Kevin C. Womack

CONDITION ASSESSMENT OF FULL-SCALE BRIDGE BENTS: The Forced-Vibration Technique

Recent research has been conducted at Utah State University regarding the effects of various states of damage on the dynamic characteristics of reinforced concrete bridge bents with the objective of evaluating forced vibration testing as a non-destructive condition assessment tool. The Utah Department of Transportation is currently involved in a very large and intense highway project, the redesign and reconstruction of the I-15 corridor through Salt Lake City. The primary access to downtown Salt Lake from I-15 was the 600 South viaduct. This viaduct was a 2-lane, elevated roadway constructed primarily of reinforced concrete. During demolition, three reinforced concrete bents were left standing and intact. These bents served as the research specimens.

Condition assessement of a six-span full-scale bridge using forced vibration

Forced Vibration testing of four different damage states of a full scale, six span, reinforced concrete bridge, were conducted by Utah State University. These tests were performed to characterize the bridge based on its dynamic characteristics and to determine any correlation that may exist between the dynamic properties of a structure and the location of the inflicted damage. The test structure was a mature bridge being prepared for demolition as part of the Interstate 15 reconstruction through Salt Lake City. An eccentric mass shaker was utilized to excite the structure at several different natural frequencies for which data was collected through an array of seismometers. This data was processed to determine the natural frequencies and mode shapes of the structure. Investigation showed a decrease in the natural frequencies of the structure as well as noticeable changes in the mode shapes of the structure as a result of localized damage to the structure.

Determination of the Residual Prestress Force of In-Service Girders using Non-Destructive Testing

There continues to be a need for the accurate determination of the effective prestress force in precast, prestressed concrete bridge girders. In general, design codes lead to conservative estimations of in-service prestress forces which in turn can lead to permit and load posting requirements. The focus of this research is on the development of a nondestructive method to more accurately determine this effective prestress force. The research results are based on the testing of eight AASHTO Type II bridge girders that were in service for approximately 40 years. On average, the non-destructive tests results were within 94% of the results based on cracking tests; a technique that has more traditionally been used to directly determining residual prestress force. The residual prestress force was also compared with values obtained according to current code procedures. On average, the AASHTO LRFD-04 and 07 detailed methods overestimated the measured prestress force, by 12% by way of the cracking tests and 17% by way of the non-destructive tests. On average, the AASHTO Lump Sum Method agreed with measured residual prestress force obtained with the cracking tests and overestimated the measured values by 6% in comparison to the non-destructive tests. The details of the testing and proposed methodology are presented in this paper.

Full-scale reinforced concrete bridge bent condition assessment using forced vibration testing

Recent research has been conducted at Utah State University regarding the ability to assess the condition of a reinforced concrete bridge bent. Three full-scale, in-situ, reinforced concrete bridge bents were tested and modeled through varying states of damage. Each bent was initially tested in an undamaged state using a horizontal sine-sweep test. The forced-vibration testing was achieved using an eccentric mass shaker. The structures were tested in the frequencies ranging from 1.0 Hz to 20.0 Hz in increments of 0.05 Hz. A known amount of damage was inflicted upon each bent for two separate states. The sine-sweep test was re-administered for each damage state. The changes in dynamic characteristics, such as frequencies and mode shapes, were noted from state to state. Detailed finite element models were constructed to match the changes in dynamic characteristics of each bent for each state. This was achieved by matching the severity and location of the structural damage of the model to the field structures. Decreases in structural stiffness were detected with modal analysis. The models were consistent in matching the changes of the field structures. A method was devised for locating possible regions of structural damage.

Finite element modeling of a six-span bridge

Periodic inspection and testing of a structure is necessary to ensure its structural integrity and reliability. Visual inspection alone is not adequate to determine structural integrity. Internal damage and the damage in inaccessible areas are hard to detect in such investigations. Structural identification is a technique that may help to overcome these shortcomings. Developing the right analytical model of the structure plays a key role in ensuring that the model can be used in structural identification. Finite element modeling is the most commonly used tool in structural modeling. Finite element models usually have a high sensitivity to different structural parameters like stiffness, damping etc. Arriving at the correct values for these parameters is an important factor due to the models high sensitivity to these values. Different kinds of iterative optimization algorithms have been developed to arrive at these values. This paper discusses one such algorithm and shows the application of this algorithm to the finite element model of a six span bridge.

Modal analysis of a three-span curved steel girder bridge

The purpose of this study was to investigate the application of modal analysis to ascertain changes in the boundary conditions (or structural damage) of a complicated bridge structure. Reconstruction of the Interstate 15 corridor through Salt Lake City, Utah had provided an opportunity for destructive testing to be conducted on a three-span, continuous curved steel girder bridge. Forced Vibration testing using an eccentric mass shaker was conducted on the bridge in three phases. Each phase represented a change in boundary conditions. The initial testing was done with the bridge in the as-built condition with the continuous deck at the abutments and frozen bronze bearings. The second phase of testing occurred after the bridge deck was cut way from the approach slabs. For the third phase of testing, the bearings at the ends of the girders were replaced with teflon pads and the bearings over the two intermediate piers were jacked up and greased. The results of the study show that modal analysis is capable of determining changes in a structures natural frequencies and mode shapes due to a change in the boundary conditions. By extrapolation this would indicate that modal analysis would work as an effective non-destructive evaluation tool.

Use of velocity transducers in low-frequency modal testing of structures

One challenge in performing modal testing of structures at low frequencies is obtaining deflection sensors that provide reasonable output in this frequency range. Deflection sensors that require a fixed datum are generally not practical on full-scale structures. Sensors that use an inertial reference such as accelerometers and geophones are necessary. Velocity transducers are typically used at frequencies above their resonant frequency where their amplitude and phase response is flat. They can be used at frequencies near and below their resonant frequency. Thought, in this region, their response is frequency dependent. The advantage of using velocity transducers at low frequencies is that velocity is equal to displacement times frequency, the output of a velocity transducers falls off more gradually with decreasing frequency than the output of an accelerometer. Using velocity transducers at low frequencies requires characterization of the low frequency response of the transducer. A velocity transducers response can be modeled accurately as a single- degree-of-freedom, damped resonator. Laboratory methods of calibrating velocity transducers to determine the coefficients that describe their behavior are presented. Methods of correcting the amplitude and phase measurements to account for the transducers response are demonstrated. These procedures are illustrated on modal testing data from highway bridges.

Comparison of an impact source and a monochromatic source for modal testing of bridge structures

Modal testing requires imparting energy to a structure over a range of frequencies. This energy can be applied to the structure one frequency at a time using a monochromatic source, or a broadband energy source can apply energy at many frequencies simultaneously. An example of a monochromatic source is a rotating eccentric-mass shaker, and an example of a broadband source is an impact. A 250-kg, instrumented pendulum was developed at Utah State University to apply impulsive forces to bridge structures and measure the applied forcing function. This paper presents the basic design of this device. Data measured on three highway bridge bent structures are presented and the procedures used to analyze this data are presented. The results of testing with an impact source are compared to the results obtained using an 89-kN rotating eccentric-mass shaker. This paper shows that the rotating eccentric-mass shaker provides very high-quality data over a limited frequency range, while the 250-kg impulsive source provides somewhat poorer quality data, but over a much wider frequency band. The impulsive source also requires less testing time, and simpler data analysis than the rotating eccentric-mass shaker.