STRENGTHENING AND LOAD TESTING OF THREE BRIDGES IN BOONE COUNTY, MO

Transcription

1 STRENGTHENING AND LOAD TESTING OF THREE BRIDGES IN BOONE COUNTY, MO S. Schiebel 1, R. Parretti 1, A. Nanni 2, and M. Huck 3 ABSTRACT Three bridges in Boone County, Missouri (Brown School Road Bridge, Coats Lane Bridge, and Creasy Springs Bridge) were selected for strengthening with CFRP (Carbon-Fiber Reinforced Polymer) laminates in both shear and flexure. The objective of the rehabilitation program was to remove the 15 Ton load posting that had been imposed on each of the bridges. In order to verify the results of the upgrade, load tests were performed before and after strengthening on two of the bridges (Brown School Road Bridge and Coats Lane Bridge). This paper presents the procedures followed in the design, installation and load testing of the bridges. A recommendation regarding the removal of the load posting is made. 1 Co-Force America, Inc., 800 West 14 th Street, Rolla, MO Center for Infrastructure Engineering Studies, University of Missouri Rolla 3 Harrington and Cortelyou, Inc., 127 West 10 th Street, Kansas City, MO

2 INTRODUCTION FRP Background In recent years, the technology of strengthening reinforced concrete (RC) and prestressed concrete (PC) members with externally bonded FRP laminates has been widely investigated and reported (Nanni 1993, Nanni and Dolan 1993, El-Badry 1996, Dolan et al. 1999, Burgoyne 2001, ACI ). Similar to steel plate bonding, FRP laminate bonding involves adhering thin flexible fiber plies to the concrete surface with a thermoset resin. This technique, known as manual lay-up, may be used to increase the shear and flexural capacity of beams and slabs and to provide confinement in columns. The advantages of this technology include speed and ease of installation, durability of the material system, light weight, and performance. Bridge Configuration The three bridges subject of this project are located in Boone County, Missouri, and are all simply supported spans composed of precast RC channel sections. The bridge at Brown School Road has a span of 6.13 m (20.13 ft.) and is composed of seven channel sections with one solid slab section on either side. The solid slab sections were added in 1986 and have a capacity sufficient for HS20 truckloads. Coats Lane Bridge and Creasy Springs Bridge have a span length of m (38.84 ft.) and 5.92 m (19.43 ft.), respectively, and are both composed of eight channel sections. The RC channel sections have a total depth of 457 mm (18 in.) for Brown School and Creasy Springs and 610 mm (24 in.) for Coats Lane. They are 965 mm (38 in.) wide and the thickness of the flange is 100 mm (4 in.). The legs are 152 mm (6 in.) wide at the bottom and 203 mm (8 in.) wide where they meet the flange. 17

3 STRENGTHENING DESIGN Existing Conditions The existing capacity of the bridges was determined using basic principals of RC theory. The concrete strength was taken as 34.5 MPa (5,000 psi) and the steel yield strength was taken as 276 MPa (40 ksi). Given the steel reinforcement areas (A s ) shown in the second column of Tables 1 and 2, and based on reinforcement depth and spacing as shown in the third column, the existing factored moment and shear capacities (φm n and φv n ) for a single RC channel section in each bridge were computed and shown in the tables. Visual inspection of the bridges revealed no significant deterioration of the concrete channels apart from some hairline cracks. Flexural and Shear Strengthening Design Design loads are based on an HS20 truck load as given by AASHTO design guidelines (1996) for precast multi-girder bridge decks. The moment and shear demand (M u and V u ) imposed by an HS20 and the corresponding deficiencies are given in Tables 1 and 2. From this, the level of strengthening required ( M and V) is determined. The FRP material selected for this application utilizes a high-strength carbon fiber, unidirectional fabric with the following characteristics: thickness, t f = mm ( in.); elastic modulus, E f = GPa (33 Msi); and strain at rupture, ε fu = (mm/mm). The fabric is impregnated by manual lay-up with a two-component epoxy resin. Based on such properties, the level of strengthening required for these bridges could be achieved as described below. 18

4 Flexural strengthening is attained by applying strips of CFRP laminate to the soffit of the channel legs with the fibers oriented in the longitudinal direction (along the length of the bridge). For Brown School Road Bridge and Creasy Springs Bridge, a 127-mm (5-in.) wide strip is applied over the entire length of the span, whereas the Coats Lane Bridge uses a 100-mm (4-in.) wide strip (see Figure 1 for schematic views of the flexural strengthening layout for this bridge). The last column in Table 1 displays the new flexural capacity (ΦM n ) of each bridge after strengthening. Strengthening the bridges for shear loads was achieved by applying strips of CFRP laminate (with fiber perpendicular to the direction of the bridge) to the sides of the web on each channel leg. Due to the way sections are joined side-by-side, the CFRP strips could only be applied to one side of each channel leg. Each strip would extend from the channel web across the soffit of the leg, covering the longitudinal FRP strip. Shear strengthening for all three bridges consists of 610 mm (24 in.) wide CFRP strips spaced at 864 mm (34 in.) center-to-center (see Figure 2 for schematic a view of the shear strengthening layout). The last column in Table 2 displays the new shear capacity (ΦV n ) of each bridge after strengthening. Effect of Strengthening on Bridge Load Rating The evaluation included review of the bridge construction drawings, visual inspection, and use of established state and federal guidelines (AASHTO 1994). According to MoDOT s current load rating guidelines (1996), any bridge built, rehabilitated, or reevaluated is to be rated using the Load Factor Method rating at two load levels, the maximum load level (Operating Rating) and a lower load level (Inventory Rating). The Operating Rating is the maximum permissible load that should be allowed on the bridge. Exceeding this level could damage the bridge. The Inventory 19

5 Rating is the load level the bridge can carry on a daily basis without damage and is taken as 60% of the Operating Rating. Table 3 summarizes the effect of the strengthening on the load-rating factor (RF) of the three bridges. The method for determining the rating factor is that outlined by AASHTO in the Manual for Condition Evaluation of Bridges (AASHTO 1994). The following equation was used: RF C A1 D = (1) A L(1 + I) 2 where: RF is the Rating Factor, C is the capacity of the member, D is the dead load effect on the member, L is the live load effect on the member, I is the impact factor to be used with the live load effect (0.3), A 1 is the factor for dead loads, and A 2 is the factor for live loads. Since the Load Factor Method is being used, A1 is taken as 1.3, and A 2 varies depending on the desired rating level: for Inventory rating, A 2 = 2.17, and for Operating rating, A 2 = 1.3. If the rating factor determined in Equation 1 is greater than 1.0 (RF>1.0), then the bridge can be rated safe for the target live load. To determine the rating (RT) of the bridge the following equation is used: RT = ( RF) W (2) 20

6 In the above equation, W is the weight of the nominal truck used to determine the live load effect (L) and it is 36 Tons for an HS20 truck. As seen from the results in the table, the strengthening has effectively increased the Rating Factor for all bridges in both flexure and shear to a value greater than 1. This result allows for the recommendation to remove the load posting of the three bridges. INSTALLATION A brief summary of the installation procedure used in FRP manual lay-up is given below. This procedure was followed in all bridges for both shear and flexural strengthening. Surface Preparation: the concrete surface is cleaned and prepared for installation through the use of sand blasting to remove laitance and open the concrete pores. Primer: a coat of low viscosity epoxy primer is applied to the concrete surface to penetrate the open pores so to hatch and strengthen the most external layer of concrete. Putty: a thin coat of epoxy-based putty is smoothed over the surface to fill in any small voids, cracks or uneven regions. Saturant: a layer of epoxy saturant is applied to receive the fibers. Fiber Sheets: the first ply of carbon fibers is then applied. The ply is rolled into the saturant layer to insure good impregnation. Saturant: the application of the second layer of saturant is meant to fully impregnate the fibers. The laminate is rolled thoroughly to ensure good penetration of the resin around the fibers. 21

7 The photos of Figure 2 show details of the completed installation for one of the bridges. LOAD TESTING A load-testing program was carried out on two of the bridges before and after strengthening. The bridge on Creasy Springs Road was not load tested due to its similarity to the bridge on Brown School Road. In this paper, only the results relative to Coats Lane Bridge are reported for brevity as similar results were obtained in the case of Brown School Road Bridge. A diagnostic load test does not seek to evaluate the safety or the load carrying capacity of the structure, but, rather, it is designed to verify its performance under service load conditions. A diagnostic load test is to determine the flexural performance, but cannot assess shear performance or ultimate capacity. Instrumentation, Procedure and Analysis Under known loads, the flexural response of the bridges was measured in terms of steel and CFRP reinforcement strains, concrete strains, and member deflections. The testing apparatus used for each load test consisted of strain gauges and linear variable differential transducers (LVDTs) installed at locations as schematically shown in Figure 3. LVDTs mounted horizontally in the longitudinal direction were used to record deformations in both the compression zone and at the level of the tensile reinforcing steel (see Figure 4). These horizontal deformations were then used to compute smeared strain values over the gauge length of the LVDTs. Instrumentation installed for the post-strengthening tests was identical to the 22

8 installation for the pre-strengthening tests except that longitudinal strain was also recorded in the CFRP laminate applied to the soffit of the RC channel members. Two H20 trucks were used to load the bridges as HS20 trucks were not available. This was not a problem since the diagnostic load test operates in the elastic range of a structure where proportionality governs. The two trucks made two passes on each of the bridges driving side-byside along the length of the span (see Figure 5), and stopping at certain predetermined locations (see Figure 6). The trucks stopped with their rear (heaviest) axle positioned at five locations along the span of each of the bridges. The trucks rested at each location for approximately two minutes before proceeding to the next location. Table 4 gives the maximum service moment and shear induced in the channel-members due to the two selected trucks with the rear axle at the mid-span location. The fraction of the wheel load carried by the channel member was determined in accordance with AASHTO s guidelines (Article ). The effect of impact was not examined during the load testing. Pre-strengthened Load Testing The vertical deflection histogram resulting from the load testing is given in Figure 7. The three separate lines display the deflections taken by the LVDTs: Channel 4 is at the centerline of the bridge with Channel 3 just next to it, while Channel 1 lies at the edge of the bridge. The deflection histogram is a stepped function with five steps corresponding to the trucks stops at the five locations selected on the bridge. Because the trucks passed over the bridge twice there are two cycles displayed. Theoretically, the second cycle should be the same as the first cycle. In Figure 7, the second cycle displays slightly higher deflections than the first one due to the 23

9 residual deflection: common response in structures when not enough time is given for the structure to return to its initial state. The decrease in deflections from the center of the bridge to the outside edge shows that most of the load from the trucks was concentrated towards the centerline. Strain data were used to back-calculate the applied moment and compare it with that based on the applied load distributed according to AASHTO guidelines. Figure 8 below displays the moment histogram. The maximum moment computed with this procedure is approximately 84.0 kn-m (62.0 kip-ft). The theoretical live load moment for a single channel section is kn-m (95.5 kip-ft) based on AASHTO wheel load distribution guidelines and an H20 truck. There are two reasons to explain this difference: 1) the lateral distribution of the wheel loads across the width of the bridge as proposed by AASHTO is a conservative estimate; and 2) the strain data are dependent on the location of the measurement and the crack pattern present in that region (i.e., concrete is not a homogenous material in tension). Discussion The recorded deflections for the pre-strengthened test compare well with theoretical results. Branson s classical procedure (Branson 1977) was used in which the effective moment of inertia, I e, was calculated for a channel section based on an applied moment as given by the AASHTO wheel load distribution factor. The mid-span deflection was then calculated based on this effective moment of inertia. The results of the theoretical analysis are compared with the experimental values in Table 4. As expected, the analytical value is slightly larger than the experimental one. This is an indication that the tests were successful in terms of execution and that the structure had a predictable performance. The difference between the experimental and 24

10 analytical values is due to two reasons: 1) the computation of deflection based on I e is conservative; and 2) the assumption of simply supported beam is a conservative approximation since some level of rotational restraint is present at the supports. Further analysis was carried out and was intended to show the possible level of rotational restraint (i.e., fixity). The degree of fixity was determined by computing the value of beam-end moments so that the analytical deflection equals the maximum measured deflection at mid-span. The degree of fixity is a percentage that represents the condition of the end supports ranging between simply supported (0%) and fixed-fixed (100%). Using the same I e used for the analytical deflection calculation, the degree of fixity for the Coats Lane Bridge is 8%. A similar study on another RC bridge determined a degree of fixity of roughly 15% (Alkhrdaji et al.1999). If this degree of fixity is taken into account, the expected induced live-load moment at mid-span becomes 78.1 kip-ft (from the previous value of kn-m (95.5 kip-ft) of simply supported beam). This value is roughly 26% greater than the live-load moment back calculated from strain values (84.0 kn-m (62.0 kip-ft)). The difference between service moment according to AASHTO and moment back calculated from strain measurement is a further indication that the design for strengthening is based on conservative assumptions since it is based upon the AASHTO wheel load distribution coefficients. Post-strengthened Load Testing Strain gauges were applied to the FRP laminates on the soffit of the channels directly below the LVDTs. Figure 9 shows the horizontal LVDTs and the application of a strain gauge. With the 25

11 same instrumentation setup and loads, the deflection histogram for the post-strengthened test is given in Figure 10. This five-step deflection histogram is very similar to that of Figure 7, obtained for the un-strengthened structure. It is noted that the trends are almost identical and that the maximum deflection values for Channels 4 and 3 (i.e., the ones at locations receiving the largest load) are slightly smaller. This is an indication that the FRP is adding some stiffness to the structure by restraining the opening of the cracks. Figure 11 gives the moment histogram for the post-strengthened load test back calculated from strain readings of two LVDTs and one strain gage. The maximum live load moment is approximately 50.2 kn-m (37.0 kip-ft), which is roughly 40% less than that computed during the pre-strengthened load test. Since the moment back calculated from strain data before and after strengthening should remain the same under the same applied load, this difference may appear to be large but it can be logically explained. The horizontal LVDTs are set up to measure a deformation over a long gauge length (e.g., 305 mm (12 in.)). This allows for a smeared strain value that can take into account cracks present in that region of measurement. The strain gauges applied to the FRP are only 10 mm (0.4 in.) in length and record a local strain. It is possible that once the FRP is applied, it prevents the opening of any cracks that are being spanned by the horizontal LVDTs and if the strain gauge on the FRP were not applied over the crack (i.e. to either side of the crack), the recorded strain in the FRP would also be minimal. 26

12 Figure 12 shows a comparison of the moment-deflection results from both before and after strengthening. The plotted moments are those that were calculated based on the AASHTO load distribution factors. The deflections are those that were recorded during the two tests at each of the truck positions (average of two cycles). For any given moment (or applied load) the deflection decreased after the strengthening was applied. This reflects the increased stiffness of the member due to the addition of the FRP laminates. CONCLUSIONS AND RECOMMENDATIONS This report describes design, installation and validation of the rehabilitation project involving three reinforced concrete bridges in Boone County, Missouri. The bridges were similar in materials, construction method and geometry. The strengthening was accomplished with the use of externally bonded FRP laminates installed by manual lay-up. Based on the results of the analytical calculations of the structural components as applied to the AASHTO rating equation and the validation by load testing, a recommendation to remove the load posting can be substantiated. A single channel section was selected for each of the three bridges to determine its load rating based on an HS20 truckload. For both bending moment and shear, the load-rating factor was increased to a value of over 1.0 for each bridge member. The load tests (i.e., pre- and post-strengthening for two bridges) did not display any unusual behavior in either bridge tested. The results based on both deflection and recorded moments are reasonable and acceptable. 27

13 The maximum recorded deflection for both bridges before strengthening was installed was roughly 33% lower than the predicted value. This indicates a conservative estimate of the flexural stiffness (i.e., I e ) coupled with a difference in the boundary conditions from simply supported. An analysis of the bridge on Coats Lane determined the degree of fixity to be approximately 8%. The deflections for both bridges slightly decreased after the strengthening was installed. This reflects the added flexural stiffness due to the CFRP laminates. The moments back calculated from strain values are smaller than and within the expected range of the corresponding ones based on AASHTO wheel load distribution factor. This reflects and confirms the conservative nature of the design. Based on these tests, the bridges are behaving well under service loads. REFERENCES ACI Committee 440 (2001). Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening of Concrete Structures, American Concrete Institute, Farmington Hills, MI, (in press). AASHTO (1994). Manual for Condition Evaluation of Bridges, 2 nd Edition, pp AASHTO (1996). Standard Specifications for Highway Bridges, 16 th Edition, 760 pp. 28

14 Alkhrdaji, T., Nanni, A., Chen, G., Barker, M. (1999). Destructive and Non-Destructive Testing of Bridge J857, Phelps County Missouri, Center for Infrastructure Engineering Studies, University of Missouri-Rolla, Rolla, Missouri, pp Branson, D.E. (1977). Deformation of Concrete Structures, McGraw-Hill, New York Burgoyne, C., Ed., (2001). Fifth International Symposium on Fiber Reinforced Polymer Reinforcement for Reinforced Concrete Structures (FRPRCS-5), Thomas Telford, London, UK, Vol. 1 and 2, 1151 pp. Dolan, C. W., Rizkalla, S. H., and Nanni, A., Eds., (1999). Fourth International Symposium on Fiber Reinforced Polymer Reinforcement for Reinforced Concrete Structures (FRPRCS- 4), ACI SP-188, American Concrete Institute, Farmington Hills, Michigan pp. El-Badry, M., Ed., (1996). "Advanced Composite Materials in Bridges and Structures," Proceedings ACMBS-II, Montreal, Canada, August 1996, pp Nanni, A., Ed. (1993). Fiber-Reinforced-Plastic (FRP) Reinforcement for Concrete Structures: Properties and Applications, Developments in Civil Engineering, Vol. 42, Elsevier, Amsterdam, The Netherlands, pp

15 Nanni, A. and Dolan, C.W., Eds., (1993). " First International Symposium on Fiber Reinforced Polymer Reinforcement for Reinforced Concrete Structures (FRPRCS-1)," ACI SP-138, American Concrete Institute, Detroit, MI, pp Missouri Department of Transportation (1996). MoDOT Bridge Load Rating Manual, Jefferson City, Missouri, pp ACKNOWLEDGEMENTS The authors would like to thank the following for their help on this project: David Nichols, County Engineer of Boone County, MO, for his vision, Structural Preservation System (SPS), Chicago, IL, for the installation, and Master Builders Technologies, Cleveland, OH, for supplying the FRP system. 30

16 LIST OF TABLES Table 1 - Existing Moment Capacity and Demand (HS20 Truck Load) of One Channel Table 2 Existing Shear Capacity and Demand (HS20 Truck Load) of One Channel Table 3 Inventory Rating Factor for a Single Channel Section in Flexure and Shear Table 4 Comparison of Analytical and Experimental Deflections (H20 Load) LIST OF FIGURES Figure 1 Strengthening Configuration for Coats Lane Bridge Figure 2 Completed Installation Figure 3 Loading and Instrumentation (mid-span cross-section) Figure 4 Instrument Setup (Pre-Strengthened) Figure 5 Truck Loads on Coats Lane Bridge Figure 6 Position of Rear Axle for Typical Span Figure 7 Coats Lane Bridge Deflection Histogram (Pre-Strengthening) Figure 8 Moment Histogram for Coats Lane Bridge (Pre-Strengthening) Figure 9 Horizontal LVDTs and Application of a Strain Gauge Figure 10 Coats Lane Bridge Deflection Histogram (Post-Strengthening) Figure 11 Moment Histogram for Coats Lane Bridge (Post-Strengthening) Figure 12 Comparison of Moment-Deflection Results (Coats Lane)

17 Table 3 - Existing Moment Capacity and Demand (HS20 Truck Load) of One Channel Steel Effective Required φm n Reinf. A s Depth, d φm n M u % Strengthening M (after strengthening) (in 2 ) (in.) (kip-ft) (kip-ft) Deficiency (kip-ft) (kip-ft) Brown School Coats Lane None 509 Creasy Springs Note: 1 in=25.4 mm;1 kip-ft=1.356 kn-m. 32

18 Table 4 Existing Shear Capacity and Demand (HS20 Truck Load) of One Channel Steel Concrete Steel Required φv n Reinf. A s Spacing V c V s φv n V u % Strengthening, V (after strengthening) (in 2 ) (in) (kips) (kips) (kips) (kips) Deficiency (kips) (kips) Brown School Coats Lane Creasy Springs Note: 1 in=25.4 mm; 1 kips=4.448 kn. 33

19 Table 5 Inventory Rating Factor for a Single Channel Section in Flexure and Shear Flexure Shear RF RF Rating (RT) RF RF Rating (RT) Pre-Strength. Post-Strength. (Tons) Pre-Strength. Post-Strength. (Tons) Brown School Road Coats Lane Creasy Springs

20 Table 6 Comparison of Analytical and Experimental Deflections (H20 Load) Maximum Moment Maximum Deflection (in.) Live-load * (kip-ft) Service * (kip-ft) Analytical Experimental Coats Lane Bridge *for a typical channel-member, based on AASHTO distribution factor Note: 1 kip-ft=1.356 kn-m. 35

21 Shear Strengthening (exterior channel) 24 - in. wide MBrace CF Spaced at 34 - in c/c, 10 - in clear edge/edge Sheets U - wrapped on web. Sheets are terminated in from channel interface. 6 Flexural Strengthening 4-in. wide MBrace CF-130 Run the entire span Centered on 6-in web bottom. Shear Strengthening (interior channel) 24-in. wide MBrace CF Spaced at 34 - in c/c, 10 - in clear edge/edge Sheets attached to web side and continuing on web bottom. Sheets are terminated 0.5-in from channel interface ½ Figure 3 Strengthening Configuration for Coats Lane Bridge Note: 1 in=25.4 mm; 1 ft= m. 36

22 (a) Overview (b) Detail Figure 4 Completed Installation 37

23 Lane A Lane B Channel 8 Channel 7 Channel 6 Channel 5 Channel 4 Channel 3 Channel 2 Channel 1 Varies from 23.9' 22.2 to 25.4' Vertical LVDT C L LVDT Strain Gauge Figure 5 Loading and Instrumentation (mid-span cross-section) 38

24 (a) Overall (b) detail of horizontal LVDTs Figure 6 Instrument Setup (Pre-Strengthened) 39

25 Figure 7 Truck Loads on Coats Lane Bridge 40

26 Wheel Locations Wheel Paths 1 6L 1 6L 1 6L 1 6L 1 6L 1 6L L Figure 8 Position of Rear Axle for Typical Span 41

27 0.25 Deflection (in.) Channel 4 Channel 3 Channel 4 Channel Channel 1 Channel 1 0 0:00:00 0:02:53 0:05:46 0:08:38 0:11:31 0:14:24 0:17:17 0:20:10 0:23:02 0:25:55 0:28:48 Time Figure 9 Coats Lane Bridge Deflection Histogram (Pre-Strengthening) Note: 1 in=25.4 mm. 42

28 Channel Moment (kip-ft) Channel :00:00 0:02:53 0:05:46 0:08:38 0:11:31 0:14:24 0:17:17 0:20:10 0:23:02 0:25:55 0:28:48 Time Figure 10 Moment Histogram for Coats Lane Bridge (Pre-Strengthening) Note: 1 kip-ft=1.356 kn-m. 43

29 Figure 11 Horizontal LVDTs and Application of a Strain Gauge 44

30 Channel 3 Channel Channel 4 Deflection (in.) 0.1 Channel 1 Channel Channel 1 0 0:00:00 0:02:53 0:05:46 0:08:38 0:11:31 0:14:24 0:17:17 0:20:10 0:23: Time Figure 12 Coats Lane Bridge Deflection Histogram (Post-Strengthening) Note: 1 in=25.4 mm. 45

31 Moment (kip-ft) Channel 3 Channel 4 Channel Channel :00:00 0:02:53 0:05:46 0:08:38 0:11:31 0:14:24 0:17:17 0:20:10 0:23:02 Time Figure 13 Moment Histogram for Coats Lane Bridge (Post-Strengthening) Note: 1 kip-ft=1.356 kn-m. 46

32 AASHTO Moment (kip-ft) Pre-Strengthening Post-Strengthening Measured Deflection (in.) Figure 14 Comparison of Moment-Deflection Results (Coats Lane) Note: 1 in=25.4 mm; 1 kip-ft=1.356 kn-m 1