Charles Sikorsky

Field Verification of the Damage Index Method in a Concrete Box‐Girder Bridge via Visual Inspection

The structural condition of a concrete box-girder bridge is monitored twice by detecting and localizing potential damage in the bridge superstructure. Experimental field data were collected on the bridge in December 1997 and 9 months later in September 1998. Modal parameters for the structure are extracted from the measured frequency-response functions, and the resulting resonant frequencies and modeshapes are fed into a proven systems identification procedure. Modal parameters from the identified baseline structure and the modal parameters determined in the field are used as input to a field-tested nondestructive damage-evaluation method (the damage index method) to localize damage in the bridge superstructure. To provide some evidence of the veracity of the predictions of the possible damage locations, a visual inspection was performed on the bridge in May 1999, and surface cracks on the deck were recorded. A comparison of the predicted damage locations on the superstructure with the surface cracks documented via visual inspection is provided. The results indicate that a strong correlation exists between the predicted damage locations and the observed surface crack pattern.

Review and Comparison of Fracture Mechanics-based Bond Strength Models for FRP-strengthened Structures

This study presents a critical review on fracture mechanics-based bond strength models related to the use of externally bonded fiber reinforced polymer (FRP) composites in the rehabilitation of concrete. The predictions from a set of typically used bond strength models are compared with each other through a set of parametric studies. Given that the models use a diverse set of parameters, in order to enable an equitable comparison, parameters are carefully chosen as related to (a) characteristics of concrete, (b) characteristics of the FRP, and (c) characteristics related to concrete cover and epoxy mortar. The corresponding reliability and accuracy is also confirmed by comparing with test data. A comparison of some of the major models in this class with experimental data shows that the accuracy in prediction of Pmax and Le varies substantially with very few of the models actually incorporating the effects of critical bond parameters such as adhesive thickness and characteristics of the FRP–concrete interphase. Furthermore, the direct application of the results of these models into a sectional analysis provides an underestimation of the capacity of FRP-rehabilitated RC beams, with the exception of the ACI-440 approach which in most cases overpredicts results indicating its non-conservative nature. The use of procedures recommended in ACI-440 and as proposed herein are shown to provide a more accurate description, with the latter providing a means of determining the mode of failure through incorporation of the energetics of fracture interaction between cracking in concrete and redistribution of forces to the FRP. Critical aspects still needing study are also identified.

Use of Interwire Ultrasonic Leakage to Quantify Loss of Prestress in Multiwire Tendons

This paper presents a technique developed on the basis of ultrasonic guided waves to monitor prestress levels in multiwire prestressing strands. The transducer layout identified for stress measurement is composed of an ultrasound excitation provided by a piezoelectric element bonded on a peripheral wire. Ultrasound detection is performed on the central and peripheral wires at the strand’s end. The ultrasonic feature used for stress monitoring is the interwire leakage between the peripheral and the central wire, occurring across the strand anchorage. A semianalytical finite-element analysis is first used to predict modal and forced wave solutions in seven-wire strands as a function of the applied prestress level. The numerical analysis accounts for the changing interwire contact as a function of applied loads and predicts the attenuation occurring in loaded strand when the wave travels across the anchorage. Results of load monitoring in free strands during laboratory tests are then presented. Finally, a st...

Documentation of Changes in Modal Properties of a Concrete Box-Girder Bridge Due to Environmental and Internal Conditions

This paper presents the results of 2 modal tests performed on a concrete box-girder bridge. The 2 modal tests were performed sequentially with a 9-month time interval between tests. This sequence of tests form the initial phase of a modal-based level IV nondestructive damage-evaluation process implemented to establish the rate of structural deterioration and remaining service life of the tested structure. A level IV process detects, locates, and sizes damage and determines the impact of the damage on the performance of the structure. A novel feature of the field testing is the execution of an incremental and minimally intrusive (to traffic flow) modal test procedure. The instrumentation, experimental test plan, and resulting modal analysis are discussed. The changes in the 2 modal data sets are also discussed.

Web-Based Structural Health Monitoring of an FRP Composite Bridge

This paper describes how wireless technology was used in a structural health monitoring scheme to monitor the long-term performance of fiber reinforced polymer (FRP) composite bridge structures. The scheme was implemented on the Kings Stormwater Channel Bridge, located on a major state highway in California. The bridge was constructed using FRP composite girders and deck panels. The data collected by a comprehensive array of sensors are transmitted wirelessly, and processed in real-time remotely. Computer-based automated analysis algorithms process the incoming data to provide an assessment of structural response. Effects, due to time-based deterioration, and irregularities are determined using modal parameters, in terms of damage localization indices and an estimated damage severity. The results, made available via a web-based interface, enable appropriate action to be authorized for preliminary maintenance or emergency response prior to actual on-site inspection. This approach shows promise for monitoring and assessment of structural systems.

Long-term structural health monitoring system for a FRP composite highway bridge structure

This article discusses the implementation of a long-term structural health monitoring system for a FRP highway bridge using vibration-based monitoring techniques. The design and installation of such a system is prompted by the need to present a quantitative assessment of structural response to owners and operators in order to provide early warnings for major changes in response in the case of impending catastrophic failures. The bridge under discussion is instrumented with accelerometers as well as corresponding data acquisition and transmission equipment for automatic data collection. Ambient vibration based techniques are used to extract modal parameters from measured bridge response. Information regarding the change of internal structural characteristics is then obtained through a mode shape curvature based approach. Structural stiffness change is based on the analysis of collected data. A finite element model updating technique is used to provide an improved baseline model. The updated model is then used for simulation of damage effects.

Operational modal analysis for vibration-based structural health monitoring of civil structures

Abstract: Work to date on structural health monitoring systems for civil structures has been useful, but resembles existing bridge management systems. These management systems focus on processing collected data, but are unable to measure or evaluate the rate of structural deterioration for a specific bridge. In the last decade and a half or so, starting from the early 1990s, Operational modal analysis (OMA) has drawn significant attention in the civil engineering field as an attractive way to tackle this problem. OMA utilizes only response measurements of the structure under operational or ambient conditions to identify modal parameters. Compared with traditional experimental modal analysis (EMA), OMA does not require expensive excitation sources and can be applied to structures while they are in operation. The latter attribute is particularly attractive for vibration-based bridge health monitoring applications because the target bridge does not need to be closed to traffic to perform the modal parameter identification. This chapter discusses the use of OMA in long-term vibration-based structural health monitoring applications.

Structural Health Monitoring of CFRP Strengthened Bridge Decks Using Ambient Vibrations

In order to localize and quantify the effects of externally bonded carbon fiber reinforced polymer (CFRP) composites on a deteriorating reinforced concrete (RC) bridge structure, a vibration based monitoring approach is implemented. Modal data, including mode shapes of the deck slab, are extracted under the output-only configuration using time domain decomposition (TDD) techniques from ambient vibration tests conducted prior to rehabilitation, and at periodic intervals over a period of about two years after rehabilitation. Damage indices and fractional stiffness changes are determined by bay to enable assessment of CFRP rehabilitation effectiveness and response changes as a function of time.

Nonlinear ultrasonic guided waves for stress monitoring in prestressing tendons for post-tensioned concrete structures

Many bridges, including 90% of the California inventory, are post-tensioned box-girders concrete structures. Prestressing tendons are the main load-carrying components of these and other post-tensioned structures. Despite their criticality, much research is needed to develop and deploy techniques able to provide real-time information on the level of prestress in order to detect dangerous stress losses. In collaboration with Caltrans, UCSD is investigating the combination of ultrasonic guided waves and embedded sensors to provide both prestress level monitoring and defect detection capabilities in concrete-embedded PS tendons. This paper presents a technique based on nonlinear ultrasonic guided waves in the 100 kHz - 2 MHz range for monitoring prestress levels in 7-wire PS tendons. The technique relies on the fact that an axial stress on the tendon generates a proportional radial stress between adjacent wires (interwire stress). In turn, the interwire stress modulates nonlinear effects in ultrasonic wave propagation through both the presence of finite strains and the interwire contact. The nonlinear ultrasonic behavior of the tendon under changing levels of prestress is monitored by tracking higher-order harmonics at (nω) arising under a fundamental guided-wave excitation at (ω). Experimental results will be presented to identify (a) ranges of fundamental excitations at (ω) producing maximum nonlinear response, and (b) optimum lay-out of the transmitting and the receiving transducers within the test tendons. Compared to alternative methods based on linear ultrasonic features, the proposed nonlinear ultrasonic technique appears more sensitive to prestress levels and more robust against changing excitation power at the transmitting transducer or changing transducer/tendon bond conditions.

Load monitoring in multiwire strands by interwire ultrasonic measurements

Nearly 90% of the bridges in California are post-tensioned box-girders. Prestressing (PS) tendons are the main load-carrying components of these and other post-tensioned structures. Despite their criticality, much research is needed to develop and deploy techniques able to provide real-time information on the level of prestress and on the presence of structural defects (e.g. corrosion and broken wires) in the PS tendons. In collaboration with Caltrans, UCSD is investigating the combination of ultrasonic guided waves and embedded sensors as an approach to provide both prestress level monitoring and defect detection capabilities in concrete-embedded PS tendons. This paper will focus on the prestress level monitoring by first discussing the behavior of ultrasonic guided waves propagating in seven-wire, 0.6-in diameter twisted strands typically used in post-tensioned concrete structures. A semi-analytical finite element analysis is used to predict modal and forced wave solutions as a function of the applied prestress level. This analysis accounts for the changing inter-wire contact as a function of applied loads. A feature shown sensitive to load levels is the inter-wire energy leakage. In order to monitor such feature, the method uses low-profile piezoelectric sensors able to probe the individual, 0.2-in wires comprising the strand. Results of load monitoring in free and embedded strands during laboratory tests will be presented.