United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

National Wood

in Transportation

Information Center

Research

Paper

FPL–RP–572

Field Performance of

Timber Bridges

17. Ciphers Stress-Laminated

Deck Bridge

James P. Wacker

James A. Kainz

Michael A. Ritter
Abstract

In September 1989, the Ciphers bridge was constructed

within the Beltrami Island State Forest in Roseau County,

Minnesota. The bridge superstructure is a two-span continuous stress-laminated deck that is approximately 12.19 m

long, 5.49 m wide, and 305 mm deep (40 ft long, 18 ft

wide, and 12 in. deep). The bridge is one of the first to

utilize red pine sawn lumber for a stress-laminated deck

application. The performance of the bridge was monitored

continuously for 24 months beginning July 1993, approximately 46 months after installation. Performance monitoring

involved evaluating data relative to the moisture content of

the wood deck, the force level of stressing bars, and the

behavior of the bridge under static load conditions. In addition, temperatures were collected from the bridge superstructure and ambient air. Based on field evaluations, the Ciphers

bridge is performing satisfactorily with no structural or

serviceability deficiencies.

Keywords: Timber, bridge, red pine. Minnesota

October 1998

Wacker, James P.; Kainz, James A.; Ritter, Michael A. 1998. Field performance of timber bridges—17. Ciphers stress-laminated deck bridge.

Res. Pap. FPL–RP–572. 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

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

Madison, WI 53705–2398. Laboratory publications are sent to hundreds of

libraries in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the

University of Wisconsin.

The use of trade or firm names is for information only and does not imply

endorsement by the U.S. Department of Agriculture of any product or

service.

The United States Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national

origin, gender, religion, age, disability, political beliefs, sexual orientation,

or marital or familial status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communication of program information (braille, large print, audiotape, etc.)

should contact the USDA’s TARGET Center at (202) 720–2600 (voice and

TDD). To file a complaint of discrimination, write USDA, Director, Office

of Civil Rights, Room 326-W, Whitten Building, 14th and Independence

Avenue, SW, Washington, DC 20250–9410, or call (202) 720–5964 (voice

and TDD). USDA is an equal employment opportunity employer.

Acknowledgments

We express appreciation to the following individuals who

assisted with the Ciphers bridge monitoring project: John

Stanton (Warroad office) of the Minnesota Department of

Natural Resources for assistance collecting field data;

Rich Dahlman (St. Paul office) and Larry Nixon (Grand

Rapids office) of the Minnesota Department of Natural

Resources for assistance with project administration; and

Maureen Mathias (former Forest Products Laboratory

technician) for assistance with data reduction.

Contents

Page

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

Background and Development........................................1

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

Design, Construction, and Cost .....................................2

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

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

Cost ......................................................................4

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

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

Bar Force and Deck Temperature.................................5

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

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

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

Results and Discussion.................................................6

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

Bar Force and Deck Temperature.................................7

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

Load Test 1.........................................................8

Load Test 2.........................................................8

Analytical Evaluation................................................9

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

Wood Components...............................................9

Wearing Surface....................................................9

Stress-Laminating System....................................12

Conclusions .............................................................12

References................................................................12

Appendix A—Information Sheet...................................14

Appendix B—Load Testing Results .............................15
Field Performance of Timber Bridges

17. Ciphers Stress-Laminated Deck Bridge

James P. Wacker, General Engineer

James A. Kainz, General Engineer

Michael A. Ritter, Research Engineer

Forest Products Laboratory, Madison, Wisconsin

Introduction

Two national programs have offered an incentive to state and

local governments to choose timber for their highway bridges

by providing funds to assist with design and construction

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

as the Timber Bridge Initiative (TBI). The objective of this

legislation was to establish a national program to provide

effective and efficient utilization of wood as a structural material for highway bridges (USDA 1991). Responsibility for

the development, implementation, and administration of the

TBI was assigned to the USDA Forest Service. To implement a program, the Forest Service established three primary

emphasis areas: demonstration bridges, technology transfer,

and research. Responsibility for the demonstration bridge and

technology transfer programs was assigned to the National

Wood In Transportation Information Center (NWITIC),

formerly the Timber Bridge Information Resource Center, in

Morgantown, West Virginia. Under the demonstration

bridge program, the NWITIC provides matching funds to

local governments to construct demonstration timber bridges

and encourage innovation through the use of new or previously underutilized wood products, bridge designs, and

design applications.

Responsibility for the research portion of the TBI was

assigned to the USDA Forest Service, Forest Products Laboratory (FPL), a national wood utilization research laboratory.

As part of this broad research program, FPL assumed a lead

role in assisting local governments in evaluating the field

performance of demonstration bridges, many of which employ design innovations or materials that have not been

previously evaluated. Through such assistance, FPL is able

to collect, analyze, and distribute information on the field

performance of timber bridges, thus providing a basis for

validating or revising design criteria and further improving

efficiency and economy in bridge design, fabrication, and

construction.

In addition to the TBI, the U.S. Congress passed the Intermodal Surface Transportation Efficiency Act (ISTEA) in

1991, which included provisions for a timber bridge program

aimed at improving the utilization of wood transportation

structures. Responsibility for the development, implementation, and administration of the ISTEA timber bridge program was assigned to the Federal Highway Administration

(FHWA) and included demonstration timber bridge, technology transfer, and research programs. Because many aspects of the FHWA research program paralleled those underway at FPL, a joint effort was initiated to combine the

respective research of the two agencies into a central research

program. As a result, the FPL and FHWA merged resources

to jointly develop and administer a national timber bridge

research program.

This report, seventeenth in a series, documents the field

performance of the Ciphers bridge located in Roseau County

of Northern Minnesota. It describes the design, construction,

and a 2-year field evaluation study of the Ciphers bridge. The

bridge is a single-lane stress-laminated deck bridge with a

total length of approximately 12.19 m (40 ft). Built in the

summer of 1989, this bridge is unique in that it is the first

known bridge application to utilize red pine sawn lumber in

a stress-laminated deck superstructure. The owner of the

bridge received matching funds through the TBI demonstration bridge program. An information sheet on the specific

characteristics of the Ciphers bridge is given in Appendix A.

Background and Development

The Ciphers bridge is located approximately 22.5 km

(14 miles) southeast of the city of Warroad in Roseau

County, Minnesota, within the boundaries of the Beltrami

Island State Forest (Fig. 1). The bridge is located along the

Tangnes Trail where it crosses the East Branch of the Warroad River. This single-lane gravel road provides access to

the forest for logging and fire control activities. The estimated average daily traffic is 60, consisting of passenger

and logging vehicles.

Before replacement in 1989, the Ciphers bridge was a flatbed

railcar with a wooden deck (Fig. 2). Originally constructed

in the 1950s, the bridge was vandalized with fire in 1988.

The damage caused by the fire was severe and necessitated

replacement of the bridge.
2

Through a cooperative effort betweenthe Minnesota Department of Natural Resources (MN–DNR) and the Department

of Forest Products at the University of Minnesota, a proposal

was submitted to the USDA Forest Service for partial funding under the TBI for the replacement structure. The project

proposed a stress-laminated deck superstructure that used red

pine sawn lumber for the deck laminations. In 1989, the

project received funding and the final design and construction

plans for the Ciphers bridge were completed.

After construction in the summer of 1989, the performance of

the bridge was monitored by the MN–DNR in cooperation

with the University of Minnesota for approximately 2 years

(Franck 1991). During this initial monitoring, the bridge

exhibited substantial bar force loss during the winter seasons.

These bar force losses were of concern to the MN–DNR

because of winter hauling operations on the bridge. To gather

additional information about the performance of the Ciphers

bridge, especially during the cold winter months, the MN–

DNR contacted FPL for assistance. Subsequently, FPL and

MN–DNR mutually developed a field evaluation program for

the Ciphers bridge.

Objective and Scope

The objective of this project was to evaluate the field performance of the Ciphers bridge for 2 years, beginning in July

1993, approximately 46 months after bridge installation. The

project scope included data collection and analysis related to

the wood moisture content, stressing bar force, bridge behavior under static truck loading, and general structure condition. In addition, data were collected to determine the effects

of extremely cold temperatures on the bridge. Results of this

project will be used to formulate recommendations for future

design and construction of similar stress-laminated bridges.

Design, Construction,

and Cost

The Ciphers bridge project was completed as part of a

mutual effort between several agencies and individuals. An

overview of the design, construction, and cost of the project

follows.

Figure 1—Location of Ciphers bridge.

Figure 2—Ciphers bridge before replacement in 1989.
3

Design

Design of the Ciphers bridge was completed during 1989 by

the MN–DNR in cooperation with the University of Minnesota–Department of Forest Products. The stress-laminated

deck superstructure was designed according to USDA Forest

Service standards (Ritter 1990) under development at the

time and met American Association of State Highway and

Transportation Officials (AASHTO) load requirements

(AASHTO 1989). The design included procedures for prefabricating into two half-width panels that were interconnected

during construction.

The design geometry provided for a stress-laminated deck

superstructure 12.19 m long, 5.49 m wide, and 305 mm

deep (40 ft long, 18 ft wide, and 12 in. deep) with a single

support at mid-length (Fig. 3). Design calculations were

based on a 11.76-m (38.6-ft) bridge length (center–center of

end bearing) in a two-span continuous orientation. Each span

was designed for 5.88 m (19.3 ft) (center–center of bearing)

between supports. Deck laminations were specified as 102 by

305 mm, S1S to 98 mm (nominal 4 by 12 in., S1S to 3-7/8

in.), red pine, grade No. 2 and better, pressure treated with

creosote. The deck lamination moisture content 

Figure 3—Design configuration of the Ciphers bridge.
4

was specified to be 19% maximum. Because the deck laminations were not available in lengths required to span the

entire bridge, the deck design included lamination butt joints

in a repetitive pattern. Transversely, butt joints were limited

to no more than one in every fourth adjacent lamination.

Longitudinally, rows of butt joints were spaced at

1.22-m (4-ft) intervals.

For stress laminating, the design specified the use of 25-mm-

(nominal 1-in.-) diameter ASTM A722 high strength,

threaded steel bars with an ultimate strength of 1.034 GPa

(150 × 103

 lb/in2) (ASTM 1988). The bars were spaced at

1.22-m (4-ft) intervals, beginning 610 mm (2 ft) from the

ends of the bridge. The design tension bar force of 300 kN

(67,500 lb) provided an interlaminar compressive stress of

807 kPa (117 lb/in2). Bars were anchored with a discreteplate system consisting of 279- by 533- by 25-mm (11- by

21- by 1-in.) steel bearing plates, 152- by 305- by 38-mm

(6- by 12- by 1-1/2-in.) steel anchor plates, and hexagonal

nuts. All components of the stressing system and other steel

hardware were galvanized for corrosion

protection.

All components of the rail and curb system were designated

as visually graded Douglas Fir sawn lumber. The bridge rail

was specified as 152 by 254 mm (nominal 6 by 10 in.); rail

posts were specified as 203 by 254 mm (nominal 8 by

10 in.) spaced at 1.22-m (4-ft) intervals along the bridge.

The curbs were specified as 203 by 305 mm (nominal 8

by 12 in.).

To protect the deck of the bridge from deterioration, a 51- to

76-mm- (2- to 3-in.-) thick asphalt wearing surface was

designated. To offset deck deadload deflection, a positive

deck camber in the longitudinal direction was specified by

raising the center pier elevation 102 mm (4 in.) with respect

to the abutment elevations.

Construction

Construction of the Ciphers bridge by a local contractor was

completed in the summer of 1989. The existing railroad

flatbed car bridge was removed and demolished. New timber

pile abutments were installed while the bridge superstructure

was being preassembled at the fabrication facility.

The deck was preassembled into two half-width panels at the

fabrication facility to simplify transportation to the site. At

the bridge site, an overhead crane lifted and placed the preassembled deck panels next to each other on the abutments.

The stressing bars extended only through the half-width

panels and were interconnected with couplers along the deck

interface. After removal of anchorage plates from every other

bar on each panel at the interface, the coupled bars were

tensioned to the required design force level to achieve a

single deck unit from the two half-width panels (Fig. 4).

The initial bridge stressing was conducted with a single

hydraulic jack stressing system. Beginning at one end of the

bridge, prestress was introduced by tensioning alternating

bars to the required design tension force in a successive

manner at both sides of the bridge. This bar tensioning

procedure was repeated several times at the time of the initial

stressing to achieve uniformity in the bar forces. Subsequent

stressings followed similar procedures and were performed

2 and 8 weeks after the initial stressing.

Attachment of the deck to the substructure was made with

through-bolts passing through the deck and abutment/pier

cap and was performed after the second bridge stressing. At

the third bridge stressing, approximately 8 weeks after construction was completed, the rail and curb system was installed. In addition, the asphalt wearing surface was applied

approximately 10 weeks after the initial bridge stressing.

A view of the completed bridge, at the initiation of FPL

monitoring, after approximately 46 months in service, is

shown in Figure 5.

Cost

Cost for the fabrication and construction of the Ciphers

bridge project, including substructure, superstructure, and

approach roadway work, totaled $54,350. The portion of this

cost pertaining to the superstructure was estimated at

$38,045 (70% of the total cost). Cost figures were not available for the design of the Ciphers bridge project. Based on a

total deck area of 63.6 m2 (685 ft2), the unit cost of the

Ciphers bridge superstructure was estimated at $598/m2

($55/ft2).

Evaluation Methodology

To evaluate the structural performance of the Ciphers bridge,

the MN–DNR contacted FPL for assistance. Through mutual

Figure 4—Ciphers bridge was prefabricated in two

half-width panels that formed the deck after the

initial bridge stressing.
5

agreement, a 2-year monitoring plan was developed by FPL

and implemented through a Cooperative Research and

Development Agreement with the MN–DNR. The plan

called for performance monitoring of the deck moisture content, stressing bar force, static load test behavior, and general

bridge condition. The evaluation methodology used procedures and equipment previously developed by FPL on similar structures in the past (Ritter and others 1991). In addition, the monitoring plan included an evaluation of the effects

of cold temperatures on the performance of the deck superstructure.

Moisture Content

An electrical-resistance moisture meter was used to collect

moisture content data in accordance with ASTM D4444–84

(ASTM 1990). To monitor deck moisture content, 76-mm-

(3-in.-) long, insulated probe pins were driven approximately

64 mm (2.5 in.) into the underside of the deck at several

locations. After recording the reading from the meter, adjustments were made for temperature and wood species accordingly (FORINTEK 1984). Moisture content measurements were collected at the initiation and conclusion of the

monitoring period. In addition to electrical-resistance meter

readings, core samples were removed from the deck of the

bridge to determine moisture content by the ovendry method

in accordance with ASTM D4442–84 (ASTM 1984). The

core sample locations were adjacent to the electricalresistance locations in all cases.

Bar Force and Deck Temperature

To monitor bar force changes in the steel stressing bars, three

load cells were installed at the beginning of the monitoring

period. The hollow-core load cells were placed over the

stressing bar, between the bearing and anchorage plates, and

compressed as the stressing bars were tensioned. Load cell

measurements were converted to corresponding bar force

levels, based on the laboratory calibrations. At the conclusion of the monitoring period, load cells were removed and

recalibrated.

To monitor temperatures inside the deck superstructure,

several copper-constantine thermocouple wires were embedded into the deck at various depths. In addition, thermocouple wires were placed beneath the deck, not in direct sunlight, to measure the surrounding air temperature. On several

occasions, the accuracy of the thermocouple readings was

checked with a hand-held thermometer.

A remote data acquisition system was used to collect bar

force and deck temperature data (Fig. 6). A battery-powered

datalogger was preset to collect bar force and temperature

readings each hour. Data were stored in solid-state memory

modules that were interchanged by local MN–DNR personnel and sent to FPL on approximately a monthly basis

throughout the monitoring period.

Load Test Behavior

Static-load testing of the Ciphers bridge was conducted at the

beginning and end of the monitoring period to determine the

response of the bridge to highway truck loads. In addition,

predicted deflections based on analytical modeling were

determined for each load test. Load testing consisted of

positioning a fully loaded truck on each span of the bridge

deck and measuring the resulting deflections at a series of

locations along the centerspan and abutment cross-sections.

Bridge deflections were measured prior to testing (unloaded),

after placement of the test truck for each load case (loaded),

and at the conclusion of testing (unloaded). Bridge deflections from an unloaded to loaded condition were measured

by hanging calibrated rules on the underside of the deck and

reading the values with a surveying level.

For each load test, the test truck consisted of a flatbed truck

with a bulldozer loaded on the truckbed. The adjustable

truckbed was positioned such that the total weight of the

bulldozer was directly over the rear truck axles. The placement of the test truck on the bridge deck for each load case

Figure 5—Ciphers bridge after 46 months in service:

side view (top); end view (bottom).
6

was identical for both load tests. In the longitudinal direction, the test truck was positioned such that the midpoint

between the rear axles was directly over the centerspan of

each span tested. Because each span was relatively short, the

longitudinal positioning of the test truck resulted in the front

axle being just off the bridge. In the transverse direction, the

test truck was positioned at three different locations for each

span (Fig. 7). For load case 1, the test truck was centered on

the centerline of the roadway. For load case 2, the test truck

was placed adjacent to the upstream curb, with the inside

wheel line at the centerline of the bridge. For load case 3, the

test truck was placed adjacent to the downstream curb with

the inside wheel line at the centerline of the bridge.

Load test 1 was conducted July 12, 1993, with a gross

vehicle weight of 174 kN (39,020 lb) for the test truck. Load

test 2 was completed June 29, 1995 with a gross vehicle

weight of 176 kN (39,660 lb) for the test truck (Fig. 8).

Analytical Evaluation

At the conclusion of load testing, the behavior of the bridge

was modeled for load test conditions and AASHTO

HS 20–44 loading using an orthotropic plate computer

program developed at FPL. In addition, the AASHTO

HS20–44 predicted deflection was computed for each load

test using the recommended design method given in Guide

Specifications for the Design of Stress-Laminated Wood

Decks (AASHTO 1991).

Condition Assessment

General condition of the bridge was assessed at the initiation

and conclusion of the monitoring period. The assessments

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

of specific interest included the condition of the wood components, wearing surface, and the stress-laminating system.

Results and Discussion

Performance monitoring of the Ciphers bridge was initiated

in July 1993, approximately 46 months after bridge construction, and continued for 24 months until June 1995.

Figure 6—Remote data acquisition system used at the

Ciphers bridge: datalogger attached to the bridge rail

(top); load cell installed over stressing bar (bottom).

Figure 7—Transverse test truck positions used for both load tests, as

viewed from the south end of the bridge

(looking north). For all load cases, the

test truck was placed longitudinally

with the rear axles at the centerspan

and the front axle off the end of the

bridge.
7

Moisture Content

Results indicate that the average moisture content of the deck

laminations is decreasing but remains substantially greater

than that recommended by design. For the electricalresistance meter readings, the average moisture content reading decreased from 30 to 26%. For the ovendry analyses, the

average moisture content reading decreased from 45% to

39%.

These results indicate that the moisture content of the deck

laminations is still decreasing from levels recorded as high as

50% at the time of construction (Franck 1991). The interior

portion of the deck laminations (ovendry analyses) were, on

average, at a higher moisture content than the exterior portion (electrical-resistance meter), as a result of the gradual air

drying of the deck in service. All moisture content readings

were in excess of the long-term maximum level of 19%

included in the design specifications. This elevated moisture

content had an adverse affect on the bar tension forces and is

discussed in the following section.

Bar Force and Deck Temperature

Although the Ciphers bridge had been in service for nearly

4 years and the bars were re-tensioned to 258 kN

(58 × 103 lb), or 690 kPa (100 lb/in2) interlaminar deck

compression, at the initiation of monitoring, substantial bar

force loss was observed during the monitoring period and

was attributed to stress relaxation in the deck laminations

caused by high moisture content. Daily bar force and deck

temperature data are presented in Figure 9. The bar forces

decreased rapidly during the first few weeks but leveled off at

approximately 156 kN (35 × 103 lb), or 414 kPa (60 lb/in2)

interlaminar deck compression, for the remainder of the

monitoring period. However, during both winter seasons,

bar force temporarily decreased an additional 67 kN

(15 × 103

 lb). The bar force remained greater than 80 kN

(18 × 103 lb), or 207 kPa (30 lb/in2) interlaminar deck

compression, for the duration of the monitoring period.

Temporary bar force losses were observed during both winter

seasons because the rate of thermal contraction for the deck

laminations exceeded that of the steel stressing bars. In both

winters, the correlation between the interior deck temperature

and the bar force loss was most pronounced when the interior

deck temperature remained less than 0°C (32°F) (freezing

point of water). The bar force loss observed during the winter

of 1994 is presented in Figure 10. During the first winter,

temperatures at the deck interior remained below −18°C

(0°F) for several weeks and reached as low as −32°C (−25°F)

during January. The bar force loss observed during the winter

of 1995 is presented in Figure 11. During the second winter,

temperatures at the interior of the deck remained at approximately −18°C (0°F) for several weeks and reached as low as

−22°C (−8°F) during February. As temperatures began to

increase during the spring, bar forces recovered to levels

observed the previous fall.

The rate of thermal contraction of the wood laminations

while compressed together in a stress-laminated deck configuration is strongly dependent upon moisture content,

while the rate for the steel bars is unaffected. A recent laboratory study of red pine stress-laminated deck sections under

extreme cold temperatures also reported substantial bar force

loss when the lamination moisture content was greater than

30% (Wacker and others 1996). However, when the lamination moisture content was reduced to 17%, the amount of bar

force loss reported was significantly less. For the Ciphers

bridge, the interior lamination moisture content remained at

or above fiber saturation (approximately 30%) for the entire

monitoring period and magnified the bar force loss during the

winter seasons. Additional laboratory and field results are

forthcoming on bar force retention in stress-laminated deck

bridges in cold environments.

Behavior Under Static Load

Results of the static-load testing and analytical evaluation are

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

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

residual deflection was measured between load cases or at the

conclusion of testing. In addition, no measurable deflection

was observed at the bridge supports. The average bridge

prestress during testing was approximately 620 kPa

(90 lb/in2

) for load test 1 and 414 kPa (60 lb/in2) for load

test 2.

Figure 8—Test truck axle weights and spacings used

for load testing.
8

Load Test 1

Measured and predicted transverse deflections for load test 1

are shown in Figure 12. For load case 1 on the south span,

the maximum measured deflection near the centerline of the

bridge was 6 mm (0.23 in.) (Fig. 12a). For load cases 2 and

3 on the south span, the maximum measured deflections

occurred near the outside truck wheel line and were both

6 mm (0.25 in.) (Fig. 12b,c). For load case 1 on the north

span, the maximum measured deflection near the centerline

of the bridge was 6 mm (0.25 in.) (Fig. 12d). For load

cases 2 and 3 on the north span, the maximum measured

deflections occurred near the outside truck wheel line and

were 7 and 6 mm (0.28 and 0.24 in.), respectively

(Fig 12e,f).

Load Test 2

Measured and predicted transverse deflections for load test 2

are shown in Figure 13. For load case 1 on the south span,

the maximum measured deflection near the bridge centerline

was 6 mm (0.23 in.) (Fig. 13a). For load cases 2 and 3 on

the south span, the maximum measured deflections occurred

near the outside truck wheel line and were 6 mm (0.24 and

0.23 in., respectively) (Fig. 13b,c). For load case 1 on the

North span, the maximum measured deflection near the

centerline of the bridge was 6 mm (0.24 in.) (Fig. 13d). For

load cases 2 and 3 on the north span, the maximum measured deflections occurred near the outside truck wheel line

and both were 6 mm (0.24 in.) (Fig 13e,f). Additional load

test figures that further present load test data are given in

Appendix B.

Figure 9—Bar force and deck temperature summary: Average daily load cell readings (top); interior deck

temperatures (bottom). Note that the gap in the data was due to a computer hardware failure that occurred

during summer when bar forces are not significantly affected by temperature.
9

Analytical Evaluation

Predicted deflections given in Figures 12 and 13 provide a

good approximation of the measured deflection for each load

case. Minor differences between measured and predicted

deflection are noticeable at the edges of the bridge and are

caused, in part, by the edge-stiffening effect of the sawn lumber curb assemblies.

Deflections for the AASHTO HS20–44 design loading are

not provided because they were not significantly different

from the predictions for each load test. However, the maximum predicted deflection values for each load case under

AASHTO HS20–44 loading conditions and the design

deflection are presented in Table 1 for each load test. Deflections were converted to an equivalent span/deflection ratio

based on the appropriate span length (center–center of

bearing). The HS20–44 predicted maximum deflections are

significantly less than predicted by design and are below the

new recommended design deflection limit of L/500 established after design and construction of the Ciphers bridge.

Condition Assessment

General condition assessments indicated that the structural

and serviceability aspects of the Ciphers bridge were satisfactory.

Wood Components

Visual inspection of the wood components of the bridge

indicated no signs of distress. Preservative accumulation on

the surface of rail members was evident during the summer

months, but did not cause significant leaching problems.

Checking on the end grain top-surface of the rail posts was

minor and did not penetrate the preservative envelope.

Wearing Surface

The asphalt wearing surface was observed to be in satisfactory condition at the conclusion of monitoring, approximately 6 years after bridge installation. There was no

cracking evident on the asphalt wearing surface.

Figure 10—Bar force and deck temperature summary,

Winter 1994.

Figure 11—Bar force and deck temperature summary,

Winter 1995.
10

Figure 12—Load test 1 comparing measured and predicted centerspan deflection (looking north). Bridge

cross-sections and vehicle positions are presented to aid interpretation only and are not to scale.
11

Figure 13—Load test 2 comparing measured and predicted centerspan deflection (looking north). Bridge

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

Stress-Laminating System

The steel stressing bars and steel plate anchorages were in

good condition. The exposed portion of the steel stressing

bars were slightly corroded in areas were the anchor nuts had

removed the galvanized protective coating. The affected areas

should be wire-brushed to remove the corrosion and recoated

with galvanizing paint.

Conclusions

Based on the results of a 2-year field evaluation program,

performance of the Ciphers bridge is satisfactory. However, as

a result of the northern location, the bridge is susceptible to

substantial bar force loss during the winter season because of

its high lamination moisture content. Low bar force in a

stress-laminated deck leads to structural and serviceability

deficiencies that may prevent the bridge from providing

satisfactory performance in the future. The following conclusions are based on our findings:

• The sawn lumber components slightly decreased in average moisture content during the monitoring period. After

approximately 6 years in-service, the average moisture

content of the deck is approximately 30%.

• The majority of bar force loss during the monitoring

period was attributed to stress relaxation of the deck

laminations and totaled approximately 89 kN

(20 × 103

 lb), or 33% of the restressing force of 258 kN

(58 × 103 lb) introduced at the initiation of monitoring.

However, during winter seasons the bar force loss was

accelerated due to low temperatures and high lamination

moisture content. This temperature-induced bar force loss

was up to an additional 67 kN (15 × 103

 lb) for short

periods when the temperature dropped below the freezing

point of water during the winter season but fully recovered

when temperatures increased.

• For the entire monitoring period, bar force remained above

the minimum required design levels and did not compromise the structural efficiency of the bridge. However, future

inspections of the Ciphers bridge should include hydraulic

equipment measurements to ensure that bar forces remain

above 80 kN (18 × 103 lb). These bar force measurements

are strongly recommended prior to any winter hauling

operations across the bridge.

• Static-load testing and analysis indicate that the Ciphers

bridge is performing in a linear-elastic manner. When

subjected to truck loading at interlaminar compression

levels of 620 kPa (90 lb/in2) and 414 kPa (60 lb/in2), the

bridge deck deflection was substantially below limitations

determined during design.

• The predicted maximum deflection under AASHTO

HS20–44 loading was 7 mm (0.28 in.), or 1/810 of the

bridge span measured center–center of bearing.

• Visual inspection indicated no signs of deterioration of the

wood components. Minor corrosion was observed on the

exposed portions of the steel stressing bars and should be

removed and re-coated with galvanized paint.

Table 1—Predicted maximum midspan deflections and corresponding

span/deflection ratios using HS20–44 loading conditions for both load tests

Estimated

interlaminar

Predicted HS20–44 maximum midspan deflections

and span/deflection ratios

Load compression Load South span North span

test (kPa) (lb/in2) case Modela AASHTOb Modela AASHTOb

1 620 (90) 1 5 mm

L/1052

21 mm

L/275

6 mm

L/987

21 mm

L/275

2,3 6 mm

L/884

21 mm

L/275

7 mm

L/841

21 mm

L/275

2 414 (60) 1 6 mm

L/1004

21 mm

L/275

6 mm

L/946

21 mm

L/275

2,3 7 mm

L/850

21 mm

L/275

7 mm

L/810

21 mm

L/275

a

Using orthotropic plate prediction model with L = 5.60 m (18.4 ft) (south span)

 and L = 5.76 m (18.9 ft) (north span) based on as-built measurements.

b

Using recommended method in Guide Specification for Stress-Laminated

 Decks (AASHTO 1991) with L = 5.79 m (19 ft).
13

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.

ASTM. 1984. Standard test methods for direct moisture

content measurement of wood and wood-based materials.

ASTM D4442–84. Philadelphia, PA: American Society for

Testing and Materials. 5 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.

FORINTEK. 1984. Moisture content correction tables for

the resistance-type moisture meter. Eastern Species. SP511E.

Ottawa, Canada: FORINTEK Corp. 37 p.

Franck, Bruno M. 1991. Long term behavior of a stresslaminated timber deck. In: Proceedings, International

Association of Bridge and Structural Engineers (IABSE)

symposium; 1991, September; Leningrad, Russia.

Ritter, Michael A. 1990. Timber bridges: design, construction, inspection and maintenance. EM 770–8. Washington,

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

Engineering Staff. 944 p.

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 2–5;

London, United Kingdom. High Wycombe, United Kingdom: Timber Research and Development Association:

3.319–3.326. Vol. 3.

USDA. 1991. The timber bridge initiative, fiscal year 1991

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

Forest Service, State and Private Forestry, Northeastern

Area. 12 p.

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

1996. Field performance of timber bridges—8. Lynches

Woods Park stress-laminated deck bridge. Res. Pap.

FPL–RP–551. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 17 p.
14

Appendix A—Information Sheet

General

Name: Ciphers bridge

Location: Beltrami Island State Forest

(Roseau County, Minnesota)

Date of Construction: September 1989

Owner: Minnesota Department of Natural Resources

Design Configuration

Structure Type: Stress-laminated deck with butt joints

Butt Joint Frequency: 1 in 4 laminations transversely

separated 1.2 m (4 ft) longitudinally

Total Length (out–out): 12.19 m (40 ft)

Skew: 0 degrees

Number of Spans: 2 (continuous over an intermediate

support)

Span Lengths (center-to-center bearings): 5.88 m (19 ft;

19.3 ft, as-built)

Width (out–out): 5.49 m (18 ft; 17.8 ft, as-built)

Width (curb–curb): 4.88 m (16 ft; 15.8 ft, as-built)

Number of Traffic Lanes: 1

Design Loading: AASHTO HS20–44

Wearing Surface Type: Asphalt pavement; 25 to 51 mm

(1 to 2 in.) thick

Material and Configuration

Wood Laminations:

Species: Red pine

Size: 102 by 305 mm (4 by 12 in.

(3-7/8 by 12 in. actual))

Grade: No. 2 and better

Moisture Condition: approximately 30% at initiation

of monitoring

Preservative Treatment: Creosote

Stressing Elements:

Type: High strength steel threaded bar, conforming

to ASTM A722, Type II

Diameter: 25 mm (nominal 1 in.)

Number: 9

Design Force: 320 kN (72 × 103

 lb)

Spacing: 1.22 m (4 ft) center–center, 610 mm (2 ft)

from bridge ends

Anchorage Configuration:

Discrete Steel Plates: 279 by 533 by 25 mm

(11 by 21 by 1 in.) bearing

152 by 305 by 38 mm

(6 by 12 by 12 in.) anchor

Rail and Curb System:

Design: AASHTO 44.5 kN (10, 000 lb) static load

Species: Douglas-fir

Grade: No. 1

Member Sizes:

Rails: 152 by 254 mm (6 by 10 in.)

Posts: 203 by 254 mm (8 by 10 in.)

Curbs: 203 by 305 mm (8 by 12 in.)
15

Appendix B—Load Testing Results

The following transverse deflection figures present additional data comparisons from both load tests. Figures B1 and B2

compare the deflection measured at each span for Load Test 1. Figures B3 and B4 compare the deflection measured

at each span for Load Test 2. Both sets of figures serve to validate the deflection data collected during load tests.

Figure B1—Load test 1 comparing measured centerspan deflection of the north and south span from load

cases 1, 2, and 3 (looking north). Bridge crosssections and vehicle positions are presented to aid

interpretation only and are not to scale.

Figure B2—Load test 1 comparing measured centerspan deflection from load cases 2 and 3 (looking

north). Bridge cross-sections and vehicle positions

are presented to aid interpretation only and are not

to scale.
16

Figure B3—Load test 2 comparing measured centerspan deflection of the north and south span from load

cases 1, 2, and 3 (looking north). Bridge crosssections and vehicle positions are presented to aid

interpretation only and are not to scale.

Figure B4—Load test 2 comparing measured centerspan deflection from load cases 2 and 3 (looking

north). Bridge cross-sections and vehicle positions

are presented to aid interpretation only and are not

to scale.