Field Performance 

of Timber Bridges

20. Gray Stress-Laminated 

Deck Bridge 

Habib J. Dagher 

Frank M. Altimore 

Vincent Caccese 

Michael A. Ritter 

Richard Hebert 

United States

Department of 

Agriculture 

Forest Service 

Forest 

Products

Laboratory 

National Wood 

in Transportation 

Information

Center 

Research 

Paper

FPL−RP−592

In cooperation 

with the 

United States 

Department of 

Transportation 

Federal

Highway 

Administration
Abstract 

The Gray bridge was constructed in the fall of 1991 in 

Gray, Maine. The bridge is a single-span, two-lane, stresslaminated deck structure that is approximately 24 ft long and 

23 ft wide. It was constructed from chromated-copperarsenate- (CCA-) treated eastern hemlock grown in Maine. 

This report presents information on the design, construction, 

and field performance of this bridge. The field performance 

of the bridge was monitored for 61⁄2 years, beginning shortly

after construction. During the field monitoring program, data 

were collected related to wood moisture content, force level 

of stressing bars, behavior under static truck loading, and 

overall structural condition. With the exception of having 

to be retensioned approximately every 3 years, the bridge 

is performing well, with no structural or serviceability

deficiencies. 

Keywords: Timber, bridge, wood, stress-laminated 

January 2001 

Dagher, Habib J.; Altimore, Frank M.; Caccese, Vincent; Ritter, Michael A;

Hebert, Richard. 2001. Field performance of timber bridges--20. Gray

stress-laminated deck bridge. Res. Pap. FPL-RP-592. Madison, WI:

U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 

15 p. 

A limited number of free copies of this publication are available to the

public from the Forest Products Laboratory, One Gifford Pinchot Drive, 

Madison, WI 53705–2398. Laboratory publications are sent to hundreds 

of libraries in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the

University of Wisconsin. 

The use of trade or firm names is for information only and does not imply

endorsement by the U.S. Department of Agriculture of any product or

service. 

The United States Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national

origin, sex, religion, age, disability, political beliefs, sexual orientation, or 

marital or familial status. (Not all prohibited bases apply to all programs.) 

Persons with disabilities who require alternative means for communication

of program information (Braille, large print, audiotape, etc.) should contact

the USDA’s TARGET Center at (202) 720–2600 (voice and TDD). To file a 

complaint of discrimination, write USDA, Director, Office of Civil Rights, 

Room 326-W, Whitten Building, 1400 Independence Avenue, SW, Washington, DC 20250–9410, or call (202) 720–5964 (voice and TDD). USDA

is an equal opportunity provider and employer.

Acknowledgments

This project was a cooperative effort of the University of

Maine, the U.S. Department of Transportation, the Maine 

Department of Transportation, and the USDA Forest Service, 

Forest Products Laboratory. We express sincere appreciation 

to the following individuals who contributed to this project:

Jim Chandler, John Buxton, Mike Eldridge, and Warren 

Huggins of the Maine Department of Transportation; Mark 

Hews and Dick Wood of Threshold to Maine RC&D; Peter 

Lammert of the Maine Forest Service; and Harold Bumby of 

Maine Wood Treaters. 

Contents

 Page

Introduction............................................................................1

Background............................................................................2

Objective and Scope ..............................................................2

Design and Construction........................................................2

Design................................................................................2

Construction.......................................................................4

Evaluation Methodology........................................................6

Moisture Content ...............................................................6

Bar Force ...........................................................................6

Behavior Under Static Load...............................................8

Condition Assessment........................................................9

Results and Discussion...........................................................9

Moisture Content ...............................................................9

Bar Force .........................................................................10

Behavior Under Static Load.............................................10

Load Test Comparison.....................................................11

Predicted Response..........................................................11

Condition Assessment......................................................13

Conclusions..........................................................................13

References............................................................................14

Appendix—Information Sheet .............................................15
Field Performance of Timber Bridges 

20. Gray Stress-Laminated Deck Bridge 

Habib J. Dagher, Director of Advanced Engineered Wood Composites Center 

and Professor of Civil Engineering 

University of Maine, Orono, Maine 

Frank M. Altimore, Engineering Research Consultant 

Greenville, Maine 

Vincent Caccese, Associate Professor of Mechanical Engineering 

University of Maine, Orono, Maine 

Michael A. Ritter, Research Engineer 

Forest Products Laboratory, Madison, Wisconsin 

Richard M. Hebert, Engineer 

T.Y. Lin International, Portland, Maine 

Introduction

In 1988, the U.S. Congress passed legislation known as the 

Timber Bridge Initiative (TBI). The objective of this legislation was to establish a national program to provide effective 

and efficient utilization of wood as a structural material for 

highway bridges (USDA 1995). Responsibility for the development, implementation, and administration of the TBI was 

assigned to the USDA Forest Service. To implement the 

program, the Forest Service established three primary emphasis areas: demonstration bridges, technology transfer, and 

research. Responsibility for technology transfer and demonstration bridge programs was assigned to the National Wood 

in Transportation Information Center (NWITIC), formerly 

the Timber Bridge Information Research Center, in Morgantown, West Virginia. Under the demonstration program, the 

NWITIC provides matching funds to local governments to 

construct demonstration timber bridges, which encourages 

innovation through the use of new or previously underutilized wood products, bridge designs, and/or design 

applications. 

Responsibility for the research portion of the TBI was assigned to the USDA Forest Service, Forest Products Laboratory (FPL), a national wood utilization research laboratory. 

As part of this broad research program, FPL assumed a lead 

role in assisting local governments in evaluating the field 

performance of demonstration timber bridges, many of which 

use design innovations or materials that have not been previously evaluated. Through such assistance, FPL is able to 

collect, analyze, and distribute information on the field 

performance of timber bridges, thus providing a basis for 

validating or revising design criteria and further improving 

efficiency and economy in bridge design, fabrication, and 

construction. 

In addition to the TBI, Congress passed the Intermodal 

Surface Transportation Efficiency Act (ISTEA) in 1991, 

which includes provisions for a timber bridge program aimed 

at improving the utilization of wood transportation structures. 

Responsibility for the development, implementation, and 

administration of the ISTEA timber bridge program was 

assigned to the Federal Highway Administration (FHWA). 

Because many aspects of the FHWA research program paralleled those underway at FPL, a joint effort was initiated to 

combine the respective research of the two agencies into a 

central research program. As a result, the FPL and FHWA 

merged resources to develop and administer a national timber 

bridge research program. 

This paper is 20th in a series that documents the field performance of timber bridges included in the FPL timber 

bridge monitoring program. It addresses the field performance of a chromated-copper-arsenate- (CCA-) treated stresslaminated truss bridge located in Gray, Maine. This report 

summarizes results from a 61⁄2-year field-monitoring program, which was initiated when the bridge was constructed in 

December 1991. Data were collected on wood moisture 

content, force level of stressing bars, behavior under static 

truck loading, and overall structural condition. 

The Gray bridge is a single-span, two-lane, stress-laminated 

structure that is approximately 24 ft long, 23 ft wide, and 

14 in. deep. (See Table 1 for metric conversion factors.) The 

deck was constructed from CCA-treated eastern hemlock 

grown in Maine. 
2

Background

The Gray bridge is located near Gray, Maine (Fig. 1). The 

bridge is on Merrill Road, a two-lane paved road that crosses 

Collyer brook and provides access to a local dairy farm and a 

residential area. The average daily traffic is unknown but is 

estimated to be relatively low. 

The original Gray bridge consisted of a single-lane precast 

concrete bridge supported by granite abutments. Although

the original bridge had undergone substantial deterioration, 

the primary reason for the replacement was to widen the 

structure to allow two-way traffic. 

Through a cooperative effort involving the University of 

Maine and the Maine Department of Transportation, a proposal was submitted to the USDA Forest Service to partially 

fund the Gray bridge replacement with funds from the Timber Bridge Initiative. The proposal specified a stresslaminated timber deck bridge constructed from CCA-treated 

eastern hemlock grown in Maine. 

Objective and Scope 

The objective of this project was to evaluate the field performance of the Gray bridge for 61⁄2 years. The scope included field monitoring of wood moisture content, force level 

of stressing bars, behavior under static truck loading, and 

overall structural condition of the bridge. In an effort to 

improve future design and construction methods, results 

of this project will be considered with similar monitoring 

projects. 

Design and Construction

The design and construction of the Gray bridge involved 

mutual efforts of several agencies and individuals. An overview of the design and construction of the bridge follows. 

Figure 1—Location of Gray bridge. 

Design

Design of the Gray bridge was completed by a team of engineers at the University of Maine, in cooperation with the 

Main Department of Transportation and with assistance from 

the FPL. The design featured a stress-laminated deck structure using CCA-treated No. 2 and better eastern hemlock 

laminations. For this bridge configuration, the laminations 

were placed side by side across the span. High strength steel 

bars were inserted through prebored holes in the laminations. 

The bars were tightened to provide sufficient friction to 

develop load transfer between the individual laminations 

(Figs. 2 and 3). Thus, the components were assumed to act 

together as a single unit. 

With the exception of those features related specifically to 

stress laminating, design of the Gray bridge conformed to the 

American Association of State Highway and Transportation 

Officials (AASHTO 1989) Standard Specifications for Highway Bridges for two lanes of HS20–44 loading and the 

National Forest Products Association (NFPA 1991a,b) 

National Design Specification for Wood Construction.

Table 1—Factors for converting inch–pound units of 

measurement to SI units 

Inch–pound

Conversion 

factor SI unit 

inch (in.) 25.4 millimeter (mm) 

foot (ft) 0.3048 meter (m) 

square foot ft2

 0.09 square meter (m2)

pound (lb) 4.448 Newton (N) 

lb/in2

 (stress) 6,894 Pascal (Pa) 

lb-in 0.1129 Newton meter (N-m) 

lb/ft3

 16.01 kilogram per cubic meter (kg/m3)

Gray, Maine U.S. map 

Gray

N

Gray bridge

Mayall Rd.

Rt. 115

Depot Rd. 

1⁄2 mile

Collyer Brook 

Merrill Rd.

Site plan 
3

Figure 2—Design plan for Gray bridge. 

Figure 3—Design section of Gray bridge. 
4

The Gray bridge was designed before the 1991 AASHTO 

guide specifications for stress-laminated bridge decks were 

published (AASHTO 1991). The design procedures were 

based on the 1983 Ontario Highway Bridge Design Code 

(OHBDC 1983) and research conducted at the University of 

Wisconsin (Dimakis and Oliva 1987, 1988). 

The design procedure assumes that the wheel load from an 

HS20–44 wheel line is distributed over a 60-in. width of the 

bridge. Based on this distribution width, the stresses and 

deflections were calculated using the basic principles of 

mechanics. 

The design geometry provided for a single-span superstructure 24 ft long, 23 ft wide, and 14 in. deep at a 0° skew. The 

depth of the bridge was limited to 14 in. because of clearance 

constraints at the site. The design specified No. 2 and better 

eastern hemlock laminations. Prior to fabrication, all wood 

members were cut and drilled, then pressure treated with a 

CCA–type III preservative to a minimum retention of 

0.60 lb/ft3

. Not enough full-length laminations to complete 

the bridge were available at the time of construction; therefore, butt joints were specified in some laminations. The 

design specified one butt joint in every fourth lamination 

separated by 10.5 ft. 

For stress laminating, the design specified 5/8-in.-diameter, 

epoxy-coated, high strength steel threaded bars with an ultimate strength of 150,000 lb/in2

. The design bar force of 

26,000 lb provides an interlaminar compressive stress of 

77 lb/in2

. The stressing system was designed for eleven 

5/8-in.-diameter, high strength steel threaded bars. The spacing of the bars was 24 in. on center, beginning 24 in. from 

the bridge ends. The bar anchorage system used an MC 13 by 

40 bulkhead channel with two types of bar anchorage. Five 

of the 11 bars were anchored using square 8- by 8- by 1⁄2-in. 

galvanized steel bearing plates (Fig. 4). The remaining 6 bars 

were anchored with 10-in.-diameter Belleville springs with 

a 7/8-in.-thick, 10-in.-diameter bearing plate (Fig. 5). 

The springs consist of four sets of two Key Belleville 

K1000–U–312 springs grouped to form stacks on 6 of the 

11 bars. The springs were installed to help reduce bar force 

loss from stress relaxation and reduction in wood moisture 

content. All components of the stressing system were provided with corrosion protection. The stressing bars and nuts 

were epoxy coated, and the steel bearing channels and bearing plates were galvanized. 

Design of the curb and rail system was based on a crashtested railing developed for longitudinal, spike-laminated 

timber decks in accordance with AASHTO Performance 

Level 1 criteria (FHWA 1990). The bridge curb and rail were 

specified to be roughsawn lumber, measuring 12- by 12-in. 

and 6- by 12-in., respectively, and pressure treated with a 

CCA–type III preservative to a minimum retention of 

0.60 lb/ft3

. Rail posts were designated to be CCA-treated 

roughsawn lumber measuring 8- by 12-in. and were spaced 

48 in. on center. 

To protect the bridge from moisture, one coat of adhesive 

primer was specified to be painted directly onto the wood 

deck, followed by the installation of two layers of waterproof 

membrane. The pavement was specified to consist of a 3-in.-

thick hot bituminous wearing surface. An information sheet 

on specific bridge characteristics and material specifications 

is provided in the Appendix. 

Construction

Construction of the Gray bridge was completed by a local 

contractor in the fall of 1991. Following work on the approach alignment and rehabilitation of the bridge abutments, 

construction of the bridge superstructure began October 31, 

1991, and was completed December 3. The bridge was preassembled at the site on temporary timber supports placed 

adjacent to the east bridge abutment (Fig. 6). To construct 

the deck, laminations were placed edgewise across the supports. Steel prestressing rods were inserted through the 

Figure 4—Steel channel and discrete plate bar 

anchorage configuration.

Figure 5—Steel channel and Belleville spring bar 

anchorage configuration. 
5

predrilled holes in the laminations. As the prestressing rods 

were advanced, timbers were placed over the ends of the 

rods. This process of advancing the rods and adding timber 

was continued until the entire deck was assembled (Fig. 7). 

Steel bearing channels were installed on the bridge edge, 

followed by the Belleville springs, anchorage plates, and 

nuts. 

Following installation of the stressing system, the bridge was 

stressed to the full design value of 77 lb/in2

 (Fig. 8). During 

the initial stressing procedure, the bars were sequentially 

tightened by a hydraulic jack to the desired stress value. The 

same process was used to stress the bridge at 1 week, 

6 weeks, 4 years, and 61⁄2 years after installation. After the 

deck was stressed, a 12- by 12-in. CCA-treated eastern hemlock curb was bolted to each edge of the deck. Prior to placing the deck, galvanized steel tie-down angles were bolted to 

the sides of the concrete cap of each abutment and an 

elastometric bearing pad was placed on top of each concrete 

abutment cap (Fig. 9). The entire deck was lifted from the 

temporary supports with a large overhead crane (Fig. 10). 

The deck was moved over the span (Fig. 11) and carefully 

placed on the abutments (Fig. 12). 

The superstructure was attached by bolting the bottom of the 

bridge deck to the steel tie-down angles attached to the 

abutment sides. Following attachment of the deck, the timber 

post and rails were installed (Fig. 13). 

To protect the bridge from moisture, one coat of adhesive 

primer was painted directly onto the wood deck, followed by 

the installation of two layers of waterproof membrane. The 

waterproofing membrane was wrapped over the backwalls to 

completely seal the top surface of the structure from moisture. The bridge was paved with a 3-in.-thick hot bituminous 

wearing surface. The completed Gray bridge is shown in 

Figure 14. 

Figure 6—Preassembly of bridge at site on temporary 

timber supports placed adjacent to east bridge 

abutment.

Figure 7—Insertion of prestressing rods through 

predrilled holes in laminations. 

Figure 8—After the stressing system was installed, 

the bridge was stressed to the full design value. 

Figure 9—Prior to placing the deck, galvanized steel 

tie-down angles were bolted to the sides of the concrete cap of each abutment. 
6

Evaluation Methodology 

To evaluate the field performance of the Gray bridge, a plan 

was developed by the University of Maine in cooperation 

with the FPL. The plan called for two static load tests of the 

completed structure and monitoring of the moisture content 

of the deck, stressing bar force, and general bridge condition. 

The evaluation methodology utilized procedures and equipment previously developed and used on similar structures 

(Caccese and others 1991, 1993; Dagher and others 1991, 

Ritter and others 1991). 

Moisture Content 

The moisture content of the Gray bridge was measured using 

an electrical-resistance moisture meter with 2-in. probe pins 

in accordance with ASTM D4444–84 (ASTM 1990). Measurements were obtained from several locations on the bridge 

superstructure by driving the probe pins into the wood approximately 1 in., recording the moisture content values, and 

adjusting the values for temperature and wood species. 

Moisture content readings were taken at the time of bridge 

installation and during the condition assessments. 

Bar Force 

Stressing bar force was measured with calibrated steel load 

cells developed at the University of Maine (Fig. 15) and with 

a hydraulic jack during the scheduled retensionings. The load 

cells were installed on five bars prior to the initial construction tensioning. Load cell measurements were obtained using 

a computer-controlled data acquisition system. Strain measurements were converted to units of bar tensile force by 

applying a calibration factor to the strain reading. Bar force 

measurements were also obtained from five bars prior to each 

retensioning by noting the jack pressure required to move the 

stressing nut away from the anchorage plate of each bar. The 

jack pressure was converted to bar force by applying a laboratory calibration factor to the pressure value. 

Figure 10—The entire deck was lifted from the 

temporary supports with a large overhead crane. 

Figure 11—The deck was moved by crane over 

the span. 

Figure 12—The deck was placed by crane onto the 

abutments.

Figure 13—Gray bridge following installation of the 

post and rail system. 
7

 Figure 14—Completed Gray bridge. 
8

Behavior Under Static Load 

To determine the response of the Gray bridge to highway 

truck loads, static-load testing was conducted immediately 

before the bridge was opened to traffic and approximately 

4 years afterward. In addition, the maximum predicted deflection was determined on the basis of static analysis for 

HS20–44 loading. Load testing involved positioning one or 

two fully loaded dump trucks on the bridge and measuring 

the resulting deflections at a series of locations along the 

bridge centerspan and abutment cross sections. 

Deflection measurements for load test 1 were obtained using 

a surveyor’s level and rulers attached to stiff boards mounted 

to the underside of the bridge (Fig. 16). Deflection measurements for load test 2 were obtained using displacement transducers mounted to a temporary support erected under the 

centerspan of the bridge. The transducer measurements were 

read with a voltmeter and converted to units of displacement 

by applying a laboratory calibration factor. Deflection 

measurements were obtained prior to each loading (unloaded 

bridge) and after placement of the test trucks (loaded bridge) 

for each load case. Each load case was carried out twice, and 

the results were averaged. Deflection measurements were 

also obtained at the conclusion of the load testing (unloaded 

bridge). 

Load Test 1 

Load test 1, conducted December 3, 1991, used three load 

cases and two fully loaded, three-axle dump trucks: truck A, 

with a gross vehicle weight of 56,900 lb, and truck B, with a 

gross vehicle weight of 58,700 lb (Fig. 17). For load cases 1, 

2, and 3, the trucks were positioned transversely 2 ft from the 

centerline of the bridge. For all load cases, the truck center 

of gravity was positioned at midspan and deflections were 

Figure 15—One of five load cells installed on stressing 

bars to monitor changes in bar tension. 

Figure 16—Load test 1. Bridge deflections were 

measured using a surveyor’s level and rulers attached 

to stiff boards mounted to the underside of the bridge.

Figure 17—Truck weights, axle spacings, and transverse 

load test positions for load test 1. 

Load Case 1 

 Truck B 

Load Case 2 

 Truck A 

Load Case 3 

 Truck A Truck B 

Truck A 

Truck B 

CL 2 ft 





















2 ft CL





















2 ft 

CL







 



 



 



 

2 ft CL



 



 



 



 

2 ft 

4.5 ft 13.8 ft 

21,300 lb 22,000 lb 13,600 lb 

GVW=56,900 lb 

4.8 ft 13.3 ft 

22,100 lb 22,400 lb 14,200 lb 

GVW=58,700 lb 
9

measured to within 0.06 in. One of the two trucks used for 

load test 1 is shown in Figure 18. 

Load Test 2 

Load test 2, conducted November 7, 1995, used three load 

cases and two fully loaded, three-axle dump trucks: truck A, 

with a gross vehicle weight of 61,100 lb, and truck B, with a 

gross vehicle weight of 60,650 lb (Fig. 19). For load cases 1, 

2, and 3, trucks were positioned transversely 2 ft from the 

centerline of the bridge. For all load cases, the truck center of 

gravity was positioned at midspan and deflections were 

measured to within 0.01 in. 

Condition Assessment 

The general condition of the Gray bridge was assessed on 

three occasions during the monitoring period. The first assessment was December 3, 1991, during the first load test. 

The second assessment took place November 7, 1995, during 

the second load test after approximately 4 years of service. 

The third assessment occurred April 7, 1998, after approximately 61⁄2 years of service. The condition assessments involved visual inspections, measurements, and photographic 

documentation. Items of specific interest included the bridge 

geometry, wood components, wearing surface, and stressing 

bar system. 

Results and Discussion 

The following results are based on data collected during the 

61⁄2-year monitoring program for the Gray bridge. 

Moisture Content 

The average trend in wood moisture content is presented in 

Figure 20. At the initiation of the monitoring, the average 

moisture content was approximately 17%. After 4 years of 

service, the moisture content decreased to 13%. After 

Figure 18—One of the two trucks used for load test 1. 

Figure 19—Truck weights, axle spacings, and 

transverse load test positions for load test 2. 

Figure 20—Average trend in moisture content, 

October 1991 to 1998. 

0

5

10

15

20

91 92 93 94 95 96 97 98

 Year

Average moisture content (%)

Load Case 1 

 Truck B 

Load Case 2 

 Truck A 

Load Case 3 

 Truck A Truck B 

Truck A 

Truck B 

CL 2 ft 





















2 ft CL





















2 ft 

CL







 



 



 



 

2 ft CL



 



 



 



 

2 ft 

4.4 ft 13.3 ft 

22,200 lb 22,600 lb 16,300 lb 

GVW=61,100 lb 

4.3 ft 13.5 ft 

22,100 lb 22,100 lb 16,450 lb 

GVW=60,650 lb 
10

61⁄2 years, the moisture content seemed to have stabilized at 

approximately 12%. Moisture content measurements and 

visual inspections of the wood indicated that the waterproof 

membrane and pavement crown have been effective in protecting the bridge from water. 

Bar Force 

The average trend in bar force is shown in Figure 21. For 

stress-laminated structures to perform efficiently, adequate 

bar force must be maintained to prevent interlaminar slip. 

The bar force was expected to decrease after construction; 

therefore, the bridge was retensioned to the full design value 

of 26,000 lb or 77 lb/in2

 interlaminar compression after 

1 week, 6 weeks, 4 years, and 61⁄2 years of service. 

Data collected during the first retensioning indicated that the 

average bar force had decreased 35% to approximately 

50 lb/in2

 interlaminar compression during 1 week. Data 

collected during the second retensioning indicated that the 

average bar force had decreased approximately 40% to 

45 lb/in2

 interlaminar compression during 5 weeks. 

Measurements taken 46 months after the second retensioning 

indicated that the bar force had decreased 60%, to approximately 30 lb/in2

 interlaminar compression. Subsequently, the 

bars were retensioned. Bar force measurements taken approximately 30 months after the third retensioning indicated 

that the bar force had again decreased approximately 60%, to 

30 lb/in2

 interlaminar compression. Therefore, the bars were 

tensioned again. 

Bar force was expected to decrease as a result of the combined effects of a decrease in wood moisture content and 

stress relaxation. The 5% decrease in moisture content 

caused wood shrinkage, which was probably most significant 

during the first half of the monitoring period when the greatest moisture content loss occurred. Stress relaxation in the 

laminations has been observed to cause bar force loss in 

numerous other stress-laminated bridges (Ritter and others 

1991).

The data indicated that the bar force decreased approximately 60%, to 30 lb/in2

, prior to the last two retensionings. 

The AASHTO guide specifications for stress-laminated 

bridges require a bar force of 37 lb/in2

 for this type of bridge. 

It is probable that the bar force will continue to decrease, and 

future bar retensionings will be necessary to maintain an 

adequate level of bar force in the Gray bridge. 

Behavior Under Static Load 

Results of static load testing and predicted response are 

presented in the following text. For each load case, transverse deflections are given at the centerspan of the bridge as 

viewed from the south end (looking north). No permanent 

residual deflection was measured between load cases or at 

Figure 21—Average trend in prestress level, 

October 1991 to 1998. 

the conclusion of load testing. In addition, no significant 

movement was detected at the bridge supports during testing. 

At the time of load tests 1 and 2, the average bridge 

interlaminar compressive stress was 77 and 30 lb/in2

,

respectively. 

Load Test 1 

Transverse deflections from load test 1 (which was conducted December 3, 1991) at a prestress level of 77 lb/in2

 are 

shown in Figure 22. The maximum deflection for load case 1 

occurred under the outside truck wheel line and measured 

0.21 in. (Fig. 22a). The maximum deflection for load case 2 

occurred between the truck wheel lines and measured 0.34 in. 

(Fig. 22b). The maximum deflection for load case 3 of 

0.41 in. occurred between the truck B wheel lines and represented the largest deflection for all load cases (Fig. 22c). 

Assuming accurate load test results and linear elastic behavior, the sum of the deflections resulting from individual 

truckloads should equal the deflections from both trucks 

applied simultaneously. Figure 23 shows the load test 1 

comparison of individual and simultaneous truck loading. As 

shown in this figure, the two plots are similar, with variations 

within the accuracy of the measurements. From this information, it is concluded that the bridge behavior is within the 

linear elastic range. 

Load Test 2 

Transverse deflections from load test 2, which was conducted 

November 7, 1995, are shown in Figure 24. The maximum 

deflection for load case 1 occurred between the truck wheel 

lines and measured 0.37 in. (Fig. 24a). The maximum deflection for load case 2 also occurred between the truck wheel 

lines and measured 0.32 in. (Fig. 24b). The maximum deflection for load case 3 of 0.49 in. occurred under the inside 

wheel line of truck A and represented the largest deflection 

of all load cases (Fig. 24c). 

0

5

10

15

20

91 92 93 94 95 96 97 98

 Year

Average moisture content (%)
11

Figure 25 shows the load test 2 comparison of individual 

and simultaneous truck loading. The two plots are nearly 

identical, again indicating that the bridge behavior is within 

the linear elastic range. 

Load Test Comparison 

A comparison of measured deflections for both load tests is 

presented in Figure 26 for load case 3. The plots are similar 

in shape, but the deflections measured in load test 2 are 

greater at all but a few data points than the deflections for 

load test 1. The maximum measured deflection for load test 1 

of 0.41 in. was 16% less than the maximum measured deflection of 0.49 in. for load test 2. Several factors, including 

differences in the truck size and weight and in the prestress 

level, may have contributed to the differences in deflection. 

The trucks used for load test 2 were approximately 5% heavier than the trucks used for load test 1, which would increase 

the deflections in load test 2. In addition, slight differences in 

the test truck configurations could have resulted in slight 

variations in the deflections. The 60% reduction in interlaminar compression for load test 2, which tends to reduce the 

bridge transverse stiffness and therefore increase deflections, 

contributed to the increased deflections for load test 2. The 

comparison of load test 1 and 2 showed that after adjusting 

for the 5% difference in truck weights, the maximum deflection increased only 12% when the prestress level was reduced from 77 to 30 lb/in2

.

Predicted Response 

Table 2 summarizes the maximum measured deflections for 

both load tests, e allowable deflection for design purposes, 

and the predicted deflection for AASHTO HS20–44 truck 

loading. The measured values were obtained from load 

case 3, where the maximum load test deflection occurred. 

The allowable deflection for design purposes was L/400 or 

Figure 22—Tranverse deflections for load test 1, 

measured at bridge centerspan (looking north). Bridge 

cross-sections and vehicle positions are presented for 

the purpose of interpretation only and are not drawn to 

scale.

Figure 23—Load test 1 comparison: sum of measured 

deflections from load cases 1 plus 2 compared with 

deflections from load case 3. 

Truck A Truck B 

-1.00

-0.75

-0.50

-0.25

0.00

-15 -10 -5 0 5 10 15

Distance from centerline (ft)

Deflection (in.)

LC1+2 LC3





































-1.00

-0.75

-0.50

-0.25

0.00

-15 -10 -5 0 5 10 15

(a) Load case 1 

Deflection (in.)







 



 



 



 

0.21 in.

Truck B 

-1.00

-0.75

-0.50

-0.25

0.00

-15 -10 -5 0 5 10 15

Distance from centerline (ft) 

(c) Load case 3 

Deflection (in.)





































0.41 in.

Truck A Truck B 

0.16 in.

-1.00

-0.75

-0.50

-0.25

0.00

-15 -10 -5 0 5 10 15

(b) Load case 2 

Deflection (in.)





 



 



 



 

0.34 in.

Truck A 
12

0.69 in., where L is the bridge span, measured center–center 

of bearings. The maximum predicted deflection of 0.49 in. 

was obtained by assuming that one wheel line from an 

HS20–44 truck was carried by a 60-in. width of the bridge. 

Figure 25—Load test 2 comparison: sum of deflections 

from load cases 1 plus 2 compared with deflections from 

load case 3.

Figure 26—Comparison of load test 1 and 2 deflections. 

Table 2 shows that the maximum measured and predicted 

deflections did not exceed the design limit of L/400 or 

0.69 in. The maximum predicted HS20–44 deflection of 

0.49 in. resulted in a span deflection ratio of L/540, which 

was lower than the design limit of L/400. Figure 24—Transverse deflections for load test 2, 

measured at bridge centerspan (looking north). Bridge 

cross-sections and vehicle positions are presented for 

the purpose of interpretation only and are not drawn to 

scale.

Table 2—Summary of load test, allowable, and 

predicted deflections

Load

test

Maximum 

measured

load test 

deflection

(in.)

Maximum 

allowable 

deflection

(in.)

Maximum 

predicted

HS20–44 

deflection

(in.)

1 0.41 0.69 0.49 

2 0.49 0.69 0.49 

-1.00

-0.75

-0.50

-0.25

0.00

-15 -10 -5 0 5 10 15

Distance from centerline (ft)

Deflection (in.)

LC1+2

LC3







 



 



 



 



 



 



 



 

Truck A Truck B 

-1.00

-0.75

-0.50

-0.25

0.00

-15 -10 -5 0 5 10 15

Distance from centerline (ft)

Deflection (in.)

Load Test 1, LC3

Load Test 2, LC3





































Truck A Truck B 

-1.00

-0.75

-0.50

-0.25

0.00

-15 -10 -5 0 5 10 15

(a) Load case 1 

Deflection (in.)





 



 



 



 

0.37 in.

Truck B 

0.16 in.

-1.00

-0.75

-0.50

-0.25

0.00

-15 -10 -5 0 5 10 15

(b) Load case 2 

Deflection (in.)





 



 



 



 

0.32 in.

Truck A 

-1.00

-0.75

-0.50

-0.25

0.00

-15 -10 -5 0 5 10 15

Distance from centerline (ft) 

(c) Load case 3 

Deflection (in.)





































0.49 in.

Truck A Truck B 
13

Condition Assessment 

General condition assessments indicated that the structural 

and serviceability aspects of the Gray bridge are satisfactory. 

The areas subjected to inspection were bridge geometry, 

wood components, wearing surface, and anchorage system. 

Bridge Geometry 

Width measurements taken at the initiation of monitoring 

indicated that the stress-laminated structure was 1 in. narrower at the east abutment than at the west abutment. This 

was probably due to the sequential bar tightening with a 

single jack. In addition, the bridge was approximately 2 in. 

narrower at midspan than at the abutments. This slight distortion should not affect overall bridge performance. 

Wood Components 

Visual inspection of the wood components of the bridge 

indicated no signs of deterioration. However, minor damage 

to the curb, probably from a snowplow, was noted during the 

third condition assessment. 

Wearing Surface 

The asphalt-wearing surface is in good condition, with minor 

transverse reflective cracking visible over the bridge abutments. This is typical for single-span bridges and was expected. Longitudinal asphalt rutting or cracking was not 

evident. 

Anchorage System 

The continuous steel channel anchorage system is performing 

satisfactorily. No signs of wood crushing were visible beneath the channels. Surface rust was visible on some steel 

components in areas where the epoxy coating had chipped 

off. It is recommended that these areas be brush coated with 

an approved epoxy paint. 

Conclusions

Based on the results of this research, we present the following conclusions and recommendations: 

• Data collected during this research program indicate that 

the performance of the Gray bridge is satisfactory. With 

the exception of having to be retensioned approximately 

every 3 years, the bridge is performing well with no 

structural or serviceability deficiencies evident. 

• During the 61⁄2-year field monitoring, wood moisture 

content decreased gradually from approximately 17% 

to 12%. Based on moisture content readings and visual 

inspections, it is concluded that the waterproof membrane has been effective in protecting the bridge from 

moisture. 

• The bridge was retensioned to the full design value of 

77 lb/in2

 four times during the 61⁄2-year field monitoring, 

The bar force had decreased to 30 lb/in2

 prior to each of 

the last two retensionings. 

• Static load testing indicated that the Gray bridge is 

performing in the linear elastic range when subjected to 

two 61,000-lb trucks positioned with their center of 

gravity at midspan. 

• The predicted deflection of 0.49 in. for HS20–44 loading was below the design limit of 0.69 in. or L/500, 

where L is the span length measured center–center of 

bearings. 

• For load test 1, which was conducted at a bar force level 

of 77 lb/in2

, the maximum measured deflection from two 

58,000-lb trucks positioned with their center of gravity 

at midspan was 0.41 in. This is below the design limit of 

0.69 in. or L/400. 

• For load test 2, which was conducted at a bar force level 

of 30 lb/in2

, the maximum measured deflection from two 

61,000-lb trucks positioned with their center of gravity 

at midpan was 0.49 in. This is also below the design 

limit of 0.69 in. or L/400. 

• The load test comparison of load tests 1 and 2 shows 

that, after adjusting for the slight difference in the truck 

weights, the maximum deflection increased only 12% 

when the bar force was reduced from 77 to 30 lb/in2

.

• Visual inspections indicated no signs of deterioration of 

the wood components. Surface rust is visible on some 

of the stressing system hardware in the vicinity of the 

anchorage nuts. 

• There was no measurable difference in performance 

between the stressing bars equipped with Belleville 

springs and the stressing bars that were not equipped 

with Belleville springs. 

• Assessment of wood moisture content, bar force, and 

general condition of bridge components should 

be performed on an annual basis. 

• During annual assessments, the bars should be retensioned to the full design value of 26,000 lb if the bar 

force is less than 13,000 lb. 

• Areas of the stressing system hardware where the epoxy 

coating has chipped off should be brush coated with an 

approved epoxy-based paint. 
14

References

AASHTO. 1989. Standard specifications for highway 

bridges. 14th ed. Washington, DC: American Association of 

State Highway and Transportation Officials. 

AASHTO. 1991. Guide specifications for the design of 

stress-laminated wood decks. Washington, DC: American 

Association of State Highway and Transportation Officials. 

ASTM. 1990. Use and calibration of hand-held moisture 

meters. ASTM D 4444–84. Philadelphia, PA: American 

Society for Testing and Materials. 6 p. 

Caccese, V.; Dagher, H.; Hebert R. 1991. Design and 

monitoring of a CCA-treated stressed timber bridge deck. 

Forest Products Journal. 41(11/12): 71–78. 

Caccese, V.; Dagher, H.; Light K. 1993. Remote monitoring of a stressed timber bridge deck. In: Proceedings, 1993 

ASCE Structures Congress, Irving, CA. Orono, ME: University of Maine, Departments of Mechanical and Civil 

Engineering: 7 p. 

Dagher, H.; Caccese, V.; Hebert, R.; Schwartz, D. 1991. 

Feasibility of CCA-treated stressed timber bridge decks, 

Forest Products Journal. 41(10): 60–64. 

Dimakis A. G.; Oliva M. G. 1987. Behavior of posttensioned wood bridge decks: Full scale testing and analytical correlation. Rep. 87–2. Madison, WI: University of 

Wisconsin.

Dimakis A. G.; Oliva M. G. 1988. Behavior stresslaminated timber highway bridges. Journal of Structural 

Engineering. 114/8. 

FHWA. 1990. Memorandum on crash tested bridge railings. 

August 13, 1990. File Designation HNG–14. Washington, 

DC: Federal Highway Administration. 

NFPA. 1991a. Design values for wood construction: a 

supplement of the national design specification for wood 

construction. Washington, DC: National Forest Products 

Association. 125 p. 

NFPA. 1991b. National design specification for wood 

construction. Washington, DC: National Forest Products 

Association. 51 p. 

OHBDC. 1983. Ontario highway bridge design code. Toronto, Ontario: Ontario Highway Engineering Division. 

Ritter, M.A.; Geske, E.A.; Mason, L. [and others]. 1991. 

Methods for assessing the field performance of stresslaminated timber bridges. In: Maarcroft, J., comp. Proceedings, 1991 International timber engineering conference; 

1991, September 2–5; London, England. High Wycombe, 

United Kingdom: Timber Research and Development 

Association. 

USDA. 1995. The timber bridge initiative, fiscal year 1995 

status report. Radnor, PA: U. S. Department of Agriculture, 

Forest Service, State and Private Forestry, Northeastern 

Area. 10 p. 
15

Appendix—Information Sheet 

General

Location: Gray, Maine 

Date of construction: December 1991 

Owner: Maine Department of Transportation 

Configuration

Structure type: Stress-laminated solid timber deck 

Butt joint frequency: 1 in 4 laminations transversely, 

separated 10.5 ft longitudinally 

Total length (out–out): 24 ft 

Skew: 0°

Number of spans: 1 

Span length (center–center of bearings): 23 ft 

Width (curb–curb): 23.7 ft (as built) 

Number of traffic lanes: 2 

Design loading: AASHTO HS20–44 

Camber: 0 in. 

Wearing surface: asphalt pavement, 3 in. thick 

Material and Configuration 

Truss Laminations: 

Species: Eastern hemlock 

Size: 2- by 14-in. roughsawn 

Grade: Number 2 or better 

Moisture condition: Approximately 17% at initiation 

of monitoring 

Preservative treatment: CCA–type III, minimum retention 

of 0.60 lb/ft3

Stressing Bars: 

Diameter: 5/8 in. 

Number: 11 

Design force: 26,000 lb 

Spacing (center–center): 24 in. 

Type: High strength steel threaded bar 

(Dywidag Systems International, Lincoln Park, NJ) 

Bar Anchorage Type: 

Continuous MC 13 × 40 steel channels with two types of 

bearings: Type 1: 8- by 8- by 3/4-in. steel anchorage plates 

 Type 2: 10-in.-diameter steel Belleville springs 

Rail and Curb System: 

Design: Crash-tested at AASHTO Performance Level 1 on 

a longitudinal spike-laminated deck. 

Species: Eastern hemlock 

Member sizes: Rails: 6- by 12-in. sawn timbers 

Posts: 8- by 12-in. sawn timbers 

Curbs: 12- by 12-in. sawn timbers 

Preservative treatment: CCA–type III, minimum retention 

of 0.60 lb/ft3

Waterproof Membrane System: 

Membrane waterproofing Type 10A 

Membrane waterproofing Type 108 ARN