United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL–RP–549

Field Performance of

Timber Bridges

6. Hoffman Run

Stress-Laminated Deck Bridge

Michael A. Ritter

Paula D. Hilbrich Lee

Gregory J. Porter
Abstract

The Hoffman Run bridge, located just outside Dahoga,

Pennsylvania, was constructed in October 1990. The bridge

is a simple-span, single-lane, stress-laminated deck superstructure that is approximately 26 ft long and 16 ft wide. It is

the second stress-laminated timber bridge to be constructed

of hardwood lumber in Pennsylvania. The performance of the

bridge was monitored continually for approximately

32 months, beginning shortly after installation. Performance

monitoring involved gathering and evaluating data relative to

the moisture content of the wood deck, the force level of steel

stressing bars, the deck vertical creep, and the behavior of the

bridge under static-load conditions. Furthermore, comprehensive visual inspections were executed to assess the overall

condition of the structure. Based on field evaluations, the

bridge is performing properly with no structural deficiencies,

although with respect to serviceability, the bridge has developed a slight sag at midspan.

December 1996

Ritter, Michael A.; Lee, Paula D. Hilbrich; Porter, Gregory J. 1996. Field

performance of timber bridges: 6. Hoffman Run stress-laminated deck

bridge. Res. Pap. FPL–RP–549. Madison, WI: U.S. Department of

Agriculture, Forest Service, Forest Products Laboratory. 16 p.

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

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Acknowledgments

We express sincere appreciation to the following individuals

who contributed to the success of this project:

Walt “Butch” Ashadka, Tim Lechien, and Harry Markert of

Jones Township Road Department for their assistance in

collecting field data and conducting load tests and Terry

Wipf and Doug Wood of the Iowa State University Civil

Engineering Department for assistance in completing the final

load testing.

Thanks also to the following from the USDA Forest Service,

Forest Products Laboratory: Earl Geske for installing instrumentation during the initial site visit; Earl Geske and Vyto

Malinauskas for developing and manufacturing load cell

instrumentation; Jim Wacker, Jim Kainz, and Kim Stanfill–

McMillan 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 and Construction...............................................2

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

Construction ...........................................................2

Evaluation Methodology...............................................4

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

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

Vertical Creep..........................................................5

Load Test Behavior..................................................5

Load Test 1.........................................................5

Load Test 2.........................................................6

Analytical Evaluation ............................................6

Condition Assessment ..............................................6

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

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

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

Vertical Creep..........................................................9

Load Test Behavior................................................10

Load Test 1.......................................................10

Load Test 2.......................................................10

Load Test Comparison ........................................10

Analytical Evaluation ..........................................12

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

Bridge Geometry ................................................12

Wood Condition ................................................12

Wearing Surface..................................................14

Bar Anchorage System.........................................14

Conclusions .............................................................14

Literature Cited.........................................................15

Appendix—Information Sheet......................................16
Field Performance of Timber Bridges

6. Hoffman Run Stress-Laminated Deck Bridge

Michael A. Ritter, Research Engineer

Paula D. Hilbrich Lee, General Engineer

Forest Products Laboratory, Madison, Wisconsin

Gregory J. Porter, Civil Engineer

Allegheny National Forest, Warren, Pennsylvania

Introduction

In 1988, Congress passed legislation known as the Timber

Bridge Initiative (TBI). The intent of this legislation was to

advance development and expand the use of timber as a

structural material for highway bridges. Responsibility for

administrating 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 USDA Forest Service, Forest Products Laboratory (FPL), in Madison, Wisconsin (USDA 1994). In

addition, the Forest Service National Forest System, which

manages the national forests of the country, made a commitment to demonstrate new technology in timber bridge design

and construction. Because of this commitment, many local

municipalities became aware of the technology and found it

to be a viable method for renewing aging infrastructures.

This report, the sixth in a series of bridge performance

documents, describes the development, design, construction,

and field performance of the Hoffman Run bridge near

Dahoga, Pennsylvania, Elk County, Jones Township. The

Hoffman Run bridge was constructed in October 1990 and

was a direct result of the success of the Little Salmon Creek

bridge (Ritter and others 1996) on the Allegheny National

Forest in Pennsylvania. The Hoffman Run bridge is a singlelane, simple-span, stress-laminated deck that is approximately 26 ft long and 16 ft wide. (See Table 1 for metric

conversion factors.) It is the second stress-laminated deck

superstructure constructed of hardwood lumber in

Pennsylvania. Characteristics of the bridge are

summarized in the Appendix.

Background

The Hoffman Run bridge is located just outside Dahoga,

Pennsylvania (Fig. 1). It is on County Road T364, a double-lane, gravel roadway that provides access to local residences and recreation areas on the Allegheny National Forest. 

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)

pound (lb) 4.448 newton (N)

lb/in2

 (stress) 6,894 pascal (Pa)

Traffic consists of light-passenger vehicles, recreational vehicles, school busses, and occasionally logging trucks. The

average daily traffic is estimated between 40 and

50 vehicles per day.

The original Hoffman Run bridge was constructed in the

early 1930s and consisted of a timber plank deck supported

by steel girders (Fig. 2). Inspection of the bridge in the

1980s indicated that the steel girders were in poor condition

and insufficient to carry modern traffic loads. Because of these

deficiencies, a decision was made by Jones Township to

replace the bridge.

Based on the success of the Little Salmon Creek bridge built

on the Allegheny National Forest in 1988, Jones Township

decided to replace the Hoffman Run bridge with a stresslaminated timber bridge using mixed hardwood lumber.

This type of timber bridge is an attractive alternative because

it is inexpensive and can be built using local lumber and a

township construction crew. At the time of planning for the

bridge, stress-laminated timber bridges were relatively new

in the United States. To evaluate bridge performance after

construction, both Jones Township and the Pennsylvania

Department of Transportation determined that the field performance of the bridge should be monitored to provide assurance that the performance of the system was acceptable.

Subsequently, FPL and Jones Township entered into an

agreement to complete structural monitoring of the bridge.
2

Objective and Scope

The objective of this project was to ascertain the field performance characteristics of the Hoffman Run stress-laminated

bridge by monitoring the bridge for approximately

2-1/2 years, beginning shortly after 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 activities in an effort to

improve design and construction methods for future stresslaminated timber bridges.

Design and Construction

Design

The design of the Hoffman Run bridge was based on that of

the Little Salmon Creek bridge, located in the Allegheny

National Forest. The bridge was designed before a nationally

recognized design procedure for stress-laminated timber

bridges was available in the United States. The design criteria for the bridge 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 Hoffman Run 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. 3). Mixed hardwoods, mostly beech, visually

graded No. 2 and better, were selected as the material for the

bridge. The design of the deck was based on nominal 2- by

12-in. mixed hardwood laminations, pressure treated with

creosote. The length of the laminations varied from 3 to 16 ft

in length, and butt joints in the deck laminations were

placed transverse to the bridge span in every fourth lamination, with a 4-ft longitudinal spacing between butt joints in

adjacent laminations (Fig. 4). 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 45 in. on-center,

beginning 18 in. from the bridge ends. The design bar tension force was 65,000 lb, resulting in 120 lb/in2

 of interlaminar compression, which is approximately 10 percent

greater than that of the Little Salmon Creek bridge. A discrete plate bar anchorage system was used, consisting of two

steel plates (Fig. 5). The bridge railing consisted of a sawn

lumber curb, post, and rail. Because of the low traffic volume, no wearing surface was specified.

Construction

Construction of the Hoffman Run bridge was completed

by the Jones Township Road Department. Construction

began by removing the existing superstructure and replacing the original abutments with concrete abutments and

Figure 1—Location and site layout of the Hoffman

Run bridge.
3

Figure 2—Original Hoffman Run bridge: (top) side view;

(bottom) underside of bridge and abutment. Figure 3—Design configuration of the Hoffman Run

bridge.

Figure 4—Butt joint configuration used for the Hoffman Run bridge. A butt joint was placed

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

laminations were separated by 4 ft.
4

wingwalls (Fig. 6). Superstructure assembly began at the

treating plant where treated deck laminations were nailed

together to form two deck sections, each half the bridge

width. Temporary steel bars were inserted through the laminations at several locations so that the deck sections could be

transported to the bridge site.

Upon completion of the concrete abutments and wingwalls,

the deck sections were loaded on a flatbed truck at the treating plant and transported to the bridge site (Fig. 7). At the

bridge site, the sections were unloaded from the truck and

placed side-by-side on temporary supports situated on the

approach roadway (Fig. 8). The temporary steel bars were

removed, and the stressing bars were inserted through both

sections (Fig. 9). A single hydraulic jack was used to tension the bars to the required 65,000 lb (Fig. 10). Following

bar tensioning, the entire superstructure was lifted as a unit

by a small crane and placed atop the abutments (Fig. 11).

The installation of the bridge superstructure was completed

in 1 day. After the initial bar tensioning, the bars were retensioned to 65,000 lb, approximately 1 week and 8 weeks after

installation. The curb and bridge railing were installed

shortly after the second bar tensioning. The completed bridge

is shown in Figure 12.

The as-built configuration of the Hoffman Run bridge varied

slightly from the design configuration shown in Figure 3.

After the final stressing, the width of the bridge measured

16.1 ft at the abutments and midspan.

Evaluation Methodology

Because of the experimental nature of the Hoffman Run

bridge, Jones Township 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 Jones Township. The plan called for performance monitoring of the deck moisture content, bar force in

Figure 5—Details of the discrete plate bar

anchorage configuration.

Figure 6—Completed concrete abutments and

wingwalls for the Hoffman Run bridge.

Figure 7—Two deck sections for the Hoffman Run

bridge were transported from the treating plant to

the bridge site on a flatbed truck.

Figure 8—At the bridge site, the two deck sections

were placed side-by-side atop temporary abutments

located on the approach roadway.
5

stressing bars, vertical bridge creep, load test behavior, and

condition assessment of the structure. The evaluation

methodology used procedures and equipment previously

developed by FPL (Ritter and others 1991).

Moisture Content

The moisture content of the Hoffman Run bridge was measured using an electrical-resistance 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 underside of the deck at depths of 1 to 2 in.,

recording the moisture content 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, and at the

conclusion of the monitoring.

Bar Force

Bar force for the Hoffman Run bridge was measured using

load cells developed by FPL. The load cells were installed

between the bearing plate and anchor plate on the second and

fourth stressing bars from the north abutment on the downstream side of the bridge. Load cell measurements were

obtained by Jones Township Road Department personnel

using a portable strain indicator. Strain measurements 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 load cell installation and approximately monthly thereafter.

Vertical Creep

Vertical creep was measured at the beginning and end of the

monitoring period. Measurements were obtained at midspan

on the upstream deck edge 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 Hoffman Run bridge was conducted

at the beginning and end of the monitoring period to determine the response of the bridge to 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 December 12, 1990,

approximately 2 months after bridge installation. The bridge

interlaminar compression at the time of the test was

114 lb/in2

. The test vehicle consisted of a fully loaded, twoaxle dump truck with a gross vehicle weight of 30,120 lb

(Fig. 13). The vehicle was positioned longitudinally on the

bridge so that the rear axle was centered at midspan. Transversely, the vehicle was placed for three load positions

(Fig. 14). 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. For load position 3,

the vehicle was positioned on the upstream 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 calibrated rules on

the deck underside and reading values with a surveyor’s level

to the nearest 0.06 in. (Fig. 15).

Figure 9—The temporary steel bars in the two deck

sections were removed, and stressing bars were

inserted through both deck sections.

Figure 10—Stressing bars were tensioned with

a single hydraulic jack.
6

Load Test 2

The second load test was completed on July 19, 1993,

approximately 34 months after bridge installation. At the

time of the test, the interlaminar compression was approximately 70 lb/in2

. The test vehicle consisted of a fully loaded

three-axle dump truck with a gross vehicle weight was of

46,450 lb (Fig. 16). Longitudinally, the vehicle was

positioned so that the two rear axles were centered at midspan and the front axle was off the bridge span. Transversely,

the vehicle was positioned for the same three load positions

used for load test 1 (Fig. 14 and 17). Measurements of the

bridge deflection from an unloaded to a loaded condition

were taken at midspan using string potentiometers and an

electronic data acquisition system (Fig. 18). The accuracy

of this method for repetitive readings is estimated to be

±0.005 in.

Analytical Evaluation

Past research and investigations have indicated that stresslaminated decks can be accurately modeled as orthotropic

plates (Oliva and others 1990). To further analyze the theoretical behavior of the Hoffman Run 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. A modulus of elasticity

value of 1,900,000 lb/in2

 was used for modeling based on

estimates from unpublished in-grade lumber testing previously completed by FPL.

Condition Assessment

The overall condition of the Hoffman Run bridge was assessed on two separate occasions during the monitoring

period. The evaluations occurred at the time of the load tests,

which coincided with the beginning and end of the monitoring, and entailed visual inspections, measurements, and

photographic documentation of the bridge condition. Items

of specific interest include bridge geometry, condition of the

timber deck and rail system, and condition of the stressing

bars and anchorage systems.

(a) (c)

(b) (d)

Figure 11—Completed deck placed on abutments: (a) bar tensioning completed, (b) deck lifted from temporary

abutments by small crane, (c) deck positioned over Hoffman Run, (d) superstructure placed on abutments.
7

Figure 12—Completed Hoffman Run bridge: (top) side view; (bottom) end view.
8

Results and Discussion

The performance monitoring of the Hoffman Run bridge

extended from December 1990 through July 1993. Results

and discussion of the performance data follow.

Moisture Content

Average lamination moisture content of the Hoffman Run

bridge was approximately 42 percent in December 1990,

2 months after installation. From that time, the bridge experienced a gradual decrease in moisture content to approximately 32 percent at the conclusion of the monitoring. The

10-percent decrease in moisture content occurred at a relatively constant rate during the 32 months, although there

were fluctuations of 2 to 3 percent in the measurement zone

as a result of seasonal climate changes. It is expected that the

moisture content in the interior of the laminations was

greater than the values obtained in the measurement zone as a

result of slower moisture migration through the lamination

depth.

Because of the high moisture content at the time of installation, the deck moisture content remained above the fiber

saturation moisture content, which is approximately 25 to

30 percent. Above this level, changes in the moisture content

do not cause dimensional changes in the wood laminations.

In time, the moisture content will gradually decrease and

eventually stabilize below the fiber saturation point. When

the moisture content of the bridge falls below fiber saturation,

subsequent moisture loss will result in shrinkage of the

laminations and could significantly influence bar force

retention.

Bar Force

The trend in average bar tension force for the Hoffman Run

bridge is shown in Figure 19. The final bar tensioning,

which occurred December 1990, approximately 2 months

after installation, was approximately 65,600 lb, or 101 percent of the design force. After the final bar tensioning, the bar

force began to decline. The decline continued throughout the

monitoring, although the rate of loss decreased with time. At

the conclusion of the monitoring, the average bar force was

approximately 38,000 lb, or 59 percent of the design force,

which corresponds to an average interlaminar compression of

Figure 13—Load test 1 truck configuration and

axle loads. Lighter axle is vehicle front. The transverse

vehicle track width, measured center-to-center of the

rear tires, was 6 ft.

Figure 14—Load test 1 and 2 transverse load positions

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

centered over the bridge midspan.

Figure 15—Load test 1 bridge deflections were

measured by suspending calibrated rules from

the deck underside and reading displacement

values with a surveyor’s level.

Figure 16—Load test 2 truck configuration and axle

loads. The transverse vehicle track width, measured

center-to-center of the rear tires, was 6 ft.
9

approximately 70 lb/in2 and exceeds the recommended

minimum interlaminar compression of the 40 lb/in2 (Ritter

1990). Because the moisture content of the deck laminations

remained greater than the fiber saturation point, there was no

lamination shrinkage due to moisture loss. Thus, the decline

in bar force is primarily attributed to accelerated stress

relaxation in the lumber laminations as a result of the high

moisture content level.

Vertical Creep

The Hoffman Run bridge was constructed without camber

and was approximately level between the abutments when

installed. In July 1993, at the conclusion of the monitoring

period, negative camber measured 0.5 in. on the upstream

bridge edge. The slight 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 the creep.

Figure 17—Load test 2 transverse load positions

(looking north): (top) load position 1; (middle) load

position 2; (bottom) load position 3. For all load

positions, the two rear axles were centered over the

bridge midspan with the front axle off the span.

Figure 18—Load test 2 was measured with string

potentiometers and an electronic data acquisition

system.

Figure 19—Average trend in bar tension force obtained from load cells installed on two stressing bars.
10

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

measured at the conclusion of the testing. Furthermore, there

was no detectable movement at either of the abutments.

Load Test 1

Transverse deflection values for load test 1 are shown in

Figure 20. For each of the three load positions, the deflection

results are typical of the orthotropic plate behavior of stresslaminated bridges (Ritter and others 1990). For load

position 1 (Fig. 20a), the maximum measured deflection of

0.25 in. occurred at the bridge centerline, at 1.5 ft either side

of the centerline, and beneath the wheel line on the downstream (right) side. For load position 2 (Fig. 20b), the

maximum deflection of 0.25 in. was measured at the bridge

centerline and at all data points up to and including the point

beneath the downstream wheel line. For load position 3

(Fig. 20c), the maximum deflection of 0.25 in. was measured

beneath both wheel lines and at 1.5 and 3 ft from the centerline on the upstream side of the bridge. For each case, it was

impossible to determine the exact maximum deflection

location given the accuracy of the readings.

Load Test 2

Transverse deflection values for load test 2 are shown in

Figure 21. As with load test 1, the deflections are typical of

orthotropic plate behavior. For load position 1 (Fig. 21a),

the maximum measured deflection of 0.50 in. occurred beneath the downstream wheel line and 1.5 ft from the bridge

centerline on the downstream (right) side. For load position

2 (Fig. 21b), the maximum measured deflection of 0.60 in.

occurred beneath the downstream wheel line and the adjacent

upstream data point, 17 in. from the wheel line. For load

position 3 (Fig. 21c), the maximum measured deflection of

0.52 in. was under the upstream (left) wheel line. Given the

accuracy of the measurement method, it is likely that the

deflections accurately represent the actual bridge behavior.

Load Test Comparison

A comparison of the load position deflections for load tests

1 and 2 is shown in Figure 22. For both load positions, the

plots are similar, although load test 2 deflections exceed

those of load test 1. Maximum measured deflections for both

load tests occurred at the same location for the respective

load positions and differed by 0.25 in. for load position 1,

0.35 in. for load position 2, and 0.27 in. for load position 3.

The difference in the deflections is attributable to the

17,190 lb additional rear axle load for load test 2 and the

change in longitudinal bridge stiffness as a result of the

difference in prestress levels at the time of load testing

(discussed in Analytical Evaluation section).

Assuming linear elastic behavior and uniform material

properties and loading, the bridge deflections for load

positions 2 and 3 should be a mirror image. The actual load

Figure 20—Transverse deflection for load test 1,

measured at the bridge midspan (looking north).

Bridge cross sections and vehicle positions are

shown to aid interpretation and are not to scale.
11

Figure 21—Transverse deflection for load test 2,

measured at the bridge midspan (looking north).

Bridge cross sections and vehicle positions are

shown to aid interpretation and are not to scale.

Figure 22—Comparison of the measured deflections

for load tests 1 and 2.
12

position 2 deflections and the mirror image of load position

3 deflections are shown in Figure 23 for each load test. As

shown for load test 1, there are minor differences between the

deflections, ranging from 0.01 to 0.03 in. at the bridge

edges, but the plots are essentially the same. For load test 2,

deflection plots vary somewhat in shape. The differences in

deflection for the two load positions are probably the result of

a slight difference between the wheel line loading caused by

eccentric loading of the test vehicle. Load position 1

(Fig. 21) indicates that the downstream wheel line load is

the greater of the two. When the heavier wheel line is placed

towards the edge of the bridge, as in load position 2, it

results in a larger edge deflection. For load position 3, the

heavier wheel line is placed at the centerline and the edge

deflection is not as large.

Analytical Evaluation

Comparisons of the measured load test results to the theoretical bridge response are shown in Figure 24. As illustrated, 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 25. Based on this analysis, the predicted

maximum AASHTO HS 20-44 static deflection is 0.56 in.

or approximately 1/536 of the bridge span for load test 1, and

0.60 in. or approximately 1/500 of the bridge span for load

test 2. The theoretical maximum for each load test occurs

when the truck is positioned eccentrically at the point between the wheel lines, adjacent to the eccentric wheel line.

Assuming constant bridge properties, the same bridge deflection would normally 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 difference in the maximum HS 20-44 deflection

for the two load tests is attributable to an approximate

7 percent decrease in bridge stiffness at load test 2, as a result

of the decreased level of interlaminar compression of 70 lb/in2

for load test 2 compared with 114 lb/in2 for load test 1.

Condition Assessment

Condition assessment of the Hoffman Run bridge indicates

that structural and serviceability performance are acceptable.

Inspection results for specific items follow.

Bridge Geometry

Bridge measurements during the monitoring indicated that

the width remained relatively stable, narrowing approximately 0.1 ft. The change in bridge width is probably the

result of stress relaxation in the laminations. Additional

reductions in bridge width can 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 some checking was evident

on rail members exposed to wet–dry cycles. Shortly after rail

installation, truss plates were attached to the ends of the rail

and the railposts to hinder checking (Fig. 26). The top of the

bridge rail showed minor checking, but the depth of the

checks did not appear to penetrate the preservative treatment

envelope. Checking was most pronounced in the end grain of

the timber rail posts. This would have likely been prevented

if an end grain sealer had been applied at the time of construction. There was no evidence of wood preservative loss

and no preservative or solvent accumulations on the wood

surface.

Figure 23—Comparison of measured deflections for

load test 1 (top) and 2 (bottom), showing the actual

deflection for load position 2 and the mirror image of

load position 3 (looking north).
13

Figure 24—Comparison of the measured deflections for load tests 1 and 2 to the theoretical deflection, based on

orthotropic plate analysis (looking north).
14

Wearing Surface

The Hoffman Run bridge was designed and constructed

without a wearing surface; hence, the vehicles ride directly on

the treated-wood deck. At the time of each load test, heavy

accumulations of gravel and other debris from the unpaved

approach roadway were present on the surface of the deck

(Fig. 27). Deck wear from vehicle tracking and debris abrasion was noted in several locations, although the level of

wear was slight and did not significantly reduce the deck

section or extend beyond the preservative treatment. Over

time, deck wear will continue, potentially causing a reduced

deck section as well as accelerating bridge deterioration as

the preservative treatment envelope wears away. In addition,

the large volume of debris is capable of trapping moisture on

the surface of the deck, which creates an environment more

suitable to biological attack of the wood.

Bar Anchorage System

The stressing bar anchorage system has performed as designed with no significant signs of distress. There are no

indications of the discrete plate anchorage crushing into the

exterior laminations or measurable distortion in the bearing

plate. The exposed steel stressing bars, hardware, and steel

plates showed no visible signs of corrosion or other distress.

Conclusions

After approximately 2-1/2 years in service, the Hoffman Run

bridge is exhibiting acceptable performance, although several

serviceability deficiencies are noted. These deficiencies are

primarily attributed to the high moisture content of the

lumber laminations at the time of construction and throughout the monitoring. Based on monitoring conducted since

bridge construction, the following conclusions are made:

Figure 25—Maximum predicted deflection profile at the

bridge midspan for AASHTO HS 20–24 truck loading.

Figure 26—Truss plates attached to end of bridge rail

and rail post to impede checking.

Figure 27—Debris accumulation on the bridge deck

noted during condition assessment.
15

• It is feasible to construct stress-laminated decks using

mixed hardwood lumber.

• 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 reduces field work, minimizes

traffic disruption, and may be an economical alternative

to field construction.

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

laminations of the Hoffman Run bridge decreased from

42 percent at the time of the first load test to 32 percent

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

saturation moisture content. It is anticipated that the

moisture content will continue to decrease and will

eventually fall below the fiber saturation point, resulting

in lamination shrinkage and bar force loss. For stresslaminated bridges constructed in the future, it is recommended that the average moisture content of the lumber

laminations not exceed 19 percent at time of construction.

• During the monitoring, the average bar force for the Hoffman Run bridge decreased from 65,000 lb (120 lb/in2

 interlaminar compression) to 38,000 lb (70 lb/in2

 interlaminar compression). The decline in bar force is greater than

expected and is primarily a result of accelerated stress

relaxation in the lumber laminations caused by the high

moisture content.

• Vertical creep during the monitoring period resulted in a

sag at the superstructure midspan of 0.5 in. along the upstream bridge edge. This accelerated creep is again attributable to the high lamination moisture content. It is

doubtful that the small sag will adversely affect the

serviceability of the bridge.

• Load testing and analysis indicate that the Hoffman Run

bridge exhibits linear elastic orthotropic plate behavior

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

decreased approximately 7 percent when the interlaminar

compression decreased from 114 to 70 lb/in2

. The maximum predicted bridge deflection as a result of AASHTO

HS 20-44 static truck loading is estimated to be 0.56 in.

(L/536) at 114 lb/in2 interlaminar compression and

0.60 in. (L/500) at 70 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 show 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 minimal and does not

significantly reduce the deck section or extend beneath the

preservative treatment, further deterioration is anticipated.

Literature Cited

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.

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. EM 7700–8. U.S.

Department of Agriculture, Forest Service, Engineering Staff.

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.

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. 14 p.

USDA. The timber bridge initiative, fiscal year 1994 status

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

Service, State and Private Forestry, Northeastern Area. 16 p.
16

Appendix—Information Sheet

General

Name: Hoffman Run bridge

Location: County Road T364, Dahoga, Pennsylvania

Date of Construction: October 1990

Owner: Jones Township

Design Configuration

Structure Type: Stress-laminated deck with butt joints

Butt Joint Frequency: Every 4th lamination transversely

Every 4 ft longitudinally in

 adjacent lamination

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 HS 20–44

Wearing Surface Type: None

Material and Configuration

Timber:

Species: Mixed hardwoods, mostly beech

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

Grade: No. 2 and better, visually graded

Moisture Condition: 42 percent average 2 months

 after installation at 1 to 2 in.

 depth

Preservative Treatment: Creosote

Stressing Bars:

Diameter: 1 in.

Number: 7

Design Force: 65,000 lb

Spacing: 45 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 11 by 1 in. bearing

 6 by 4 by 1 in. anchor