Remote Monitoring of Soil Pressures on Bridge Footings

Transcription

1 More Info at Open Access Database Remote Monitoring of Soil Pressures on Bridge Footings Michael DAVIDSON 1, Zhihui ZHU 2, Issam HARIK 3, Charlie SUN 4, Kevin SANDEFUR 5 1 Bridge Software Institute, University of Florida, Gainesville, Florida, USA; Phone: x1551, michael@ce.ufl.edu 2 School of Civil Engineering, Central South University, Changsha, China; zzhh0703@163.com 3 Department of Civil Engineering, University of Kentucky, Lexington, Kentucky, USA; Harik@uky.edu 4 Kentucky Transportation Center, University of Kentucky, Lexington, Kentucky, USA; charlie.sun@uky.edu 5 Division of Structural Design, Kentucky Transportation Cabinet, Frankfort, Kentucky, USA; Kevin.Sandefur@ky.gov Abstract Little consideration has previously been given to explicitly quantifying the effect that thermal stresses have on foundation systems associated with intermediate bridge piers on integral abutment bridges. As part of a larger research effort aimed at characterizing temperature-induced structural demands that can arise in these types of bridge foundation systems, a multi-span integral abutment bridge has been instrumented with temperature and foundation-pressure monitoring devices. Data generated at the bridge site was monitored remotely in real-time over the period May 2011 to May 2014, and can be viewed by the public at a dedicated website. Discussed in this paper are instrumentation devices and the installation procedures involved in establishing a remote monitoring system for the selected bridge. Additionally, an overview of the remote-monitoring website is given. The research efforts discussed herein enable elucidation, via remote monitoring, of a relatively unknown facet of bridge foundation behavior. Keywords: Thermal stresses, concrete bridge, prestressed beams, foundations, bridge pier. 1 Introduction Infrastructure containing structural members that are either partially or fully restrained against motion can develop internal stresses when those members are subjected to changes in temperature. Bridge structures constitute a substantial portion of the U.S. infrastructure, and are regularly subjected to significant temperature changes. Due to partial or full restraint conditions that are typically incorporated into the design of bridge superstructure systems (e.g., diaphragms, fixed-bearings), temperature changes in members such as bridge decks and superstructure girders can lead to the development of internal stresses, which can in turn induce stresses throughout underlying bridge piers, into foundation systems, and ultimately into the underlying soil. The American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications [1] contains provisions for determining superstructure temperature load effects on overall bridge design. Furthermore, for construction types such as integral abutment bridges, the design of intermediate piers can be strongly influenced or even primarily controlled by the AASHTO thermal loads requirements. However, little consideration has been given to explicitly quantifying the effect that thermal stresses have on the foundations of these intermediate piers. Therefore, it is critical that accurate estimates of the thermal loads (as translated to effects on foundation members) can be achieved so as to ensure proper design.

2 With the cooperation of bridge owners in the state of Kentucky (the Kentucky Transportation Cabinet, KyTC), and given the particular need for determining the impact that thermal stresses in continuous superstructures can have on the foundations of intermediate piers, a multi-span, integral abutment bridge was selected, fitted with instrumentation, and subjected to remote monitoring. Namely, foundation pressures of the New Trammel Creek Bridge on KY-100 in Allen County, in southwest Kentucky, USA are investigated in the current study. Over a period of three years (from May 2011 through May 2014), data was collected from the bridge site. Instrumentation readings are available for viewing in real-time on a website (discussed later) that is maintained by the Kentucky Transportation Center (KTC), housed at the University of Kentucky in Lexington, Kentucky, USA. 1.1 Background and Motivation International Symposium The phenomenon of thermal stresses that originate at the superstructure level, and in turn, pervade throughout bridge structures has been previously investigated for integral abutment bridges, as reported in the literature. For example, Arsoy et al. [2] investigated the effect that bridge temperature changes had on the motion and forces in abutment walls. Paul et al. [3] carried out an analytical investigation to quantify the effects of uniform superstructure temperature loading on stresses that develop in superstructure girders and abutment walls and piles. Kim and Laman [4] carried out an analytical study using a bridge finite element model, which was subjected to superstructure temperature loading, to ascertain those parameters that have the highest impact on internal forces that develop in abutment-supporting piles. While there have been numerous studies that have focused on the effect of superstructure temperature changes in relation to abutment foundation forces, very few studies have been undertaken to examine the effects that those same temperature changes can have on intermediate piers that are placed along internal spans of integral abutment bridges. 1.2 Objective The overall objective of this study is to make a comparative analysis between measurements of temperature-induced soil pressures with those pressures derived using the AASHTO design provisions. A major component in fulfilling this objective has been to instrument the New Trammel Creek Bridge with temperature, bridge pier inclination, and pressure cell instrumentation. Additionally, remote monitoring has been initiated, where data are continually updated and published to a dedicated website. The overall project undertakings and findings are reported in Zhu et al. [5]. In the current paper, the investigative efforts made toward instrumenting the bridge footings, and the layout of the remote monitoring website are detailed. 2 New Trammel Creek Bridge at KY-100 in Allen County, Kentucky 2.1 Bridge Layout The bridge selected for instrumentation as part of this study, the New Trammel Creek Bridge over KY-100 in Allen Co., is located within the state of Kentucky in the southeast United States. The bridge spans Trammel Creek, where under ordinary flow conditions, the stream edges are bounded between the two leftmost intermediate piers (Figure 1). The bridge is of integral abutment construction with monolithically cast bridge deck, stem caps, and wing

3 walls located at each of the bridge far ends. Additionally, the bridge contains three internal (intermediate) piers that aid in supporting the bridge roadway. In turn, the roadway consists of four spans. Each span of the two-lane concrete slab deck is supported by six prestressed concrete girders of varying reinforcement configurations. Figure 1. Structural configuration as excerpted from structural drawings In total, the bridge is 123 m long. Each of the three uniformly distributed bridge piers are spaced at 36.6 m from one another, and the outermost piers (Pier 1 and Pier 3) are located 24.4 m from the bridge abutments (Figure 2). The two integral end abutments are each supported by a row of driven h-piles. The intermediate piers contain three columns and partial height shear walls, where each of the pier columns is supported by a shallow foundation reinforced concrete (spread) footing. Interior Spans 3660 mm End Spans 2440 mm Pier 3 Pier Bridge Footings Figure 2. Structural configuration as constructed The spread footings supporting each pier contain concrete with 27.6 MPa 28-day compressive strength (Figure 3). The footings are oriented consistently with the overlying piers, parallel to the bridge span longitudinal direction, and with right-skew angles of 25. Each footing is 3.7 m square in plan, and 1.1 m thick. Pier columns extend directly upward

4 from, and are centered over, the footings. As discussed in detail below, Pier 1 and Pier 3 footings were fitted with pressure cell instrumentation to facilitate completion of the study objective. These footings rest on a 0.23 m thick layer of #10 crushed stone, and fill concrete is placed around the perimeter of the excavation pits up to the footing half-thickness (0.55 m). The non-instrumented footings rest directly on rock. Figure 3. Construction of Pier 1 spread footings (prior to placement of soil fill) 3 Instrumentation 3.1 Pressure Cells As shown in Figure 4, the southwest footing of Pier 1 and the northeast footing of Pier 3 were selected to be fitted with instrumentation to measure (in real-time) pressures that develop immediately beneath the two footings. In each instrumented footing, seven pressure cells were embedded 7.6 cm below the top of the No. 10 crushed stone layer. The pressure cell array is shown in Figure 5 for the selected footing of Pier 1. A similar layout was employed for the pressure cells installed beneath the selected footing of Pier 3. Each pressure cell was fitted with a data transmission cable, and all cables were fed into a 3.8 cm diameter conduit at a mid-corner of the footing gravel base. The conduit extends to a weather-resistant enclosure containing remote-monitoring data acquisition hardware. Following placement of the pressure cells, an additional 3 cm of crushed stone was placed. The 23 cm layer of crushed stone (containing the embedded pressure cells) was then leveled, compacted, and covered using a Type I geotextile fabric cover. Subsequently, the reinforced concrete footing was constructed using standard construction techniques. 3.2 Data Record As installed, the remote-monitoring data acquisition hardware was used in reading the pressure cells at 5 min. intervals over a period of 3 years (May 2011 to May 2014). Pressure measurements were (and continue to be) continuously cataloged on servers housed by the KTC. Shown in Figure 6 is the currently available data record for the centrally positioned

5 pressure cell located beneath the selected footing of Pier 1. Please refer to Zhu et al. [5] for a detailed presentation of the temperature, bridge inclination, and foundation pressure data records. Figure 4. Plan view of pier foundation with indication of instrumented footings Figure 5. Placement of pressure cells and data transmission cables beneath the Pier 1 footing 4 Remote Monitoring The current study has been undertaken as part of a larger effort to establish remote bridge monitoring capabilities for numerous, pertinent bridge sites distributed throughout the state of Kentucky, USA. As an integral facet of the remote monitoring functions, a dedicated website has been developed to allow bridge Owners, researchers, and members of the general public to have access to data that are continually being recorded at the numerous bridge sites. Providing access to these datasets enables vested parties (researchers, Owners) to remotely assess the activity of the instrumented bridges. Additionally, the web interfaces that have been developed act as a powerful tool in allowing the public to maintain research-product transparency and to engage in educational opportunities in exploring the real-time, in-service

6 behavior of bridge structures. Presented below is a brief overview of the collective web pages that are dedicated to the instrumented bridges. Additionally, the website dedicated to monitoring of temperatures, motions, and foundation pressures in the piers of the New Trammel Creek Bridge is identified. Figure 6. Record of foundation pressures at centrally located pressure cell of Pier 1 footing (note: reading No. 0 corresponds to May 11, 2011) 4.1 Remote Monitoring Website for Bridges in Kentucky, USA Remote monitoring studies in addition to that associated with monitoring of the New Trammel Creek Bridge are underway for a total of six bridge locations. For each of these six bridge sites, instrumentation and remote monitoring hardware have been installed to monitor a wide variety of relevant bridge-response data. By accessing the web link, interested parties are presented with a dynamic, interactive map at the following site: Using the interactive map, individual-bridge websites can, in turn, be accessed. Relevant to the current study is the bridge site highlighted in Figure 7, which links directly to the website dedicated entirely to the remote monitoring of the New Trammel Creek Bridge. 4.2 Remote Monitoring Website for New Trammel Creek Bridge Upon accessing the interactive map link indicated in Figure 7, users are directed to the website dedicated to monitoring of the New Trammel Creek Bridge: The home page for this specific remote monitoring effort is shown in Figure 8. A detailed walkthrough of the website is given in Zhu et al. [5].

7 Interactive map link to the New Trammel Creek Bridge website. Figure 7. Homepage of the Remote Bridge Monitoring in KY website Figure 8. Homepage of the website dedicated to monitoring of the New Trammel Creek Bridge The bridge-specific website contains background information related to the bridge location and structural configuration. Additionally, an overview of the study objective and the relevant AASHTO temperature loading provisions is available for review. Also, users may click on the links provided on the homepage to access all historical temperature, tiltmeter, and pressure cell data, as well as more recent and real-time data. Further, the data are available for examination and comparison to estimates of the corresponding AASHTO estimates of foundation pressures for Pier 1 and Pier 3 of the New Trammel Creek Bridge via a dynamic plotting interface. Please refer to the bridge-specific website listed above or Zhu et al. [5] for further details.

8 5 Summary and Concluding Remarks The overall objective of the current study was to determine the effects of bridge superstructure temperature changes on bridge substructure response. In particular, for a selected multi-span integral abutment bridge located in southwestern Kentucky in the United States, pressure cell instrumentation, bridge pier inclination measuring devices, and temperature measuring devices have been placed throughout selected piers. These devices have been installed in conjunction with solar-powered data acquisition boxes that facilitate remote monitoring of the in-service bridge foundation response to temperature changes at the superstructure level. The emphasis of the current paper has been to document installation of the pressure cell arrays that were being used over the period of May 2011 to May 2014 (and continued to the current date) to monitor in-service foundation pressures that arise beneath the selected bridge piers. Additionally, as documented in this paper, a dynamic, interactive website has been developed to provide the general public (including vested infrastructure entities) with access to the data measured on-site. Ultimately, the efforts undertaken in installing bridge response measurement devices and establishing remote monitoring procedures facilitate assessments of the current design provisions related to temperatureinduced bridge substructure response. References 1. AASHTO. (2012). LRFD Bridge Design Specifications 6 th Edition, Washington D.C. 2. Arsoy, S., Barker, R. M., Duncan, J.M., and Via, C. E. (1999). The Behavior of Integral Abutment Bridges. Virginia Transportation Research Council, Charlottesville, Virginia. 3. Paul, M., Laman, J. A., and Linzell, D. G. (2005). Thermally Induced Superstructure Stresses in Prestressed Girder Integral Abutment Bridges. Transportation Research Record: Journal of the Transportation Research Board, CD 11-S, Kim, W. and Laman, J. A. (2010). Integral Abutment Bridge Response Under Thermal Loading. Engineering Structures, Vol. 32, Zhu, Z., Davidson, M., Harik, I. E., and Sun, C. (2015). Effect of Thermal Loads on Substructures: New Trammel Creek Bridge on KY-100 in Allen Co., Kentucky. Kentucky Transportation Center Research Report, University of Kentucky, Lexington, Kentucky.