Structural Health Monitoring for Damage Detection Under Heavy Construction Loading

Transcription

1 Pavelchak and Williams Page Structural Health Monitoring for Damage Detection Under Heavy Construction Loading Revised Manuscript Submission Date: November, 0 Abstract: Paper Text:, Figures:,000 Total Word Count:, Authors: Matthew A. Pavelchak Graduate Engineer, Walter P. Moore and Associates, Inc. Diagnostics Group 0 McKinney, Suite 00, Houston, Texas 00 Phone: 0 0; Fax: 0 mpavelchak@walterpmoore.com (Corresponding Author) Mark E. Williams, Ph.D., P.E., S.E. Principal, Walter P. Moore and Associates, Inc. Diagnostics Group 0 McKinney, Suite 00, Houston, Texas 00 Phone: 0 ; Fax: 0 mwilliams@walterpmoore.com

2 Pavelchak and Williams Page 0 ABSTRACT Significant advancements in sensors and communication capabilities have increased interest in structural health monitoring as a cost effect method to monitor bridge performance and provide for pseudo real time damage detection. The objective of this paper is to present a case study of the use of structural health monitoring as part of a cost effective risk based approach to handling large construction loads on an existing bridge where lack of information prevented calculation of the bridge capacity. The bridge involved in this study was constructed in and provides access to a large privately held facility. As the 00 million dollar expansion and renovation of the existing facility was about to start, the contractor and owner identified the existing five span bridge providing the only access to this portion of the facility as a potential impediment to the fast pace construction schedule. Limited information was available regarding the reinforcement of the existing AASHTO Type I composite beams. Based upon the available information a bridge rating of HS0 could be obtained, but anticipated construction loads would significantly exceed the HS0 design vehicle. In order to maintain the construction schedule a pseudo real time structural health monitoring program was successfully implemented to monitor bridge performance under construction loading. The monitoring system consisting of MEMS inclinometers, a data logger, digital camera, and cellular modem allowed for pseudo real time remote monitoring of bridge performance with a built in alarm notification system.

3 Pavelchak and Williams Page 0 INTRODUCTION Assessment of safe live load capacity for existing bridges can be a significant challenge to owners and design professionals. It is not uncommon for information regarding original construction to be missing or incomplete. The effects of age related deterioration and distress as they relate to load carrying capacity are not easily quantified. Overly conservative bridge ratings can lead to load restrictions which can carry significant economic consequences for the areas served. This paper presents a case study of the successful use of Structural Health Monitoring (SHM) techniques for pseudo real time monitoring of bridge performance under heavy construction loadings which were anticipated to be well in excess of the bridge s conservative rated capacity. The structure instrumented in this study provides the sole means of access to a large privately held facility. The five span bridge was constructed in and handles two vehicular lanes over active railroad tracks (located in Span ) and a canal (located in Span ) as shown in Figure a and b, respectively. The bridge is considered mission critical to operation of the facility as it carries all traffic in and out of this portion of the facility and supports all principal utilities (carried on top of the bridge deck on either side of the traffic lanes) into the facility as shown in Figure. The rail line below the bridge is highly active with trains passing at approximately 0 minute intervals. (a) (b) Figure (a): Overview of Span over active railroad tracks (b): Overview of Span over canal 0 Figure : Typical bridge cross section

4 Pavelchak and Williams Page Limited information was available regarding the construction of the existing bridge which consists of AASHTO TYPE I beams with a composite cast in place concrete deck. The substructure consists of driven precast concrete piles with cast in place bents. The bridge has five equal spans of ft (. m) with a skew of approximately 0 degrees as shown in Figure. The available drawings depict the geometry of the bridge as well as typical detailing and reinforcement profiles for the cast in place elements. The drawing general notes indicate a design live loading of H0 S per AASHTO. No information regarding the mild reinforcement or prestressing of the precast beams was available for review. 0 0 Figure : Plan view of the bridge As the owner prepared to embark on an ambitious multi year expansion and renovation of the facility, the owner and general contractor quickly identified the access bridge as a primary project constraint. Due to the extensive construction planned which included significant levels of cut and fill operations as well as the use of large (and heavy) cranes, the bridge would have to carry a large number of heavy loads for construction to progress on schedule and in the most efficient manner. In conjunction with the need to carry a large number of heavy construction loads, the owner and contractor understood the severe economic consequences of the bridge use being impaired by damage from repeated construction loading. As a result a multifaceted bridge inspection, evaluation and instrumentation program was implemented to monitor bridge performance and maintain the project s construction schedule. BRIDGE INSPECTION AND LOAD RATING Prior to initiating the renovation project, a bridge inspection was performed based upon the National Bridge Inspection Standards (NBIS). The purpose of the bridge inspection was two fold: to indentify the current condition of the bridge for rating purposes and to document the condition of the bridge prior to initiation of heavy construction vehicle traffic over the bridge. The inspection arrived at a numerical condition rating for each of the primary bridge components (deck, superstructure, substructure, channel, and approaches). A number of maintenance items were also identified including minor cracking, spalling, distressed and clogged expansion joints, and end bearing anchor rods out of plumb. While the bridge inspection identified a number of maintenance items which needed to be addressed to maintain long term durability of the bridge, the spans were generally in good condition.

5 Pavelchak and Williams Page A review of available drawings indicated that the bridge was designed for H0 S per AASHTO. This vehicle rating was subsequently referred to as HS0 (). According to AASHTO, a bridge designed for HS0, which was the State Legal Load at the time of bridge construction, must carry a design vehicle of,000 lbs (, kg) with a maximum single axle load of 0,000 lbs (,0 kg) and a maximum distributed load of 0 psi (, kpa) (). This vehicle rating, also known as the Inventory Rating, was used as the basis for establishing a safe, routine load for the bridge. CONSTRUCTION LOADS The general contractor for the renovation project indicated that construction equipment unique to the project would likely exceed the pre established load rating. Certain construction equipment to be brought to the site would require special hauling permits. Some of the hauling equipment also had unique axle loading configurations. As such, structural calculations were conducted to determine the axle loads on the bridge for each type of heavy construction hauling equipment. Unfortunately, the maximum permissible live load that could be placed on the bridge, also known as the Operating Rating, was unknown due to a limited availability of original structural drawings. This dilemma left two options for consideration: load testing each type of expected heavy construction hauling equipment or pseudo real time remote monitoring of bridge performance with a built in alarm notification system. In order to not further delay the construction project, the general contractor and owner elected to implement a structural health monitoring program with the intent of reviewing the bridge performance as the construction loads traveled across the bridge. INSTRUMENTATION HARDWARE SETUP A structural health monitoring program was designed to measure the deflection characteristics of the bridge spans over time. Monitoring of deflections allowed for detection of deflection trends over time and isolation of inelastic deflections which might be associated with structural cracking, or other damage initiation. In planning the instrumentation program, a number of limitations existed which informed the selection of the hardware and software configurations. The hardware system needed to be installed on the existing structure without impacting vehicular traffic on the bridge or train traffic underneath the bridge. The available clearance between the underside of the superstructure and the top of the trains passing below was a major limitation, one that required the instrumentation to not extend more than a couple inches below the superstructure. After consideration of several instrumentation systems, digital MEMS (Micro Electrical Mechanical Systems) inclinometers were selected. These sensors have a range of ± from horizontal with a resolution of ±0.000 and can function under extreme environmental conditions within 0 to F ( 0 to C). The inclinometers were mounted on. inch ( m) rigid beams (this configuration is sometimes referred to as a tilt beam). This configuration returns an average tilt value over the length of the rigid beam. The selected inclinometers were fully digital sensors reducing the complexity of the data collection system and eliminating subsequent analog to digital conversions prior to data storage and transmission. The inclinometers return the sine of the angle of inclination which can be multiplied by the reference distance (in this case the known length of the rigid beam) to obtain the deflection of the

6 Pavelchak and Williams Page object to which it is mounted. Since the lengths of the bridge spans were all ft (. m), two. inch ( m) tilt beams were installed end to end starting at the edge of the beam seats as shown in Figure a. By summing the deflection of the two tilt beams installed on a given beam, the mid span deflection of that beam could be closely approximated as outlined in Figure b. 0 Figure (a): Tilt beam orientation (b): Deflection formula A total of inclinometers were used to instrument the bridge. As each span has six beam lines which support the vehicular lanes, every other beam was instrumented in each span with alternating beam lines instrumented in adjacent spans as shown in Figure. This configuration was selected to provide an overview of the superstructure behavior under the proposed construction loads while limiting the overall number of sensors. A more refined monitoring program could have been achieved by installing sensors on every beam which supports vehicular loads. However, in discussion with the bridge owner, they were willing to accept the greater level of uncertainty which occurs by monitoring every other beam line. Instrumentation of alternating beam lines resulted in a reduction in instrumentation costs and implementation time. Additionally, due to access restrictions, no sensors could be installed in Span (over a canal). Each instrumented beam was assigned a number as shown in Figure for identification purposes. The inclinometers were installed on the underside of the bottom flange of the beams as

7 Pavelchak and Williams Page shown in Figure. This orientation was required due to frequent diaphragm placement between beams which would have prevented placement of the inclinometers back to back on the web of the beams. The inclinometer and rigid beam configuration was only inches deep, which did not significantly impact the bridge under clearance. Figure : Instrumentation layout 0 Figure : Typical tilt beam installation The inclinometers were wired to a centrally located traffic control cabinet mounted on the edge of the bridge superstructure. Despite providing access to a large active facility, no temporary power or communications infrastructure was readily available in the vicinity of the bridge. Installing these services would have delayed the schedule and would have represented a major expense. Therefore, the

8 Pavelchak and Williams Page instrumentation had to rely on a battery system maintained by a solar panel mounted on the side of the bridge. Communication of data from the instrumentation setup to the engineering staff was made via cellular modem at regular intervals. The digital data logger, batteries, and cellular modem were installed in the traffic control cabinet and a digital camera was mounted to the exterior of the cabinet to allow for periodic photo documentation of loading events. A digital thermistor was also included in the instrumentation system to allow for correlation of results with changes in daily temperatures. Installation of the instrumentation system occurred primarily at night when traffic on the train line was minimal. The tilt beams which weigh approximately 0 lbs (. kg) were secured to the beams with shallow post installed concrete anchors at each end. Programming for the data logger was developed and uploaded prior to field deployment simplifying the installation process. Overall instrumentation of the bridge including configuration of the solar panel, cellular modem, and data logger took three days with a team of two field engineers. Obtaining access to the underside of the bridge beams and running of the instrumentation wiring represented the bulk of the installation time. PROGRAMMING AND PREPROCESSING Use of solar power and cellular communications were key parameters which impacted the programming developed for the structural health monitoring program. Operation of the cellular modem draws significant levels of power (well in excess of the power consumed by the data logger). Additionally, data transfer rates through cellular communications are relatively slow compared to land line communications. Therefore, the monitoring program was developed to limit the quantity of data which would need to be transmitted back to the offsite personnel while maintaining a system of alarms which would allow the project team to detect adverse vehicle loading conditions. Determination of the appropriate data collection rate is a key factor in designing and implementing a structural health monitoring program, and one that depends both on the goals of the project as well as the complexity of the system. In general structural health monitoring programs are designed to remain in place for long periods of time (data collection durations of 0 years are not uncommon). Over long durations enormous quantities of data can be generated by the sensors in the field, complicating data analysis and storage. Additionally, the cost of the data logger is typically the most expensive single component of the structural health monitoring system and the cost of data loggers generally increases with increased sampling rate capacity. Most data loggers function by reading one channel at a time and progressively cycle through all specified channels; therefore, the maximum collection rate for a data logger is inversely proportional to the number of sensors to be sampled. Commonly available data loggers can collect data on the order of,000 Hertz (Hz). These sampling rates can be required for quantification of dynamic behaviors such as structural load induced vibrations. The goal of the instrumentation system designed for this bridge was to monitor long term performance of the bridge by providing for timely detection of inelastic deflections. These goals placed the monitoring system in a semi static monitoring classification. Based upon the goals of the monitoring program and the limitations of the data logger, cellular communications, and the sensor count, a data sampling rate of 0. Hz was selected (each sensor sampled once every minutes). This monitoring

9 Pavelchak and Williams Page frequency allowed for timely damage detection; however, did not provide contemporaneous documentation of all elastic loading events. To reduce the quantity of data collected and transmitted to the monitoring team; the data logger was programmed to perform a number of preprocessing functions onboard. Deflection values were compiled (by addition of individual sensor deflections as depicted in Figure ), and compared to previous readings as well as threshold values within the data logger. If a data point was sampled which exceeded the preset threshold values, the data logger would closely monitor subsequent readings (for sampling cycles) to determine if the deflection was permanent. If the deflection exceeded the threshold throughout the waiting period, an alarm was triggered which would cause the system to power on the cellular modem and transmit the data to a base monitoring computer offsite which would receive the alarm and notify the engineers assigned to the project. Additionally, the system was programmed to capture a photograph of the bridge deck if an event above the threshold was detected. In the event that a threshold was not reached, the system would simply store the largest deflection reading made for each beam during 0 minute segments. Therefore, while a sampling rate of minutes was used, the data was significantly reduced in the field by recording the maximum of every ten readings. The system would then periodically cycle the cellular modem on and off throughout the day to transmit packets of data back to the base computer offsite. A number of alarm features were programmed to allow the data logger to monitor the health of the instrumentation system and alert the monitoring team to malfunctions of sensors as well as the health of the batteries. While the batteries selected for the project were designed for a long service life, like all batteries their performance decays over time eventually requiring replacement. The system was designed to monitor and report battery voltage regularly so that battery health could be monitored over time. This feature was particularly useful for preventing system outages, and to diagnose the likely cause of loss of system responsiveness by review of recent data. This system allowed for more efficient use of time. Should a sensor be damaged or the battery be depleted, staff could be dispatched with the appropriate parts and tools. During the monitoring program a sensor was damaged by bridge maintenance personnel and the problem was resolved more quickly within several days, due to the self diagnostic features of the system. The remote computer was setup in the consulting engineer s offices to continually collect and display the structural health monitoring data. This dedicated computer was configured with boot scripts which would automatically provide login credentials and launch the monitoring software whenever the computer was restarted for any reason such as temporary loss of power. The data logger was equipped with enough internal memory to hold several days of data at the specified collection frequency. In the event of loss of communication between the cellular modem and the remote PC, the data was buffered on the data logger and transmitted to the remote PC when communication was restored. MONITORING PROGRAM The monitoring program was conducted in two principal phases, at the onset of the construction project, the general contractor and subcontractors arranged to load test the bridge under typical

10 Pavelchak and Williams Page construction loads which were planned for the construction project. During this phase alternate programming was used to allow the project engineers to remotely monitor the bridge performance more closely with continuous data streaming. The construction loads were successfully detected; however, no permanent deflections were detected, and construction was allowed to progress as planned. After the initial load tests, the system was configured for long term monitoring (as discussed in the preceding section) and the incoming data stream was monitored on a regular basis by the monitoring team for a period of one year during the majority of the heavy construction loading. RESULTS AND FINDINGS As with most structural health monitoring projects, monitoring of temperature was essential to understanding the behavior of the structure over time. Initial data showed that over the course of a day the bridge experienced cyclical sweeping deflection changes. Based upon correlation with the thermal data collected it was concluded that these changes were a result of differential heating and cooling of the bridge deck during the course of a day which causes curling due to differential thermal expansion and contraction. The deflection results for a typical day are shown in Figure collected in May 00. Higher temperatures during the daytime hours generally produced negative deflections (upward deflections relative the time of instrumentation), while cooler night time temperatures produced positive deflections (downward deflections relative to the time of instrumentation). The overall magnitude of daily thermal deflection changes was typically on the order of 0. in to 0. in ( mm to 0 mm) throughout the course of the structural health monitoring program. In order to benchmark the collected data, hand calculations were performed to determine the predicted magnitude of thermal deflections based upon the design thermal gradient given by AASHTO Section.. Thermal Gradient, which showed close agreement with the observed values (). In addition to the temperature induced fluctuations, small elastic deviations can also be observed in the deflection data shown in Figure (one example highlighted by the dashed box). These small elastic deviations are associated with passing of vehicular traffic over the bridge and the bridge deflecting elastically and rebounding once the vehicle has passed. These elastic deflections typically occur simultaneously in multiple beams within the same span (as shown in the event highlighted by the dashed box) as the bridge beams distribute the applied loads.

11 Pavelchak and Williams Page B B B 0. Deflection (in) //00 :00 PM //00 :00 AM //00 :00 AM //00 :00 PM //00 :00 PM Time 0 Figure : Mid span deflection values for beams B, B, and B in span during a typical day While most of the monitoring program passed smoothly without significant incident one event of particular note was detected in which one of the beams (B as labeled in Figure ) experienced a permanent offset in its deflection value as shown in Figure and highlighted by a dashed box. The event occurred in the afternoon during the height of the construction activities. The sensors installed on beam B indicated an inelastic deflection of approximately 0. in (. mm) at midspan. After this event, the deflection for beam B rebounded slightly and then continued to follow the daily thermal trend with the new permanent offset. No other beams within the bridge experienced inelastic deflections during this loading event. The general contractor and owner were alerted to the potential for load induced damage to the bridge with specific interest in beam B. The contractor reported that during the incident in question cut and fill operations were underway and the bridge was carrying a number of loads of flowable fill into the jobsite. A team was dispatched to the bridge to inspect the span in question, and no significant cracking in the concrete beams was observed; however, the elastomeric pad supporting the beam was observed to be distressed and partially displaced which likely caused the offset in deflection values. Based upon the limited visual distress observed, the bridge was reopened to construction traffic without further incident. The damage to the elastomeric pad was reported to the owner for consideration during assessment of future bridge maintenance.

12 Pavelchak and Williams Page Deflection (in) //00 :00 PM //00 :00 AM //00 :00 AM //00 :00 PM //00 :00 PM 0 Time Figure : Mid span deflection results for instrumented beam B in span including inelastic loading event CONCLUSIONS The structural health monitoring program implemented for this bridge utilized commercially available inclinometers to provide an economical means of monitoring bridge performance. The system was able to be installed without any existing electrical or communications infrastructure at the bridge site within several working days. The system was configured with multiple alarms to allow the system to notify designated personnel of adverse events in a timely manner. All data was transmitted and stored offsite to minimize the need for personnel to visit the bridge. The preprocessing functions and alarm notifications also allowed for a significant reduction in the amount of data to be transmitted and stored while maintaining detection capabilities for adverse events. The use of structural health monitoring allowed for passage of a substantial number of construction loads in excess of the conservative rated capacity of the existing bridge while continually monitoring the bridge for damage. The monitoring system was more economical than the level of non destructive evaluation which would have been required to document the as built information which would have been required to analytically calculate the Operating Rating. Furthermore such an analysis would likely not have yielded the significant increase in the operational capacity necessary for passage of the construction loads. Through structural health monitoring construction progressed on schedule and the

13 Pavelchak and Williams Page potential for cumulative damage from construction loading was documented to protect the integrity of the bridge and keep the facility functioning throughout the construction project. REFERENCES. American Association of State, Highway, and Transportation Officials (AASHTO), LRFD Bridge Design Specifications, 0 Interim Revisions.. American Association of State, Highway, and Transportation Officials (AASHTO), Manual for Bridge Evaluation, st Edition, 00.