Structural Health Monitoring aims to characterize the performance of a structure from a combination of recorded sensor data and analytic techniques. Many methods are concerned with quantifying the elastic response of the structure, treating temperature changes as noise in the analysis. While these elastic profiles do demonstrate a portion of structural behavior, thermal loads on a structure can induce comparable strains to elastic loads. Understanding this relationship between the temperature of the structure and the resultant strain and displacement can provide in depth knowledge of the structural condition. A necessary parameter for this form of analysis is the Coefficient of Thermal Expansion (CTE). The CTE of a material relates the amount of expansion or contraction a material undergoes per degree change in temperature, and can be determined from temperature-strain relationship given that the thermal strain can be isolated. Many times with concrete, the actual amount of expansion with temperature in situ varies from the given values for the CTE due to thermally generated elastic strain, which complicates evaluation of the CTE. To accurately characterize the relationship between temperature and strain on a structure, the actual thermal behavior of the structure needs to be analyzed. This rate can vary for different parts of a structure, depending on boundary conditions. In a case of unrestrained structures, the strain in the structure should be linearly related to the temperature change. Thermal gradients in a structure can affect this relationship, as they induce curvature and deplanations in the cross section. This paper proposes a method that addresses these challenges in evaluating the CTE.
Prestressed concrete has been increasingly used in the construction of bridges due to its superiority as a building material. This has necessitated better assessment of its on-site performance. One of the most important indicators of structural integrity and performance of prestressed concrete structures is the spatial distribution of prestress forces over time, i.e. prestress losses along the structure. Time-dependent prestress losses occur due to dimensional changes in the concrete caused by creep and shrinkage, in addition to strand relaxation. Maintaining certain force levels in the strands, and thus the concrete cross-sections, is essential to ensuring stresses in the concrete do not exceed design stresses, which could cause malfunction or failure of the structure. This paper presents a novel method for monitoring prestress losses based on long-gauge fiber optic sensors embedded in the concrete during construction. The method includes the treatment of varying environmental factors such as temperature to ensure accuracy of results in on-site applications. The method is presented as applied to a segment of a post-tensioned pedestrian bridge on the Princeton University campus, Streicker Bridge. The segment is a three-span continuous girder supported on steel columns, with sensors embedded at key locations along the structure during construction in October 2009. Temperature and strain measurements have been recorded intermittently since construction. The prestress loss results are compared to estimates from design documents.
KEYWORDS: Sensors, Temperature metrology, Bridges, Fiber Bragg gratings, Data modeling, Temperature sensors, Sensor performance, Structural health monitoring, Civil engineering, Roads
Temperature monitoring has been of increased importance in recent years due to the need for temperature measurements in order to compensate other measurement parameters, such as strain, and the increased attention to understanding thermal behaviors of structures in order to assess their performance and condition. To ensure the accuracy of thermal compensation and study of thermal behavior, reliable long-term temperature measurements are required. In this paper, two methods that are aimed at validating long-term temperature measurements are created and their application is presented. The methods differ in the type of data they use for the purpose of validation. The first method relies on the existence of two independent temperature sensors at the same location. Validation is performed by comparing the measurements from the two sensors to one another, and discrepancies between the two data sets indicate malfunction or drift in at least one of the sensors. The second method is applicable to the more general case where only one temperature sensor is available at a given location. The method thus utilizes ambient temperature data from a nearby weather tower to validate measurements from the sensor. The two methods are applied to temperature measurements from FBG sensors installed on Streicker Bridge on the Princeton University campus. The methods successfully identified and characterized malfunction and drift in some of the sensors and confirmed stable measurements in other sensors.
Structural Health Monitoring seeks to characterize the performance of a structure from combinations of recorded sensor data and analytic techniques. Temperature is normally considered noise in this analysis, obstructing the goal measuring the elastic response of the structure. While these elastic loads do help characterize a portion of structural behavior, the thermal loads on a structure can induce comparable strains to these elastic loads. Characterizing a relationship between the temperature of the structure and the resultant strain and displacement can provide a deep understanding of the structural condition. In order to begin characterizing this 3-dimensional relationship, time periods with relatively steadystate, uniform temperature distributions need to be identified from the measured data. These periods of uniform temperature distribution in the structure show a thermal response as free as possible from thermal gradients across the structure. These steady-state periods help create a signature of the structure when analyzed with the relevant strain and displacement measurements of the structure. An algorithm for finding these uniform distributions was created to identify these desirable time periods with data of interest. Finding time periods with a completely uniform temperature distribution can be unreasonable, so a suitable temperature interval was chosen to produce a set of data with a reasonable approximation to a uniform distribution, while still providing a large enough set of data to produce meaningful results. These time intervals provide the necessary temperature, strain, and displacement measurements to characterize a signature for the structure, providing a more in-depth analysis in SHM.
Prestressed concrete experiences low to no tensile stresses, which results in limiting the occurrence of cracks in prestressed concrete structures. However, the nature of construction of these structures requires the concrete not to be subjected to the compressive force from the prestressing tendons until after it has gained sufficient compressive strength. Although the structure is not subjected to any dead or live load during this period, it is influenced by shrinkage and thermal variations. Thus, the concrete can experience tensile stresses before the required compressive strength has been attained, which can result in the occurrence of “pre-release” cracks. Such cracks are visually closed after the transfer of the prestressing force. However, structural capacity and behavior can be impacted if cracks are not sufficiently closed. This paper researches a method for the verification of the status of pre-release cracks after transfer of the prestressing force, and it is oriented towards achievement of Level IV Structural Health Monitoring (SHM). The method relies on measurements from parallel long-gauge fiber optic sensors embedded in the concrete prior to pouring. The same sensor network is used for the detection and characterization of cracks, as well as the monitoring of the prestressing force transfer and the determination of the extent of closure of pre-release cracks. This paper outlines the researched method and presents its application to a real-life structure, the southeast leg of Streicker Bridge on the Princeton University campus. The application structure is a curved continuous girder that was constructed in 2009. Its deck experienced four pre-release cracks that were closed beyond the critical limits based on the results of this study.
Streicker Bridge is a pedestrian bridge on the Princeton University campus. It consists of a main span and four curved
continuous girders (legs). The main span and the southeast leg of the bridge are equipped with fiber optic strain and
temperature sensors, allowing the bridge to also function as an on-campus laboratory for the Structural Health
Monitoring research group. Parallel sensors were embedded at critical cross-sections in the deck prior to the pouring of
concrete. The deck of the southeast leg experienced early age cracking within a few days of concrete pouring, which was
detected by the strain sensors. Post-tensioning was then performed and it is assumed that it closed off the cracks.
Evaluation of post-tensioning forces is complex due to the existence of the cracks, and this paper researches a procedure
to estimate the post-tensioning forces at cracked and uncracked locations. The obtained post-tensioning forces were
compared to design forces and conclusions regarding the status of post-tensioning were made. This is important as it
gives information on the actual health condition and performance of the structure. It also provides information on the
safety of the structure. The objective of this paper is to present a methodology for the evaluation of the post-tensioning
force along the deck based on strain measurements. The monitoring system, results, data analysis method, and
conclusions regarding the bridge health condition and performance are presented in this paper.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.