Field Performance 

of Timber Bridges 

18. Byron Stress-Laminated 

Truss Bridge 

Habib J. Dagher 

Frank M. Altimore 

Vincent Caccese 

Michael A. Ritter 

United States

Department of 

Agriculture 

Forest Service 

Forest 

Products 

Laboratory 

National Wood 

in Transportation 

Information 

Center 

Research 

Paper 

FPL−RP−588

 

In cooperation 

with the 

United States 

Department of 

Transportation 

Federal 

Highway 

Administration 
Abstract 

The Byron bridge was constructed in the fall of 1993 in

Byron, Maine. The bridge is a single-span, two-lane, stresslaminated truss structure approximately 46 ft long and 32 ft 

wide. The truss laminations were produced using chromatedcopper-arsenate- (CCA-) treated Southern Pine connected 

with metal plate connectors. This report includes information 

on the design, construction, and field performance of the 

bridge. Field performance was monitored for approximately

5 years, beginning shortly after bridge construction. Performance monitoring involved collecting and evaluating data 

relative to wood moisture content, force level of stressing 

bars, behavior under static truck loading, and overall structural condition. The field evaluations showed that the Byron 

bridge is performing well, with no structural or serviceability

deficiencies. 

Keywords: timber, bridge, wood, stress laminated, truss 

December 2000 

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

2000. Field performance of timber bridges. 18. Byron stress-laminated truss 

bridge. Res. Pap. FPL–RP–588. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory. 20 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 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 

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Room 326-W, Whitten Building, 1400 Independence Avenue, SW, Washington, D.C. 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 USDA Forest 

Service, Forest Products Laboratory, with the University of 

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

Department of Transportation. We express sincere appreciation to the following individuals from the Maine Department 

of Transportation for their assistance in various phases of the 

project: Joseph Larrabee, bridge design; Mark Stanewick and 

Dick Lauze, bridge construction; and Mike Eldridge and 

Warren Huggins, bridge stressing and static load tests. 

Thanks are also due to the representatives of various 

organizations: Dave Matychowiak, Wood Structures Inc., 

Biddeford, Maine; Steve Cabler, MiTek Industries, St. Louis, 

Missouri; Peter Krakoff, CPM Constructors, Freeport, 

Maine; Harold Bumby, Maine Wood Treaters, Mechanic 

Falls, Maine; Mark Milici, Dywidag Systems International, 

Lincoln Park, New Jersey; Stewart Lewis, Alpine Engineering Products, Inc., Pompano Beach, Florida; and Ronald 

Wolfe, Forest Products Laboratory, Madison, Wisconsin. 

Contents 

 Page 

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

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

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

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

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

Construction.......................................................................6

Evaluation Methodology........................................................8

Moisture Content ...............................................................8

Bar Force ...........................................................................8

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

Condition Assessment......................................................10

Results and Discussion.........................................................10

Moisture Content .............................................................10

Bar Force .........................................................................12

Behavior Under Static Load.............................................13

Condition Assessment......................................................17

Conclusions..........................................................................17

References............................................................................18

Appendix—Information Sheet.............................................20
Field Performance of Timber Bridges 

18. Byron Stress-Laminated Truss 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 

USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin 

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 bridges 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 that encourage innovation 

through the use of new or previously underutilized wood 

products, bridge designs, and 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 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 for 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 research 

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, FPL and FHWA merged 

resources to develop and administer a national timber bridge 

research program. 

This paper, 18th in a series, documents the field performance 

of timber bridges included in the FPL timber bridge monitoring program. It addresses the field performance of a stresslaminated truss bridge treated with chromated copper arsenate (CCA). This report summarizes the results from a 5-year 

field monitoring program, which was initiated when the 

bridge was constructed near Byron, Maine, in November 

1993. 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. 

The Byron bridge is a single-span, two-lane structure that is 

approximately 46 ft long, 32 ft wide, and 36 in. deep. (See 

Table 1 for metric conversion factors.) The truss laminations 

were produced from CCA-treated Southern Pine and metal 

plate connectors. This bridge is the second known stresslaminated structure to be constructed from metal plate connector truss laminations. The first known bridge of this type 

was constructed in Tuscaloosa County, Alabama, in 1992 

(Triche and others 1994). 
2 

Background 

The Byron bridge is located near Byron, Maine, on State 

Highway 17, a two-lane, paved road that crosses the west 

branch of the Swift River, providing access to several popular recreation areas and serving as a primary route for logging traffic to area mills (Fig. 1). Average daily traffic varies 

seasonally; it was estimated to be 505 vehicles in 1989, and 

will probably increase to 810 vehicles by 2009. 

The original Byron bridge, which was constructed in 1937, 

consisted of steel stringers with a nail-laminated timber deck 

supported by log crib abutments. The bridge was 40 ft long 

and 22 ft wide, and it included a rail system constructed of 

steel angles and channels. In 1969, the abutments were replaced with ones identical to those used in 1937. The timber 

deck was replaced with a noncomposite, reinforced concrete 

deck, and the existing structural steel and rail system was reused. In 1991, inspection of the bridge indicated that the 

substructure was failing, and the bridge was rated structurally 

deficient. 

Through a cooperative effort involving the Maine Department of Transportation, FPL, and the University of Maine, 

a proposal was submitted to FHWA to partially fund the 

Byron bridge replacement. The proposal specified a stresslaminated timber truss. 

Objective and Scope 

The objective of this project was to evaluate the field performance of the Byron bridge for 5 years. The scope included the development and verification of a transverse load 

distribution analysis procedure and field monitoring of wood 

moisture content, force level of stressing bars, behavior 

under static truck loading, and overall structural condition. 

The results of this project will be compared with those from 

similar monitoring projects in an effort to improve future 

design and construction methods. 

Figure 1—Location of Byron bridge. 

Design and Construction 

The design and construction of the Byron bridge involved 

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

follows. 

Design 

The design was completed by a team of engineers at the 

University of Maine in cooperation with the Maine Department of Transportation, with assistance from the FPL. The 

design features a stress-laminated timber truss structure 

(Figs. 2 and 3) with dimension lumber chords and webs 

connected with metal plate connectors. For this bridge configuration, the trusses were placed side by side across the 

span. High strength steel bars were inserted through the web 

openings and pre-bored holes in the top and bottom chords. 

The bars were tightened to provide sufficient friction to 

develop load transfer between the individual truss laminations. Thus, the components were assumed to act together as 

a single unit.

Table 1—Factors for converting inch–pound units of 

measurement to SI units 

Inch-pound unit 

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/cubic meter (kg/m3) 

mile (mi) 1.609 kilometer (km) Byron, Maine

U.S. map 

 

Byro n bridge 

 Swift river 

Andover Road 

 

Rt 17 

 N

 

 

Byron 

 

Rumford 1 mile 

Site plan 
3 

Figure 2—Design configuration of Byron bridge. 
4 

Figure 3—Profile of structural and spacer trusses. 

With the exception of those features related specifically to 

the stress-laminated truss, design of the Byron bridge conforms to the American Association of State Highway and 

Transportation Officials (AASHTO 1989) Standard Specifications for Highway Bridges for two lanes of HS25–44 

loading and the American Forest & Paper Association National Design Specification for Wood Construction (NFPA 

1991a,b). The Byron bridge was designed in the absence of a 

recognized design procedure for stress-laminated truss 

bridges. The primary design concerns were as follows: 

• Corrosion and stress-corrosion cracking of metal plate 

connectors 

• Fatigue of metal plate connectors 

• Transverse load distribution in a solid stress-laminated 

truss deck 

To reduce the rate of metal plate connector corrosion, the 

moisture content of a wooden bridge should be kept below 

19% and the metal plate connectors should be protected with 

an appropriate coating (Bruno and Weaver 1989). To address 

the fatigue and load distribution concerns, two separate 

research programs were conducted at the University of 

Maine. 
5 

The first research program examined the fatigue behavior of 

metal plate connector trusses in low-volume bridges (Dagher 

and others 1992, 1994; Dagher and West 1998). Nearly 

300 individual metal plate connector joints and 35 full-scale 

trusses were tested under high-cycle fatigue loading, resulting in fatigue design recommendations for metal plate 

connector joints. 

The second research program developed and verified a 

simplified transverse load analysis procedure for stresslaminated truss decks (Altimore 1995). The analysis procedure assumes that the AASHTO design truck wheel load is 

distributed transversely at a 45° angle through the thickness 

of the top chord of the bridge (Fig. 4). Therefore, normal to 

the direction of the span, the wheel load is distributed over 

the width of the tire plus twice the thickness of the top chord 

of the deck. For deflection calculations, the distribution 

width was increased by 15%, paralleling the AASHTO recommendation for solid stress-laminated decks (AASHTO 

1991). A two-dimensional model was used to analyze the 

trusses, with top and bottom chords modeled as continuous 

beam elements and the webs as truss elements. 

Prior to construction of the bridge, the transverse load distribution assumptions were verified to be conservative through 

laboratory testing of an 8-ft-wide by 46-ft-long model of the 

Byron bridge. Results indicated that at a bar force of 

125 lb/in2

, the transverse load distribution analysis overpredicted the maximum stress by 39% and the maximum deflection by 28%. 

The design geometry of the Byron bridge consisted of a 

single-span superstructure 46 ft long, 32 ft wide, and 36 in. 

deep at an 18° skew. The depth of the trusses was limited to 

36 in. because of clearance constraints at the site. The design-specified trusses were constructed from machine-stressrated (MSR) Southern Pine and fabricated with 20-gauge 

metal plate conncetors. Grades for the various sizes of lumber used were as follows: 2 by 12 lumber, MSR 1950f–1.7E; 

2 by 10 and 2 by 8, MSR 2250f–1.9E; and 2 by 6 and 2 by 4, 

MSR 2400f–2.0E. (See Table 2 for metric conversion of 

nominal dimensions.) Prior to fabrication, all wood members 

were cut and drilled, then pressure-treated with CCA–type III 

to a minimum retention level of 0.60 lb/ft3

. 

Two truss configurations were used: structural and spacer. 

Each structural truss had 22 diagonal webs that connected the 

top and bottom chords. The structural trusses were placed 

next to the spacer trusses. The spacer trusses had deeper top 

and bottom chords compared with those of the structural 

trusses. Each spacer truss was connected with only enough 

metal plate connector joints and vertical webs to allow it to 

be handled as a single unit. The deeper chords of the spacer 

trusses covered the metal connector plates of the structural 

trusses. This detail prevented the structural truss metal plate 

connectors from withdrawing as result of cyclic loading and 

moisture and temperature changes. 

The bridge was constructed from six rectangular modules. 

Each module was staggered by 22 in. to allow for the 18°

skew. The skew required that two types of structural trusses 

be developed to maintain continuous web openings across 

the bridge for the prestressing rods. These structural trusses 

differed in the origination of the diagonal webs. For Type I 

trusses, the diagonal webs started from the top chord, and 

for Type II trusses, the webs started from the bottom chord. 

These two truss types were alternated from module to 

module. 

Two types of spacer trusses were also developed. Chord butt 

joints of Type I spacer trusses were positioned so that they 

did not coincide with chord butt joints of Type I structural 

trusses. Similarly, the position of chord butt joints of Type II 

spacer trusses did not coincide with that of chord butt joints 

of Type II structural trusses. Consequently, one butt joint 

occurred in every two laminations. The butt joints in the top 

and bottom chords of all trusses were connected with metal 

plate connectors. 

Figure 4—Simplified depiction of load distribution. 

Table 2—Metric conversion of 

dimensional lumber sizes 

Nominal (in.) Standard (mm) 

2 by 4 38 by 89 

2 by 6 38 by 140 

2 by 8 38 by 184 

2 by 10 38 by 235 

2 by 12 38 by 286 
6 

For stress laminating, the design specified 1-in.-diameter, 

epoxy-coated, high strength, steel threaded bars with an 

ultimate strength of 150 lb/in2

. The design bar force of 

50,000 lb provided an initial interlaminar compressive stress 

of 125 lb/in2

. 

Two types of anchorage systems were used. Because of the 

18° skew, the bars in the ends of the bridge did not pass 

completely through the bridge. In this region and in part of 

the central region of the bridge, the bars passed through holes 

drilled in the top and bottom chords, and they were anchored 

by a discrete plate anchorage system. The bar ends were 

sustained on continuous C 15 by 50 steel channels through 

5- by 5- by 1.25-in. epoxy-coated anchorage plates (Fig. 5a). 

In the remaining portion of the bridge, the rods passed 

through the openings in the webs and were anchored by a 

vertical tube anchorage system. The bar ends were sustained 

on continuous C 15 by 50 steel channels through 10- by 4- by 

0.50-in. structural tubes and 5- by 5- by 1.25-in. anchorage 

plates (Fig. 5b). All components of the stressing system were 

protected from corrosion. The stressing bars, nuts, and anchorage plates were coated with epoxy. The structural steel 

tubes and continuous steel channels were specified to be 

Grade 50 all-weather steel. 

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, rail, and rail 

post were specified to be constructed of glulam, which was 

treated after gluing with a CCA–type III preservative to a 

minimum retention of 0.60 lb/ft3

. The curb and rail measured 

12- by 12-in. and 6- by 12-in., respectively. Rail posts measured 8- by 12-in. and were spaced 68 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 a selfsealing waterproof membrane. The pavement was specified 

to consist of a 1-in. leveling course and an asphalt wearing 

surface that measured 3.5 in. at the crown and 1.5 in. at the 

curb. An information sheet on specific bridge characteristics 

and material specifications is provided in the Appendix. 

Construction 

After the bridge approach was aligned and the bridge abutments were rehabilitated, construction of the Byron bridge 

began October 15, 1993; it was completed November 17, 

1993. 

The CCA-treated trusses were fabricated in Biddeford, 

Maine, and transported to the bridge site on a flatbed trailer 

in bundles banded together with light metal straps. Before the 

bridge was assembled, the galvanized metal plate connectors 

of each truss were brush painted at the site with an epoxybased paint for additional protection from corrosion (Fig. 6). 

After the paint dried, the trusses were placed by crane onto 

temporary supports to form mini-modules (Fig. 7). 

The mini-modules consisted of 10 to 12 trusses and were 

formed by nailing structural trusses to spacer trusses. Figure 8 shows the assembly of a mini-module at the bridge site. 

After the trusses were nailed together, the mini-modules were 

again banded together with light metal straps. The minimodules were lifted by a crane and placed side by side to 

form the bridge deck. They were positioned to form six 

rectangular modules to account for the 18° skew. To hold the 

trusses in position, several steel bars were inserted through 

holes in the chords as the modules were assembled. After all 

the modules were assembled, the remaining steel stressing 

bars were inserted through predrilled holes in the truss 

chords and through some web openings. Steel bearing channels were installed on the edge of the bridge, followed by the 

structural tubes, anchor plates, and nuts (Fig. 9). 

After installation of the stressing system, the bridge was 

initially stressed to half the design value (63 lb/in2

). During 

stressing, each bar was individually tightened to the desired 

stress value, using a hydraulic jack; the bars were tightened 

Figure 5—Anchorage system for Byron bridge: 

(a) continuous steel channel and discrete plate bar 

configuration; (b) continuous steel channel and 

vertical structural tube bar configuration. 
7 

sequentially, beginning at one end of the bridge (Fig. 10). 

This procedure was repeated until the bars held the desired 

stress of 63 lb/in2

. The bars were retensioned at 1 week, 

6 weeks, 9 months, 22 months, and 54 months after 

installation to the full design stress of 125 lb/in2

 using the 

same procedure. 

After initial stressing was completed, we noted that the width 

of the bridge measured 31.6 ft, 5.5 in. less than specified in 

the design. As a result, four additional trusses were added to 

increase the width of the bridge. The width of a stresslaminated deck is typically increased during fabrication to 

compensate for anticipated losses caused by high compressive stresses during the stress-laminating process. The reduction of bridge width was probably due to underestimating the 

amount of compression within the gaps created from the 

metal plate connectors. The superstructure was attached by 

bolting 5- by 5- by 0.5-in. steel angles with bolts spaced 

Figure 6—Brush painting of galvanized metal plate 

connectors of each truss at site for additional 

protection from corrosion. 

Figure 7—Placement of painted trusses onto 

temporary supports for formation of mini-modules. 

Figure 8—Preassembly of laminations into minimodules at bridge site. 

Figure 9—Installation of stressing system. 

Figure 10—Tightening of stressing bars during 

stressing. 
8 

every 24 in. to the side of the substructure abutments, then 

bolting the bridge to the steel angles (Fig. 11). 

After the superstructure was attached, construction of the 

concrete backwalls was completed. The glulam curb and rails 

were installed shortly after the backwalls were poured 

(Fig. 12). To protect the bridge from moisture, one coat of 

adhesive primer was painted directly onto the deck, followed 

by the installation of two layers of self-sealing waterproof 

membrane. The waterproof membrane was wrapped over the 

backwalls to completely seal the top surface of the structure 

from moisture (Fig. 13). The bridge was paved with a 1-in. 

leveling course and a variable thickness bituminous pavement, measuring 3.5 in. at the crown and 1.5 in. at the curb. 

The completed Byron bridge is shown in Figure 14. 

Evaluation Methodology 

To evaluate the field performance of the Byron bridge, a 

5-year monitoring 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 

truss moisture content, bar force, and general condition. 

Procedures and equipment for the evaluation were developed 

previously and had been used on similar structures (Caccese 

and others 1991, 1993; Dagher and others 1991; Ritter and 

others 1991). 

Moisture Content 

The moisture content of the Byron bridge trusses 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 

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 when the bridge was installed, during the 

second load test, and at the conclusion of the monitoring 

period. 

Bar Force 

During the scheduled retensionings, bar force was measured 

with calibrated steel load cells developed at the University of 

Maine (Fig. 15) and with a hydraulic jack. The load cells 

were installed on six bars prior to the initial construction 

stressing. 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 

anchorage 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 11—Superstructure was attached by bolting 

steel angles to sides of abutments, then bolting bridge 

to steel angles. 

Figure 12—Bridge after construction of backwalls and 

installation of rail post and curb. 

Figure 13—Installation of waterproof membrane. 
9 

Figure 14—Completed Byron bridge: (a) profile of completed bridge (looking east); (b) completed 

bridge just after paving (looking south). 
10 

Behavior Under Static Load 

Static load testing was conducted immediately before the 

bridge was opened to traffic and 21 months later to determine 

the response of the bridge to highway truck loads. In addition, the maximum predicted deflection was determined for 

each load test based on static analysis for actual and HS25–

44 loading. Load testing involved positioning one or two 

fully loaded dump trucks, weighing between 54,000 and 

65,000 lb, on the bridge and measuring the resulting deflections at a series of locations along the bridge centerspan and 

abutment cross sections. Deflection measurements were 

obtained under the centerspan of the bridge using displacement transducers mounted to a temporary support consisting 

of two 2 by 8’s nailed together and supported every 6 ft 

(Fig. 16). 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 (November 25, 1993) consisted of four load 

cases and used one fully loaded three-axle dump truck with a 

gross vehicle weight of 64,750 lb (Fig. 17). For load cases 1 

and 2, the truck was positioned transversely 2 ft from the 

centerline of the bridge. For load cases 3 and 4, the truck was 

positioned transversely 6.5 ft from the roadway centerline. 

For all load cases, the truck center of gravity was positioned 

at midspan and deflections were measured to within 0.01 in. 

Load Test 2 

Load test 2 (August 7, 1995) consisted of six load cases and 

used two fully loaded three-axle dump trucks: truck A with a 

gross vehicle weight of 54,300 lb, and truck B with a gross 

vehicle weight of 54,100 lb (Fig. 18). For load cases 1, 2, 

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

centerline of the bridge. For load cases 4, 5, and 6, the trucks 

were positioned transversely 6.5 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. Load cases 3, 4, and 6 are shown in Figure 19. 

Condition Assessment 

The general condition of the Byron bridge was assessed three 

times: (1) during the first load test (November 25, 1993), 

(2) during the second load test, after 21 months of service 

(August 7, 1995), and (3) after approximately 41⁄2 years of 

service (April 6, 1998). The condition assessments involved 

visual inspections, measurements, and photographic documentation. Items of specific interest included the bridge 

geometry, wood components, wearing surface, prestressing 

system, and metal plate connectors. 

Results and Discussion 

The following results are based on data collected during the 

5-year monitoring program for the Byron bridge. 

Moisture Content 

The average trend in wood moisture content is presented in 

Figure 20; measurements were taken in September. When 

monitoring was initiated, average wood moisture content was 

approximately 22%. After 20 months of service, moisture 

content decreased to 16%, and after 53 months, it stabilized 

at approximately 14%. Moisture content measurements and 

visual inspections of the wood indicated that the waterproof 

membrane and pavement crown were effective in protecting 

the bridge from water. 

Figure 15—Load cells were installed on stressing 

bars to monitor changes in bar tension. 

Figure 16—Bridge deflections of load tests 1 and 2 were 

measured using displacement transducers attached to a 

wood beam supported by scaffolding. 
11 

 Figure 17—Truck weights, axle spacings, and transverse load test positions for load test 1. 

 Figure 18—Truck weights, axle spacings, and transverse load test positions for load test 2. 























2 ft 

C L 





















C L 2 ft 

4.6 ft 12.9 ft 

26,310 lb 26,550 lb 11,890 lb 

GVW=64,750 lb 























6.5 ft 

C L 

Load case 1 

Load case 2 





















6.5 ft 

C L Load case 3 

Load case 4 





















2 ft C L 





















2 ft 

C L 





















2 ft 

C L 

















2 ft 

4.8 ft 13.0 ft 

20,200 lb 19,900 lb 14,200 lb 

GVW=54,300 lb 





















6.5 ft 

C L 





















6.5 ft 

C L 





















6.5 ft 

C L 

















6.5 ft 

4.8 ft 13.1 ft 

20,900 lb 20,800 lb 12,400 lb 

GVW=54,100 lb 

Load case 3 

Load case 2 

Load case 1 Load case 4 

Load case 5 

Load case 6 

Truck A Truck B 

Truck B Truck B 

Truck B Truck B 

Truck A Truck A 

Truck A Truck A 
12 

Bar Force 

The average trend in bar force is shown in Figure 21; measurements were taken in September. 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 as a result of the 

combined effects of a decrease in wood moisture content, 

stress relaxation, and seating of the metal plate connectors. 

Therefore, the bridge was retensioned to the full design value 

Figure 20—Average trend in wood moisture content. 

Figure 21—Average trend in prestress level of 

bar force. 

of 50,000 lb or 125 lb/in2 interlaminar compression after 

6 weeks, 8 months, 20 months, and 53 months of service. 

Data collected during the first retensioning indicated that the 

average bar force had decreased 45% to approximately 

70 lb/in2

 interlaminar compression during the first 6 weeks. 

Data collected during the second retensioning 61⁄2 months 

later indicated that the average bar force had decreased 60% 

to approximately 50 lb/in2

 interlaminar compression. 

Measurements taken 12 months after the second retensioning 

indicated that the bar force had decreased 68% to approximately 40 lb/in2

 interlaminar compression. The bars were 

retensioned again. Bar force measurements taken 33 months 

after this retensioning indicated that the bar force had decreased 75%, to approximately 30 lb/in2

 interlaminar compression. Therefore, the bars were tensioned again. 

The 8% decrease in moisture content caused wood shrinkage; 

shrinkage was probably most significant during the first half 

of the monitoring period, when the greatest moisture content 

loss occurred. Stress relaxation in wood laminations has 

been observed to cause bar force loss in many other stresslaminated bridges (Ritter and others 1991). Shrinkage and 

Figure 19—Truck positions for load test 2 for three 

load cases (looking north): (a) load case 3, (b) load 

case 4, (c) load case 6. 

0

5

10

15

20

25

1993 1994 1995 1996 1997 1998

Date

Average moisture content (%)

0

25

50

75

100

125

150

1993 1994 1995 1996 1997 1998

Date

50,000 lb/rod 

Average prestress level (lb/in2

)

 6 wk 8 mo 20 mo 53 mo
13 

stress relaxation are primary sources of bar force loss in solid 

stress-laminated decks. However, additional bar force loss 

was expected for the Byron bridge structure because of the 

seating of the metal plate connectors into the wood of adjacent trusses. 

At the time of the design and construction of the bridge, the 

magnitude and duration of the bar force loss were unknown. 

However, data collected during the 53 months of monitoring 

showed that the bar force has not stabilized. Although the 

rate of prestress loss decreased with each retensioning, bar 

force still decreased approximately 75% to 30 lb/in2 in the 

33 months prior to the last retensioning. Because of the 

ongoing bar force loss, it is recommended that the bar force 

be assessed annually. 

The AASHTO guide specifications for stress-laminated 

bridges apply only to solid-sawn wood slab decks and not to 

trusses; therefore, an AASHTO minimum bar force could not 

be directly obtained for this bridge. However, by assuming 

that the sum of the thickness of the top and bottom chords is 

equivalent to the thickness of the bridge deck, the AASHTO 

 guide specifications require a minimum bar force of approximately 45 lb/in2

 for this bridge. Therefore, it is recommended that the bars be retensioned to the design value of 

125 lb/in2

 whenever the bar force is below 45 lb/in2. 

Behavior Under Static Load 

For each load case, transverse deflections are given at the 

bridge centerspan as viewed from the south end (looking 

north). No permanent residual deflection was measured 

between load cases or at the conclusion of load testing. In 

addition, no significant movement was detected at the bridge 

supports during testing. At the time of both load tests, the 

average bridge interlaminar compressive stress was 

125 lb/in2

. The allowable deflection for design purposes 

was L/500 or 1.08 in. 

Load Test 1 

Transverse deflections for load test 1, which utilized one 

fully loaded dump truck and four load cases, are presented in 

Figure 22. The maximum deflection for load cases 1 and 2 

Figure 22—Transverse deflections for load test 1, measured at bridge centerspan (looking north). Bridge cross section 

and vehicle positions are for the purpose of interpretation only and are not drawn to scale. 
14 

occurred under the outside truck wheel line and measured 

0.14 and 0.13 in., respectively (Fig. 22a,b). Maximum deflections for load cases 3 and 4 occurred under the outside 

truck wheel line and measured 0.17 and 0.16 in., respectively 

(Fig. 22c,d) 

For these four load cases, the deflected shape of the centerspan cross section followed the symmetrical truck positions, 

with maximum measured deflection occurring at the same 

relative positions. 

Only one truck was available for load test 1; therefore, experimental data are not directly available for two trucks on 

the bridge. However, assuming accurate load test results and 

linear elastic behavior, the sum of the deflections resulting 

from individual truck loads should equal the deflection from 

two trucks applied simultaneously. To verify the accuracy of 

the test results, the deflections from load case 1 were compared with the mirror image of the deflections from load 

case 2, and the deflections from load case 3 were compared 

with the mirror image of the deflections from load case 4. 

As shown in Figure 23, the plots are nearly identical, verifying the accuracy of the test results. The linear elastic behavior of the bridge under truck loading was verified in load 

test 2, when two trucks were available for the load test. 

Figure 24 shows the deflections resulting from the sum of 

load cases 1 plus 2 and the deflections resulting from the sum 

of load cases 3 plus 4. For load cases 1 plus 2, maximum 

deflection (0.21 in.) occurred under the inside wheel line of 

the southbound lane. For load cases 3 plus 4, maximum 

deflection (0.17 in.) occurred under the outside wheel line of 

the northbound lane. 

Load Test 2 

Transverse deflections for load test 2, which utilized two 

fully loaded dump trucks and six load cases, are shown in 

Figure 25. The maximum deflection of 0.11 in. for load 

case 1 occurred under the outside truck wheel line (Fig. 25a), 

and the maximum deflection of 0.13 in. for load case 2 occurred under the outside truck wheel line (Fig. 25b). 

Figure 23—Measured deflections for load test 1: 

(a) actual deflection for load case 1 (LC1) and mirror 

image of load case 2 (LC2 MI); (b) LC3 actual deflection 

and LC4 MI. 

Figure 24—Sum of deflections for load test 1: (a) load 

cases 1 plus 2, (b) load cases 3 plus 4. 
15 

Figure 25—Transverse deflections for load test 2, measured at bridge centerspan (looking north). Bridge cross section 

and vehicle positions are for the purpose of interpretation only and are not drawn to scale. 
16 

Figure 26—Measured deflections for load test 2: (a) sum 

of measured deflections for LC1 and LC2 compared 

with measured deflections for LC3; (b) sum of 

measured deflections for LC4 and LC5 compared 

with measured deflections for LC6. 

The maximum deflection of 0.15 in. for load case 3 occurred 

under the outside truck wheel line. For load case 3, maximum 

deflections occurred under the inside truck wheel line of 

truck B, adjacent to the centerline of the bridge, and represented the largest deflection for all load cases (Fig. 25c). 

Maximum deflections for load cases 4 and 5 occurred at the 

bridge edge adjacent to the outside truck wheel line and 

measured 0.12 and 0.14 in., respectively (Fig. 25d,e). The 

maximum deflection of 0.13 in. for load case 6 occurred at 

the bridge edge, adjacent to the outside wheel line of truck A 

(Fig. 25f). 

For load cases 1 through 4, the deflected shape of the centerspan cross section followed the symmetrical truck positions, 

with maximum measured deflections occurring at the same 

relative positions. 

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

loads should equal the deflections from both trucks applied 

Figure 27—Comparison of deflections for load tests: 

(a) Load test 1 (LT1), LC1+3 compared with LT2, LC3; 

(b) LT1, LC3+4 compared with LT2, LC6. 

simultaneously. Figure 26 shows the load test 1 comparison 

of individual and simultaneous truck loading. As shown in 

Figure 26, the two plots are nearly identical with only minor 

variations, which are within the accuracy of the measurements. From this information, we conclude that the bridge 

behavior is within the linear elastic range. 

Load Test Comparison 

Figure 27 compares measured deflections for both load tests 

for two trucks on the bridge. The plots are similar in shape, 

but the deflections measured in load test 1 are greater than 

those measured in load test 2. At the time of testing, average 

bridge interlaminar compressive stress was 125 lb/in2

. The 

difference in the deflections is probably due to the greater 

weight of the trucks in load test 1, approximately 20% more 

than that of the trucks in load test 2. Another contributing 

factor was a slight difference in the type of trucks used for 

the tests, which could have resulted in a slight variation in the 

deflections. 

-0.40

-0.30

-0.20

-0.10

0.00

-20 -15 -10 -5 0 5 10 15 20

Deflection (in.)

LC1+2

LC3 (a)







 



 



 



 



 



 



 



 

Truck A Truck B 

-0.40

-0.30

-0.20

-0.10

0.00

-20 -15 -10 -5 0 5 10 15 20

Distance from centerline (ft)

Deflection (in.)

(b)



































 Truck A Truck B 

LC4+5

LC6

-0.40

-0.30

-0.20

-0.10

0.00

-20 -15 -10 -5 0 5 10 15 20

Distance from centerline (ft)

(b)

Deflection (in.)





































LT1

LT2

-0.40

-0.30

-0.20

-0.10

0.00

-20 -15 -10 -5 0 5 10 15 20

(a)

Deflection (in.)







 



 



 



 



 



 



 



 

LT1

LT2
17 

Predicted Response 

Table 3 summarizes the maximum measured and predicted 

deflections for both load tests and the predicted maximum 

deflections for AASHTO HS25–44 truck loading. 

The maximum measured deflections were expected to be less 

than the predicted deflections because of (a) the conservatism 

of the design procedure and (b) the performance of the load 

tests when the bridge was stressed to the full design value of 

125 lb/in2

. The maximum measured deflection for load test 1, 

0.21 in., was 0.10 in. less than the 0.31-in. deflection predicted by the analysis model for loading. The maximum 

measured deflection for load test 2, 0.15 in., was 0.11 in. less 

than the 0.26-in. deflection predicted by the analysis model 

for loading. The maximum predicted deflection of the Byron 

bridge under HS25–44 truck loading, 0.43 in., resulted in a 

span/depth ratio of approximately L/1200, which is within 

the design limit of L/500 or 1.08 in. 

Condition Assessment 

Assessment of the general condition of the Byron bridge 

indicated that its structure and serviceability are satisfactory. 

The areas subjected to inspection were bridge geometry, 

wood components, wearing surface, anchorage system, and 

metal plate connectors. 

Bridge Geometry 

Width measurements taken at the initiation of monitoring 

indicated that the stress-laminated truss structure was 2 in. 

narrower at the south abutment than at the north abutment. 

This was probably due to the sequential tightening of bars 

with a single jack. The slight distortion was reduced to 1.5 in. 

during the monitoring period and should not affect overall 

bridge performance. 

Wood Components 

Visual inspection of the wood components of the bridge 

indicated no signs of deterioration. However, checking was 

noted in the guardrail post during the second condition assessment, and each post was subsequently covered with a 

thin metal cap. In addition, 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-based paint. 

Metal Plate Connectors 

The metal plate connectors are performing well. They 

showed no signs of deterioration or rust on inspection. 

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 Byron bridge is satisfactory. With 

the exception of having to be retensioned periodically, 

there are no structural or serviceability deficiencies evident 

in the bridge. 

• The moisture content decreased gradually from approximately 22% to 14% during the 5-year field monitoring. 

Based on moisture content readings and visual inspections, 

it is concluded that the waterproof membrane and pavement crown have been effective in protecting the bridge 

from moisture. 

• During performance monitoring, the bridge was retensioned to the full design value of 125 lb/in2

 four times. 

After 53 months of monitoring, the bar force has not stabilized above the 45 lb/in2

 minimum recommended. This ongoing bar loss for more than 4 years is not typical of stresslaminated decks and is probably caused by a narrowing of 

the gaps between the truss laminations caused by the thickness of metal connector plates. The rate of bar force loss 

has decreased with each retensioning; therefore, it is expected that the bar force will eventually stabilize. 

• Based on data collected, the transverse load distribution 

analysis is conservative when used to analyze this bridge. 

This analysis assumes that normal to the direction of the 

span, the wheel load is distributed over the width of the tire 

plus twice the thickness of the top chord of the deck. For 

deflection calculations, the distribution width was increased by 15%, paralleling the AASHTO recommendation for solid stress-laminated decks (AASHTO 1991). 

Table 3—Summary of load test and predicted 

HS25–44 midspan deflections 

Load 

test 

Maximum 

measured 

load test 

deflection 

(in.) 

Maximum 

predicted 

load test 

deflection 

(in.) 

Maximum 

predicted 

HS25–44 

deflection 

(in.) 

Span/deflection ratio for 

predicted 

HS25–44 

deflection 

1 0.21 0.31 0.43 L/1200 

2 0.15 0.26 0.43 L/1200 
18 

Results from two load tests indicate that at a bar force of 

125 lb/in2

, the transverse load distribution analysis over 

predicted the maximum deflection by an average of 37%. 

The results from laboratory testing indicate that at a bar 

force of 25 lb/in2

, the transverse load distribution analysis 

over predicted the maximum stress by 39% and the maximum deflection by 28%. 

• The predicted deflection for AASHTO HS25-44 loading 

using the transverse load distribution analysis was 0.43 in. 

or L/1250. This is well below the design limit of L/500 or 

1.08 in., where L is the span length measured center-center 

of bearings. 

• For load test 1, the maximum deflection from two 

65,000-lb trucks positioned with their center of gravity at 

midspan was calculated to be 0.21 in. or L/2570 when the 

bridge was stressed to the full design value of 125 lb/in2

. 

Two trucks were not available for load test 1; therefore, 

this result was obtained by adding the deflection from one 

truck positioned in the northbound lane to the deflection 

from the same truck positioned in the southbound lane. 

• For load test 2, the maximum deflection from two 

54,000-lb trucks positioned with their center of gravity at 

midspan was measured to be 0.15 in. or L/3600 when the 

bridge was stressed to the full design value of 125 lb/in2

. 

• Static load testing indicates that the Byron bridge is performing in the linear elastic range when subjected to two 

54,000-lb trucks positioned with their center of gravity at 

midspan when the bridge was stressed to the full design 

value of 125 lb/in2

. 

• Visual inspections indicate no signs of deterioration of the 

wood or metal plate connectors. Surface rust is visible on 

some of the stressing system hardware in the vicinity of the 

anchorage nuts. 

• Assessment of the moisture content, bar force, and general 

condition of the bridge components, including the metal 

plate connectors, should be performed on an annual basis. 

• During annual assessment, the bars should be retensioned 

to the full design value of 50,000 lb if the bar force is between 10,000 and 18,000 lb or 25 and 45 lb/in2 interlaminar compression. Under no circumstances should the bar 

force be permitted to decrease below 10,000 lb or 

25 lb/in2

 interlaminar compression. 

• Areas of the stressing system hardware where the epoxy 

coating has chipped off should be brush coated with an 

approved epoxy-based paint. 

References 

AASHTO. 1989. Standard specifications for highway 

bridges. 14th ed. Washington, D.C.: American Association 

of State Highway and Transportation Officials. 

AASHTO. 1991. Guide specifications for the design of 

stress-laminated wood decks. Washington, D.C.: American 

Association of State Highway and Transportation Officials. 

Altimore, F. 1995. Stress-laminated, metal-plate-connected 

truss bridges. Orono, ME: University of Maine. Ph.D. 

dissertation. 

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

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

Society for Testing and Materials. 6 p. 

Bruno, J.; Weaver, R. 1989. Evaluation of coatings for 

metal connector plates. Pittsburgh, PA: Steel Structures 

Painting Council. 

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

monitoring of a CCA-treated stressed timber bridge deck. 

Forest Products Journal. 41(11/12): 74–78. 

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.; West, B. 1998. Fatigue tests of full-scale MPC 

trusses. Final report to U.S. Department of Agriculture, 

Forest Service, Forest Products Laboratory and Federal 

Highway Administration. Orono, ME: University of Maine, 

Department of Civil Engineering. 

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

Feasibility of CCA-treated stressed timber bridge decks. 

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

Dagher, H.; Caccese, V.; Hsu, Y. 1992. Feasibility of 

MPCs in timber bridges. Report to U.S. Department of 

Agriculture, Forest Service Forest Products Laboratory and 

Federal Highway Administration. Orono, ME: University of 

Maine, Department of Civil Engineering, 

Dagher, H.; Caccese, V.; Wolfe, R.; Hsu, Y. 1994. Fatigue 

of metal connector plates. Completion report of phase 1 to 

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

Products Laboratory and Federal Highway Administration. 

Orono, ME: University of Maine, Department of Civil 

Engineering. 

FHWA. 1990. Memorandum on crash tested bridge railings. 

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

D.C.: Federal Highway Administration. 
19 

NFPA. 1991a. Design values for wood construction: a supplement of the National Design Specification for Wood 

Construction. Washington, D.C.: National Forest Products 

Association. 125 p. 

NFPA. 1991b. National design specification for wood 

construction. Washington, D.C.: National Forest Products 

Association. 51 p. 

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

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

September 2–5; London, United Kingdom. High Wycombe, 

United Kingdom: Timber Research and Development 

Association. 

Triche, M.; Ritter, M.; Lewis, S.; Wolfe, R. 1994. Design 

and field performance of a metal plate connected wood truss 

bridge. In: Proceedings, Structures Congress XII. New York, 

NY: American Society of Civil Engineers. 6 p. 

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. 
20 

Appendix—Information Sheet 

General 

Name: Byron bridge 

Location: Byron, Maine 

Date of construction: November 1993 

Owner: Maine Department of Transportation 

Design Configuration 

Structure type: Stress-laminated trusses with metal plate 

connectors 

Butt-joint frequency: 1 in 2 laminations transversely 

separated 22 in. longitudinally 

Total length (out–out): 45.9 ft 

Skew: 18° 

Number of spans: 1 

Span length (center–center of bearings): 44 ft 

Width (out–out): 31.6 ft (as built) 

Number of traffic lanes: 2 

Design loading: AASHTO HS25–44 

Camber: 0 in. 

Wearing surface: asphalt pavement, 1.5 to 3.5 in. thick 

Material and Configuration 

Truss Laminations: 

Species: machine-stress-rated (MSR) Southern Pine 

Size and grade: 2 by 12 in. MSR 1950f–1.7E 

2 by 10 in. MSR 2250f–1.9E 

2 by 8 in. MSR 2250f–1.9E 

2 by 6 in. MSR 2400f–2.0E 

Moisture condition: approximately 22% at time 

of construction 

Preservative treatment: CCA–type III 

Metal plate connectors: galvanized, 20-gauge plates 

(MiTek Industries, St. Louis, 

MO), brush painted with Series 27 

FC typoxy (Tnemec Company, 

Inc., Kansas City, MO) 

Stressing Bars: 

Diameter: 1.0 in. 

Number: 28 partial-width bars, 18 full-width bars 

Design force: partial-width bars, 25,000 lb; 

full-width bars, 50,000 lb 

Spacing (center–center): partial-width bars, 2 bars every 

22 in.; full-width bars, 1 bar 

every 22 in. 

Type: High strength steel-threaded bar, epoxy coated 

(Dywidag Systems International, Lincoln Park, NJ) 

Bar Anchorage Type: 

Continuous C 12 × 30 grade-50 all-weather steel channels 

on top and bottom chords with two types of bearings: 

Type 1: 6- by 6- by 1.25-in. steel anchorage plates 

Type 2: 10- by 4- by 0.50-in. grade-50 all-weather 

steel tubes and 6- by 6- by 1.25-in. steel anchorage plates 

Rail and Curb System: 

Design: crash-tested at AASHTO Performance Level 1 

on longitudinal spike-laminated deck 

Species: Southern Pine 

Member sizes: Rails: 6- by 12-in. glulam 

Posts: 8- by 12-in. glulam 

Curbs: 12- by 12-in. sawn lumber 

Preservative treatment: CCA–type III 

Waterproof Membrane System: 

80A adhesive primer 

M400A self-sealing waterproof membrane