Concrete international / JULY 2004 1

BY NABIL F. GRACE, JOHN J. ROLLER, FREDERICK C. NAVARRE, RICHARD B. NACEY, AND WAYNE BONUS

Fibrous composite materials are currently used to reinforce concrete beams, girders, and/or slabs in

innovative infrastructure throughout the world.1-3

The Bridge Street Bridge Deployment Project4 in Southfield, MI, is an example of one such innovative project where bridge members are reinforced and prestressed fully with carbon fiber-reinforced polymer (CFRP) tendons and strands. This article will focus on the details and findings related to a field load test conducted on one of the project’s bridges, Structure B, after construction completion.

The Bridge Street Bridge Deployment Project (Fig. 1) consists of two parallel, independent bridges (Structure A and Structure B) over the Rouge River in the city of Southfield, MI. Both structures are designed to accommodate two traffic lanes and incorporate three, 21-m-long (69 ft), 15-degree-skewed spans. Components of Structure A are a new substructure and superstructure, which incorporates five equally spaced conventional AASHTO-I beams in each of the three spans, with a continuous cast-in-place concrete deck slab. Structure B consists of 12 double tee (DT) beams (4 beams per span), each prestressed using internal pretensioned Leadline* tendons and externally post-tensioned in the longitudinal and transverse directions, with carbon fiber composite

Load Testing a CFRP-Reinforced

Bridge Bridge Street Bridge load distribution behavior accurately

predicted by AASHTO design specs

cable (CFCC)† strands. Figure 2 shows the external unbonded CFCC post-tensioning strands installed underneath the bridge. Construction Technology Laboratories, Inc. (CTL), Skokie, IL, conducted the initial inspection and load test of Structure B.

BRIDGE DESIGN DETAILS Several research investigations5-12 performed at Lawrence

Technological University, Southfield, MI, were the basis for the design of the bridge. Hubbell, Roth and Clark, Inc. (HRC), Bloomfield Hills, MI, was the engineer of record for the Bridge Street Bridge. HRC designed the traffic lanes for

*Trademark name of Leadline tendons manufactured by Mitsubishi

Chemical Corp., Tokyo, Japan† Trademark name of CFCC strands manufactured by Tokyo Rope

Manufacturing Co. Ltd., Tokyo, JapanFig. 1: Bridge Street Bridge in Southfield, MI, consists of two parallel, independent bridges over the Rouge River

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Structures A and B using the provisions, to the extent possible, of the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifi-cations for Highway Bridges13 and LRFD Bridge Design Specifications.14 As shown in Fig. 3, superstructure dead loads for Structure B included: DT beams, composite CFRP-reinforced concrete topping, surfacing mixture, pedestrian sidewalk, barrier wall, and bridge parapet and railing. Live load design was based on the Michigan MS-23 (AASHTO HS 25) standard truck loading requirements.

HRC calculated the live load distribution factor for design of the DT beams two different ways, using provisions defined in AASHTO specifications.13,14 The provisions defined in Section 3.23.4 of the AASHTO Standard Specifi-cation13 resulted in a distribution factor of approximately 0.64 lanes/beam. Provisions defined in Section 4.6.2.2 of

Fig. 2: External, unbonded CFCC post-tensioning strands underneath Structure B of the Bridge Street Bridge

the AASHTO LRFD Bridge Design Specification14 resulted in a live load distribution factor equal to approximately 0.63 lanes/beam. Applying the AASHTO LRFD provisions in a more conservative approach, where the longitudinal stiffness contribution from the beam flange was ignored, resulted in a live load distribution factor equal to approxi-mately 0.81 lanes/beam. Ultimately, HRC decided to use a distribution factor equal to 0.80 lanes/beam to calculate the design live load moment for the DT beams of Structure B.

The Michigan MS-23 standard truck loading resulted in a maximum live load plus impact (LL+I) moment equal to 2027.9 kN-m (1500 kip-ft) per lane. Therefore, HRC designed each DT beam for a LL+I moment equal to 1622.3 kN-m (1200 kip-ft), or 80% of the maximum LL+I moment.

CONSTRUCTION OF STRUCTURE B Prestressed Systems, Inc., Windsor,

ON, fabricated the 12 DT beams for Structure B of the Bridge Street Bridge. Figure 4 shows a cross section through a typical DT beam near midspan. Each beam incorporated 60 internal 10-mm-diameter (0.4 in.) pretensioned CFRP tendons (30 in each beam stem). In addition to the tendons, each beam also incorporated four draped longitudinal, external, 40-mm-diameter (1.6 in.) post-tensioned CFCC strands. Other longitudinal and transverse nonprestressed reinforcement in the beam flange and diaphragms were also made of CFRP. The epoxy-coated, stainless steel stirrups used for shear reinforcement in each beam stem were the only internal steel reinforce-ment incorporated in the beams.

Each of the three bridge spans of Structure B incorporated four DT beams, as shown in Fig. 3 and 5. Each DT beam had five intermediate transverse diaphragms (D2 to D6) and two end transverse diaphragms (D1 and D7). Additional CFCC strands were installed in the diaphragms and used to post-tension the four beams in each span together transversely.

After erection, workers placed a minimum 76-mm-thick (3 in.) concrete topping, reinforced with NEFMAC*

Fig. 3: Cross section through Structure B of the Bridge Street Bridge

*Trademark name of NEFMAC grids

manufactured by Autocon Companies, Inc.,

ON, Canada

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grids, over each span. The concrete topping was discontinuous over the bridge supports and was anchored to the DT beams by the hooked stirrup ends that protruded from the girders’ top flanges. Finally, they installed a 38-mm-thick (1.5 in.) latex-modified surfacing mixture over the concrete topping within the 6100-mm-wide (20 ft) roadway.

INSTRUMENTATION During fabrication, six of the

12 DT beams for Structure B were instrumented with various sensors to measure concrete strains, beam deflections, and force levels in the external CFCC post-tensioned strands. Figure 5 shows the six DT beams selected for instrumentation (Beams C, G, J, K, L, and M). Once workers erected the DT beams at the bridge site, CTL installed numerous additional sensors throughout Structure B during the various stages of construction. CTL connected all sensors installed in Structure B to a dedicated on-site data acquisition system used to monitor the long-term behavior of the bridge.

INSPECTION AND LOAD TESTING

After construction completion, CTL conducted a visual inspection and static load test of Structure B. The purpose of the visual inspection of Structure B was to document the size and location of any concrete cracks visible to the unaided eye both before and after the load test. The visual inspection included the top and underside of the superstructure. With the exception of the beam diaphragms that provided anchorage for the external longitudinal post-tensioning tendons (D2 and D6), there were no structural cracks found during the inspection. After the static load test was completed, there were no apparent new cracks and no discernable difference observed in the existing diaphragm cracks.

After CTL completed the initial visual inspection of Structure B, we conducted a static load test on November 28, 2001. The objective of the static load test was to evaluate structural performance under live load conditions approximating design service load levels. Static loads

were applied to each span of Structure B (individually) using two identical tandem rear-axle dump trucks provided by the city of Southfield. The specified empty gross weight of each dump truck was 118.8 kN (26,700 lb). Prior to the test, each dump truck was filled with granular material to achieve a total target vehicle weight of approximately 258 kN (58,000 lb).

After filling, the Chief Weigh Master for the Road Commission for Oakland County measured the actual weight of each truck. During this exercise, the Chief Weigh Master used portable scales, accurate to within ±1% of the reading, to measure the load distributed to each of the three axles. Details of each axle load for both

Fig. 4: Cross section near midspan through a typical DT beam for Structure B of the Bridge Street Bridge

Fig. 5: Plan view of Structure A and B for the Bridge Street Bridge project

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trucks can be found elsewhere. The filled weights of both trucks were within 2.0 kN (450 lb) of the 258 kN (58,000 lb) target weight.

The load test consisted of four different stages of loading. In each loading stage, we positioned the two dump trucks back-to-back in one lane near midspan (Fig. 6). In the first two load stages, we located the two loaded

Fig. 6: Two dump trucks, back-to-back near mid-span for load testing of Structure B

Fig. 7: Example of truck positions for Load Stage 1—maximum positive moment in west lane of north span

dump trucks in the west lane of the north and south spans (individually) to generate maximum positive bending moment at midspan (Fig. 7). The third and fourth load stages consisted of positioning the two loaded dump trucks in the east lane of the north and middle spans (individually) to generate maximum positive bending moment at midspan.

During each load stage, the rear-most axles of the two dump trucks were spaced about 2591 mm (8.5 ft) apart and located 1295 mm (4 ft) from the midspan centerline. This loading configuration produced a moment at midspan of the lane equal to about 90% of the 2028 kN-m (1500 kip-ft) design service live load moment (including impact). The resulting midspan lane moments in the north, middle, and south spans were 1850, 1774, and 1837 kN-m (1370, 1310, and 1360 kip-ft), respectively.

During the load test, we used the existing bridge instrumentation and data acquisition system to measure the response to the applied loads. The protocol for each of the four load stages was instrument readings: Prior to moving the trucks onto the span (initial

reading);

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Immediately after the trucks were moved into the required positions;

After approximately 5 min with the trucks in the required positions; and

About 5 min after the load was removed (final reading).

STATIC LOAD TEST RESULTS The response of Structure B to the applied static loads

was evaluated based primarily on measured concrete strains and deflection data at midspan of each instrumented beam. Therefore, although the instrumentation array for Structure B included more than 450 individual sensors, the discussion of measured data from the load test presented in this article is limited to concrete strains and deflections measured at midspan of each instrumented beam. Deflection measurements, taken manually at midspan during the load test using precision surveying equipment, corroborated the measured strain data.

As shown in Fig. 5, every beam in the north span (J, K, L, M) and the third beam from the west in each of the south and middle spans (C and G) included a full set of instrumentation. Figure 8 presents the measured data from Load Stages No. 1 and 2. We observed a similar response for Load Stages No. 3 and 4. The reported data represent the measured response due to the application of the truck loads and do not include the effects of the pre-existing dead loads.

Measured midspan strain profiles presented in Fig. 8 indicate that the applied load was effectively distributed to all four beams in the north span. Average measured strains near the web bottom at midspan of the four beams ranged from 23 to 45 microstrain and decreased from west to east. In addition, the measured midspan strain profile for Beam C in the south span is nearly identical to the strain profile for the corresponding north span beam (Beam L). This observation indicates that the north and south spans of Structure B exhibited similar load distribution behavior.

Measured midspan strain profiles corresponding to Load Stages No. 3 and 4 indicate that the applied load was distributed effectively to all four beams in the north span. Average measured strains near the web bottom at midspan of the four beams in the north span ranged from

15 to 51 microstrain, decreased from east to west, and were consistent with the corresponding measured deflections. Additional stiffness contributed by the sidewalk likely accounted for the slight variation between the peak strains measured during Load Stages No. 1 and 3. It was also observed that the measured midspan strain profile for Beam G in the middle span is almost identical to the strain profile for the corresponding north span beam (Beam L). This observation indicates that the north and middle spans of Structure B also exhibited similar load distribution behavior.

EVALUATION OF LOAD DISTRIBUTION BEHAVIOR

Comparison of the average measured concrete strains in the web bottom of Beams J to M during Load Stages No. 1 and 3 can be used to provide an indication of load distribution in the north bridge span. Figure 9 shows such comparison for the combined effects of Load Stages No. 1 and 3. Based on the magnitude of the average measured bottom web strains, the cumulative percentages of the

Fig. 8: Load Stages 1 and 2: measured responses for Beams J-M and C

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Fig. 9: Load distribution based on combined percentage of strain, north span

applied lane loads distributed to each of the four beams in the north span (Beams J, K, L, and M) would be 46.3, 43.7, 51.2, and 58.9%, respectively. Using this rationale, it can be theorized that no more than 60% of the total live load per lane would be distributed to any individual beam in Structure B. This percentage is in good agreement with the calculated AASHTO distribution factors equal to approximately 0.60 lanes/beam, resulting from the condition where the full composite beam section is used in the determination of the longitudinal stiffness.

STRUCTURE ACES TEST Based on results from the load test of Structure B,

it is concluded that: 1) the applied loads per lane were effectively distributed to all four beams of each bridge span; 2) the three spans of Structure B exhibit similar load distribution behavior; and 3) the actual load distribution behavior is consistent with the distribution factors derived from the provisions of the AASHTO design specifications. Based on the measured data from the load test of Structure B, it is concluded that the provisions of the AASHTO specifications can be used to predict load distribution

behavior of bridge superstructure configurations similar to Structure B.

AcknowledgmentsThe success of this project is due to the energy and talent

of many people, each of whom played a significant role. These

include various researchers, designers, manufacturers, suppliers,

and builders. The project benefited from the congressional

support of Michigan Representatives Joseph Knollenberg and

Sander Levin. The site construction was funded in the 1998 fiscal

year through the Federal Highway Administration (FHWA) as

one of the TEA-21 High Priority Projects. Further funding for

instrumentation and monitoring was provided under the

Innovative Bridge Research and Construction Program of TEA-21.

The Bridge Street Industrial Park Subdivision property

owners approved a Special Assessment District, created by the

City of Southfield, which contributed to the funding of the

project. In addition, the project was awarded a Michigan

Strategic Fund Grant from the Michigan Economic Development

Corporation. Furthermore, the National Science Foundation

funded the original research upon which the project design

concept was based, and for analysis presented in previous

publications. Finally, the Mayor of the City of Southfield, the City

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Council, and the City Administration are to be commended for

their vision of the future and their courage to venture into this

unconventional technology in bridge construction.

References1. ACI Committee 440, “State-of-the-Art Report on Fiber

Reinforced Plastic Reinforcement for Concrete Structures (440R-96),”

American Concrete Institute, Farmington Hills, MI, 1996, 65 pp.

2. Rizkalla, S. H., “ A New Generation of Civil Engineering

Structures and Bridges,” Proceedings of the Third International

Symposium on Non-Metallic (FRPRC) Reinforcement for Concrete

Structures, Sapporo, Japan, V. 1, Oct. 1997, pp. 113-128.

3. Fam, A. Z.; Rizkalla, S. H.; and Tadros, G., “Behavior of CFRP

Prestressing and Shear Reinforcements for Concrete Highway

Bridges,” ACI Structural Journal, V. 94, No. 1, Jan.-Feb. 1997, pp. 77-86.

4. Grace, N. F.; Navarre, F. C.; Nacey, R. B.; Bonus, W.; and

Collavino, L., “Design-Construction of Bridge Street Bridge-First CFRP

Bridge in the United States,” PCI Journal, V. 47, No. 5, Sept.-Oct. 2002,

pp. 20-35.

5. Grace, N. F.; Abdel-Sayed, G.; Sakla, S.; and Wahba, J., “Finite

Element Analysis of Bridge Street Bridge, City of Southfield,

Michigan,” LTU Research Project #36, Report Submitted to Hubbell

Roth & Clark, Inc. Consulting Engineers, 1997, Bloomfield Hills, MI.

6. Grace, N. F., and Abdel-Sayed, G., “Behavior of Externally

Draped CFRP Tendons in Prestressed Concrete Bridges,” PCI Journal,

V. 43, No. 5, Sept.-Oct. 1998, pp. 88-101.

7. Grace, N. F., and Abdel-Sayed, G., “Behavior of Carbon Fiber

Reinforced Prestressed Concrete Skew Bridge,” ACI Structural

Journal, V. 97, No. 1, Jan.-Feb. 2000, pp. 26-34.

8. Grace, N. F., “Response of Continuous CFRP Prestressed

Concrete Bridges Under Static and Repeated Loadings,” PCI Journal,

V. 45, No. 6, Nov.-Dec. 2000, pp. 84-102.

9. Grace, N. F., “Transfer Length of CFRP/CFCC Strands for Double-

T Girders,” PCI Journal, V. 45, No. 5, Sept.-Oct., 2000, pp. 110-126.

10. Grace, N. F.; Enomoto, T.; and Yagi, K., “Behavior of CFCC and

CFRP Leadline Prestressing System in Bridge Construction,” PCI

Journal, V. 47, No. 3, 2002, pp. 90-103.

11. “AASHTO Standard Specifications for Highway Bridges,” 16th

Edition, American Association of State Highways and Transportation

Officials, Washington, DC, 1996.

12. “AASHTO LRFD Bridge Design Specifications,” 2nd Edition,

American Association of State Highways and Transportation

Officials, Washington DC, 1998.

13. Roller, J. J., and Elremaily, A. F., “Instrumentation and

Structural Testing of Full-Scale Double-Tee Beam, Bridge Street

Bridge, City of Southfield, Michigan,” Final Report to the City of

Southfield, Michigan, Construction Technology Laboratories, Inc.,

April 13, 2001, 46 pp.

14. Grace, N. F.; Enomoto, T.; Abdel-Sayed, G.; Yagi, K.; and

Collavino, L., “Evaluation of CFRP/CFCC Full-Scale DT-Beam:

Experimental Study and Analysis,” PCI Journal, V. 48, No. 4, July-Aug.

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Selected for reader interest by the editors.

ACI member Nabil F. Grace is a professor and Chair of the Civil Engineering Department at Lawrence Technological Uni ver si ty, Southfield, MI. He is a member of ACI Com mit tees 343, Evaluation of Concrete Bridges and Bridge Elements, and 440, Fiber Reinforced Polymer Reinforcement.

John J. Roller, a licensed structural and professional engineer, is Principal Structural Engineer at Construction Technology Laboratories, Inc., Skokie, IL. Roller has worked for CTL for slightly more than 20 years.

ACI member Frederick C. Navarre, Senior Associate and Chief Structural Engineer, Hubbell, Roth & Clark, Inc., Bloomfield Hills, MI, served as Project Engineer for the Bridge Street Bridge Project. He received his bachelor’s degree in civil en gi neer ing from Wayne State Uni ver si ty, Detroit, MI.

Richard B. Nacey, Senior Project Engineer, Hubbell, Roth & Clark, Inc., Bloomfield Hills, MI, served as Project Structural Engineer for the Bridge Street Bridge Project. He received his bach e lor’s degree in civil engineering from Michigan State Uni ver si ty, East Lansing, MI, and his master’s degree in civil engineering from Wayne State University.

Wayne Bonus is the Ad min is tra tive Engineer for the City of Southfield, MI. He has served as project manager on a variety of diverse projects during his 14 years with the city, including the city’s Project Engineer for the Bridge Street Bridge Project. He received his BS in civil engineering from Wayne State Uni ver si ty.