United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL-RP-515

Field Performance of

Timber Bridges

1. Teal River Stress-Laminated

Deck Bridge

James P. Wacker

Michael A. Ritter
Contents

Page

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

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

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

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

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

Construction ................ 4

cost ................... 7

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

Lamination Stiffness ............. 7

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

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

Creep ................... 8

Load Test Behavior ............. 8

LoadTest 1 ............... 8

LoadTest 2 ............... 9

Predicted Behavior Under HS20-44 Loading . 9

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

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

Lamination Stiffness ............. 10

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

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

Creep ................... 12

Load Test Behavior ............. 12

Load Test 1 ............... 12

Load Test 2 ............... 12

Predicted Behavior Under HS20-44 Loading . 12

Condition Assessment ............ 14

Wood Components ............ 14

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

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

Stressing Bars and Hardware ........ 14

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

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

Appendix A ................. 17

Appendix B ................. 19
Abstract Acknowledgments

The Teal River bridge was constructed in late 1989 in

Sawyer County, Wisconsin, as a part of the demonstration timber bridge program of the USDA Forest

Service. The bridge is a stress-laminated deck structure

with a 32.5-ft length and a 23.7-ft width. The design

is unique in that it is the first known stress-laminated

timber bridge in the United States to be constructed

of full-span glued-laminated timber beams, rather than

the traditionally used sawn lumber laminations. The

performance of the bridge was continuously monitored

for 2 years, beginning at the time of installation. This

performance monitoring involved gathering data relative to the moisture content of the wood deck, the force

level of stressing bars, the deck dead-load deflection,

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 2 years of field evaluations, the

bridge is performing well with no structural or serviceability deficiencies.

Keywords: Timber, bridge, wood, stress laminated,

glued-laminated timber

December 1992

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.

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.

We express sincere appreciation to the following who

assisted in the structural monitoring of the Teal River

Bridge:

Rob Fallon, Art Pond, Ralph Chafer, and Tom

Thompson of the Chequamegon National Forest

Engineering Department for obtaining field readings

and conducting load tests.

Don Ladenthin and Dan Slisz of the Sawyer County

(Wisconsin) Highway Department for conducting load

tests.

Dave Summy, Regional Bridge Engineer, USDA Forest

Service, Eastern Regional Office, for reviewing and

interpreting data.

The staff of Sentinel Structures, Inc. (Peshtigo,

Wisconsin) for testing glulam timber beams.

Earl Geske and Bob Ross of the Forest Products

Laboratory, Engineering Mechanics Laboratory,

for developing the instrumentation used for bridge

evaluation.

The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin.
Field Performance of Timber Bridges

1. Teal River Stress-Laminated Deck Bridge

James P. Wacker, General Engineer

Michael A. Ritter, Research Engineer

Forest Products Laboratory, Madison, Wisconsin

Introduction

In 1988, the U.S. Congress passed legislation known

as the Timber Bridge Initiative. The objective of

this legislation was to establish and annually fund a

national timber bridge program to provide effective

utilization of wood as a structural material for highway bridges. Responsibility for the development, implementation, and administration of the timber bridge

program was assigned to the USDA Forest Service. A

key element of this program is a demonstration bridge

program, which provides matching funds to local governments to demonstrate timber bridge technology

through the construction of demonstration bridges

(USDA 1991). A primary objective of the demonstration bridge program is to encourage innovation through

the use of new or previously underutilized wood products, bridge designs, and design applications. In so

doing, bridge designers and users will become more

aware of the attributes of wood as a bridge material, and new, economical, structurally efficient timber bridge systems should result. In addition, it is contemplated that timber use in bridges will be expanded

to include several abundant but underutilized wood

species.

As a national wood utilization research laboratory

within the USDA Forest Service, the Forest Products

Laboratory (FPL) has taken a lead role in assisting

local governments in evaluating the field performance

of demonstration bridges, many of which employ

design innovations that were not previously evaluated. This has involved the development and implementation of a comprehensive national bridge monitoring program. The objective of this monitoring

program is to collect, analyze, and distribute information on the field performance of timber bridges. This

Table 1—Factors for converting English

units of measurement to SI units

Conversion

English unit factor SI unit

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

foot20.09 square meter (m2)

pound (lb) 0.14 Newton (N)

lb/in2

 (stress) 6,894 Pascal (Pa)

lb/ft2 (weight) 4.88 kilogram/meter2 (kg/m2)

information will provide a basis for validating or revising design criteria and further improving efficiency and

economy in bridge design, fabrication, and construction. This report is one in a series documenting field

performance of timber bridges built as a part of the

Timber Bridge Initiative.

This report describes the development, design,

construction, and field performance of the Teal River

bridge located in Sawyer County in northwestern

Wisconsin. The bridge, built in 1989, is a two-lane,

single-span, stress-laminated deck with a length of

32.5 ft. (See Table 1 for metric conversion factors.)

The bridge design is unique in that it is the first known

U.S. application that utilizes full-span structural gluedlaminated (glulam) timber beams in a stress-laminated

deck. In 1991, this bridge design was awarded first

place in a National Timber Bridge Design Competition

in the “Under 40 ft Individual Span Vehicular Bridge”

category. An information sheet on the Teal River

bridge is provided in Appendix A.
Objective and Scope

The objective of this project was to evaluate the field

performance of the Teal River bridge for 2 years,

beginning at bridge installation. The project scope

included data collection and analysis related to the

wood moisture content, stressing bar force, deck creep,

bridge behavior under static truck loading, and general

structure performance. The results of this project will

be used to formulate recommendations for the design

and construction of similar bridges in the future.

Background

The Teal River bridge site is located approximately

20 miles east of Hayward, Wisconsin, in Sawyer County

(Fig. 1). It is on County Highway S, a two-lane paved

road that crosses the Teal River just upstream of where

it flows into the Chippewa River. This road is located

within the boundary of the Chequamegon National

Forest and provides access to several popular recreation

areas. The road is on the Chequamegon National

Forest transportation network and is a primary route

for logging traffic. The estimated average daily traffic

over this section of the road is 100 vehicles per day.

The Teal River bridge was originally constructed in

1925 and consisted of steel stringers with a concrete

deck supported by concrete abutments. The bridge

was 34 ft long, 16 ft wide, and included a rail system

constructed of steel angles. In 1988, inspections of the

bridge indicated that the concrete deck was in poor

condition and the steel girders were badly corroded,

although not to a point where restricted load limits

were required. In addition, the railing system was

substandard, and the narrow bridge width on a twolane road raised safety concerns. It was apparent to

the Sawyer County Highway Department that major

rehabilitation or replacement of the structure would be

required in the near future.

Subsequent to the bridge inspection, Sawyer County

officials determined that the Teal River bridge would

be replaced. Through a cooperative effort involving

Sawyer County, the North Twenty Resource Conservation and Development Council, and the Chequamegon

National Forest, a project proposal was submitted to

the USDA Forest Service for partial funding the Teal

River bridge replacement as a demonstration bridge

under the Timber Bridge Initiative. The proposal included a stress-laminated deck constructed of Red

Oak sawn lumber harvested from Wisconsin forests.

In 1989, the project was approved as proposed, and

matching funds were provided through the USDA

Forest Service, Timber Bridge Information Resource

Center in Morgantown, West Virginia. In finalizing

2

Figure 1—Location of the Teal River bridge.

project plans, it was found that Wisconsin Red Oak

lumber of the size required for the bridge was not readily available. As an alternative design, the use of Wisconsin red pine lumber was investigated, but again, the

material was not readily available in the required sizes.

Consequently, the FPL was contacted for assistance

in developing material options for the bridge, where

the use of wood products native to Wisconsin was a

primary consideration. Preliminary investigations indicated that glulam timber beams presented the most

feasible alternative for design. Using this approach, red

pine lumber could be laminated with structural waterproof adhesives to form beams of the size required for

the bridge laminations.
Design, Construction, and Cost

The design and construction of the Teal River bridge

was completed by the Chequamegon National Forest

engineering staff in cooperation with the Sawyer

County Highway Department officials. Assistance

was provided by the bridge design office of the Forest

Service Eastern Regional Office and the FPL. An

overview of the design, construction, and cost of the

bridge superstructure is presented.

Design

Design of the Teal River bridge was a two-part process,

involving the development of the glulam timber beams

followed by the design of the bridge superstructure.

As with most stress-laminated timber bridge decks,

it was anticipated that stiffness rather than strength

would be the primary design concern. To serve as

a basis for designing the glulam timber beams, a

preliminary analysis of the Teal River bridge indicated

that an acceptable deck depth could be obtained if a

minimum design modulus of elasticity of 1.6 x 106

lb/in2 could be achieved. The primary objective in

developing the glulam timber beams was to meet

this stiffness requirement utilizing Wisconsin-grown

red pine. Analytical analysis conducted at the FPL

indicated that a glulam timber beam manufactured

entirely of red pine was potentially feasible. However,

little information was available on the properties of

red pine lumber, and no information was available on

Figure 2—Layup of the Southern Pine-red pine

beam laminations used for the Teal River bridge.

red pine glulam timber to confidently establish design

values. Given this uncertainty, the FPL engineers

proposed that Southern Pine lumber be combined

with red pine lumber to provide additional beam

stiffness. No design information on Southern Pinered pine glulam timber beams was available in current

American Institute of Timber Construction (AITC)

standards (AITC 1987). However, the concept of

developing beams using lower stiffness species for the

inner lumber laminations and higher stiffness species

for the outer lumber laminations had been used for

other species in this standard. This same concept

was used to develop a proposed beam design with

Southern Pine outer laminations and red pine inner

laminations (Appendix B). The proposed beam design

was subsequently approved by AITC and was used in

the Teal River bridge (Fig. 2).

After the glulam timber design values were developed,

design of the Teal River bridge was completed by

the engineering staff of the Chequamegon National

Forest using criteria developed at the University of

Wisconsin-Madison and the FPL (Ritter 1990).

The design geometry of the deck provided for a

32.5-ft length, a 24-ft width, and a 13.75-in. thickness

(Fig. 3). This required the use of 91 Southern Pinered pine glulam timber beams, each measuring

approximately 3.125 in. wide. The outside beam

along each deck edge was designed to be Red Oak

glulam timber. The Red Oak would provide additional

strength in distributing the force in the stressing bars

into the deck without damaging the Southern Pinered pine beams. All glulam beams were used as bridge

laminations to form a continuous deck and hereafter are

referred to as beam laminations.

All beam laminations were designed to be continuous

between supports, without butt joints. The deck

was also provided with a curb and bridge rail system

to meet American Association of State Highway

and Transportation Officials (AASHTO) static-load

design requirements. Following fabrication, all wood

components were specified to be pressure treated with

pentachlorophenol in accordance with American Wood

Preservers’ Association (AWPA) Standard C14 (AWPA

1991).

The stressing system for the Teal River bridge was

designed to provide a uniform compressive stress of

100 lb/in2 between the beam laminations. It was assumed that approximately 60 percent of this compression will be lost during the lifetime of the bridge

as a result of transverse stress relaxation of the wood

laminations. The remaining 40 percent, or 40 lb/in2

of compression between the laminations, provides a

safety factor greater than 2.0 against relative lamination movement caused by transverse shear or

3
bending. To provide this interlaminar compression,

1-in.-diameter high-strength stressing bars were spaced

44 in. on center. The bars were specified to comply

with the requirements of ASTM A 722 (ASTM 1988)

and to provide a minimum ultimate tensile strength

of 1.5 x 105 lb/in2. The bar anchorage system was

the discrete plate anchorage system, consisting of

12- by 12-in. steel-bearing plates with 4 by 6.5-in.

steel anchorage plates (Fig. 4). To provide protection

from deterioration, all steel components were galvanized, including hardware,, stressing bars, and anchorage plates.

Construction

Construction of the Teal River bridge was completed

by Sawyer County and Chequamegon National Forest

personnel in the fall of 1989. Following work on

the approach roadway and widening of the existing

concrete abutments, construction of the bridge

superstructure began November 14 and was completed

November 15. However, several additional days were

required for construction of the bridge and approach

railing. During this period, the construction site was

subjected to inclement weather conditions, including

snow and cold temperatures. Although this weather

tended to slow the construction, it was completed on

schedule with little difficulty.

Figure 3—Design configuration of the Teal River

bridge.

Construction of the bridge began with the arrival of the

beam laminations at the bridge site. The 91 Southern

Pine-red pine beam laminations were transported to

the site on a flatbed trailer in banded panels consisting

of 15 to 16 beam laminations per panel (Fig. 5).

Each beam lamination measured 3.125 in. wide by

13.75 in. deep by 32.5 ft long. Panels were placed on

the abutments by a crane, bands were removed, and

the beam laminations were positioned by the work crew

(Fig. 6). After placement of the Southern Pine-red pine

beam laminations, a Red Oak beam lamination was

placed along each deck edge. During placement, it was

Figure 4-Details of the discrete plate bar

anchorage system.
discovered that the abutment configuration restricted

the bridge width and would not allow placement of

all the beam laminations. Consequently, one Southern

Pine-red pine beam lamination was removed, resulting

in a finished bridge width of approximately 23.7 ft

(design width was 24 ft).

After all beam laminations were in place, steel stressing

bars were manually inserted through the predrilled

holes in the beam laminations; bearing plates and

anchor plates were installed, and nuts were hand

tightened. Several stressing bars were then partially

tensioned to bring all beam laminations in contact, and

the initial deck stressing began. This was accomplished

with a hydraulic jacking system, consisting of a

hydraulic pump, a single hollow core jack, and a

stressing chair (Fig. 7). Using this system, force

applied by the jack is transferred through the stressing

chair to the bar bearing plate. The bar is pulled away

from the deck until the design force (60,500 lb) is

reached. Then, the anchorage nut is tightened with a

wrench to lockoff the tension force in the bar. Starting

at one bridge end, each stressing bar of the Teal River

bridge was tensioned in this manner (Fig. 8). After

all bars were tensioned, each was retensioned to insure

that the stress level in the deck was uniform and at the

required design level.

Following the initial stressing, the timber curb and rail

system was installed, and a glulam timber approach rail

was constructed. Approximately 1 week after the initial

stressing, the bridge was restressed to compensate

for anticipated losses in the bar force (Ritter 1990).

Approximately 7 weeks after the second deck stressing,

the third and final deck stressings were completed. An

asphalt wearing surface was subsequently applied in

June 1990. The completed bridge is shown in Figure 9.

Figure 5—Arrival of beam laminations at bridge

site; 15 to 16 beam laminations were banded

into panels to facilitate transportation and

construction. (M92 0009-3)

Figure 7—Hydraulic jacking system used to

tension bars.

Figure 6—Placement of beam laminations on

abutments. (M92 0009-4)

Figure 8—Tensioning bars with hydraulic jacking

system. (M92 0009-6)

5
Figure 9—Completed Teal River bridge. (M92 0001-26, M92 0001-24)

6
Cost

Costs for the design, fabrication, and construction

of the Teal River bridge superstructure and rail

system were $4,000 for survey and design, $26,485 for

materials, and $5,400 for labor and equipment for a

total cost of $35,885. Based on a total deck area of

780 ft2, the cost per square foot was approximately

$46.

Evaluation Methodology

To evaluate the structural performance of the Teal

River bridge, Sawyer County representatives contacted

the FPL for assistance. Through mutual agreement,

a bridge monitoring plan was developed by the FPL

and implemented as a cooperative research effort

with Sawyer County and the Chequamegon National

Forest. The plan called for stiffness testing of the

beam laminations prior to bridge construction and for

performance monitoring of the deck moisture content,

bar force in stressing bars, bridge creep, load test

behavior, and condition assessments of the structure for

the first 2 years in service. The evaluation methodology

utilized procedures and equipment previously developed

(Ritter and others 1991) and is discussed in the

following sections.

Figure 10—Measurement of the beam

lamination MOE at the bridge construction

site using the transverse vibration technique.

(M92 0009-7)

preservatives. These tests involved a limited number

of beam laminations that were previously tested at the

manufacturing plant and tagged for identification after

treating. The testing methods used in the field were

the same as those used at the manufacturing plant and

were conducted immediately prior to deck assembly

(Fig. 10).

Moisture Content

Lamination Stiffness

The glulam timber beam developed for the beam

laminations of the Teal River bridge was a new

combination. Thus, stiffness tests were completed to

verify design assumptions. This testing was conducted

at both the manufacturing plant and the bridge site.

At the manufacturing plant, modulus of elasticity

(MOE) tests were performed on the lumber before

gluing and on the completed glulam timber beams prior

to preservative treatment. Three different methods

were used to determine MOE values (Ross and Pellerin

1991). The first method, known as the static-load

method, involved placing a known load at the lumber

or beam midspan and measuring the deflection with

a dial gauge. The second method, known as the

transverse vibration technique, involved impacting the

lumber or beam to induce a transverse vibration and

measuring the natural frequency with a computer (Ross

and others 1991). The third method employed stresswave technology and involved inducing a longitudinal

stress wave in the lumber or beam and measuring the

time-of-flight of the wave along the lamination length.

In addition to the MOE testing at the manufacturing

plant, tests were also performed at the bridge site after

the glued beam laminations were treated with wood

Changes in the moisture content level of stresslaminated timber decks can significantly affect the

performance of the structure. If moisture decreases,

the deck can shrink, resulting in a decrease in stressing

bar force. Conversely, if moisture increases, swelling of

the timber can occur and cause an increase in stressing

bar force. Changes in moisture content level can also

affect the deck stiffness, creep, and transverse stress

relaxation.

To measure the moisture content of the Teal River

bridge deck, an electrical-resistance moisture meter

with 3-in. pins was used. Measurements were obtained

on a monthly basis by driving probe pins into the

deck underside at depths of 2 to 3 in., recording the

moisture content value from the unit, then adjusting

the values for temperature and wood species. At the

3-in. maximum pin penetration depth, measurements

were taken in the lower Southern Pine portion of the

beam laminations.

Bar Force

For stress-laminated bridges to perform properly, an

adequate level of interlaminar compression must be

maintained between the bridge laminations. This

compression is placed in the bridge by tensioning the

7
Figure 11—A load cell placed on a stressing

bar to measure changes in bar tension.

(M92 0009-8)

Figure 12—Reading a load cell with a portable

strain indicator. (M92 0001-27A)

stressing bars to high levels and maintaining a portion

of this force during the life of the structure. Thus, the

force level in the bars provides a direct indication of

the interlaminar compression in the bridge. For the

Teal River bridge, the initial interlaminar compression

of 100 lb/in2 required a force in each stressing bar

of 60,500 lb. If the force level decreases more than

approximately 80 percent (i.e., less than 20 percent

remaining), structural and serviceability problems can

occur.

To monitor bar force, load cells developed by the FPL

were installed on two of the nine stressing bars when

the bridge was assembled. These cells consisted of a

steel cylinder that was placed between the stressing

bar bearing plate and anchorage plate (Fig. 11). Each

cell was provided with two 90° strain gage rosettes

that measured the strain in the load cell. Strain

measurements were then converted to force levels to

determine the force remaining in the bar.

Load cell measurements were obtained by connecting

a portable strain indicator to a plug on the load

cell body (Fig. 12). Measurements were taken on a

biweekly basis for the first year and monthly thereafter.

Approximately midway through the monitoring period,

1 year after bridge construction, the load cells were

unloaded and checked for zero balance shift.

Creep

As a structural material, wood can deform permanently

as a result of long-term sustained loads. For stresslaminated bridges, creep caused by structure dead

load is an important consideration, because excessive

creep can result in a sag in the superstructure (Ritter

and others 1990). Creep of the Teal River bridge was

measured on a monthly basis with a displacement

rule attached to the deck underside at midspan.

Vertical movement over time was recorded relative to

a stringline attached near the abutments. In addition,

a surveying level and rod were used periodically to

confirm stringline data.

Load Test Behavior

Static-load testing of stress-laminated bridges is an

important part of a comprehensive bridge monitoring

program. The information obtained from these tests

is used to refine and improve design procedures and

evaluate the effects of various design variables on bridge

performance. To determine the load test behavior of

the Teal River bridge, load tests were conducted at

7 and 17 months after installation. Each test consisted

of positioning fully loaded trucks on the bridge deck

and measuring the resulting deflections at a series of

locations along the bridge centerspan, quarter points,

and abutments. Measurements of bridge deflections

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

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

Load Test 1

For the first load test on June 19, 1990, the vehicle

used was a three-axle loaded dump truck with a

gross vehicle weight of 79,840 lb (Fig. 13). The

vehicle was positioned longitudinally on the bridge

so that the centroid of the vehicle aligned with the

bridge centerspan for each of three transverse load

positions (Fig. 14). For load position 1, the vehicle

was positioned facing north in the downstream lane

with the center of the outside wheel line 2 ft from the

curb face. For load position 2, the vehicle faced south

in the upstream lane with the center of the inside wheel

line 2 ft from the bridge centerline. Load position 3

was the same as load position 1, except the vehicle

faced south in the upstream lane. Measurements of

bridge deflections from an unloaded to loaded condition

8
Figure 13—Load Test 1 vehicle configuration

and axle loads. The transverse vehicle track

width was 6 ft, measured center-to-center of

the rear tires.

Load position 1

Load position 2

Load position 3

Figure 14 - Transverse load positions used for

Load Test 1. Vehicles in the right lane face

North, into the page; those in the left lane face

South, out of the page. For all load positions,

the longitudinal centroid of the vehicle was

placed over the bridge centerspan.

were obtained by placing a surveying rod on the deck

underside and reading values with a surveyor’s level

to the nearest 0.005 ft (Fig. 15). The accuracy of

this method for repetitive readings is estimated to be

±0.06 in.

Load Test 2

The second load test on April 26, 1991, involved two

test vehicles; Truck 11 with a gross vehicle weight of

Figure 15—Measuring bridge deflection with

surveyor’s rod. (M92 0009-9)

59,880 lb and Truck 13 with a gross vehicle weight

of 59,600 lb (Fig. 16). For this test, the vehicles

were positioned longitudinally with the two rear

axles centered over the bridge centerspan, and three

transverse load positions were used (Fig. 17). For

load position 1, Truck 11 was positioned in the

downstream lane with the center of the inside wheel

line 2 ft from the bridge centerline. Load position 2

involved Truck 13 in the upstream lane, also with the

center of the inside wheel line 2 ft from the bridge

centerline (Fig. 18). Load position 3 utilized both

vehicles placed in the same respective positions used

for load positions 1 and 2 (Fig. 19). Measurements

of bridge deflections were obtained by suspending

calibrated rules from the deck underside and reading

values to the nearest 0.06 in. with a surveyor’s level.

The accuracy of measurements is estimated to be

±0.03 in.

Predicted Behavior Under HS 20-44 Loading

Previous research showed that stress-laminated decks

can be accurately modeled as orthotropic plates (Oliva

and others 1990). To further analyze the behavior of

the Teal River Bridge, an orthotropic model currently

being developed and verified at the FPL was used to

predict the deflection of AASHTO HS 20-44 loading

and to evaluate changes in bridge stiffness.

Condition Assessment

The general condition of the bridge was assessed

on four different occasions during the monitoring

period. The first assessment occurred at the time

of installation. The second and third assessments

took place at the time of load testing. The final

assessment occurred during a site visit near the end

of the monitoring period. These assessments involved

visual inspections, measurements, and photograph

9
Truck 11

Truck 13

Figure 16—Load Test 2 vehicle configurations

and axle loads. The transverse vehicle track

width for both vehicles was 6 ft, measured

center-to-center of the rear tires.

Truck 11

Load position 1

Truck 13

Load position 2

Truck 13 Truck 11

Load position 3

Figure 17—Transverse load positions used for

Load Test 2. Vehicles in the right lane face

North, into the page; those in the left lane face

South, out of the page. For all load positions,

the two rear ax/es were centered over the bridge

centerspan.

Figure 18—Load position 2 with a single vehicle

in the upstream lane. (M92 0009-12)

Figure 19—Load position 3 with both vehicles

on the bridge. (M92 0009-13)

documentation of the bridge condition. Items of

specific interest included the condition of the timber

deck and rail system, asphalt wearing surface, stressing

bars, and anchorage systems.

Results and Discussion

The performance monitoring of the Teal River bridge

extended from November 1989 through October 1991.

The following presents results of the performance data.

Lamination Stiffness

Results of MOE tests on lumber prior to gluing are

given in Table 2. For Southern Pine, the measured

values were slightly greater than the assumed values

given in Appendix B. For red pine, the measured values

were approximately 3.5 percent greater than assumed

values.

10
Tests of 30 laminated beams at the manufacturing

plant using the transverse vibration technique resulted

in an average MOE of 1.78 x 106 lb/in2. Additional

testing at the plant using the static-load method

resulted in an average MOE of 1.77 x 106

 lb/in2 for

24 laminated beams. Measured MOE values for the

five beams tested at both the manufacturing plant and

bridge construction site are given in Table 3.

For tests conducted at the manufacturing plant and the

bridge site, the stress-wave technique did not provide

reliable data and was not used for evaluation.

The average modulus of elasticity of the laminated

beams exceeded the target value of 1.6 x 106 lb/in2

by approximately 10 percent. This was due primarily

because lumber properties for both Southern Pine and

red pine exceeded those assumed in the original design.

Moisture Content

The trend of average moisture content changes in

the bridge deck is shown in Figure 20. The beamlaminations were initially installed at an average

moisture content of less than 10 percent. Since installation, moisture content gradually increased

to an average level of approximately 13 percent

at the conclusion of the monitoring period. Moisture content fluctuated following seasonal climate

changes, with a maximum average moisture content of

approximately 15 percent occurring in the fall of 1990

(Fig. 20). These changes will continue to occur over

the life of the structure and were generally most apparent in the outer 1 to 3 in. of the deck, where moisture

measurements were taken. Moisture changes in the interior portion of the deck were gradual, where the moisture content will continue to increase until an equilibrium level is reached. It is anticipated that the average

moisture content of the deck will eventually stabilize at

an equilibrium value of 18 to 20 percent (McCutcheon

and others 1986), although short-term seasonal changes

will continue to occur, primarily in the outer 2 to 3 in.

of the deck.

Bar Force

The average trend in bar tension force measured from

the average of load cells on two stressing bars is shown

in Figure 21. As indicated, the first two stressings were

at the design force of 60,500 lb. The final stressing,

which occurred about 8 weeks after installation, was

approximately 10 percent greater than the design force

level. As is typical for stress-laminated decks, the

rate of bar force loss as a result of transverse stress

relaxation decreased substantially with each restressing

(Fig. 21).

Table 2—Results of modulus of elasticity (MOE)

measurements on lumber prior to gluing

Species and lumber grade

Pieces Average

tested MOE

(number) (x 106 lb/in2)

Southern Pine No. 1 Dense 5 2.07

Southern Pine No. 2 Dense 10 1.93

Red pine No. 2 20 1.49

Table 3—Measured MOE values for five laminated

beams tested at the manufacturiug plant and the

bridge construction site

MOE (x 106 lb/in2)

Manufacturing plant Bridge sitea

Beam Transverse Static Static Transverse Static

number vibration a

loadaloadb

vibration load

6 1.87 1.67 1.75 1.83 1.73

7 1.85 1.73 1.90 1.78 1.66

11 1.80 1.71 1.79 1.78 1.61

12 1.70 1.67 1.76 1.91 1.52

14 1.77 1.88 1.54

aPerformed by FPL personnel.

bPerformed by Sentinel Structures, Inc. personnel.

c—, no data.

Figure 21 also reflects the minor fluctuations in

bar force as a result of moisture changes and stress

relaxation in the deck. However, force losses were

minimal, and the average bar force was within

10 percent of the design force at the end of the

monitoring period. This can be attributed to several

factors, the most significant of which was the initial low

moisture content level of the beam laminations. As the

deck slowly gained moisture in reaching an equilibrium

moisture content, the wood swelled slightly, which

tended to offset force losses as a result of transverse

stress relaxation. Other stress-laminated decks installed

at relatively high moisture content levels had bar force

losses as much as 80 percent during 2 years (Ritter and

others 1990). On a comparative basis, the Teal River

bridge vividly illustrates the advantage of low wood

moisture content levels at the time of construction in

reducing the rate of bar force loss.

11
Creep

The beam laminations for the Teal River bridge were

manufactured with a positive camber of approximately

2 in. After the dead-load deflection of about 0.4 in.,

measurements indicated that approximately 0.1 in. of

vertical creep occurred at centerspan during 2 years. At

the conclusion of the monitoring period, approximately

1.5 in. of positive camber remained in the deck at

centerspan.

Load Test Behavior

Results for both load tests and the predicted response

of the bridge under AASHTO HS20-44 loading

are presented. In each case, transverse deflection

measurements are given at the bridge centerspan as

viewed from the south end (looking north). To aid

visual interpretation, deflection values are presented

as fourth-order polynominal curve fits to the measured

data points. For each load test, no permanent residual

deformation was measured at the conclusion of the

testing. Additionally, movement at either of the

abutments was not detected.

Load Test 1

Transverse deflection values for Load Test 1 are shown

in Figure 22. For load position 1 (Fig. 22a), the

maximum deflection of 0.84 in. occurred under the

outside wheel line, 3 ft from the downstream deck

edge. For load position 2 (Fig. 22b), the maximum

deflection of 0.85 in. was measured 10 in. from the

outside wheel line, toward the upstream deck edge.

For load position 3 (Fig. 22c), the maximum deflection

of 0.90 in. occurred under the outside wheel line, 3 ft

from the upstream deck edge.

In comparing the results for load positions 1 and 3, the

symmetry of the load positions should have resulted in

the same maximum deflection magnitude and location

for both cases, assuming linear elastic deck behavior

and uniform deck stiffness. As the load measurement

accuracy using a surveyor’s rod was within the 0.06 in.

difference for the two load cases, it is probable that the

maximum deflections for load positions 1 and 3 were

approximately equal.

Load Test 2

Transverse deflection values for Load Test 2 are shown

in Figure 23. With a single truck on the bridge, both

load positions 1 and 2 (Figure 23a,b) produced a

maximum deflection of 0.59 in. approximately midway

between the truck wheel lines. With both vehicles on

the bridge, the maximum deflection for load position 3

(Fig. 23c) was 0.94 in. under the inside wheel line of

Truck 11.

12

Figure 20—Trend of average moisture content

changes in the bridge deck.

Figure 21—Average trend in bar tension force

obtained from load cells installed on two

stressing bars.

The measured deflection points for load positions 1

plus 2 compared to a fitted curve for load position

3 are shown in Figure 24. Assuming linear elastic

behavior, the deflection resulting from the sum of the

two individual truck loads should equal the deflection

from both trucks applied simultaneously. As observed

in Figure 24, the two curves are similar with only minor

variations. These variations are within the accuracy of

the measurement methods. From this information, it is

concluded that bridge behavior under the applied loads

is within the expected linear elastic range.

Predicted Behavior Under HS20-44 Loading

Based on an analysis of both load tests, the maximum

deflection of the Teal River bridge subjected to two

lanes of HS20-44 loading was estimated at 0.99 in.,

or where the bridge span is 31 ft measured centerto-center of bearings (Fig. 25). Further orthotropic

plate analysis of Load Test 2 compared to Load Test 1
Figure 22—Transverse deflection for Load Test

1, measured at the bridge centerspan. Bridge

cross-sections and vehicle positions are shown to

aid interpretation and are not to scale.

Figure 23—Transverse deflection for load Test

2, measured at the bridge centerspan. Bridge

cross-sections and vehicle positions are shown to

aid interpretation and are not to scale.

13
Figure 24—Load Test 2 showing the sum of

measured deflections from load positions 1 and

2 compared to a fitted curve of the measured

deflections for load position 3.

indicated that for equivalent HS20-44 loading, no

significant decrease occurred in bridge stiffness during

the monitoring period. This was expected because

neither a significant decrease in bar force nor an

increase in deck moisture content occurred during the

monitoring period.

Condition Assessment

Condition assessments of the Teal River bridge

indicated that structural and serviceability performance

were good. Inspection results for specific items follow.

Wood Components

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 Southern Pine glulam timber

rail posts. This would have likely been prevented if a

bituminous 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 of the member. Inspection showed no evidence

of wood preservative loss and no preservative or solvent

accumulations on the wood surface. The Red Oak edge

beams showed a lightening in color from a medium tan

to a very light tan as a result of sunlight exposure.

Wearing Surface

Inspection of the asphalt wearing surface indicated

minor transverse cracking in a random pattern.

Figure 25—Predicted deflection profile at the

bridge centerspan for two HS20-44 trucks, each

positioned 2 ft on either side of the bridge

longitudinal centerline.

This was attributed to a deficiency in the asphalt

mix or application procedures, because the same

cracking was observed on both approach roadways,

which were paved at the same time as the bridge

deck. Aside from the minor cracking, the asphalt

was in good condition and showed no other signs of

distress.

Anchorage System

The stressing bar anchorage system performed

as designed with no significant signs of distress.

Inspection indicated no crushing of the discrete plate

anchorage into the outside Red Oak beams and no

measurable distortion in the bearing plate. A very

gradual compression deformation was noted along the

deck edge for a distance of several feet on either side of

several anchorages. This deformation was difficult to

detect visually and was likely the result of transverse

stress relaxation in the beam laminations.

Stressing Bars and Hardware

The exposed steel stressing bars and hardware showed

no visible signs of corrosion, except at the ends of

stressing bars. At bar ends, minor corrosion appeared

where the galvanized coating had been stripped from

the bar exposing uncoated steel. This occurred because

the nuts were not adequately oversized to compensate

for galvanizing and were forced on the bars during

construction. This problem would not have occurred

if nuts had been properly oversized to compensate for

galvanizing or if a cold galvanizing compound had been

applied to the bar to replace the removed coating.

14
Conclusions

After 2 years in service, the Teal River bridge is

exhibiting excellent performance and should provide

many more years of acceptable service. Based on

the extensive monitoring conducted since bridge

fabrication, the following conclusions are given:

1. It is both feasible and economically practical to

manufacture structural glued-laminated (glulam)

timber beams for bridge applications using a

combination of red pine and Southern Pine lumber.

Based on an evaluation of this project, it is probable

that beams could be manufactured entirely from red

pine, provided appropriate quality control measures

are implemented to insure stiffness requirements for

the specific design.

2. Stress-laminated decks can be constructed using

glulam timber beams for the deck laminations.

The ability to manufacture the glulam timber

beam laminations as continuous members greatly

facilitates transportation and construction because

butt joints are not required.

3. The use of Red Oak outside edge laminations

facilitates good performance of discrete plate

stressing bar anchorages. Red Oak provides

sufficient strength to adequately distribute the bar

force into the deck without wood crushing or anchor

plate deformation.

4. The average trend in deck moisture content indicates

that global moisture content changes have been

occurring very slowly with an average increase of

approximately 3 percent during the 2 years. Cyclic

seasonal variations in moisture content have been

occurring more rapidly and at greater magnitudes in

the outer 2 to 3 in. of the exposed deck.

5. Stressing bar force remained at a relatively high

level during the P-year monitoring with less than

a lo-percent average decrease in bar force below

the initial level. This is attributable primarily to

low average moisture content levels of the bridge

laminations at installation, which were less than

the anticipated equilibrium moisture content for

the site. Loss in bar force caused by transverse

stress relaxation in the wood has been minimized

by dimensional increases in the deck as moisture

content increases toward an equilibrium level. The

lower moisture content level has also reduced the

rate of stress relaxation within the deck and in the

vicinity of bar anchorage plates.

6. Creep of the bridge deck has been minimal with approximately 0.10 in. of vertical displacement during

the 2 years. A positive camber of approximately

1.5 in. has remained at centerspan.

7.

8.

9.

Load testing and analysis indicate that the Teal

River bridge is performing as a linear elastic

orthotropic plate when subjected to highway

loading. The maximum deflection caused by two

lanes of AASHTO HS20-44 loading is estimated to

be 0.99 in.

The deck stiffness has not been appreciably changed.

This is attributable to the high level of prestress

maintained in the deck during the monitoring

period.

Wood checking is evident in the exposed end grain

of bridge rail posts and other components. It is

likely this would not have occurred if a bituminous

sealer had been applied to the end grain at the time

of construction.

10. The ends of some stressing bars show signs of minor

corrosion at locations where the galvanizing was

removed during construction. This would not have

occurred if the stressing nuts had been oversized

to compensate for the thickness of the galvanized

coating.

References

15
16
Appendix A—Information Sheet

General

Name: Teal River Bridge

Location: Along Highway S in the northern part of

Sawyer County, Wisconsin

Date of Construction: November 15, 1989

Owner: Sawyer County Highway Department

Design Configuration

Structure Type: Stress-laminated deck consisting of

full-span beam laminations (see Table 1 for metric

conversion).

Total Length (out-out): 32 ft 6 in.

Skew: None

Number of Spans: 1

Span Lengths (center-to-center bearings): 31 ft 6 in.

Width (out-out): 24 ft 0 in. (23 ft 8 in. as-built)

Width (curb-curb): 22 ft 0 in. (21 ft 8 in. as-built)

Number of Traffic Lanes: 2

Design Loading: HS20-44

Material and Configuration

Timber:

Species: Glulam combination of Southern Pine

exterior laminations and red pine interior laminations;

Red Oak exterior beams

Size (actual): 13-3/4 in. by 3-1/8 in.

Grade: (Southern Pine) No. 1 and No. 2 Dense, (red

pine) No. 2, (Red Oak) No. 1

Moisture Condition: Approximately 10 percent at

installation

Preservative Treatment: Pentachlorophenol in oil

Stressing Rods:

Diameter: 1 in.

Number: 9

Design Force: 60,500 lb

Spacing: 44 in.

Type: High strength steel thread bar with course

right-hand thread, conforming to ASTM A 722.

Anchorage Type and Configuration:

Steel Plates: 12 in. by 12 in. by 1-1/4 in. Bearing

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

Butt Joint Frequency: None

Wearing Surface Type: Asphalt pavement; 1 to 2 in.

thickness.

17
Appendix B—Glulam Timber

Beam Combination

Development of the glulam timber beam combination

for the Teal River bridge was based on the procedures

outlined in ASTM D 3737 (ASTM 1990b), which were

used to develop AITC 117-Manufacturing (AITC 1988).

Material property information for Southern Pine was

obtained from AITC 117-Manufacturing. Property

information for red pine was assumed as follows (see

Table 1 for metric conversion).

Modulus of Elasticity: 1.1 x 106

 lb/in2, which is the

average bending MOE found in the In-Grade Testing

Program (Green and Evans 1987) for No. 2 red pine

and is slightly leas than the value presently listed in the

National Design Specification (NFPA 1988).

Bending stress index: 1,700 lb/in2, as applicable to all

softwood species with an assigned MOE more than

1.0 x 106 lb/in2 for No. 3 grade.

Knot properties: Same as used in developing AITC

117-Manufacturing for No. 3 grade softwoods.

Values for red pine were believed to represent conservative estimates for use in developing a glulam timber

combination. Values used for Southern Pine and red

pine are summarized in Table 4.

The beam lamination combination used for the Teal

River bridge (Fig. 2) was determined to be adequate

for a design bending stress of 2,000 lb/in2 and a design

MOE of 1.6 x 106 lb/in2 if a 302-20 grade tension

lamination was used for the outer tension ply (AITC

117-Manufacturing).

In addition to the MOE testing completed at the

manufacturing plant and the construction site, one

laminated beam was shipped to the FPL and tested

in the laboratory using ASTM D198 (ASTM 1990a)

techniques. Results indicate a MOE of 1.71 x 106

lb/in2 and a MOR of 7,360 lb/in2.

Table 4—Information used in developing glulam beam combinations

Knots (percent) Bending

MOE Minimum stress

(x 106 Near strength index

Species and lumber grade lb/in2

) Average maximum ratio (lb/in2

)

Southern Pine No. 1 Dense 2.00 3.3 35.6 0.710 3,500

Southern Pine No. 2 Dense 1.80 7.9 51.5 0.540 3,500

Red pine No. 2 1.10 20.0 80.0 0.450 1,700

19