United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL–RP–547

Field Performance of

Timber Bridges

5. Little Salmon Creek

Stress-Laminated Deck Bridge

Michael A. Ritter

James A. Kainz

Gregory J. Porter
Abstract

The Little Salmon Creek bridge was constructed in November 1988 on the Allegheny National Forest in Pennsylvania.

The bridge is a simple span, single-lane, stress-laminated

deck superstructure that is approximately 26-ft long and 16-ft

wide. The bridge is unique in that it is the first known

stress-laminated timber bridge to be constructed of hardwood

lumber. The performance of the bridge was monitored continuously for approximately 4 years, beginning at the time of

installation. Performance monitoring involved gathering and

evaluating data relative to the moisture content of the wood

deck, the force level of stressing bars, the deck vertical creep,

and the behavior of the bridge under static-load conditions.

In addition, comprehensive visual inspections were conducted to assess the overall condition of the structure. Based

on field evaluations, the bridge is performing well with no

structural deficiencies, although the bridge has developed a

slight sag as a result of vertical creep.

Keywords: Timber, bridge, wood, stress laminated

May 1996

Ritter, Michael A.; Kainz, James A.; Porter, Gregory J. 1996. Field

performance of timber bridges—5. Little Salmon Creek stress-laminated

deck bridge. Res. Pap. FPL–RP–547. Madison, WI: U.S. Department of

Agriculture, Forest Service, Forest Products Laboratory. 15 p.

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

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

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

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The Forest Products Laboratory is maintained in cooperation with the

University of Wisconsin.

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religion, age, disability, political beliefs, and marital or familial status.

Persons with disabilities who require alternative means of communication

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Washington, DC 20250, or call (202) 720–7327 (voice), or (202) 720–1127

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

Acknowledgments

We express sincere appreciation to the Construction and

Maintenance Crew of the Allegheny National Forest for providing the load test vehicle and assisting with load testing

and Terry Wipf and Doug Wood of the Iowa State University Civil and Construction Engineering Department for assistance in completing the final load test.

Thanks also to the following from the USDA Forest Service,

Forest Products Laboratory: Bill McCutcheon and Earl

Geske for installing instrumentation during the initial site

visit; Earl Geske and Vyto Malinauskas for developing and

manufacturing load cell instrumentation; Paula Hilbrich Lee

and Jim Wacker for assistance with data collection, analysis,

and presentation; 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 .....................................2

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

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

Cost ......................................................................3

Evaluation Methodology...............................................3

Moisture Content.....................................................5

Bar Force................................................................5

Creep.....................................................................7

Load Test Behavior..................................................7

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

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

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

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

Creep.....................................................................9

Load Test Behavior..................................................9

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

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

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

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

5. Little Salmon Creek Stress-Laminated Deck Bridge

Michael A. Ritter, Research Engineer

James A. Kainz, General Engineer

Forest Products Laboratory, Madison, Wisconsin

Gregory J. Porter, Civil Engineer

Allegheny National Forest, Warren, Pennsylvania

Introduction

In 1988, Congress passed the Timber Bridge Initiative (TBI)

to further develop and extend the use of timber as a bridge

material. Responsibility for administration of the TBI was

delegated to the USDA Forest Service and included a demonstration timber bridge program managed by the Timber

Bridge Information Resource Center (TBIRC) in Morgantown, West Virginia, and a research program at the Forest

Products Laboratory (FPL) in Madison, Wisconsin (USDA

1994). In addition, the Forest Service National Forest System, which administers the National Forests of the country,

made a commitment to demonstrate new technology in timber bridge design and construction. A large percentage of the

National Forests is located in rural communities where local

economies depend on natural resource management and utilization. Traditionally, the National Forest System maintains

jurisdiction over thousands of bridges, a significant percentage of which are timber bridges.

This report is the fifth in a series that documents the results

from the FPL bridge monitoring program. This paper describes the development, design, construction, and field performance of the Little Salmon Creek bridge on the Allegheny

National Forest in Pennsylvania. The bridge was constructed

in November 1988 as part of a commitment by the Forest

Service National Forest System to demonstrate new and

emerging timber bridge technology. The bridge is a singlelane, single-span, stress-laminated deck that is approximately

26-ft long and 16-ft wide. (See Table 1 for metric conversion

factors.) The bridge is unique in that it is the first known

stress-laminated deck superstructure constructed of hardwood

lumber. An information sheet on the Little Salmon Creek

bridge is provided in the Appendix.

Background

The Little Salmon Creek bridge is located within the

Allegheny National Forest, approximately 40 miles south of

Warren, Pennsylvania (Fig. 1). The bridge is on Forest

Road 145 (Salmon Creek Road), a single-lane, gravel roadway that provides access to popular recreation areas in the

Allegheny National Forest. Traffic is mostly light passenger

vehicles and occasionally logging trucks. The average traffic

is estimated between 15 to 20 vehicles per day.

The original Little Salmon Creek bridge was constructed in

the early 1930s and consisted of a reinforced concrete slab.

Inspection of the bridge in the mid-1980s indicated that the

concrete deck was in poor condition and insufficient to carry

required traffic loads. The bridge was also only 9 ft wide, and

increased width was required to meet acceptable geometric

standards established by the forest. Based on these deficiencies, a decision was made by the Allegheny National Forest

to replace the bridge.

Several options for replacement of the Little Salmon Creek

bridge were considered by the Allegheny National Forest

engineering staff. A timber bridge was determined the best

option because it would provide an opportunity to demonstrate and evaluate new and emerging timber bridge technology. In addition, a timber bridge could be built using lumber from the Allegheny National Forest and a Forest Service

construction crew. Thus, the decision was made to construct

a stress-laminated deck using local Northern Red Oak lumber. It was further determined that the field performance of the

bridge should be monitored after bridge installation to provide assurance that the performance of the newly developed

stress-laminated timber bridge system was satisfactory.

Subsequently, FPL and the Allegheny National Forest entered into an agreement to complete structural monitoring of

the bridge as part of a developing national timber bridge

monitoring program at FPL.

Table 1—Factors for converting English units of

measurement to SI units

English 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)

mile 1,609 meter (m)

pound (lb) 0.14 Newton (N)

lb/in2

 (stress) 6,894 Pascal (Pa)
2

U.S. map

Pennsylvania

Site plan

Figure 1—Location maps of the Little Salmon Creek

bridge.

Objective and Scope

The objective of this project was to determine the field performance characteristics of the Little Salmon Creek stresslaminated bridge by monitoring the bridge for approximately

4 years, beginning at bridge installation. The project scope

included data collection and analysis related to the wood

moisture content, stressing bar force, vertical bridge creep,

behavior under static truck loading, and general structure

condition. 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.

Design, Construction,

and Cost

The design and construction of the Little Salmon Creek

bridge were completed by the engineering staff of the Allegheny National Forest. An overview of the design, construction, and cost of the bridge superstructure is presented.

Design

The Little Salmon Creek bridge was designed before a nationally recognized design procedure for stress-laminated

timber bridges was available in the United States. The

design criteria for those aspects relating directly to stress

laminating were based on a draft version of Timber Bridges:

Design, Construction, Inspection, and Maintenance (Ritter

1990). The remainder of the design was based on Standard

Specifications for Highway Bridges, published by the

American Association of State Highway and Transportation

Officials (AASHTO 1983).

The Little Salmon Creek bridge was designed for AASHTO

HS 20-44 loading with a span length of 25 ft center-to-center

of bearings, a width of 16 ft, and a nominal deck thickness of

12 in. (Fig. 2). Visually graded No. 2 Northern Red Oak

was selected as the bridge material. At the time of design,

tabulated design values for Northern Red Oak were not included in the AASHTO specifications or in the National

Design Specification for Wood Construction (NDS) (NFPA

1988). Based on proposed design values for the species and

lumber grade prepared by Northeastern Lumber Manufacturers

Association (NELMA), tabulated design values of

1,375 lb/in2

 for bending and 1,350,000 lb/in2 for modulus of

elasticity (MOE) were used. These values were adjusted for

wet-use conditions using NDS wet-use factors of 0.86 for

bending and 0.97 for MOE. Final bridge design was controlled by a live-load deflection limit for AASHTO HS

20-44 loading, defined as 1/360 of the bridge span measured

center-to-center of bearings. The design live-load deflection

was 0.85 in. or 1/353 of the bridge span.

Design of the Little Salmon Creek deck was based on nominal 2- by 12-in. Northern Red Oak laminations, pressure

treated with creosote after fabrication. The length of the

laminations varied from 3 to 16 ft, and butt joints in the

deck laminations were placed transversely in every fourth
3

lamination, with a 4-ft longitudinal spacing between butt

joints in adjacent laminations (Fig. 3). The stressing system

was designed for seven 1-in.-diameter, high strength,

threaded-steel bars conforming to the requirements of ASTM

A722 (ASTM 1988). Average bar spacing was 46 in. oncenter, beginning 18 in. from the bridge ends. The design

bar tension force was 60,000 lb, which resulted in 109 lb/in2

of interlaminar compression. The bar anchorage system was

the discrete plate anchorage, consisting of two steel plates

(Fig. 4). The bridge railing consisted of a Northern Red Oak

sawn lumber curb, post, and rail. The railing was designed

for 50 percent of the AASHTO static-load design criteria,

which is typical for Forest Service bridges on single-lane

roads, and was extended beyond the bridge ends to serve as

the approach railing. Because of the low level of traffic, no

wearing surface was specified.

Construction

Lumber for the Little Salmon Creek bridge was obtained

from trees on the Allegheny National Forest that had been

killed by insect attack. The trees were felled, transported to

a local saw mill, and sawn into lumber for the deck laminations, railing, and substructure. After processing, the material

was visually graded by a NELMA certified grader and fabrication was completed. From the saw mill, the lumber was

transported to a local treating plant for pressure treatment

with creosote.

Construction of the Little Salmon Creek bridge began with

building a temporary bypass, consisting of three metal culverts. After completion of the bypass, the existing structure

was removed, and excavation and construction of the two

post and sill abutments were completed (Fig. 5). Superstructure assembly began at the treating plant where treated deck

laminations were nailed together to form two deck sections,

each half the width of the bridge. Temporary steel bars were

then inserted through the laminations at several locations so

that the deck sections could be transported to the bridge site.

After completion of the substructure, the deck sections were

loaded on a flatbed truck at the treating plant and were transported to the bridge site (Fig. 6). At the site, the sections

were unloaded from the truck and placed side-by-side on

temporary supports on the approach roadway. The temporary

steel bars were removed, and stressing bars were inserted

through both sections (Fig. 7). The stressing bars were tensioned to the required 60,000-lb force using a single hydraulic jack. After bar tensioning, the entire superstructure was

lifted as a unit by a small crane onto the abutments (Fig. 8).

Attachment of the superstructure was completed by bolting

the deck to a steel angle that was then bolted to the side of

the substructure cap (Fig. 9).

The installation of the bridge superstructure was completed

in 1 day under winter weather conditions, which included

snow and freezing temperatures. After the initial bar tensioning, the bars were again retensioned to 60,000 lb approximately 1 and 6 weeks after installation. The curb, bridge

railing, and approach railing were installed shortly after the

bars were tensioned for the second time. The completed

bridge is shown in Figure 10.

The as-built configuration of the Little Salmon Creek bridge

varied slightly from the design configurations shown in

Figure 2. After the final stressing, the bridge width measured

16.2 ft at the abutments and 15.9 ft at midspan.

Cost

Final costs for materials, fabrication, and construction of the

Little Salmon Creek bridge superstructure, including curb

and railing, are given in Table 2. Based on a deck area of

416 ft2, the cost total was approximately $37/ft2.

Evaluation Methodology

As a result of the experimental nature of the Little Salmon

Creek bridge, Allegheny National Forest representatives contacted FPL for assistance in evaluating the structural performance of the bridge. Through mutual agreement, a bridge

monitoring plan was developed by FPL and implemented as

a cooperative effort with the Allegheny National Forest. 

Figure 2—Design configuration of the Little Salmon

Creek bridge.
4

Figure 3—Butt joint configuration used for the Little Salmon Creek bridge. A butt joint was placed

transversely to the bridge span in every fourth lamination. Longitudinally, butt joints in adjacent

laminations were separated by 4 ft.

Figure 4—Discrete plate bar anchorage configuration.

Figure 5—Construction of the post and sill abutments for

the Little Salmon Creek bridge.
5

The plan called for the performance monitoring of the deck

moisture content, bar force in the stressing bars, vertical

bridge creep, load test behavior, and condition assessments

of the structure. The evaluation methodology utilized procedures and equipment previously developed at FPL (Ritter

and others 1991).

Moisture Content

The moisture content of the Little Salmon Creek bridge was

measured on the underside of the deck, using an electricalresistance moisture meter with 3-in. probe pins in accordance

with ASTM D4444-84 procedures (ASTM 1990). Measurements were obtained by driving the pins into the deck at

depths of 1 to 2 in., recording the moisture content value

from the unit, and adjusting the values for temperature and

wood species. Moisture content measurements were taken at

the time of bridge installation, periodically during the monitoring period, and at the conclusion of the monitoring.

Bar Force

Bar force for the Little Salmon Creek bridge was measured

using load cells that were developed at FPL. The load cells

were installed between the bearing plate and anchor plate on

the second and fourth stressing bars from the north abutment.

Load cell measurements were obtained by Allegheny National Forest personnel using a portable strain indicator.

Strain measurements from the indicator were converted to

units of bar tensile force by applying a laboratory calibration

factor to the strain indicator reading. Bar force measurements

were taken on a biweekly basis for several months following

construction and approximately bimonthly thereafter.

Approximately 2 years into the monitoring period, the load

cells were replaced with new load cells of a similar design.

At the conclusion of the monitoring period, the load cells

were removed, checked for zero balance shift, and recalibrated

to determine time-related changes in the initial load cell

calibration. The bar tensile force was also checked with a

hydraulic jack several times during the monitoring period to

verify load cell readings.

Figure 6—The two deck sections for the Little Salmon

Creek bridge were transported from the treating plant

to the bridge site on a flatbed truck.

Figure 7—At the bridge site, the temporary steel bars

in the two deck sections were removed and stressing

bars were inserted through both deck sections.

Figure 8—After the stressing bars were tensioned,

the entire superstructure was lifted on the abutments

by a small crane.

Figure 9—Attachment of the superstructure was by

bolting the deck to steel angles that were then bolted

to the side of the substructure cap.
6

Figure 10—Completed Little Salmon Creek bridge.
7

Creep

Vertical creep of the Little Salmon Creek bridge was measured periodically during the monitoring period. Measurements were obtained at midspan on both the upstream and

downstream deck edges, using a stringline between bearings

and a calibrated rule. The stringline served as a horizontal

benchmark, and the relative deck elevation at midspan was

measured with the rule.

Load Test Behavior

Static-load testing of the Little Salmon Creek bridge was

conducted twice during the monitoring period to determine

the response of the bridge to full truck loading. Each test

consisted of positioning a fully loaded truck on the bridge

deck and measuring the resulting deflections at a series of

transverse locations at midspan. Measurements of bridge

deflections were taken prior to testing (unloaded), for each

load position (loaded), and at the conclusion of testing

(unloaded). In addition, analytical assessments were conducted to determine the theoretical bridge response.

Load Test 1

The first load test was completed September 10, 1990, approximately 11 months into the monitoring period. The

bridge interlaminar compression at the time of the test was

63 lb/in2

, or approximately 58 percent of the 109 lb/in2 design level. The test vehicle consisted of a fully loaded, threeaxle dump truck with a gross vehicle weight of 53,700 lb

(Fig. 11). The vehicle was positioned longitudinally on the

bridge so that the two rear axles were centered at midspan

and the front axle was off the bridge span. Transversely, the

vehicle was placed for two load positions (Fig. 12). For load

position 1, the vehicle was centered on the bridge width. For

load position 2, the vehicle was positioned on the downstream side, with the center of the inside wheel line over the

bridge centerline. Measurements of the bridge deflection from

an unloaded to loaded condition were obtained by placing a

calibrated rule on the deck underside and reading values with

a surveyor’s level to the nearest 0.005 ft (Fig. 13). The accuracy of this method for repetitive readings is estimated to be

±0.06 in.

Load Test 2

The second load test was completed July 18, 1993,

44 months into the monitoring period. At the time of the

test, the interlaminar compression was at the full design level

of 109 lb/in2

. The vehicle used for load test 1 was also used

for load test 2, but the vehicle gross weight was reduced to

46,450 lb (Fig. 14). As with load test 1, the vehicle was

positioned longitudinally so that the two rear axles were

centered at midspan and the front axle was off the bridge

span. Transversely, three load positions were used (Fig. 15).

Load positions 1 and 2 placed the vehicle at the center of the

bridge width and on the downstream side, respectively, as in

load test 1. For load position 3, the vehicle was placed along

the upstream bridge edge, with the center of the inside wheel

line over the bridge centerline. Measurements of the bridge

deflection from an unloaded to loaded condition were

Table 2—Estimated costs for the Little Salmon

Creek bridge

Item Cost ($)

Wood

Stumpage, cutting, skidding, sawing, and

delivery to the preservative treatment plant

3,000

Stress grading 400

Fabrication and pressure treating

Cutting, drilling, preservative treatment,

and delivery to project site

3,200

Steel

Stressing bars, plates, nuts, bolts, lag

screws, and nails

3,900

Miscellaneous

Gravel, seeding, and erosion control

200

Crane rental for 1 day 1,000

Construction equipment and labor

construction crew, backhoe, dump truck

3,700

Total 15,400

Figure 11—Load test 1 truck configuration and axle

loads. The transverse vehicle track width, measured

center-to-center of the rear tires, was 6 ft.

Load Position 1

Load Position 2

Figure 12—Load test 1 transverse load positions

(looking north). For all load positions, the two rear

axles were centered over the bridge midspan with

the front axle off the span.
8

measured using string potentiometers and an electronic data

acquisition system (Fig. 16). The accuracy of this method for

repetitive readings is estimated to be ±0.0005 in.

Analytical Evaluation

Previous research has shown that stress-laminated decks can

be accurately modeled as orthotropic plates (Oliva and others

1990). To further analyze the theoretical behavior of the

Little Salmon Creek bridge, an orthotropic plate computer

model developed at FPL was used to analyze the load test

results and predict the bridge deflection for AASHTO

HS 20-44 truck loading.

Condition Assessment

The general condition of the Little Salmon Creek bridge was

assessed on four separate occasions during the monitoring

period. The first assessment occurred shortly after installation. The second and third assessments took place during the

first load test and during an intermediate site visit, respectively. The final assessment occurred at the time of the final

load test, which concluded the monitoring activities. The

assessments involved visual inspections, measurements, and

photographic documentation of the bridge’s condition.

Items of specific interest included the bridge geometry, the

condition of the timber deck and rail system, and the condition of the stressing bars and anchorage systems.

Results and Discussion

The performance monitoring of the Little Salmon Creek

bridge extended from November 1988 through July 1993.

Figure 13—Load test 1 bridge deflections were measured

by suspending a calibrated rule from the deck underside

and reading displacement values with a surveyor’s level.

Figure 14—Load test 2 truck configuration and axle

loads. The transverse vehicle track width, measured

center-to-center of the rear tires, was 6 ft.

Figure 15—Load test 2 transverse load positions

(looking north). For all load positions, the two rear

axles were centered over the bridge midspan with

the front axle off the span.

Figure 16—Load test 2 bridge deflections were measured

with string potentiometers attached to the underside of the

bridge. The transducers were supported by a wood beam

and tripods.
9

Moisture Content

The Little Salmon Creek bridge was installed at an average

lamination moisture content of approximately 39 percent.

Since installation, the bridge experienced a gradual decrease

in moisture content, to approximately 31 percent at the conclusion of the monitoring program. The 8 percent decrease in

moisture content occurred at a relatively constant rate during

the 44 months, although moisture content fluctuated 2 to 3

percent in the measurement zone as a result of seasonal climatic changes. It is expected that the moisture content at the

interior of the laminations is greater than the values obtained

in the measurement zone because of slower moisture migration through the lamination depth.

Because of the high initial moisture content at installation,

the deck moisture content remains above the fiber saturation

moisture content of 25 to 30 percent. Above this level,

changes in the moisture content have little effect on dimensional stability. In time, the moisture content will gradually

decrease below the fiber saturation point and the moisture

content will stabilize. When the moisture content falls below

fiber saturation, subsequent moisture loss will result in

shrinkage of the laminations and could significantly affect bar

force retention.

Bar Force

The general trend in average bar force for the Little Salmon

Creek bridge is shown in Figure 17. The first two bar tensionings, separated by 1 week, were approximately to the

design force of 60,000 lb (approximately 109 lb/in2 interlaminar compression). The final bar tensioning, which

occurred in December 1988, 6 weeks after installation, was

approximately 56,000 lb, or 93 percent of the design force.

After the final bar tensioning, the bar force remained relatively stable until October 1989 when it began to decline.

The decline continued throughout the monitoring period,

although the rate of loss decreased with time. At the

conclusion of the monitoring period, the average bar force

was approximately 28,000 lb, or 47 percent of the design

force, which corresponds to an average interlaminar compression of approximately 51 lb/in2

. Because the moisture content of the deck laminations remained above the fiber saturation point, there was no lamination shrinkage. Thus, the

decline in bar force is primarily attributable to stress relaxation in the lumber laminations as a result of the high moisture content. Although the average bar force remained above

the minimum recommended interlaminar compression of

40 lb/in2 (Ritter 1990) at the end of the monitoring period,

the bars were retensioned at that time because bar tensioning

equipment was readily available.

Creep

The Little Salmon Creek bridge was constructed without

camber and was approximately level with the abutments

when installed. In September 1990, approximately 1 year

into the monitoring period, the bridge had sagged below the

level position, and 1.7 in. of negative camber was measured

along the downstream bridge edge. In July 1993, at the conclusion of the monitoring period, negative camber measured

2.5 in. on the downstream bridge edge and 2 in. on the upstream bridge edge (Fig. 18). The visible sag in the bridge is

due to creep and is not typical of most stress-laminated timber bridges. It is likely that the high moisture content of the

laminations and cyclic moisture changes in the exposed deck

surface contributed significantly to this high creep level.

Load Test Behavior

Results for both load tests are presented in this section. In

each case, transverse deflection plots are shown at the bridge

midspan as viewed from the south end (looking north). For

each load test, no permanent residual deformation was measurable at the conclusion of the testing. In addition, there was

no detectable movement at either of the abutments.

Design prestress = 60,000 lb

70

60

50

40

30

20

10

0

Bar force (x 103 lb)

Date

Sep 88 Oct 89 Nov 90 Dec 91 Jan 93

Figure 17—Average trend in bar tension force obtained

from load cells installed on two stressing bars.

Figure 18—The 2.5 in. of superstructure sag on the

downstream bridge side is visually evident in the bridge

deck and the railing.
10

Load Test 1

Transverse deflection for load test 1 is shown in Figure 19.

For each of the two load positions, deflection is typical of the

orthotropic plate behavior of stress-laminated bridges

(Ritter and others 1990). For load position 1 (Fig. 19a), the

maximum measured deflection of 0.73 in. occurred between

the wheel lines, 1.5 ft from the bridge centerline on the upstream (right) side. For load position 2 (Fig. 19b), the

maximum measured deflection of 0.69 in. was also between

the wheel lines, 1.5 ft from the bridge centerline on the

downstream (left) side. For both load positions, the deflection of the three data points between the wheel lines (ranging

from 0.71 to 0.73 in. for load position 1 and 0.68 to 0.69 in.

for load position 2) was approximately the same. Thus, it is

not possible to determine the exact maximum deflection location given the accuracy of the readings.

Load Test 2

Transverse deflection for load test 2 is shown in Figure 20.

As with load test 1, deflection is typical for orthotropic plate

behavior. For load position 1 (Fig. 20a), the maximum

measured deflection of 0.56 in. occurred between the wheel

lines, 1.5 ft from the bridge centerline on the upstream (right)

side. For load position 2 (Fig. 20b), the maximum measured

deflection of 0.59 in. was also between the wheel lines, 1.5 ft

from the bridge centerline on the downstream (left) side. For

load position 3 (Fig. 20c), the maximum measured deflection of 0.62 in. was under the upstream (right) wheel line.

Given the improved accuracy of the measurement method

compared with that used for load test 1, it is likely that the

measured deflection accurately represents the actual bridge

behavior.

Assuming uniform material properties and loading, the

bridge deflections for load positions 2 and 3 should be a mirror image. Figure 21 shows the actual load position 2 deflection and the mirror image of load position 3 deflection. As

shown, minor differences are indicated between the deflections, ranging from 0.03 to 0.05 in. at the bridge centerline

and at the edges. The differences in deflection for the two

load positions are most likely the result of a slight difference

between the wheel-line loading caused by eccentric loading of

the test vehicle.

Load Test Comparison

A comparison of load positions 1 and 2 deflection measurements for load tests 1 and 2 is shown in Figure 22. For

both load positions, the measurements are similar, although

load test 1 deflections exceed those of load test 2. Maximum

measured deflections for both load tests occurred at the same

location for the respective load positions and differed by

0.17 in. for load position 1 and 0.10 in. for load position 2.

The difference in the deflections is attributable to the

6,650 lb additional rear-axle load for load test 1 and the

change in longitudinal bridge stiffness caused by the difference in prestress levels at the time of load testing.

Analytical Evaluation

Comparisons of the measured load test results to the theoretical bridge response are shown in Figure 23. As shown,

the theoretical bridge deflection is very close to that measured. Using the same load test analytical parameters, the

theoretical deflection for AASHTO HS 20-44 truck loading

is shown in Figure 24. Based on this analysis, the predicted

maximum AASHTO HS 20-44 static deflection is 0.70 in.

(or approximately 1/429 of the bridge span) for load test 1

and 0.63 in. (or approximately 1/476 of the bridge span) for

load test 2. Assuming constant bridge properties, the same

bridge deflection would be expected for the same loading.

However, it is known that an increase in interlaminar compression in stress-laminated bridges with butt joints results

in an increase in longitudinal bridge stiffness (Oliva and others 1990). The differences in the maximum deflection for the

two load tests are attributable to a 13-percent increase in

bridge stiffness at load test 2, as a result of the increased level

of interlaminar compression of 109 lb/in2

 for load test 2

compared with 63 lb/in2 for load test 1.

Figure 19—Transverse deflection for load test 1, measured at the bridge midspan (looking north). Bridge crosssections and vehicle positions are shown to aid interpretation and are not to scale.
11

Figure 20—Transverse deflection for load test 2, measured at the bridge midspan (looking north). Bridge crosssections and vehicle positions are shown to aid interpretation and are not to scale.

Figure 21—Comparison of load test 2 measured deflections, showing the actual deflection for load position 2

and the mirror image of load position 3 (looking north).

Figure 22—Comparison of the measured deflections for

load tests 1 and 2 (looking north), for load positions 1 (a)

and 2 (b).
12

d 2

Figure 23—Comparison of the measured deflections for load tests 1 and 2, compared with the theoretical deflection

using orthotropic plate analysis (looking north).
13

Condition Assessment

Condition assessments of the Little Salmon Creek bridge

indicated that structural and serviceability performance was

acceptable. Inspection results for specific items follow.

Bridge Geometry

Bridge width measurements during the monitoring period

indicated that the bridge width narrowed approximately 0.4 ft

(Table 3). The change in bridge width is most likely the

result of stress-relaxation in the oak laminations. Additional

reductions in bridge width may be expected as stress relaxation continues and the lamination moisture content falls below the fiber saturation point.

Wood Condition

Inspection of the wood components of the bridge showed no

signs of deterioration, although minor checking was evident

on rail members exposed to wet–dry cycles. Checking was

most pronounced in the end grain of the timber rail posts.

This may have been prevented if an end grain sealer had been

applied at the time of construction. In addition, the top of the

bridge rail showed minor checking, but the depth of the

checks did not appear to penetrate the preservative treatment

envelope. Evidence did not indicate a loss in wood preservative, and preservative or solvent accumulations were not present on the wood surface.

Wearing Surface

The Little Salmon Creek bridge was designed and constructed without a wearing surface; hence, vehicles ride directly on the treated-wood deck. During each site visit,

gravel and other debris from the unpaved approach roadway

were present on the deck surface (Fig. 25). Deck wear from

vehicle tracking and debris abrasion was noted in several

locations, although the level of wear was minor and did not

significantly reduce the deck section or extend below the

preservative treatment. Over time, it is anticipated that deck

wear will continue and could result in reduced deck section

and accelerated deterioration as the preservative treatment

envelope wears away.

Stressing Bars and Hardware

The exposed steel stressing bars and hardware showed no

visible signs of corrosion or other distress.

Bar Anchorage System

The stressing bar anchorage system has performed as designed with no significant signs of distress. There is no indication of the discrete plate anchorage crushing into the outside oak laminations and no measurable distortion in the

bearing plate. A gradual deck compression deformation of

0.13 to 0.25 in. was noted in the outside laminations in the

vicinity of the bearing plates. The deformation is most likely

due to stress relaxation in the wood, which was more pronounced due to the high moisture content of the laminations.

Conclusions

After approximately 4 years inservice, the Little Salmon

Creek bridge is exhibiting acceptable performance, although

several serviceability deficiencies were noted. These deficiencies are primarily attributed to the high moisture content of

the lumber laminations at the time of construction and

Figure 24—Maximum predicted deflection profile at the

bridge midspan for AASHTO HS 20-44 truck loading.

Table 3—Bridge width measurements during

the monitoring period

Bridge Width (ft)

Date Abutments Midspan

March 21, 1989 16.2 15.9

September 10, 1990 15.9 15.7

July 18, 1993

 (after bar retensioning)

15.8 15.5

Figure 25—Debris accumulations on the bridge’s

 deck noted during condition assessment.
14

throughout the monitoring period. Based on monitoring

conducted since bridge construction, we make the following

conclusions and observations:

• It is feasible to construct stress-laminated decks using

Northern Red Oak lumber laminations.

• Prefabricating panels for stress-laminated decks and joining them at the construction site is a viable method of

bridge construction. Assembling the panels at the construction site and lifting the entire superstructure into

place with a small crane minimizes field work and traffic

disruption and may be an economical alternative to field

construction.

• The cost of materials, fabrication, and construction of the

Little Salmon Creek bridge superstructure, including

curbs and railings, was approximately $37/ft2

. Because

this was the first bridge of its type built in the United

States, the cost should decrease as more bridges are built

and people become familiar with the bridge system and

materials.

• The average moisture content in the outer 1 to 2 in. of the

laminations of the Little Salmon Creek bridge decreased

from 39 percent at the time of construction to 31 percent

at the end of the monitoring period, but remains above the

fiber saturation moisture content of 25 to 30 percent. It is

anticipated that the moisture content will continue to decrease below the fiber saturation point, resulting in lamination shrinkage and bar force loss. For future bridges of

this type, we recommend that the average moisture content of the lumber laminations not exceed 19 percent at

the time of construction.

• During the monitoring period, the average bar force for the

Little Salmon Creek bridge decreased from 60,000 lb (109

lb/in2

 interlaminar compression) to 28,000 lb (51 lb/in2

interlaminar compression). The decline in bar force is

greater than expected and is primarily attributed to stress

relaxation in the lumber laminations caused by the high

lamination moisture content.

• Vertical creep over the monitoring period resulted in a sag

at the superstructure midspan of 2.5 in. along the downstream bridge edge and 2 in. along the upstream bridge

edge. This is much greater than expected and is again attributed to the high lamination moisture content.

• Load testing and analyses indicate that the Little Salmon

Creek bridge is performing as a linear elastic orthotropic

plate when subjected to static truck loading. Based on an

analytical comparison of load test results at different levels

of interlaminar compression, the longitudinal bridge stiffness increased approximately 13 percent when the interlaminar compression increased from 63 to 109 lb/in2

. The

maximum predicted bridge deflection as a result of

AASHTO HS 20-44 static truck loading is estimated to

be 0.70 in. (L/429) at 63 lb/in2 interlaminar compression

and 0.63 in. (L/476) at 109 lb/in2 interlaminar compression.

• Visual inspections of the bridge indicate that the performance of wood and steel components is satisfactory. The

exposed steel stressing bars and hardware showed no visible signs of corrosion or other distress, and the discrete

plate bar anchorage is not distorted or crushing into the

lumber laminations. The lack of a wearing surface has resulted in deck wear from vehicle tracking and debris abrasion, although the level of wear is minor and does not

significantly reduce the deck section or extend below the

preservative treatment. It is expected that wear will continue, and the placement of a watertight asphalt wearing

surface is recommended to protect the deck.

References

AASHTO. 1983. Standard specifications for highway

bridges. 13th ed. Washington, DC: American Association of

State Highway and Transportation Officials.

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.

NFPA. 1988. Design values for wood construction, a

supplement to the 1986 edition of the national design

specification. Washington, DC. National Forest Products

Association. 40 p.

Oliva, M.G.; Dimakis, 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. Washington, DC: U.S.

Department of Agriculture, Forest Service, Engineering Staff,

EM 7700−8. 944 p.

Ritter, M.A.; Geske, E.A.; Mason, L. [and others]. 1990.

Performance of stress-laminated bridges. Wood Design Focus

1(3): 12−16.

Ritter, M.A.; Geske, E.A.; McCutcheon, W.J. [and others]. 1991. Methods for assessing the field performance of

stress-laminated timber bridges. In: Proceedings, 1991 international timber engineering conference; 1991, September;

London, England.

USDA. 1994. The timber bridge initiative, fiscal year 1994

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

Forest Service, State and Private Forestry, Northeastern

Area. 14 p.
15

Appendix—Information Sheet

General

Name: Little Salmon Creek bridge

Location: Forest Road No. 145, Allegheny National

Forest, Pennsylvania

Date of Construction: November 1988

Owner: Allegheny National Forest

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): 26 ft

Skew: None

Number of Spans: 1

Span Lengths (center-to-center bearings): 25 ft

Width (out-out): 16 ft

Width (curb-curb): 14.5 ft

Number of Traffic Lanes: 1

Design Loading: AASHTO HS20−44

Wearing Surface Type: None

Material and Configuration

Timber:

Species: Northern Red Oak

Size (actual): 1-1/2 to 1-5/8 in. wide; 12 in. deep

Grade: Visually graded No. 2 and better

Moisture Condition: 39 percent average at installation

at 1 to 2 in. depth

Preservative Treatment: Creosote

Stressing Bars:

Diameter: 1 in.

Number: 7

Design Force: 60,000 lb

Spacing: 46 in. average center-to-center beginning

18 in. from bridge ends

Type: High strength, steel thread bar with coarse

right-hand thread, conforming to ASTM A722

Anchorage Type and Configuration:

Steel Plates: 12 by 9.25 by 1 in. bearing

5.5 by 3.25 by 1.25 in. anchor