United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL–RP–539

Field Performance of

Timber Bridges

4. Graves Crossing

Stress-Laminated Deck Bridge

James P. Wacker

Michael A. Ritter
Abstract

The Graves Crossing bridge was constructed October 1991 in

Antrim County, Michigan, as part of the demonstration

timber bridge program sponsored by the USDA Forest

Service. The bridge is a two-span continuous, stresslaminated deck superstructure and it is 36-ft long and 26-ft

wide. The bridge is one of the first stress-laminated deck

bridges to be built of sawn lumber treated with chromated

copper arsenate (CCA) preservative. The performance of the

bridge was continuously monitored for 2 years, beginning at

the time of installation. This performance monitoring involved gathering and evaluating data relative to the stiffness

of the lumber laminations, the moisture content of the wood

deck, the force level in the stressing bars, and the behavior of

the bridge under static truck loading. In addition, comprehensive visual inspections were conducted to assess the overall

condition of the structure. Based on 2 years of field observations, the bridge is performing well with no structural or

serviceability deficiencies.

Keywords: Timber, bridge, wood, stress laminated

June 1995

Wacker, James P.; Ritter, Michael A. 1995. Field performance of timber

bridges: 4. Graves Crossing stress-laminated deck bridge. Res. Pap.

FPL–RP–539. 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

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

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

1,000 libraries in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the

University of Wisconsin.

The United States Department of Agriculture (USDA) Forest Service is a

diverse organization committed to equal opportunity in employment and

program delivery. USDA prohibits discrimination on the basis of race,

color, national origin, sex, religion, age, disability, political affiliation, and

familial status. Persons believing they have been discriminated against

should contact the Secretary, U.S. Department of Agriculture, Washington,

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

Acknowledgments

We express sincere appreciation to the following who

contributed to the success of this project:

John Pemberton, Fred Hunt, Ed Long, John Fleet, Brian

Grody, and Ron Chapman of the Antrim County Road

Commission for their assistance in collecting field data and

conducting load tests; the Huron Pines Resource, Conservation, and Development Council for assistance with planning, development, and execution of the project; and the staff

of Woodstock Corporation in West Branch, Michigan, for

assistance in evaluating the modulus of elasticity of the

lumber laminations.

Thanks also to the following from the USDA Forest Service,

Forest Products Laboratory: Paula Hilbrich Lee, James

Kainz, Kim Stanfill–McMillan, and Maureen Mathias for

assistance in data analysis and report preparation; Robert

Ross for assistance in evaluating modulus of elasticity of the

lumber laminations; Earl Geske and Vyto Malinauskas for

developing and manufacturing load cells; 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 ...........................................................4

Cost ......................................................................6

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

Lamination Stiffness.................................................6

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

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

Behavior Under Static Load ........................................6

Static-Load Testing ...............................................9

Analytical Assessment...........................................9

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

Results and Discussion...............................................11

Lamination Stiffness...............................................11

Moisture Content...................................................11

Bar Force..............................................................11

Behavior Under Static Load ......................................12

Static-Load Testing .............................................12

Analytical Assessment.........................................12

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

Deck Camber .....................................................13

Wood Components..............................................13

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

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

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

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

Appendix—Information Sheet......................................17
Introduction

In an effort to improve rural transportation networks and

revitalize rural economies, the U.S. Congress established the

Timber Bridge Initiative (TBI) in 1988 as part of a comprehensive legislation package aimed at enhancing rural

America. Administrative responsibility for the program was

assigned to the USDA Forest Service and implemented

through three distinct program areas aimed at improving the

utilization of timber as a bridge material (USDA 1993).

These program areas include a demonstration program to construct timber bridges, a research program to develop and

refine new timber bridge technologies, and a technology

transfer program to disseminate available information to

bridge engineers and builders.

Administrative responsibility for the demonstration bridge

program was assigned to the State and Private Forestry

(S&PF) branch of the USDA Forest Service. The Northeast

Area office of S&PF has been awarding grants for demonstration bridges on an annual basis since 1989. An evaluation

panel selects the grant recipients based on proposals submitted by state and local governments. The research program is

administered within the USDA Forest Service by the Forest

Products Laboratory (FPL). In conjunction with a wide range

of timber bridge studies being conducted in the laboratory and

the field, the FPL established a nationwide bridge monitoring

program. The objective of this program is to collect, analyze,

and distribute information on the field performance of timber

bridges. The technology transfer program is coordinated

through the Timber Bridge Information Resource Center

(TBIRC) in Morgantown, West Virginia. Information

resources on all aspects of timber bridges are available

through the Morgantown office.

This report is fourth in a series that documents the results

from the FPL bridge monitoring program. The report includes sections on the development, design, construction,

and field performance of the Graves Crossing bridge. Built

in 1991, this bridge is a two-lane, two-span continuous,

stress-laminated deck with an overall length of 36 ft. (See

Table 1 for metric conversion factors.) The bridge is one of

the first stress-laminated deck bridge to be built of sawn

lumber treated with chromated copper arsenate (CCA). An

information sheet on the Graves Crossing bridge is provided

in the Appendix.

Background

The Graves Crossing bridge is located approximately

35 miles west of Gaylord, Michigan, in Antrim County

(Fig. 1). The bridge is on a two-lane gravel roadway that

crosses the Jordan River at Graves Crossing, approximately

15 miles upstream of its terminus into Lake Charlevoix. The

gravel road provides access for several private residences and a

4,000-acre state forest preserve. The average daily traffic

(ADT) for the road is estimated to be 100 vehicles.

Before replacement in 1991, Graves Crossing consisted of a

series of four corrugated steel culverts with a 3-ton posted

load limit (Fig. 2). The culverts were in poor condition and

insufficient to meet the hydraulic flow requirements at the

site. Past roadway washouts and severe scour problems required that the culverts be replaced with a new bridge structure designed for greater hydraulic capacity. In addition, a new

bridge capable of supporting standard highway loads was

needed to provide safe access for fire-fighting vehicles, school

buses, and logging trucks. Replacement of the existing culverts with a skewed bridge was determined to be the best alternative, because it would allow alignment of the abutments

with the natural stream channel and reduce adverse impacts on

the Jordan River.

Field Performance Of Timber Bridges

4. Graves Crossing Stress-Laminated Deck Bridge

James P. Wacker, General Engineer

Michael A. Ritter, Research Engineer

Forest Products Laboratory, Madison, Wisconsin

Table 1—Factors for converting English units of

measurement to SI units

English unit

Conversion

factor SI unit

acre 4,046 square meter (m2)

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)

ton (short, 2,000 lb) 907 kilogram (kg)
2

Through a cooperative effort between the Huron Pines

Resource Conservation and Development Council and the

Antrim County Road Commission, a proposal was submitted to the USDA Forest Service for partial funding of the

Graves Crossing replacement as a demonstration bridge under

the Timber Bridge Initiative (USDA 1993). The project proposed a stress-laminated deck utilizing local Red Pine lumber

treated with CCA preservative. In 1990, the project received

funding through the TBIRC, and plans for the design and

construction of the Graves Crossing bridge were finalized.

Subsequently, FPL was contacted to provide assistance in

developing and implementing a field evaluation program to

monitor the performance of the bridge.

Objective and Scope

The objective of this project was to evaluate the field performance of the Graves Crossing stress-laminated deck bridge

for 2 years, beginning at bridge installation. The scope of the

project included data collection and analysis related to the

lamination stiffness, wood moisture content, stressing bar

force, 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 Graves Crossing bridge project was a mutual effort

among several agencies and individuals. An overview of the

design, construction, and cost of the project is presented.

Design

Design of the Graves Crossing bridge was completed by an

engineering consultant retained by the bridge owner. Except

for features relating to stress laminating, the bridge was designed in compliance with the American Association of State

Highway and Transportation Officials (AASHTO) Standard

Specifications for Highway Bridges (AASHTO 1989) for two

lanes of HS20-44 loading. Specific design requirements for

stress laminating were based on a draft version of the

AASHTO Guide Specifications for the Design of Stresslaminated Wood Decks (AASHTO 1991).

The design geometry of the Graves Crossing bridge provided

for a two-span continuous deck, 36 ft long, 26 ft wide, and

12 in. deep (Fig. 3). The stress-laminated deck consists of

Grand

Traverse

Bay

General area

Site plan

Bridge

location

Michigan's Lower Peninsula

U.S. map

➤

Mancelona

Hwy 131

Grayling

Kalkaska

North

➣

Traverse City

Gaylord

66

32

72

72

Hwy 31

66

Jordan River

Graves Crossing Rd.

State Forest

Preserve

★

North

➣

➤

0.1 mile

75

Figure 1—Location maps for the Graves Crossing bridge.

Figure 2—Original Graves Crossing site prior

to bridge construction.
3

 Figure 3—Design configuration of the Graves Crossing bridge.
4

4-in.-thick Red Pine lumber treated with CCA preservative in

accord with AWPA Standard C14 (AWPA 1989). Because

none of the laminations extended the bridge length, butt

joints were employed in the deck. Butt joints were spaced

across the width and length at intervals of four laminations

and 4 ft, respectively.

Design values for the Red Pine laminations were based on

the National Design Specification for Wood Construction

(AFPA 1986, 1988) for lumber visually graded No. 2 in

accordance with Northeastern Lumber Manufacturing Association (NELMA) rules. Tabulated design values were for the

Northern Pine species combination and were 950 lb/in2

 for

bending, 1,300,000 lb/in2 for modulus of elasticity (MOE),

and 435 lb/in2 for compression perpendicular-to-grain. Wetuse adjustment factors were applied to all design values.

The lumber laminations were stress-laminated with threaded

steel bars to provide the interlaminar compression required to

develop load transfer between adjacent laminations. The design specified the use of 1-in-diameter, high strength,

threaded and galvanized steel bars with an ultimate strength of

150,000 lb/in2 that met the requirements of ASTM A722

(ASTM 1988); these bars were spaced 4 ft along the bridge

length. The design tension force for the bars was 58,000 lb,

which provides an interlaminar compressive stress of 100

lb/in2. Galvanized steel bearing plates measuring 12 in. long

by 16 in. wide by 1 in. thick and galvanized steel anchor

plates measuring 4 by 6-1/2 by 1-1/4 in. were specified along

with spherical hex nuts to anchor the bar along the deck

edges.

Design of the bridge rail and curb system was based on a

crash-tested design conforming to AASHTO Performance

Level 1 criteria (FHWA 1990). The bridge rail was specified

as glued-laminated Southern Pine measuring 6-3/4 in. wide

by 10 in. deep that was continuous over the bridge length.

The rail posts were designated as visually graded, Dense

Select Structural, Douglas Fir sawn lumber measuring 8 in.

wide by 12 in. deep and spaced 6 ft on-center along the bridge

edges.

A 3-in.-thick asphalt wearing surface in conjunction with a

waterproof geotextile membrane were also specified.

Construction

Construction of the Graves Crossing bridge was contractually

administered by the Antrim County Road Commission in the

fall of 1991. The existing steel culverts were removed and

coffer dams were installed, while timber piles were driven and

timber abutments and a center bent were constructed (Fig. 4).

A temporary bridge structure was installed to provide uninterrupted traffic flow during bridge construction (Fig. 5).

The lumber laminations for the superstructure were delivered

to the site in 4-ft-wide strapped bundles that extended the full

length of the bridge. These bundles were placed on the completed substructure by a small crane (Fig. 6). Before removing the metal straps from the bundles, steel stressing bars

were inserted through the prebored holes and anchorage plates

and nuts were attached. The bridge laminations were intentionally oversized in length because of the bridge skew and

were trimmed on-site with a chainsaw (Fig. 7). The field cuts

exposed untreated end-grain areas of the laminations that were

field-treated with copper napthanate preservative. A preferable

method is to cut the laminations to the correct length prior to

preservative treatment.

Figure 4—Coffer dam used during installation of

 timber piling, abutments, and bent.

Figure 5—This temporary (Bailey-type) bridge

allowed uninterrupted traffic flow across the

bridge during the construction of the Graves

Crossing bridge.
5

After anchoring the centerline of the deck to the substructure

with drift pins, hydraulic equipment was used to tension the

stressing bars. The initial tensioning was completed immediately after the deck laminations were set on the abutments.

Tensioning was completed at each successive bar with a single hydraulic jack system to a force level of 58,000 lb

(Fig. 8). During this initial tensioning, the bars were tensioned four separate times to achieve a uniform 100 lb/in2

interlaminar compressive stress. Subsequent tensioning used

the same procedure and force level and was conducted at 1,

10, and 13 weeks after the initial tensioning.

The bridge rail and curb system was installed 3 days after

initial tensioning of the bars. Shortly thereafter, a treatedlumber sidewalk was installed at the downstream side of the

bridge to provide safe pedestrian access to an adjacent campground (Fig. 9). Approximately 1 month after the initial bar

tensioning, the waterproof geotextile membrane and 3-in.-

thick asphalt pavement were installed on the deck (Fig. 10).

The completed Graves Crossing bridge is shown in

Figure 11.

During construction, the west abutment of the Graves

Crossing bridge was installed with a slight skew in relation

to the east abutment and the center bent. Thus, the as-built

configuration differed slightly from design configuration and

is shown in Figure 12. Span 2 measures approximately 8 in.

longer than span 1 (based on average center-center of

Figure 6—Small crane used to place the strapped

lamination bundles onto the abutments.

Figure 7—All deck laminations were trimmed to

length with a chainsaw, and the exposed endgrain was field treated with copper napthanate

preservative.

Figure 8—Initial tensioning of the stressing bars

was completed with a hollow-core hydraulic

cylinder and pump at each successive bar,

beginning at one end of the bridge.

Figure 9—Completed sidewalk structure attached

to the superstructure on the downstream side of

the bridge.
6

bearing); span 1, measured along the upstream and downstream edges, differs by approximately 5 in. The shortest

span length (center-center of bearing) measured 16.75 ft at

the upstream edge of span 1.

Cost

The total cost for the Graves Crossing bridge was approximately $150,000 and included design, fabrication, material,

and construction of the substructure, superstructure, and

approach roadways. Contract costs for the design of the

superstructure totaled $8,000, and costs for the material,

fabrication, and construction of the superstructure totaled

$50,000. Based on 936 ft2 of deck surface area, the cost for

the Graves Crossing bridge superstructure was $62/ft2.

Evaluation Methodology

To evaluate structural performance of the Graves Crossing

bridge, the Antrim County Road Commission contacted FPL

for assistance. Through mutual agreement, a 2-year bridge

monitoring plan was developed by the FPL and implemented

through a Cooperative Research and Development Agreement

with the Antrim County Road Commission. The plan called

for stiffness testing of the lumber laminations prior to construction and monitoring of several key performance indicators. This included lamination stiffness, moisture content,

bar force, static load behavior, and general structure condition. At the initiation of field monitoring, FPL representatives visited the bridge site to install instrumentation and

train Antrim County personnel in data collection procedures

for moisture content and bar force measurements. Load tests

and general condition assessments were conducted by FPL

personnel during site visits. The evaluation methodology

utilized procedures and equipment previously developed

(Ritter and others 1991; Wacker and Ritter 1992) and is

discussed in the following sections.

Lamination Stiffness

Although several highway bridges have been constructed of

Red Pine lumber, additional information on lumber MOE

was considered necessary to verify assumed design values and

obtain accurate information for load test analyses. To accomplish this, stiffness testing was completed at the fabrication

plant on a representative sample of the Red Pine laminations

prior to preservative treatment. A transverse vibration technique (Ross and others 1991) was used to measure the flatwise MOE of 50 deck laminations: 10 each in lengths of 6,

10, 14, 16, and 18 ft. Flatwise MOE values were converted

to equivalent edgewise MOE values using a conversion factor

of 0.965 (Williams and others 1994).

Moisture Content

Electrical-resistance moisture content readings were collected

from the Graves Crossing superstructure at installation in

accordance with ASTM standard requirements (ASTM 1990).

In addition, lumber samples were obtained from field cuts at

installation and used to measure moisture content by the

ovendry method (ASTM 1992). After installation, the general

trend in moisture content during the monitoring period was

characterized by electrical-resistance measurements taken from

the superstructure on a bimonthly basis. Measurements were

taken at a series of locations on the deck’s underside at probe

penetrations of approximately 1 in. (ASTM 1990). When

necessary, adjustments for species and temperature were

applied to determine actual moisture content values (Forintek

1984).

Bar Force

Periodic measurements of stressing bar force were obtained to

monitor loss and ensure that adequate bar force was maintained during the monitoring period. Calibrated load cells

were installed on four of the nine stressing bars prior to the

initial stressing (Fig. 13). Load cell measurements were collected on a monthly basis with a portable strain indicator

(Fig. 14) and converted from units of strain to bar force based

on laboratory calibrations. In addition, the accuracy of the

load cells was validated with recalibrations and hydraulic force

checks performed at the end of the monitoring period (Ritter

and others 1991) .

Behavior Under Static Load

A static-load test of the Graves Crossing bridge was conducted approximately 6 months into the monitoring period to

determine the bridge response under static-loading conditions.

In addition, an analytical assessment was completed to predict

the bridge response using computer modeling.

Figure 10—Asphalt pavement wearing surface

was applied to the deck approximately 4 weeks

after installation.
7

Figure 11—Completed Graves Crossing bridge: (a) side view, (b) end view.
8

 Figure 12—As-built configuration of the Graves Crossing bridge.
9

Static-Load Testing

The load test of the Graves Crossing bridge consisted of positioning loaded dump trucks on the bridge and measuring the

resulting deflections at a series of locations along a transverse

cross section at the midpoint of each span and near the bridge

supports. A surveyor's level was utilized to read deflectionvalues from calibrated rules suspended from the bridge underside to the nearest 0.04 in. Deflection measurements were

obtained prior to testing (unloaded), after placement of the

test vehicle (loaded), and at the conclusion of testing

(unloaded).

The load test was performed April 20, 1992, with two trucks:

truck 20 with a gross vehicle weight of 31,250 lb and truck

22 with a gross vehicle weight of 30,850 lb (Fig. 15). Each

of the two spans was tested separately using designated positions in the longitudinal and transverse directions to produce

the maximum deflection in accordance with AASHTO recommendations (AASHTO 1989). For each span, three load

cases were completed by longitudinally positioning the midpoint of the rear truck axle over the skewed centerspan cross

section (Fig. 16). For load case 1, truck 20 was positioned

with a wheel line 2 ft from the roadway centerline in the

downstream lane. For load case 2, truck 22 was positioned

with a wheel line 2 ft from the roadway centerline in the upstream lane. For load case 3, trucks 20 and 22 were simultaneously positioned in the same locations as load cases 1 and

2, but facing in the opposite direction. Figure 17 shows the

three load positions completed for span 1.

Analytical Assessment

At the completion of load testing, actual truck loading and

AASHTO HS20-44 loading conditions were analyzed by an

orthotropic plate computer model to predict the centerspan

deflection values.

Condition Assessment

The general condition of the Graves Crossing bridge was

assessed on three occasions during the monitoring period.

The first assessment occurred at installation when monitoring

instrumentation was installed. The second assessment

occurred at the time of load testing, approximately 6 months

into the monitoring period. The final assessment occurred at

the end of the monitoring period, approximately 24 months

Figure 13—Load cells were placed between

anchor plates at four locations prior to the initial

tensioning of the stressing bars.

Figure 14—Antrim County personnel collecting

load cell readings with a portable strain indicator

shortly after the initial tensioning of the stressing

bars.

Figure 15—Load test vehicle axle loads and configuration.
Figure 16—Load test vehicle positioning for all load cases.
11

after installation. These assessments involved visual inspections, measurements, and photographic documentation of the

bridge condition. Items of specific interest included deck

camber, wood components, wearing surface, and stressing bar

anchorage system.

Results and Discussion

Performance of the Graves Crossing bridge was monitored for

2 years, beginning October 1991 and ending October 1993.

Lamination Stiffness

Results of stiffness testing of the Red Pine lumber yielded an

average edgewise MOE of 1.14 × 106 lb/in2. The design

edgewise MOE adjusted for wet-use conditions was

1.26 × 106 lb/in2. Thus, the actual MOE of the Red Pine

lumber was approximately 10 percent less than the nominal

design value.

Moisture Content

The general trend of electrical-resistance moisture content

measurements is shown in Figure 18. Results of the electrical-resistance moisture content measurements taken at installation indicated that the lower 1 in. of the deck laminations

was at approximately 25 percent moisture content. Results of

ovendry analyses performed on samples collected at installation indicated that the interior portion of the deck laminations, where the CCA preservative had not penetrated, was at

approximately 20 percent moisture content.

During the monitoring period, the lower 1-in. of the deck

laminations maintained approximately 25 percent moisture

content, despite small seasonal fluctuations related to the

local environmental conditions. Average moisture content of

the deck laminations did not significantly change during the

2-year monitoring.

Bar Force

The general trend in bar force measurements is shown in

Figure 19. Bar tensioning was conducted on four occasions

during the first several months after installation. After each

Figure 17—Load cases 1,2, and 3 used during

load testing of span 1 (west): (a) load case 1, (b)

load case 2, (c) load case 3.

Figure 18—General trend in moisture content

based on electrical-resistance measurements.
12

successive bar retensioning, the rate of bar force loss decreased. After the final retensioning in January 1992, the

average bar force decreased to 30,000 lb, or 50 lb/in2 interlaminar compressive stress, during the next 12 months. During the last 8 months, the bar forces stabilized at approximately 30,000 lb, or 50 lb/in2

 interlaminar compressive

stress. Stressing bar forces measured with hydraulic equipment at the end of the monitoring period confirmed the

30,000-lb load measured with the load cells.

The majority of the bar force loss during the monitoring

period is attributable to stress-relaxation of the lumber laminations and occurred within 1 year of the final deck stressing

in January 1992. Other factors that contributed to bar force

losses were relatively minor and included fluctuations in

lamination moisture content, slight deck deformation under

steel anchorage plates, and thermal effects. It is anticipated

that the bar force will drop below the 40 percent of design

level during the next several years. At that time, retensioning

the bars to a full design load of 58,000 lb is recommended.

Behavior Under Static Load

Results of the static-load testing and analytical assessment

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

the west end (looking east). No permanent residual deflection

was measured between load cases or at the conclusion of load

testing. In addition, no measureable deflection was observed

at the bridge supports during load testing. At the time of the

tests, the average bar force was 46,000 lb and is equivalent to

a deck prestress of 80 lb/in2.

Static-Load Testing

Figure 20 presents transverse deflection for the three load

cases for each span. For span 1, the maximum deflection

measured for load case 1 was 0.19 in., occurring under the

wheel line near the downstream curb (Fig. 20a). Maximum

deflection measured for load case 2 was 0.13 in., occurring

under both wheel lines (Fig. 20b). Maximum deflection

measured for load case 3 was 0.19 in., occurring under the

wheel line near the outside wheel line of truck 20 (Fig. 20c).

Similarly at span 2, the maximum deflection measured for

load case 1 was 0.16 in., occurring under the wheel line near

the downstream curb (Fig. 20d). Maximum deflection measured for load case 2 was 0.19 in., occurring between the

wheel lines (Fig. 20e). Maximum deflection measured for

load case 3 was 0.19 in., occurring under both wheel lines

closest to the bridge centerline (Fig. 20f).

If both spans of the two-span continuous bridge were equal in

length, the measured deflections for the same truck loading

should be the same. However, small differences in the measured deflections for each span were recorded for the same truck

loading. These small differences are attributed to the variations in the span lengths previously noted (Fig. 12 ) and are

within the precision of the measurement technique.

A comparison between load case 3, with the summation of

load cases 1 and 2 for each span is given in Figure 21. The

near exact overlay of the plots illustrates the linearly elastic

behavior of the bridge deck under static loading.

Analytical Assessment

The predicted deflection based on the orthotropic plate analysis and measured deflection from load case 3 of the static-load

testing are shown in Figure 22. The predicted deflections are

equal or slightly greater than measured deflections at most

data point locations. For span 1, the measured deflection beneath the outer wheel line of truck 20 was slightly greater

than predicted. For span 2, the measured deflection between

the wheel lines of both wheel lines was slightly greater than

predicted. In general, the analytical prediction represents a

close approximation of the actual deflections, because the

minor differences noted are within the precision of the measurement technique.

The predicted deflection for two HS20-44 vehicles positioned

similar to load case 3 is presented in Figure 23. The maximum predicted deflection of 0.32 in. occurs at the bridge centerline and is equivalent to 1/650th of the bridge span, which

measures 17.3-ft center-to-center of bearings.

Condition Assessment

General condition assessments indicated that the structural

and serviceability aspects of the Graves Crossing bridge were

satisfactory. Results of the specific areas inspected follow.

Figure 19—General trend in stressing bar forces

based on load cell measurements.
13

Figure 20—Transverse deflection measured at bridge centerspan (looking east) of the Graves Crossing bridge.

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

Deck Camber

Measurements of the deck camber at installation indicated

that the deck was relatively flat with no measurable camber

(positive or negative) for either span. Measurements of the

deck camber at the end of the monitoring indicated that the

deck remained flat, and no vertical creep occurred at either

span.

Wood Components

Visual inspection of the wood components of the bridge indicated no signs of deterioration or damage. Minor checking

was evident on the top surface of the lumber rail posts,

which encounter rapid wetting and drying cycles as a result of

the exposure of the end-grain. Potential rail post deterioration

caused by excessive checking could be prevented by applying
14

a bituminous seal to the end-grain top surface, but the seal is

typically not considered in design. All bolted connections

remained tight with no signs of wood member crushing

beneath the connectors.

Wearing Surface

The asphalt wearing surface appeared in good condition at the

conclusion of the monitoring. Substantial sand and gravel

debris had accumulated on the wearing surface, because the

Graves Crossing bridge is situated at the bottom of a slight

vertical sag curve with unpaved approaches. These roadway

drainage conditions may lead to future asphalt damage, unless

the wearing surface is periodically cleaned of all debris.

Anchorage System

Visual inspections of the stressing bar anchorage system

indicated satisfactory performance. Wood crushing was not

evident in the area beneath the steel bearing plates, with only

slight deformations visible in the edge deck laminations.

Corrosion was not visible on any of the galvanized steel

components.

Figure 21—Comparison of the sum of measured

deflections from load cases 1 and 2 to the

measured deflections from load case 3.

Figure 22—Comparison of the measured deflections from load case 3 and the predicted deflection using orthotropic plate analysis.

Figure 23—Predicted deflection at the bridge

centerspan for two HS-20 vehicles, each

positioned 2 ft from the roadway centerline (load

case 3).
15

Conclusions

Based on the monitoring results from the Graves Crossing

bridge, we conclude the following:

• After 2 years in service, the CCA-treated, Red Pine stresslaminated deck is performing well with no structural or

serviceability deficiencies.

• Results from the lamination MOE tests performed at the

fabrication plant yielded an average edgewise MOE of

1.14 × 106 lb/in2. This was slightly less than the assumed

design value for visually graded No. 2 Red Pine lumber.

• The general trend in moisture content indicates that the deck

is experiencing slight seasonal fluctuations but remains

relatively unchanged at 25 percent during the 2-year

monitoring.

• The general trend in stressing bar force indicates that losses

during the 2-year monitoring totaled approximately

30,000 lb, or 50 percent of the design force of 58,000 lb.

The majority of bar force losses occurred within 1 year after

the final restressing of the deck and was attributed to stress

relaxation of the Red Pine laminations. It is anticipated that

the Graves Crossing bridge will require retensioning of the

stressing bars to the full design level in the near future.

• Deck camber measurements indicate that no measurable

vertical creep occurred in either span of the bridge.

• Static-load testing and orthotropic plate analysis predicts a

maximum deflection under HS20-44 loading conditions of

0.32 in., or 1/650th of the bridge span length.

• Visual inspections indicate no signs of deterioration of the

wood or steel components.

References

AASHTO. 1989. Standard specifications for highway

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

State Highway and Transportation Officials.

AASHTO. 1991. Guide specifications for the design of

stress-laminated wood decks. Washington, DC: American

Association of State Highway and Transportation Officials.

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.

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.

FHWA. 1990. Memorandum on crash tested bridge railings. August 13, 1990. File Designation HNG–14. Washington, DC: Federal Highway Administration.

FORINTEK. 1984. Moisture content correction tables for

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

Forintek Corp. 37 p.

Ritter, M.A.; Geske, E.A.; Mason, L.; [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.

Ross, Robert J.; Geske, Earl A.; Larson,

Gary R.; Murphy, Joseph F. 1991. Transverse vibration nondestructive testing using a personal computer. Res.

Pap. FPL–RP–502. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 17 p.

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.

Wacker, James P.; Ritter, Michael A. 1992. Field

performance of timber bridges: 1. Teal River stress-laminated

deck bridge. Res. Pap. FPL–RP–515. Madison, WI: U.S.

Department of Agriculture, Forest Service, Forest Products

Laboratory. 19 p.

Williams, G.D.; Bohnhoff, D.R.; Moody, R.C.

1994. Bending properties of four-layer nail-laminated posts.

Res. Pap. FPL–RP–528. Madison, WI: U.S. Department of

Agriculture, Forest Service, Forest Products Laboratory.

16 p.
16
17

Appendix—Information Sheet

General

Name: Graves Crossing bridge

Location: Antrim County, Michigan

Date of Construction: October 1991

Owner: Antrim County Road Commission

Design Configuration

Structure Type: Stress-laminated deck

Butt Joint Frequency: Every 4th lamination transversely

Every 4 ft longitudinally in adjacent

 laminations

Total Length (out-out): 36.0 ft

Skew: 6 degrees

Number of Spans: 2 (continuous over an intermediate

 support)

Span Lengths (center-center of bearings): 17.5 ft

Width (out-out): 26.0 ft

Width (curb-curb): 24.4 ft

Number of Traffic Lanes: 2

Design Loading: AASHTO HS20-44

Wearing Surface Type: 2- to 3-in.-thick asphalt pavement

Material and Configuration

Timber:

Species: Red Pine

Size (actual): 4 by 12 in.

Grade: Visual No. 2

Moisture Condition: Approximately 25 percent at

 installation

Preservative Treatment: Chromated Copper Arsenate

Stressing Bars:

Diameter: 1 in.

Number: 9

Design Force: 57,600 lb

Spacing (center-center): 4 ft

Type: High strength steel thread bar with coarse right-

 hand thread, conforming to ASTM A722

Anchorage Type and Configuration:

Steel Plates: 12 by 16 by 1 in. bearing

 4 by 6-1/2 by 1-1/4 in. anchor