experiment. Next, data preprocessing strategies including data inspection, time window selection, band-pass filtering, averaging, and windowing are proposed to reduce data errors. Three separate postprocessing methods, including Peak Picking, PolyMAX, and Complex Mode Indicator Function, are executed independently to verify the reliability of the data processing results. Experimental results for both the towers and suspended spans are correlated with simulations from three-dimensional finite-element analysis of the long-span bridge for St-Id.

Structural identification: Opportunities and challenges

Civil engineeringmaster builders havebeen constructingmasterpieces for millennia, long before the recent advent of systems engineering. However, since the 1950s, the planning, financing, design, construction, operation, and maintenance of civil engineered constructed systems, e.g., buildings, bridges, airports, plants, tunnels, dams, antenna towers, storage tanks, power transmission towers, highways, railroads, and pipelines, became the elements of highly complex, intertwined, and interdependent systems in dense urban areas. Such highly complex and multidomain systems, termed infrastructures, include government, education, healthcare, transportation, water, communication, and energy [U.S. Department of Homeland Security (DHS) 2010]. As urban populations grew, demands for infrastructure services increased. Meanwhile, the engineered elements of infrastructures aged and deteriorated, and their operational and structural capacity started to fall short of the demands. Their fragility was recognized as the failure of one infrastructure element that precipitated cascading consequential failures of additional elements from different infrastructures. Failures of critical infrastructure because of natural or manufactured hazards reiterate this connectivity. For example, on January 2, 1998, “a century-oldwatermain ruptured under lower FifthAvenue in New York City, creating a car-swallowing, curb-to-curb sinkhole andwatery chaos in a bustling neighborhoodwhose streets resembled Venice for a few hours. Then, as the rivers receded, a gas main broke and the crater spewed forth a tower of orange flames. No one was injured . . . but water damaged scores of lobbies, storefronts, and basements for blocks around, 40 residents were evacuated, hundreds of offices and businesses were closed, subways were halted, traffic was rerouted, and gas, water, electric, steam heat, and telephone services were disrupted for many” (McFadden 1998). Three infamous 21st century examples further demonstrate the unexpected cascading consequences of infrastructure failure: • In the case of the World Trade Center collapse on September 11, 2001, while airplane impact was a design consideration for the Towers, the consequential explosion and fire associated with an airplane impact were neglected in the design. Catastrophic and disproportionate collapse of the Towers because of fire in the upper floorswas completely unexpected. TheNIST investigation (NIST 2005) into the collapses led to new code provisions. • In the city of New Orleans on August 31, 2005, the storm surge because of Hurricane Katrina caused more than 50 breaches in drainage canal levees and navigational canal levees and precipitated the worst engineering disaster in the history of the United States. Such an event had been expected, but neither the consequences nor the preparation needed for effective emergency response were properly estimated (ASCE 2007). • An hour after the March 11, 2011 Tohoku earthquake off the coast of Japan, the tsunami wave breached the protective walls at the Fukushima Nuclear Power Plant and destroyed backup diesel power systems, leading to partial meltdowns at several reactors. The diesel generators were situated in a low spot on the assumption that the tsunami walls were high enough to protect against any likely tsunami. Subsequently, ancient stone markers indicating higher tsunami events were reported (Associated Press 2011).

Subjective and Objective Evaluations of Bridge Damage

A decommissioned, 40+-year-old reinforced concrete deck on a steel girder bridge was subjected to a series of induced damages, nondestructive field tests, and visual evaluations to compare objective and subjective methods of bridge-condition assessment. Prior subjective evaluations of bridge condition often produced highly variable results. For example, inspectors with different backgrounds and field experience disagreed on how severely certain forms of deterioration and damage influenced bridge behavior and safety so, consequently, different assessments of bridge condition were generated. Furthermore, a load rating of the “as-is” state of the bridge (e.g., the state before any induced damages), according to current Ohio Department of Transportation procedures, indicated that the bridge could only support truckloads of 227 804 N (51,192 lbf). However, the objective data acquired during nondestructive field testing of the bridge, which was subjected to truckloads of 282 130 N (63,400 lbf), revealed maximum superstructure deflections and live-load stresses of 0.190 cm (0.075 in.) and 15 985 kPa (2,320 psi)—values well within AASHTO limits. These values also imply that the bridge can support loads much greater than those indicated in the load rating. Comparing subjective and objective assessments for the induced damage scenarios yielded similar results. Essentially, data revealed that subjective methods of bridge evaluation and assessment were unable to properly characterize intrinsic bridge mechanisms and the influence that such mechanisms have on bridge behavior. Condition assessment of a typical reinforced concrete deck on a steel girder bridge should therefore include objective evaluations of bridge condition and behavior.

Seismic Vulnerability Evaluation of Existing Buildings

A study of building seismic vulnerability evaluation methods in the U.S. and Japan lead to a constructive critique of the FEMA-178. Research is formulated to extend FEMA-178 so that types of construction which have not yet experienced damaging earthquakes may be reliably evaluated. An element-level analytical model accurately simulating the critical response and failure mechanisms of the soil-foundation-structure in its current state emerges as a key. Developing such a model may prove difficult in case a facility has unusual and/or irregular attributes as well as damage and/or deterioration. Researchers efforts to construct and experimentally identify a 3D analytical model of a 27-story RC flat-plate building with closed thin-walled core systems are described. Forced-excitation modal testing is explored and is shown to be a reliable tool. It is exemplified that structural identification as well as other nondestructive evaluation tools may become essential for reliably evaluating seismic vulnerability of construction which have not yet revealed all of their “weak-links”.

Active vibration control of a 250 foot span steel truss highway bridge

This paper describes a full scale implementation of an active vibration control system on a 250 foot span steel truss highway bridge. An adaptive modal filter based modal control scheme is utilized in conjunction with a prototype long stroke electromagnetic reaction mass control actuator to achieve up to 20 db attenuation of modal response amplitudes.<<ETX>>

Condition and Reliability Assessment of Constructed Facilities

Nondestructive and destructive dynamic field testing and structural identification studies on actual constructed facilities are presented. The specimens discussed here include a 27-story RC flat-slab building, an RC-slab bridge, two 80-year-old steel-truss bridges, and three RC-slab on steel-girder bridges of various ages. The seismic vulnerability of the mid-rise building was evaluated and the test bridges were rated by code procedures as well as by field-calibrated comprehensive 3-D FE models developed by structural identification. Experimentally measured and analytically simulated modal flexibilities of the bridges were correlated with deflections obtained under proof-load-level truck-load tests. The rating factors obtained by field-calibrated models exceeded the corresponding operating rating factors by two and a half to four times for all of the test bridges.

Structural Identification of a Long-Span Truss Bridge

An ongoing research project involving structural identification of the Commodore Barry Bridge, a major long-span truss bridge over the Delaware River, is described. Structural identification is an approach in which a constructed facility and its loading environment are objectively characterized by field observations, measurements, and controlled experiments in conjunction with an analytical model. This process is a necessary precursor to performing health monitoring of the bridge. Long-span bridges have attributes that make utilization of experimental and analytical techniques on them quite different than for short-span bridges. The concept of structural identification and the methods used in applying it to a long-span bridge are presented and discussed. The structural characteristics of the bridge are described and conceptualized. Development of the three-dimensional analytical model and the model characteristics are summarized. Static and dynamic analyses are conducted to help locate anomalies and errors in the model. The experimental techniques necessary for structural identification of a long-span bridge are defined. A limited-scale health-monitoring system, which integrates operational data with structural performance and loading environment data, was designed and installed on the bridge. Mechanical and electrical characteristics of the monitor system and issues related to management of the data from this system are discussed. The monitoring system currently has over 80 channels of different sensor types collecting various data from the bridge. In addition, data from the system can be viewed from a remote location in real time.

Structural identification for condition assessment of civil infrastructure

The experimental aspects of structural identification for global and local condition assessment of constructed facilities are summarized. Experimental techniques suitable for meaningful field testing have been explored on an operating reinforced concrete deck-on-steel stringer highway bridge which served as a test bed. Modal analysis and instrumented monitoring applications conducted on this bridge serve to exemplify the experimental steps of a possible global structural identification methodology for health monitoring of civil infrastructure. The results given indicate that this is a feasible approach to for condition assessment and structural health monitoring.

Issues in implementation of structural monitoring to constructed facilities for serviceability with damageability considerations

A comprehensive market survey and laboratory evaluation was conducted for the Ohio Department of Transportation and the Federal Highway Administration to identify the most promising sensors and data-acquisition systems for infrastructure application. A pilot system for highway bridge monitoring was implemented on a typical steel-stringer bridge in Cincinnati for high-speed traffic and long-term environmental monitoring. Static tests were performed with known truck loads to confirm monitoring results and to calibrate finite-element and section analysis models of the bridge. A complete and accurate characterization of the as-is structural condition has been related to the structural capacities and reliability. This multi-disciplinary research improves understanding of the actual loading environment and the corresponding bridge responses. Instrumented monitoring is expected to complement inspection methods, provide an objective measure of the state-of-health, and alert officials to bridge deterioration or failure.