Load rating and testing of highway bridges is important in determining the capacity of the structure. Experimental load rating utilizes strain transducers placed at critical locations of the superstructure to measure normal strains. These strains are then used in computing diagnostic performance measures (neutral axis of bending, load distribution factor) and ultimately a load rating. However, it has been shown that experimentally obtained strain measurements contain uncertainties associated with the accuracy and precision of the sensor and sensing system. These uncertainties propagate through to the diagnostic indicators that in turn transmit into the load rating calculation. This paper will analyze the effect that measurement uncertainties have on the experimental load rating results of a 3 span multi-girder/stringer steel and concrete bridge. The focus of this paper will be limited to the uncertainty associated with the experimental distribution factor estimate. For the testing discussed, strain readings were gathered at the midspan of each span of both exterior girders and the center girder. Test vehicles of known weight were positioned at specified locations on each span to generate maximum strain response for each of the five girders. The strain uncertainties were used in conjunction with a propagation formula developed by the authors to determine the standard uncertainty in the distribution factor estimates. This distribution factor uncertainty is then introduced into the load rating computation to determine the possible range of the load rating. The results show the importance of understanding measurement uncertainty in experimental load testing.
The U.S. infrastructure system is well behind the needs of the 21st century and in dire need of improvements. The American Society of Civil Engineers (ASCE) graded America’s Infrastructure as a “D+” in its recent 2013 Report Card. Bridges are a major component of the infrastructure system and were awarded a “C+”. Nearly 25 percent of the nation’s bridges are categorized as deficient by the Federal Highway Administration (FWHA). Most bridges were designed with an expected service life of roughly 50 years and today the average age of a bridge is 42 years. Finding alternative methods of condition assessment which captures the true performance of the bridge is of high importance. This paper discusses the monitoring of two multi-girder/stringer bridges at different ages of service life. Normal strain measurements were used to calculate the load distribution factor at the midspan of the bridge under controlled loading conditions. Controlled progressive damage was implemented to one of the superstructures to determine if the damage could be detected using the distribution factor. An uncertainty analysis, based on the accuracy and precision of the normal strain measurement, was undertaken to determine how effective it is to use the distribution factor measurement as a damage indicator. The analysis indicates that this load testing parameter may be an effective measure for detecting damage.
With the advent and development of low-cost wireless structural health monitoring systems, the task of routinely
assessing the in-service condition of highway bridges through distributed sensor-based measurements is an increasingly
feasible component of bridge safety and management practice. Bridge monitoring encompasses placement of often a
limited number of distributed sensors across a relatively large and complex structural system. Consequently, the
selection of proper sensor locations is imperative to extraction of the most value from the recorded measurements. An
experimental investigation is presented wherein sensor placement on the superstructure girders or primary beams is
contrasted to the response measured on the surface of the bridge deck. The effect on the dataset richness, as evidenced
by the modal content, is presented and conclusions regarding optimal placement for this structure type are presented. To
affirm the plausibility of the observed responses and conclusions drawn, a finite element analysis is also performed on a
model developed from the as-built drawings.
With nearly one quarter of today's highway bridges rated as structurally deficient or functionally obsolete, it is ever more
important to quantify the safe level of performance using in-situ structural health monitoring techniques. This paper
discusses the experimental testing of a single simply supported span composite superstructure under various controlled
progressive damage cases. The single span is part of a three span bridge located in northern New York. Using strain
transducers mounted at the midspan and support locations, changes in load distribution factors and neutral axis locations
are calculated to detect changes in load shedding behavior and structural capacity of the bridge span as diaphragm
connections were severed and support conditions were altered to different levels. The results of the testing show that
changes in the bridge response can be tracked as damage is introduced to the superstructure. The measurements are used
to identify the diagnostic load testing parameters that can be used as structural health indicators that can in turn be used
to complement current inspection protocols. Furthermore, the measurements also help provide a basis for future
development of a performance index to be used in conjunction with existing condition rating measures for improved
bridge condition assessment.
Discussed in this paper is the deployment of a universal and low-cost dense wireless sensor system for structural
monitoring, load rating and condition assessment of bridges. The wireless sensor system developed is designed
specifically for diagnostic bridge monitoring, providing independent conditioning for both accelerometers and strain
transducers in addition to high-rate wireless data transmission. The system was field deployed on a three span simply
supported bridge superstructure, where strain and acceleration measurements were obtained simultaneously and in realtime
at critical locations under several loading conditions, providing reliable quantitative information as to the actual
performance level of the bridge. Monitoring was also conducted as the bridge was subjected to various controlled
damage scenarios on the final day of testing. Select cases of detected damage using strain and modal based analysis are
presented.
With the increased demand placed on aging infrastructure, there is great interest in new condition assessment tools for
bridges. The routine deterioration that bridges undergo causes a loss in the intended performance that, if undetected or
unattended, can eventually lead to structural failure. Currently the primary method of bridge condition assessment
involves a qualitative bridge inspection routine based on visual observations. Discussed in this paper are methods of in-situ
quantitative bridge condition assessment using a dense wireless sensor array. At the core of the wireless system is
an integrated network which collects data from a variety of sensors in real-time and provides analysis, assessment and
decision-making tools. The advanced wireless sensor system, developed at Clarkson University for diagnostic bridge
monitoring, provides independent conditioning for both accelerometers and strain transducers with high-rate wireless
data transmission in a large-scale sensor network. Results from a field deployment of a dense wireless sensor network
on a bridge located in New York State are presented. The field deployment and testing aid to quantify the current bridge
response as well as demonstrate the ability of the system to perform bridge monitoring and condition assessment.
KEYWORDS: Bridges, Sensors, Sensor networks, Damage detection, Analog electronics, Digital filtering, Cements, Linear filtering, Instrument modeling, System identification
The development of low-cost wireless sensor networks has resulted in resurgence in the development of ambient
vibration monitoring methods to assess the in-service condition of highway bridges. However, a reliable approach
towards assessing the health of an in-service bridge and identifying and localizing damage without a priori knowledge of
the vibration response history has yet to be formulated. A two-part study is in progress to evaluate and develop existing
and proposed damage detection schemes. The first phase utilizes a laboratory bridge model to investigate the vibration
response characteristics induced through introduction of changes to structural members, connections, and support
conditions. A second phase of the study will validate the damage detection methods developed from the laboratory
testing with progressive damage testing of an in-service highway bridge scheduled for replacement. The laboratory
bridge features a four meter span, one meter wide, steel frame with a steel and cement board deck composed of sheet
layers to regulate mass loading and simulate deck wear. Bolted connections and elastomeric bearings provide a means
for prescribing variable local stiffness and damping effects to the laboratory model. A wireless sensor network
consisting of fifty-six accelerometers accommodated by twenty-eight local nodes facilitates simultaneous, real-time and
high-rate acquisition of the vibrations throughout the bridge structure. Measurement redundancy is provided by an array
of wired linear displacement sensors as well as a scanning laser vibrometer. This paper presents the laboratory model
and damage scenarios, a brief description of the developed wireless sensor network platform, an overview of available
test and measurement instrumentation within the laboratory, and baseline measurements of dynamic response of the
laboratory bridge model.
KEYWORDS: Sensors, Bridges, Transceivers, Sensor networks, Digital filtering, Structural health monitoring, Transducers, Microcontrollers, Data communications, Analog electronics
The introduction and development of wireless sensor network technology has resulted in rapid growth within the
field of structural health monitoring (SHM), as the dramatic cable costs associated with instrumentation of large
civil structures is potentially alleviated. Traditionally, condition assessment of bridge structures is accomplished
through the use of either vibration measurements or strain sensing. One approach is through quantifying dynamic
characteristics and mode shapes developed through the use of relatively dense arrays of accelerometers. Another
widely utilized method of condition assessment is bridge load rating, which is enabled through the use of strain
sensors. The Wireless Sensor Solution (WSS) developed specifically for diagnostic bridge monitoring provides a
hybrid system that interfaces with both accelerometers and strain sensors to facilitate vibration-based bridge
evaluation as well as load rating and static analysis on a universal platform.
This paper presents the development and testing of a wireless bridge monitoring system designed within the
Laboratory for Intelligent Infrastructure and Transportation Technologies (LIITT) at Clarkson University. The
system interfaces with low-cost MEMS accelerometers using custom signal conditioning for amplification and
filtering tailored to the spectrum of typical bridge vibrations, specifically from ambient excitation. Additionally, a
signal conditioning and high resolution ADC interface is provided for strain gauge sensors. To permit compensation
for the influence of temperature, thermistor-based temperature sensing is also enabled. In addition to the hardware
description, this paper presents features of the software applications and host interface developed for flexible, user-friendly
in-network control of and acquisition from the sensor nodes. The architecture of the software radio protocol
is also discussed along with results of field deployments including relatively large-scale networks and throughput
rates sufficient for bridge monitoring.
Discussed in this paper is the implementation of a wireless sensor system for performance monitoring of bridges.
The advanced wireless sensor system, developed at Clarkson University's Laboratory for Intelligent Infrastructure
and Transportation Technologies (LIITT), allows for structural monitoring of bridges. A short-span integral-abutment
bridge located in New York State is instrumented with a wireless sensor system measuring acceleration,
and strain to monitor the behavior of the structure under various loading conditions including ambient,
environmental and traffic loading. Strain and acceleration measurements are recorded simultaneously and in real
time to validate various performance characteristics of the bridge, including load distribution along an interior
girder, as well as additional stiffness factors (end fixity and composite action of the beams and bridge deck), using
existing bridge load testing and condition evaluation guidelines used by the New York State Department of
Transportation (NYSDOT) and American Association of State Highway and Transportation Officials (AASHTO).
Additionally, acceleration measurements are used to extract the superstructure's first five natural frequencies and
corresponding mode shapes. Results are compared to a developed Finite Element Method (FEM) model based on
the bridge as built drawings.
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