United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Timber Bridge

Information

Resource Center

Research

Paper

FPL–RP–551

Field Performance of

Timber Bridges

8. Lynches Woods Park

Stress-Laminated Deck Bridge

James P. Wacker

Michael A. Ritter

Don Conger
Abstract

The Lynches Woods Park bridge was constructed during the

summer of 1990 in Newberry, South Carolina. It is a singlespan, single-lane, stress-laminated deck superstructure that

measures approximately 30 ft long, 16 ft wide, and 14 in.

deep. The bridge is unique in that is one of the first known

stress-laminated deck bridges to be constructed of Southern

Pine lumber treated with chromated copper arsenate. The

performance of the bridge was continuously monitored for

approximately 3 years, beginning 10 months after installation. Performance monitoring involved gathering and

analyzing data relative to the wood moisture content, force

level in the stressing bars, and behavior under static-load

conditions. In addition, comprehensive visual inspections

were conducted to assess the overall structure condition.

Based on the field evaluations, the bridge is performing well

with no structural or serviceability deficiencies.

Keywords: Timber, bridge, wood, stress laminated

July 1996

Wacker, James P.; Ritter, Michael A.; Conger, Don. 1996. Field

performance of timber bridges—8. Lynches Woods Park stress-laminated

deck bridge. Res. Pap. FPL–RP–551. Madison, WI:

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

Laboratory. 17 p.

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

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(TTD). USDA is an equal employment opportunity employer.

Acknowledgments

We express sincere appreciation to the following who contributed to the success of this project: The Newberry County

Highway Department for assistance with conducting load

tests and Ron Boozer of the Francis Marion and Sumter

National Forest Engineering staff for collecting bar force and

moisture content field data.

Thanks also to the following individuals from the USDA

Forest Service, Forest Products Laboratory: James Kainz,

Paula Hilbrich Lee, Kim Stanfill–McMillan, and Maureen

Mathias for assistance with data analysis and report preparation and the Publishing and Photography Teams for assistance in preparing this report.

Contents

Page

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

Background................................................................1

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

Design, Construction, and Cost .....................................3

Design ...................................................................3

Construction ...........................................................3

Cost ......................................................................7

Evaluation Methodology...............................................7

Moisture Content.....................................................7

Bar Force................................................................7

Behavior Under Static Load .......................................7

Condition Assessment ..............................................8

Results and Discussion.................................................8

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

Bar Force................................................................9

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

Analytical Evaluation..............................................11

Condition Assessment ............................................12

Conclusions .............................................................15

References................................................................15

Appendix—Information Sheet......................................17
Field Performance of Timber Bridges

8. Lynches Woods Park Stress-Laminated Deck Bridge

James P. Wacker, General Engineer

Michael A. Ritter, Research Engineer

Forest Products Laboratory, Madison, Wisconsin

Don Conger, Engineering Technician

Newberry County Highway Department, Newberry, South Carolina

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. Responsibility for the development, implementation, and administration of the timber bridge program was assigned to the USDA Forest Service. Within the

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

research. The demonstration bridge program, which is administered by the Forest Service Timber Bridge Information

Resource Center (TBIRC) in Morgantown, West Virginia,

provides competitive matching funds to local governments to

demonstrate timber bridge technology through the construction of demonstration bridges (USDA 1995). The TBIRC

also maintains a technology transfer program to provide

assistance and state-of-the-art information related to timber

bridges. One objective of these programs is to encourage the

use of new or previously underutilized wood products, bridge

designs, and design applications.

Responsibility for the research portion of the TBI program

was assigned to the USDA Forest Service, Forest Products

Laboratory (FPL), a national wood utilization research facility. The FPL currently conducts research in numerous areas

related to timber bridges and other wood transportation

structures (Ritter and others 1994). As part of this research

program, FPL has implemented an extensive program to

assist local governments in evaluating the field performance

of timber bridges, many of which employ design innovations

or materials that have not been previously evaluated.

Through this program, FPL is able to collect, analyze, and

distribute information on the field performance of timber

bridges, providing a basis for validating or revising design

criteria and further improving efficiency and economy in

bridge design, fabrication, and construction.

This paper is the eighth in a series that documents the field

performance of timber bridges included in the FPL timber

Table 1—Factors for converting English units of

measurement to SI units

English unit

Conversion

factor SI unit

acre 0.0040 square meter (m2)

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)

mile 0.0016 meter (m)

bridge monitoring program. It addresses the field performance

of the Lynches Woods Park bridge built in Newberry, South

Carolina, in 1990. The bridge was constructed through the

TBI demonstration bridge program and is a single-span,

single-lane, stress-laminated deck superstructure that measures approximately 30 ft long, 16 ft wide, and

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

Constructed of Southern Pine lumber, the Lynches Woods

Park bridge is unique because it is one of the first known

stress-laminated deck bridges to be constructed of lumber

treated with chromated copper arsenate (CCA). An information sheet on the Lynches Woods Park bridge is provided in

the Appendix.

Background

The bridge is located within the 268-acre Lynches Woods

Park in the City of Newberry, South Carolina (Fig. 1). The

bridge is on a single-lane, unpaved roadway where it crosses

the Rocky Branch river, approximately 2.5 miles from the

park entrance. This roadway provides the only vehicle access

to the park and consists primarily of passenger cars and

maintenance vehicles. The traffic volume across the bridge is

estimated at 20 vehicles per day.
2

Before replacement in 1990, the crossing at the Rocky

Branch river consisted of a 20-ft-long, nail-laminated lumber

deck, supported by a timber substructure. During the spring

of 1988, the Rocky Branch river flooded and the bridge

superstructure was washed out. Because of this washout and

extensive damage to the timber abutments, a temporary low

water crossing was installed until a new bridge could be

designed and constructed (Fig. 2). Based on an evaluation of

alternatives for the replacement structure, it was determined

that the new bridge would be a timber deck superstructure,

supported by concrete abutments. It was further determined

that the length and elevation of the replacement structure

would be increased from those of the original structure to

provide increased hydraulic capacity.

Through a cooperative effort between the Crossroads of

History Resource, Conservation, and Development Council

and Newberry County, a proposal was submitted to the

USDA Forest Service for partial funding of the Lynches

Woods Park replacement as a demonstration bridge under the

Timber Bridge Initiative (USDA 1995). The project proposed a stress-laminated deck timber bridge constructed of

Southern Pine lumber treated with CCA. In 1989, the project proposal was funded through the TBIRC, and plans for

the design and construction of the Lynches Woods Park

project were finalized.

In March 1991, approximately 8 months after the bridge was

installed, the Newberry County Highway Department contacted FPL for assistance in monitoring the performance of

the Lynches Woods Park bridge. Subsequently, FPL and the

Newberry County Highway Department mutually developed

a cooperative research and development agreement to complete structural monitoring of the bridge as part of the ongoing timber bridge monitoring program at FPL.

Objective and Scope

The objective of this project was to evaluate the field performance of the Lynches Woods Park bridge for a minimum

of 2 years, beginning 10 months after construction. The

scope of the project included data collection and analysis

related to wood moisture content, stressing bar force, static

truck loading, and general structure performance. The results

of this project will be considered with similar monitoring

projects in an effort to improve design and construction

methods for future stress-laminated timber bridges.

Figure 1—Location maps for Lynches Woods Park bridge.

Figure 2—Prior to bridge construction, the Lynches

Woods Park bridge site was a low-water crossing.
3

Design, Construction, and Cost

The design and construction phases of this project involved

a mutual effort among several agencies and individuals. The

following presents an overview of the design, construction,

and costs of the Lynches Woods Park bridge.

Design

Design of the Lynches Woods Park bridge was completed by

the Newberry County Highway Department personnel with

assistance from the engineering staff of the Francis Marion

and Sumter National Forests. With the exception of those

design aspects dealing with stress laminating, the bridge

superstructure was designed according to American Association of State and Highway Transportation Officials

(AASHTO) Standard Specifications for Highway Bridges

for AASHTO HS20–44 loading requirements (AASHTO

1989). Specific requirements for stress laminating were based

on a Forest Service, Eastern Region, standard stresslaminated bridge design.

The design geometry of the Lynches Woods Park bridge

provided for a single-span, simply supported stress-laminated

lumber deck, 30 ft long, 16 ft wide, and 14 in. deep (Fig. 3).

Design calculations were based on a 29-ft center-to-center of

bearing span length and a 14.3-ft clear roadway width. The

lumber laminations for the stress-laminated deck were designated as nominal 4-in.-thick Southern Pine lumber, treated

with CCA in accordance with AWPA C14 (AWPA 1989).

The availability of 4- by 14-in. Southern Pine lumber at the

full-span length was cost prohibitive. Thus, the deck was

designed with butt joints to reduce the required lamination

length. Butt joints were spaced so that only one would occur

in every four adjacent laminations transversely and at 4-ft

intervals in the longitudinal direction (Ritter 1990).

Design values for the Southern Pine laminations were based

on the National Design Specification for Wood Construction

for lumber visually graded No. 1 Dense Structural, in

accordance with the Southern Pine Inspection Bureau (SPIB)

grading rules (AFPA 1986, 1988). Tabulated design values

for the Southern Pine laminations were 1,550 lb/in2

 for

bending, 1,600,000 lb/in2 for modulus of elasticity (MOE),

and 440 lb/in2 for compression perpendicular to grain.

The stress-laminating system for the Lynches Woods Park

bridge utilized threaded steel bars placed through the laminations and tensioned to provide the required interlaminar

compression and load transfer between the individual laminations. The design specified the use of 1-in.-diameter, high

strength, galvanized steel bars with an ultimate strength of

150,000 lb/in2, in accordance with the requirements of

ASTM A722 (ASTM 1988). The bars were spaced 4 ft oncenter, beginning 1 ft from the bridge ends. To prevent

chemical interaction between the wood preservative and the

bar galvanizing, the bars were encased in polyvinyl chloride

(PVC) tubing. The design force for the bars was 72,000 lb,

which resulted in an interlaminar compression of approximately 107 lb/in2

. Bar anchorage at the edges of the bridge

was a discrete plate anchorage system, consisting of steel

bearing and anchorage plates (Fig. 4).

Design of the bridge railing and curb system was based on

AASHTO static-load requirements (AASHTO 1989). All

rail and curb members were CCA-treated, Southern Pine

sawn lumber, visually graded No. 2 Dense Structural (AFPA

1986, 1988). Rail members were specified as nominal

6 by 12 in., with a splice at the bridge midspan. The railposts were specified as nominal 8 by 8 in. and were spaced at

a maximum of 6 ft on-center. The railing was offset from the

posts with nominal 4-in.-thick offset blocks. All rail and

curb members were surfaced on four sides (S4S) to ensure

dimensional tolerance and uniform contact between adjacent

members during construction.

A wearing surface was not specified for the Lynches Woods

Park bridge because of the anticipated low traffic volume.

Construction

Construction of the Lynches Woods Park Bridge was completed in the summer of 1990 under the supervision of personnel from Newberry County and the Francis and Marion

National Forests. Labor for bridge construction was provided

by a local state prison. The construction process began with

the removal of the existing low water crossing and the erection of reinforced-concrete abutments (Fig. 5). Lumber components for the bridge superstructure were then delivered to

the bridge site on a flatbed truck. Assembly of the superstructure was completed in-place, with temporary scaffolding

support between the abutments (Fig. 6). With scaffolding

providing full support at butt joint locations, the individual

laminations were hand-placed by the work crew and spiked

together to ensure proper alignment of the bar holes. The

PVC tubing was then placed through prebored bar holes to

facilitate bar insertion and provide bar corrosion protection.

Full-width stressing bars were inserted through the PVC

tubing, and anchorage plates and nuts were attached.

The stressing bars for the Lynches Woods Park bridge were

tensioned on four separate occasions. At each bar tensioning,

the bars were fully tensioned to the design force of 72,000 lb

using a single hydraulic jack. The initial bar tensioning was

completed just prior to removal of the construction scaffolding by tensioning the first bar at one bridge end, then sequentially tensioning each successive bar along the bridge

length. The bars were then retensioned several times using

the same procedure until a uniform force was achieved in all

bars. On the three subsequent bar tensionings, conducted at

1, 4, and 8 weeks after the initial tensioning, a similar bar

tensioning procedure was followed (Fig. 7).

After the second bar tensioning, the deck was anchored to the

concrete substructure, and the rail and curb system was

installed. The deck-to-substructure connection was made

with steel angle brackets that were attached to the deck underside and bolted to the concrete abutment walls (Fig. 8).

The curb system was assembled for alignment and then

bolted to the deck through field-drilled holes that were

treated with wood preservative (Fig. 9). After the curb was in

place, the railing system was attached and installation was

completed. The completed Lynches Woods Park bridge is

shown in Figure 10.
4

Figure 3—Design configuration of the Lynches Woods Park bridge.
5

Figure 4—Design configuration of the stressing bar

anchorage.

Figure 5—Installation of reinforced-concrete abutments.

Figure 6—Temporary scaffolding was placed during

construction to support laminations before the deck

was stress laminated.

Figure 7—Bar tensioning, performed 8 weeks after

construction, using a single hydraulic jack.

Figure 8—Attachment of the superstructure to the

substructure by bolting the deck to a steel angle that is

bolted to the side of the concrete substructure backwall.

Figure 9—Attachment of the sawn-lumber curb to the

bridge deck.
6

 Figure 10—The completed Lynches Woods Park bridge, approximately 14 months after installation.
7

Cost

The total fabrication and construction cost for the Lynches

Woods Park bridge superstructure including the railing

system was $19,720. Based on a total deck surface area of

480 ft2, the cost was approximately $41/ft2.

Evaluation Methodology

To evaluate the structural performance of the Lynches Woods

Park bridge, Newberry County personnel contacted FPL for

assistance. Through mutual agreement, a 2-year monitoring

plan was developed by the FPL and implemented through a

cooperative research and development agreement with Newberry County. The plan called for monitoring several key

performance indicators, such as moisture content, bar force,

static-load behavior, and general structure condition. At the

initiation of field monitoring, approximately 10 months after

the bridge was constructed, an FPL representative visited the

bridge site to install instrumentation and train Newberry

County personnel in data collection procedures for moisture

content and bar force measurements. Load tests and general

condition assessments were completed by FPL personnel

during site visits. The evaluation methodology utilized

procedures and equipment previously developed by Ritter

and others (1991) and is discussed in the following sections.

In addition, information recorded by Newberry County personnel at bridge installation was used to augment FPL

monitoring data.

Moisture Content

Moisture content readings were collected from the deck

superstructure on a monthly basis throughout the monitoring

period. The measurements were taken with an electricalresistance meter, in accordance with ASTM standard requirements (ASTM 1990). The moisture content readings

were obtained at a series of locations on the underside of the

deck at a pin penetration of approximately 2 in. (Fig. 11).

Field readings were adjusted for wood species and temperature to determine the actual moisture content values

(Forintek 1984). In addition, several core samples were

collected at the end of the monitoring period to measure the

moisture content by the ovendry method (ASTM 1992).

Bar Force

Bar force data were obtained on a biweekly basis throughout

the monitoring period. Measurements were obtained from

load cells installed on two of the eight bars using a portable

strain indicator (Fig. 12). The load cell strain readings were

converted into equivalent units of bar force, based on load

cell calibrations completed in the laboratory prior to bridge

monitoring. At the end of the monitoring period, the accuracy of the load cells was verified with hydraulic force checks

at the bridge site and load cell recalibration in the laboratory

(Ritter and others 1991).

Behavior Under Static Load

Static-load tests of the Lynches Woods Park Bridge were

conducted at the beginning and ending of the monitoring

period to determine the bridge response under static-loading

conditions. In addition, an analytical assessment was completed to evaluate the bridge response using computer

modeling.

Load Testing

Static-load testing of the Lynches Woods Park bridge consisted of positioning a loaded dump truck on the bridge and

measuring the resulting deflections at a series of locations at

midspan and near the abutments. For each load test, the

vehicle was placed in three transverse positions (Fig. 13).

Longitudinal vehicle placement was based on the truck

configuration and was different for each load test. Measurements were obtained by suspending calibrated rules from the

deck underside and reading their relative movement with a

Figure 11—Electrical-resistance moisture readings

were taken from the underside of the deck at a depth

of approximately 2 in.

Figure 12—Load cell readings were obtained with a

portable strain indicator.
8

surveying level. Deflection measurements were obtained prior

to testing (unloaded), after placement of the test vehicle

(loaded), and at the conclusion of testing (unloaded).

Load Test 1

The first load test was conducted on May 9, 1991, at the

initiation of field monitoring. The bridge interlaminar compression at the time of the testing was approximately

40 lb/in2

. The test vehicle consisted of a fully loaded, twoaxle dump truck with a gross vehicle weight of 32,960 lb

(Fig. 14). In the longitudinal direction, the vehicle was

positioned with the load centriod at the bridge midspan.

Deflection measurements for this test were obtained to the

nearest 0.06 in.

Load Test 2

The second load test was conducted on February 28, 1994,

at the conclusion of field monitoring. For this load test, the

interlaminar compression level was approximately 40 lb/in2

or 90 lb/in2 (after retensioning the stressing bars). The load

test vehicle was a fully loaded, three-axle dump truck with a

gross vehicle weight of 48,580 lb (Figs. 14, 15). In the

longitudinal direction, the vehicle was positioned so that the

rear axles were centered at midspan and the front axle was off

the bridge. Deflection measurements for this test were obtained to the nearest 0.04 in.

Analytical Evaluation

At the completion of load testing, the theoretical bridge

behavior was modeled for actual truck loading and AASHTO

HS20-44 loading using an orthotropic plate computer program developed at FPL.

Condition Assessment

The general condition of the Lynches Woods Park Bridge

was assessed on three occasions during the monitoring

period. The first assessment occurred at the beginning of the

monitoring when instrumentation was installed and initial

load testing was completed. The second assessment occurred

approximately 3 months into the monitoring period. The

final assessment occurred at the end of the monitoring period

when final load testing was completed. These assessments

involved visual inspections, measurements, and photographic documentation of the bridge’s condition. Items of

specific interest included deck camber, wood components,

the stressing bar and anchorage system, and wood

preservative.

Results and Discussion

Performance monitoring of the Lynches Woods Park Bridge

was initiated in May 1991, approximately 10 months after

bridge construction, and continued for 34 months until

February 1994. A discussion of the monitoring results follows.

Moisture Content

The average moisture content trend for the monitoring period

is shown in Figure 16. Based on data collected by Newberry

Figure 13—The three transverse load

cases used for static-load tests.

Figure 14—Axle weights and spacings for the trucks

used in load test 1 (top) and load test 2 (bottom).
9

County personnel just prior to construction, the bridge was

installed at an initial average moisture content of 26 percent.

At the beginning of the monitoring period, approximately

10 months after installation, the initial electrical-resistance

moisture content measurements indicated an average moisture content of 25 percent. Throughout the monitoring,

climatic changes caused the average moisture content of the

laminations to fluctuate from 20 to 25 percent. During an

unusually dry period during the summer of 1992, the average

moisture content briefly dropped below 20 percent. However,

at the conclusion of monitoring, the average moisture content

had rebounded to approximately 25 percent. In addition,

ovendry moisture core samples obtained at the conclusion

of monitoring indicated an average moisture content of

24 percent.

Although the moisture content of the Lynches Woods Park

bridge was more than the recommended maximum of

19 percent at installation, it did not affect overall performance

of the structure. In addition, rapid wetting and drying cycles

did not occur as noted in previous reports on exposed structures treated with waterborne preservatives.

Bar Force

The average trend in bar force measurements during the

monitoring period is shown in Figure 17. At the initiation

of monitoring, the average measured bar force was

approximately 29,000 lb. Thus, during the 8 months since

the final bar tensioning, the average bar force decreased apFigure 15—Vehicle positions for load test 2, as viewed

from the south end of the bridge. From top to bottom,

load cases 1, 2, and 3 are shown.

Figure 16—The average trend in moisture content

readings taken at the lower 2–3 in. of the deck during

monitoring, beginning 10 months after bridge

installation.

Figure 17—The average trend in bar force

measurements since the initiation of monitoring,

beginning 10 months after bridge installation.
10

the final bar tensioning, the average bar force decreased approximately 60 percent (43,000 lb) prior to the initiation of

monitoring. Periodic measurements during the monitoring

showed that the average bar force was relatively stable at

approximately 25,000 lb, or 37 lb/in2

 interlaminar compression. At the conclusion of the monitoring, the average bar

force was approximately 24,000 lb, or 35 lb/in2 interlaminar

compression. Although not required for performance reasons,

the bars were retensioned at the end of the monitoring period

because of equipment availability and to allow load testing of

the bridge at different interlaminar compression levels.

Although the overall bar force was relatively low at the

initiation of monitoring, the bar force retention for the

Lynches Woods Park bridge was satisfactory. Factors affecting the bar force were moisture content changes and stress

relaxation. The moisture content changes were small and

occurred in the outer portion of the laminations; therefore, the

effect on bar force was minimal. The majority of bar force

loss, both before and during the monitoring period, was due

primarily to stress relaxation of the lumber laminations.

Behavior Under Static Load

Results of the static-load testing and analytical evaluation are

presented. For each load case, transverse deflection measurements are given at the bridge midspan as viewed from the

south end (looking north). For both load tests, no permanent

residual deflection was measured between load cases or at the

conclusion of testing. In addition, no measurable deflection

was observed at the bridge supports.

Load Test 1

Transverse deflection for load test 1 is shown in Figure 18.

For load case 1 (Fig. 18a) and load case 3 (Fig. 18c), the

symmetry of the load cases resulted in the same maximum

measured deflection of 0.50 in. under the wheel line nearest

the bridge edge. For load case 2 (Fig. 18b), with the vehicle

centered on the bridge cross-section, the maximum measured

deflection of 0.44 in. occurred at the bridge centerline. In all

cases, the deflection is typical of the linear elastic orthotropic

plate behavior of stress-laminated bridges, and the locations

of maximum measured deflection correspond to those observed in other similar bridges (Ritter and others 1991).

Load Test 2

Transverse deflection for load test 2 at interlaminar compression levels of 40 lb/in2

 and 90 lb/in2 is shown in Figure 19.

At the 40 lb/in2 level, the maximum deflection for load case

1 (Fig. 19a) and load case 3 (Fig. 19c) occurred under the

wheel line nearest the bridge edge and measured 0.50 and

0.55 in., respectively. For load case 2 (Fig. 19b), the maximum measured deflection of 0.45 in. occurred at the bridge

centerline. For all load cases, the maximum measured deflections are very similar to those of load test 1, with no difference in location and only minor differences in magnitude.

Such a small difference in magnitude was not expected because of the increased loading for load test 2, which should

have resulted in a substantially greater deflection magnitude.

Figure 18—Transverse deflection for load test 1,

measured at the bridge midspan (looking north). Bridge

cross sections and vehicle positions are presented

to aid interpretation only and are not to scale.
11

This difference in magnitude is discussed further in the Analytical Evaluation section.

At 90 lb/in2 interlaminar compression, the magnitude and

location of the maximum measured deflections changed

slightly from those measured at the 40 lb/in2 level. For load

case 1 (Fig. 19a) and load case 3 (Fig. 19c), the maximum

measured deflection occurred at the bridge edge, rather than

under the wheel line nearest the bridge edge, and measured

0.53 and 0.52 in., respectively. For load case 2 (Fig. 19b),

the maximum measured deflection of 0.42 in. occurred at the

bridge centerline, the same location measured for the 40

lb/in2 level and for load test 1. Given the 50-lb/in2 increase

in interlaminar compression, we expected that the bridge

stiffness would increase slightly (Oliva and others 1990).

When comparing the two interlaminar compression levels,

the relative deflection curves are very similar but an increase

in stiffness, indicated by reduced deflection, is clearly evident

only for load case 3. It is likely that a global increase in

longitudinal bridge stiffness at the 90 lb/in2 interlaminar

compression existed for all load cases. However, it is not

possible to draw definitive conclusions from these tests,

given the accuracy of the deflection measurements.

Analytical Evaluation

A comparison of load test 1 measured deflections to the

theoretical bridge response based on orthotropic plate analysis is shown in Figure 20. For modeling purposes, we assumed a deck lamination MOE of 1,600,000 lb/in2

 for the

Southern Pine material, based on tabulated design values

(AFPA 1988). As shown in Figure 20, the theoretical bridge

deflection is very close to that measured, although minor

differences between the measured and theoretical deflection are

evident in several locations.

A comparison of the measured and theoretical deflections for

load test 2 at interlaminar compression levels of 40 and

90 lb/in2 are shown in Figure 21. In completing the analysis,

we found that the assumed material MOE of 1,600,000 lb/in2

used for load test 1 resulted in poor theoretical correlation

with measured results and consistently overstated the deflections for load test 2. Based on these results, it was evident

that a larger longitudinal stiffness was necessary for load test

2 to achieve a measured theoretical correlation similar to that

obtained for load test 1. Given that both load tests 1 and 2

were conducted when the interlaminar compression was

40 lb/in2, we eliminated bridge prestress effects on the transverse deck properties and butt joints as a possible cause of

longitudinal stiffness change. In addition, the deck section

and lamination moisture content were unchanged and did not

significantly affect longitudinal stiffness. As shown in Figure 21, the correlation between the measured deflection and

that computed using an MOE of 2,100,000 lb/in2 is very

good and similar to the load test 1 correlation with a material MOE of 1,600,000 lb/in2

. Although the exact cause of

the apparent 30 percent increase in MOE is unknown at this

time, it is suggested that a chemical interaction between the

CCA treatment and the wood laminations could cause a

modification to the wood structure that results in an

Figure 19—Transverse deflection for load test 2,

measured at the bridge midspan (looking north), at

interlaminar compression levels of 40 and 90 lb/in2

.

Bridge cross sections and vehicle positions are presented to aid interpretation only and are not to scale.
12

increase in MOE. An MOE change of this extent has not

been observed in research studies of treated materials. However, the same type of phenomenon has been observed on

other bridges built with CCA-treated lumber that are currently being monitored by FPL; conclusive findings will be

forthcoming.

Theoretical deflection for AASHTO HS20-44 truck loading

is shown in Figure 22 for load cases 1 and 2. (Load case 3 is

a mirror image of load case 1.) Deflections correspond to the

load test conditions previously discussed and are based on

the same relative analytical input parameters for interlaminar

compression and MOE. As shown for each set of load test

conditions, the maximum HS20-44 theoretical deflection

occurred for load case 1 rather than load case 2, and the

location of maximum HS20-44 deflection for each load case

corresponds to the location where maximum load test deflections were measured. As summarized in Table 2, the maximum theoretical HS20-44 deflection of 0.86 in., or L/404 as

expressed as a fraction of the bridge span, occurred under load

test 1 conditions. For load test 2, with an MOE of

2,100,000 lb/in2

, the maximum theoretical deflection of

0.69 in. or L/503 was obtained at an interlaminar compression of 40 lb/in2

. After bar tensioning to 90 lb/in2, the load

test 2 maximum theoretical HS20-44 deflection was 0.64-in.

or L/543. Thus, the 50-lb/in2 increase in interlaminar compression for load test 2 resulted in an approximate 7-percent

decrease in the theoretical HS20-44 deflection.

Condition Assessment

Condition assessment of the Lynches Woods Park bridge

indicates that all structural and serviceability aspects of the

structure are satisfactory. Results for specific areas follow.

Deck Camber

An initial deck camber of 1.25 in. was introduced during

bridge construction by slightly raising the center scaffolding

support with respect to the abutments. Measurements at the

initiation of the monitoring period indicated approximately

0.50-in. positive deck camber at both the upstream and

downstream edges. Thus, approximately 0.75 in. of positive

camber was lost due to the bridge dead-load deflection and

creep during the 10 months prior to the initiation of monitoring. Measurements at the conclusion of the monitoring

period indicated no measurable change in the deck elevation,

and approximately 0.50 in. of positive camber remained.

Wood Components

Visual inspection of the wood components in the Lynches

Woods Park bridge indicated no sign of deterioration or

damage. Because the location is sheltered by tall trees most

of the year, drying of wood members was minimal, and

components appeared to remain at a relatively high moisture

content. Thus, end-grain and side-grain checking in rail and

curb members, typical for most sawn lumber highway structures, was minimal for this bridge.

Figure 20—Comparison of load test 1 measured and

predicted deflections using orthotropic plate analysis.
13

Figure 21—Comparison of load test 2 measured and predicted deflections using orthotropic plate analysis.
14

Stressing Bar and Anchorage System

Inspections of the stressing bar system indicated satisfactory

performance. Galvanized steel hardware that was exposed to

the environment was in good condition, with no visible sign

of corrosion or deterioration. At the conclusion of monitoring, several bearing plates were removed to allow visual

inspection of the stressing bars and bearing plate areas

 (Fig. 23). The condition of several bars was examined

inside the PVC tubing and signs of corrosion were not evident. In addition, there was no visible crushing of the discrete plate anchorage into the outside laminations. However,

slight deformations were present in the outside laminations

along each bridge edge. These deformations produced a

gentle depression in the vicinity of the plate anchorages that

measured from 1/8 to 1/16 in. (Fig. 24). These depressions

were the result of stress relaxation of the deck laminations

and did not affect the performance of the structure.

Figure 22—Predicted midspan deflection for load

cases 1 and 2 using HS20-44 loading and orthotropic

plate analysis.

Table 2—Maximum theoretical HS20-44 deflections

Maximum theoretical

HS20-44 deflection (in.)

Load test/condition

Load

case 1

Load

case 2

Load test 1

40 lb/in2

 interlaminar

compression

MOE = 1,600,000 lb/in2

0.86 0.78

Load test 2

40 lb/in2

 interlaminar

compression

MOE = 2,100,000 lb/in2

0.69 0.62

Load test 2

90 lb/in2

 interlaminar

compression

MOE = 2,100,000 lb/in2

0.64 0.55

Figure 23—Visual inspection of outer lamination after

removal of bearing plate showed no signs of bar

corrosion.

Figure 24—Gradual deformation of the laminations in

the vicinity of the bar anchorages. Note the small gap

between the aluminum straight edge and the deck

edge.
15

Wood Preservative

The performance of CCA preservative treatment used on

sawn lumber components was satisfactory, and the integrity

of the preservative envelope remained intact during the monitoring period. Dimensional stability of the treated wood was

good, because the shaded and humid environment at the site

prevented rapid wetting and drying cycles that leads to dimensional changes in exposed structures. Based on visual

inspections, there was no notable difference between the

performance of this bridge and other similar bridges treated

with oil-based preservatives.

Conclusions

Based on the monitoring results from the Lynches Woods

Park bridge, we make the following conclusions and recommendations:

• After 34 months in service, the performance of the

Lynches Woods Park bridge is satisfactory. There are no

visible signs of deterioration that would prevent the

bridge from providing several additional years of acceptable service.

• It is both feasible and practical to design and construct

stress-laminated timber decks using CCA-treated Southern Pine lumber.

• Stress-laminated decks can be constructed in-place by

manually placing laminations and using temporary scaffolding to support the butt-jointed laminations prior to

bridge stressing. This technique is most viable when

large construction equipment is not readily available and

manual labor is cost effective.

• The general trend in moisture content in the lower 2 in. of

the laminations indicates no significant changes during

the monitoring period. At the conclusion of monitoring,

the lumber laminations remain at approximately 25 percent moisture content.

• The general trend in bar force measurements indicates that

an adequate level of bar force was maintained throughout

the monitoring period. With an initial tension of 72,000

lb, bar forces decreased approximately 43,000 lb (or 60

percent) during the 8 months prior to monitoring. During

the monitoring period, bar force measurements were relatively stable at approximately 25,000 lb, or 37 lb/in2

 interlaminar compression. At the conclusion of monitoring,

the stressing bars were tensioned to the 90 lb/in2 interlaminar compression level.

• Loss of camber as a result of initial dead load and deck

creep totaled 0.75 in. and occurred prior to the initiation

of monitoring. No measurable camber loss was detected

during the monitoring period. A positive camber of approximately 0.50 in. remained at the conclusion of monitoring.

• Load testing and analysis indicated that the Lynches

Woods Park bridge is performing as a linear-elastic orthotropic plate when subjected to static-truck loading.

• Based on load test analyses, the bridge appears considerably stiffer after only 2-1/2 years in service. At the same

level of 40 lb/in2

 interlaminar compression, the correlation

between the actual and theoretical deflection was excellent

using an MOE of 1,600,000 lb/in2 for load test 1 and an

MOE of 2,100,000 lb/in2 for load test 2. Although the exact cause of the apparent increase in MOE is unknown at

this time, the phenomenon has been observed on other

bridges currently being monitored by FPL and conclusive

findings will be forthcoming.

• The maximum theoretical deflections for a single lane of

AASHTO HS20-44 loading were below the design limit

of 1/360 for the span length in all cases. For load test 1 at

40 lb/in2 interlaminar compression, the maximum deflection is estimated to be 0.86 in., which is approximately

1/404 of the span length measured center-to-center of bearings. For load test 2 at 40 lb/in2

 interlaminar compression, the maximum deflection is estimated to be 0.69 in.,

which is approximately 1/503 of the span length. For load

test 2 at 90 lb/in2 interlaminar compression, the maximum deflection is estimated to be 0.64-in., which is approximately 1/543 of the span length.

• Based on visual inspections, there are no indications of

deterioration in the steel hardware or wood components.

In addition, there was no notable difference between the

performance of this CCA-treated bridge and other similar

bridges treated with oil-based preservatives.

References

AASHTO. 1989. Standard specifications for highway

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

State Highway and Transportation Officials.

AFPA. 1986. National design specification for wood construction. Washington, DC: American Forest and Paper

Association (formerly the National Forest Products Association, NFPA). 87 p.

AFPA. 1988. Design values for wood construction, a supplement to the 1986 ed. of the national design specification.

Washington, DC: American Forest and Paper Association.

36 p.

ASTM. 1988. Standard specification for uncoated, highstrength steel bar for prestressing concrete; ASTM A722-86.

Philadelphia, PA: American Society for Testing and Materials. 3 p.

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

meters; ASTM D 4444-84. Philadelphia, PA: American

Society for Testing and Materials. 6 p.
16

ASTM. 1992. Standard test methods for direct moisture

content measurement of wood and wood-base materials;

ASTM D 4442-92. Philadelphia, PA: American Society for

Testing and Materials. 5 p.

AWPA. 1989. Standards. Woodstock, MD: American

Wood-Preservers’ Association. 200 p.

Forintek. 1984. Moisture content correction tables for the

resistance-type moisture meter; SP511E. Ottawa, Canada:

Forintek Corp. 37 p.

Oliva, M.G.; Dimakes, A.G.; Ritter, M.A.; Tuomi, R.L.

1990. Stress-laminated wood bridge decks: experimental and

analytical evaluations. Res. Pap. FPL–RP–495. Madison,

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

Products Laboratory. 24 p.

Ritter, Michael A. 1990. Timber bridges: design, construction, inspection, and maintenance. EM 7700-8. Washington,

DC: U.S. Department of Agriculture, Forest Service, Engineering Staff. 944 p.

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

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

1991 September 2–5; London, United Kingdom. High

Wycombe, United Kingdom: Timber Research and

Development Association: 319–326. Vol. 3.

Ritter, M.A.; Moody, R.C.; Duwadi, S.R. 1994.

U.S. timber bridge research. In: PTEC 94 Timber shaping

the future: Proceedings of Pacific timber engineering

conference; 1994 July 11–15, Gold Coast, Australia.

Fortitude Valley MAC, Queensland, Australia: Timber

Research Development and Advisory Council: 148–155.

Vol. 2.

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.
17

Appendix—Information Sheet

General

Name: Lynches Woods Park bridge

Location: City of Newberry, South Carolina

Date of Construction: July 1990

Owner: Newberry County, South Carolina

Design Configuration

Structure Type: Stress-laminated deck with butt joints

Butt Joint Frequency: 1 in 4 laminations transverse

with joints in adjacent laminations separated 4 ft longitudinally

Total Length (out–out): 30 ft, 29.9 ft (as-built)

Skew: None

Number of Spans: 1

Span Length (center–center of bearings): 29 ft

28.9 ft (as-built)

Width (out–out): 16 ft, 15.8 ft (as-built)

Width (curb–curb): 14.3 ft, 14.5 ft (as-built)

Number of Traffic Lanes: 1

Design Loading: AASHTO HS20–44

Wearing Surface Type: None

Material and Configuration

Timber:

Species: Southern Pine

Size: 3-7/8 by 14 in.

Grade: No. 1 Dense Structural

Moisture Condition: Approximately 26 percent

at installation

Preservative Treatment: Chromated copper arsenate

(CCA)

Stressing Bars:

Diameter: 1 in.

Number: 8

Design Force: 72,000 lb

Spacing (center–center): 4 ft, beginning

1 ft from bridge ends

Type: High strength, steel bar with course

right-hand thread, conforming to

ASTM A722

Rail and Curb System:

Design: AASHTO static loading

Species: Southern Pine

Grade: No. 2 Dense Structural

Member Sizes: Rails: nominal 6 by 12 in.

Posts: nominal 8 by 8 in.

Curbs: nominal 6 by 10 in.

Preservative Treatment: Chromated copper arsenate

 (CCA)

Anchorage Type and Configuration:

Discrete steel plates: 11.5 by 13.5 by 1 in. bearing

4 by 6.5 by 1.25 in. anchor