United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

National Wood

in Transportation

Information Center

Research

Paper

FPL–RP–563

Field Performance of

Timber Bridges

13. Mohawk Canal

Stress-Laminated Bridge

Paula D. Hilbrich Lee

Xanthi Lauderdale

In cooperation

with the

United States

Department of

Transportation

Federal

Highway

Administration
Abstract

The Mohawk Canal bridge was constructed in August 1994,

just outside Roll, Arizona. It is a simple-span, double-lane,

stress-laminated deck superstructure, approximately 6.4 m

(21 ft) long and 10.4 m (34 ft) wide and constructed with

Combination 16F-V3 Douglas Fir glued-laminated timber

beam laminations. The performance of the bridge was monitored continuously for 2 years, beginning shortly after installation. Performance monitoring involved gathering and

evaluating data relative to the moisture content of the wood

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

creep of the deck, and the behavior of the bridge under static

load conditions. Furthermore, comprehensive visual inspections were conducted to assess the overall condition of the

structure. Based on field evaluations, the bridge is performing properly with no structural deficiencies.

Keywords: Stress laminated, timber, bridge, wood,

performance, glued laminated

Acknowledgments

We express sincere appreciation to the following individuals

from the Yuma County Department of Development Services

who contributed to the success of this project: Robert Taylor

for assistance in collecting field data and Louie Rivera, John

Taylor, Jesus Larious, Santos Guerrero, Jr., and Richard

Rodriguez for assistance in conducting load tests.

Thanks also to the following from the USDA Forest Service,

Forest Products Laboratory: Vyto Malinauskas for assembling load cell instrumentation; Mike Ritter and Jim Wacker

for assistance with data collection and analysis; and the

Information Services Team for assistance in preparing this

report.

September 1997

Lee, Paula D. Hilbrich; Lauderdale, Xanthi. 1997. Field performance of

timber bridges—13. Mohawk Canal stress-laminated deck bridge. Res.

Pap. FPL–RP–563. Madison, WI: U.S. Department of Agriculture, Forest

Service, Forest Products Laboratory. 18 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

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University of Wisconsin.

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

service.

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

religion, age, disability, political beliefs, and marital or familial status. (Not

all prohibited bases apply to all programs.) Persons with disabilities who

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USDA is an equal employment opportunity employer.

Contents

Page

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

Background................................................................1

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

Design and Construction...............................................2

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

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

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

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

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

Vertical Creep..........................................................6

Load Test Behavior..................................................6

Load Test 1.........................................................6

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

Analytical Evaluation ............................................7

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

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

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

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

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

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

Load Test 1.........................................................9

Load Test 2.........................................................9

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

Condition Assessment ............................................11

Bridge Geometry ................................................11

Wood Condition ................................................11

Wearing Surface..................................................11

Bar Anchorage System.........................................11

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

References................................................................16

Appendix—Information Sheet......................................18
Field Performance of Timber Bridges

13. Mohawk Canal Stress-Laminated Deck Bridge

Paula D. Hilbrich Lee, General Engineer

Forest Products Laboratory, Madison, Wisconsin

Xanthi Lauderdale, Civil Engineer Assistant

Yuma County Department of Development Services, Yuma, Arizona

Introduction

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 timber bridge program to

encourage the effective and efficient use of wood as a structural material for highway bridges. Responsibility for developing, implementing, and administrating the TBI was delegated to the USDA Forest Service. The program included

three emphasis areas: technology transfer, demonstration

bridges, and research. The Forest Service National Wood in

Transportation Information Center (NWITIC) (formerly the

Timber Bridge Information Resource Center) in Morgantown, West Virginia, manages the technology transfer program and administers the demonstration bridge program.

The demonstration bridge program provides matching funds

on a competitive basis to local governments for the construction of timber bridges that illustrate the use of new or previously underutilized wood products, bridge designs, or design

applications (S&PF 1995).

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 in

Madison, Wisconsin. As part of the 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. This provides 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 describes the development, design, construction,

and field performance of the Mohawk Canal bridge in Yuma

County, Arizona. The bridge constructed in August 1994, is

a double-lane, single-span, stress-laminated deck approximately 6.4 m (21 ft) long and 10.4 m (34 ft) wide. The

laminations are Combination 16F-V3 Douglas Fir, gluedlaminated timber (glulam) beams. Characteristics of the

bridge are summarized in the Appendix.

Background

The bridge site is located approximately 56 km (35 miles)

east of Yuma, Arizona, in the Wellton–Mohawk Valley

District, near Roll, Arizona (Fig. 1). It is on County 5th

Street, a double-lane, dirt roadway that provides access to

local residences and agricultural areas and crosses over the

Mohawk Canal. The average traffic is estimated to be 200

vehicles per day and consists of passenger vehicles, school

busses, and farm vehicles and machinery.

The existing structure, constructed in 1955, was a timber

stringer bridge with a timber plank deck, supported by concrete abutments (Fig. 2). The original bridge was approximately 4.6 m (15 ft) long and 6.7 m (22 ft) wide. Inspection

of the bridge in 1988 indicated that the deck was in poor

condition. Several planks were broken or deteriorated, and

the bridge was restricted to a load limit of 89 kN (10 tons).

In addition, the railing system on the south side was broken.

It was apparent that major rehabilitation or replacement of the

structure would be required. After evaluating the structural

deficiencies, a decision was made by Yuma County officials

to replace the bridge.
2

Motivated by the TBI, Yuma County staff submitted a

proposal for a stress-laminated demonstration bridge using

laminations composed of Combination 16F-V3 Douglas Fir

glulam beams. The project was accepted and partially funded

as a NWITIC demonstration bridge in 1991. Because the

stress-laminated deck was a newly developed system, it was

determined that the field performance of the bridge should be

monitored after installation to provide assurance of acceptable

behavior. Subsequently, an agreement was developed to

include the Mohawk Canal bridge in the FPL/FHWA bridge

monitoring program.

Objective and Scope

The objective of this project was to ascertain the field performance characteristics of the Mohawk Canal stresslaminated bridge by monitoring the structure for approximately 2 years, beginning shortly after bridge installation.

The scope of the project included data collection and analysis

related to the moisture content of the wood, stressing bar

force, vertical creep of the bridge, behavior under static truck

loading, and general structure condition. The results of this

project will be evaluated with similar monitoring activities

in an effort to improve design and construction methods for

future stress-laminated timber bridges.

Design and Construction

Design and fabrication of the Mohawk Canal bridge were

completed by contract. Construction was directed by the

Yuma County Department of Development Services staff and

completed by a county construction crew. An overview of the

design and construction process follows.

Figure 1—Location of the Mohawk Canal bridge.

Figure 2—Original Mohawk Canal bridge:

side view (top), end view (bottom).
3

Design

The Mohawk Canal bridge was designed and fabricated by

contract. Design criteria for the bridge aspects relating directly to stress laminating were based on Guide Specifications for the Design of Stress-laminated Wood Decks, published by the American Association of State Highway and

Transportation Officials (AASHTO) (AASHTO 1991). All

other aspects of the superstructure design were based on the

Standard Specifications for Highway Bridges, also published by AASHTO (1989).

The bridge was designed for AASHTO HS20–44 loading

(AASHTO 1989), a span length of 6.1 m (20 ft) center-tocenter of bearings, a width of 10.3 m (33.9 ft), and a skew of

15 degrees (Fig. 3). Combination 16F-V3 Douglas Fir

glulam beams were selected as the deck laminations. The

design was based on 130- by 305-mm- (5.13- by 12-in.-)

wide laminations, pressure treated with pentachlorophenol

and heavy oil. Full-length laminations with 10 mm

(0.38 in.) of camber were specified. The stressing system was

designed for 25-mm- (1-in.-) diameter high strength, threaded

steel bars, conforming to the requirements of ASTM A722

(ASTM 1988).

Because of the skew, only three bars were continuous across

the entire deck. The remaining bars (two on each side) extended only partially across the deck, with one end embedded in the deck. The average bar spacing was 1.4 m (54 in.)

on–center, beginning approximately 457 mm (18 in.) from

the ends of the bridge. The design bar tension force was

289 kN (65,000 lb), resulting in approximately 706 kPa

(102 lb/in2

) of interlaminar compression; a discrete plate

anchorage system was used (Fig. 4). The railing of the

bridge consisted of a glulam timber rail without curb. In

1995, this rail configuration was approved by FHWA as a

crash-tested railing for AASHTO performance level 1 criteria

(Ritter and others 1995a). An asphalt wearing surface, consisting of compacted asphalt cold mix, was specified.

Construction

Construction of the Mohawk Canal bridge began with removal of the existing superstructure and construction of new

concrete abutments (Fig. 5). As illustrated in Figure 5, the

Douglas Fir glulam beams were delivered and stored on site

several days before the superstructure assembly began. In late

August 1994, after work on the approach roadway and abutments was completed, construction of the superstructure

commenced. The laminations were lifted and positioned

above the abutments by a crane (Fig. 6a). As the laminations

were lowered onto the abutments, the county construction

crew guided their placement (Fig. 6b). Because of the bridge

skew angle, lamination order and placement were critical for

proper hole alignment. An additional lamination placement

Figure 3—Design configuration of the

Mohawk Canal bridge.

Figure 4—Detail of the discrete plate bar anchorage

configuration.
4

requirement was the deck-to-substructure anchorage, which

required an anchor to pass through a lamination every

781 mm (30.75 in.) (Fig. 6b). Because perfect alignment of

deck-to-substructure connection could not be obtained,

additional holes were field drilled to align the connection.

These holes penetrated the preservative envelope; therefore,

they were treated with a wood preservative in the field.

As lamination placement progressed, embedded stressing

bars were inserted through the deck and the interior anchorages were installed. After all deck laminations were in place,

the three stressing bars that traverse the entire deck were

inserted, steel bearing and anchorage plates were placed on

the bars, and the anchor nuts were hand tightened. The steel

post supports were attached to the sides of the deck and the

railposts were installed (Fig. 7). It took several days to

complete the installation of the superstructure.

Because stressing equipment was not available, the bars were

tensioned approximately 1 month following assembly of the

superstructure. During this time, traffic was not permitted on

the bridge. A single hydraulic jack was used to tension the

bars to the required 289-kN (65,000-lb) design force.

Approximately 2 weeks following the first tensioning, the

bars were retensioned to the design force. The bars were

tensioned a third time, 10 weeks after the initial tensioning,

to approximately 321 kN (72,200 lb). The railing was installed, and the asphalt wearing surface was applied shortly

before the second bar tensioning. The as-built configuration

of the Mohawk Canal bridge varied slightly from the design

configuration in Figure 3. The measured deck thickness was

298 mm (11.75 in.,) which varied slightly from the design

value of 305 mm (12 in.). The completed bridge is shown in

Figure 8. 

Figure 5—Construction of east concrete abutment

and spread footing. The Douglas Fir glulam beams

were stored on site for several days.

Figure 6—The Mohawk Canal bridge was assembled by

a county crew: (a) Laminations were lifted by crane

from the on-site storage location and positioned above

the abutments. (b) Workers guided placement of laminations as they were lowered onto the abutments.

Figure 7—After deck assembly, steel post supports

were attached to the deck and railposts were inserted.
5

Figure 8—Completed Mohawk Canal bridge: side view (top), end view (bottom).
6

Evaluation Methodology

To evaluate the structural performance of the Mohawk Canal

bridge, Yuma County representatives contacted FPL for

assistance and requested to be included in the FPL/FHWA

bridge monitoring program. Through mutual agreement, a

monitoring plan was developed and implemented as a cooperative effort with Yuma County. The plan called for the

performance monitoring of the moisture content of the deck,

the stressing bar force, vertical creep, load test behavior, and

condition assessment of the structure. The evaluation methodology utilized procedures and equipment previously

developed by FPL (Ritter and others 1991).

Moisture Content

The moisture content of the bridge was measured in accordance with ASTM D4444-84 procedures (ASTM 1990)

using an electrical-resistance moisture meter. Measurements

were obtained by driving the insulated probe pins into the

underside of the deck at a 25- to 51-mm (1- to 2-in.) depth,

recording the moisture content from the unit, and adjusting

the values for temperature and wood species (FORINTEK

1984). Moisture content measurements were taken at the

beginning, midpoint, and conclusion of the monitoring

period.

Bar Force

Bar force was measured using load cells developed by FPL.

The load cells were installed between the bearing and anchor

plate on the second, third, and fourth stressing bars from the

east abutment along the upstream edge of the bridge. Load

cell measurements were obtained using a portable strain

indicator by Yuma County Department of Development

Services personnel on a weekly basis for 3 months following

load cell installation and approximately bi-weekly thereafter.

Strain measurements were converted to units of bar tensile

force by applying a laboratory conversion factor.

Vertical Creep

Vertical creep was measured at the beginning and end of the

monitoring period. Measurements were obtained by attaching

a stringline to the bearings to create a horizontal benchmark

and at midspan, measuring the elevation of the deck with

respect to the benchmark using a calibrated rule.

Load Test Behavior

Static load testing was conducted at the beginning and end of

the monitoring period. Each test consisted of positioning

fully loaded trucks on the bridge and measuring the resulting

deflections at a series of transverse locations at midspan.

Deflection measurements were taken prior to testing (unloaded), for the first three load positions (loaded), halfway

through testing (unloaded), for the last three load positions

(loaded), and at the conclusion of testing (unloaded). Measurements were obtained by suspending calibrated rules from

the underside of the deck and reading values to the nearest

0.1 mm (0.004 in.) with a surveyor’s level (Fig. 9).

Accuracy of the measurements is estimated to be ±0.1 mm

(0.004 in.).

Load Test 1

The first load test was completed November 28, 1994,

3 months following installation of the bridge. The bridge

interlaminar compression at the time of the test was approximately 785 kPa (114 lb/in2

). The test vehicles, trucks

A and B, were three-axle dump trucks with gross vehicle

weights of 209.3 and 202.3 kN (47,050 and 45,470 lb),

respectively (Fig. 10). The vehicles were positioned longitudinally on the bridge so that the rear axles were centered

about the skewed midspan, with the front axles off the span

(Fig. 11). Transversely, six load positions were used

(Fig. 12).

Load Test 2

The second load test was completed November 13, 1996. At

the time of the test, the interlaminar compression level was

approximately 547 kPa (79 lb/in2). The test vehicles consisted of fully loaded, five-axle, and two-axle trucks: trucks A

and B with gross vehicle weights of 335.8 and 145.2 kN

(75,500 and 32,650 lb), respectively (Fig. 13). The vehicles

were positioned on the bridge longitudinally, with the rear

axle(s) centered about the skewed midspan and transversely

in the same six load positions used for load test 1 (Figs. 11,

12, 14).

Figure 9—Load test deflection measurements were

obtained by reading values with a surveyor’s level

from calibrated rules suspended from the underside

of the deck.
7

Analytical Evaluation

Following completion of the load tests, analytical assessments were conducted to determine the theoretical bridge

response. Previous research has shown that stress-laminated

decks can be accurately modeled as orthotropic plates (Ritter

and others 1995b). To further investigate the theoretical

behavior of the Mohawk Canal bridge, an orthotropic plate

computer model currently being developed at FPL was

employed to analyze the load test results and determine the

theoretical bridge deflection for AASHTO HS20–44 truck

loading. A modulus of elasticity (MOE) value of 13,790

MPa (2,000,000 lb/in2) was used for modeling. Although

this value is 3,447 MPa (500,000 lb/in2) greater than the

established design value for Douglas Fir glulam Combination 16F-V3, it accurately models the bridge behavior

(NFPA 1991). In addition, the larger MOE value is expected

to be correct because for the lower bending strength glulam

combinations, the beams are often manufactured from wood

greater in quality than that required.

Figure 10—Load test 1 truck configurations and axle

loads. The transverse vehicle track width, measured

center-to-center of the rear tires, was 1.8 m (6 ft).

Figure 11—For each load position, the vehicles were

positioned longitudinally on the bridge with the rear

axles centered about the skewed midspan.

Figure 12—Transverse load positions (looking east).

For all load positions, the rear axles were centered

about the skewed midspan.
8

Condition Assessment

The overall condition of the bridge was assessed at the beginning and end of the monitoring period. The evaluations

entailed visual inspections, measurements, and photographic

documentation. Items of specific interest included the bridge

geometry, condition of the timber deck and rail system, and

condition of the stressing bars and anchorage system.

Results and Discussion

Performance monitoring of the Mohawk Canal bridge extended from November 28, 1994, through November 13,

1996. Results and discussion of the performance data follow.

Moisture Content

The average lamination moisture content of the bridge was

8% in November 1994, 3 months after superstructure installation. The average lamination moisture content 1 and

2 years later was 8% and 7.3%, respectively. The measurements indicated little change during the 2-year monitoring.

The glulam beam laminations were installed at a very low

moisture content, and because the local climate is extremely

dry, the moisture content of the deck underwent little change.

The moisture content may fluctuate with seasonal climatic

changes but it is anticipated that the level will remain stable

as a result of the warm, dry desert environment. Dimensional

instability as a result of moisture content fluctuation is not

expected.

Bar Force

The average bar force for the Mohawk Canal bridge is shown

in Figure 15. It also indicates the design force and the force

level required to maintain the minimum recommended interlaminar compression level of 275.8 kPa (40 lb/in2

) (Ritter

1990). Following the third bar tensioning on November 28,

1994, the average bar force was approximately 321.2 kN

(72,200 lb), or 111% of the design force. For the next

13 months, the bar force declined at an approximately linear

rate. In January 1996, the bar force had decreased to approximately 219.3 kN (49,300 lb), which corresponds to an

interlaminar compression level of 536 kPa (78 lb/in2). At

this time, the average bar force began to increase for an approximate half year, then resumed its overall decline. This

slight increase may be attributed to fluctuations in the

moisture content of the wood deck. However, as a result of

the frequency of the moisture content readings, this cannot be

Figure 13—Load test 2 truck configurations and axle

loads. The transverse vehicle track width, measured

center-to-center of the rear tires, was 1.8 m (6 ft)

Figure 14—Transverse load positions 3 (top)

and 5 (bottom) used for load test 2.
9

determined with certainty. The top surface of the deck may

absorb moisture from rainfall because there is no waterproof

membrane beneath the asphalt and the bottom surface may

absorb moisture from evaporation of the canal. At the conclusion of the monitoring period, the average bar force was

approximately 224.2 kN (50,400 lb), or 78% of the design

force, which corresponds to an average interlaminar compression of approximately 547 kPa (79 lb/in2

).

Vertical Creep

At the time of the first load test, approximately 13 mm

(0.5 in.) of positive camber was measured with a string line

and vertical rule along the edges of the bridge. At the conclusion of the monitoring period, measurements indicated that

no vertical creep of the bridge had occurred.

Load Test Behavior

Static load test results and the theoretical bridge response

under load test and AASHTO HS20–44 loading are presented in this section. All transverse deflections with the

locations and magnitudes of the maximum measured deflections are shown at the midspan of the bridge, as viewed from

the west end (looking east). For each load test, no permanent

residual deformation was measured at the conclusion of the

testing, and no movement was detected at either of the abutments. The interlaminar compression was approximately 785

and 547 kPa (114 and 79 lb/in2) for load test 1 and 2, respectively. Measured deflections from each load test are typical of

orthotropic plate behavior of stress-laminated bridges (Ritter

and others 1990).

Load Test 1

Transverse deflections for load test 1 are shown in Figure 16.

As indicated, the maximum deflections for load positions 1

and 2 occurred between the wheel lines, 610 mm (24 in.)

south of the upstream (left) wheel line. For load positions 4

and 5, the maximum deflections were measured beneath the

outside wheel line of the test vehicle. An absolute maximum

deflection of 7.5 mm (0.30 in.) was measured beneath the

downstream wheel line of truck A for load position 3 and

beneath the upstream edge of the bridge for load position 6.

Assuming linear elastic behavior, uniform material properties, proper vehicle placement, and accurate deflection measurements, the summation of the deflections resulting from

two individual truck loads applied separately should equal

the deflection resulting from both trucks applied simultaneously. This is illustrated in Figure 17, where the sum of

load positions 1 and 2 and load positions 4 and 5 are compared with load positions 3 and 6, respectively. The deflections are similar, with minor variations that are within the

accuracy of the measurement methods, indicating that the

bridge is behaving in a linear elastic manner under the

applied loads.

Load Test 2

Transverse deflections for load test 2 are shown in Figure 18.

As with load test 1, the deflections are typical of orthotropic

plate behavior. The maximum measured deflection for load

position 1 occurred between the wheel lines. For load position 2, the maximum deflection occurred beneath the upstream (left) wheel line. As with load test 1, the maximum

deflections for load position 4 and 5 occurred beneath the

outside wheel line of the test vehicle. For load position 6,

the maximum deflection occurred between the wheel lines of

truck A. The absolute maximum deflection of 6.7 mm

(0.26 in.) occurred during load position 3, beneath the

downstream (right) wheel line of truck A.

As illustrated with the measured deflections of load test 1,

the summation of deflections resulting from two separately

applied loads should equal the deflection of both loads applied together, if uniform material properties, proper vehicle

placement, and accurate deflection measurements are assumed. This principal of superposition is illustrated in

Figure 19, where the sum of load positions 1 and 2 and load

positions 4 and 5 are compared with load positions 3 and 6,

respectively. As with load test 1, the deflections are virtually

the same, with slight variations within the accuracy of the

measurement methods, demonstrating that the behavior of

the bridge is within the linear elastic range under the applied

loads.

Analytical Evaluation

Comparisons of the measured load test deflections to the

theoretical bridge response are shown in Figures 20 and 21

for load test 1 and 2, respectively. As illustrated, the theoretical bridge deflection is very close to that measured, with

the greatest differences occurring at the edges of the bridge.

Employing the same analytical parameters used to determine

the theoretical bridge response, the theoretical deflection for

Figure 15—Average bar force.
10

Figure 16—Transverse deflections for load test 1, measured at the bridge midspan (looking east).

Bridge cross-sections and vehicle positions are shown to aid interpretation and are not to scale.
11

AASHTO HS20–44 truck loading is shown in Figure 22.

Based on this analysis, the maximum AASHTO HS20–44

static deflection occurred at centerline when two HS20–44

vehicles were placed centrically on the bridge. The resulting

theoretical maximum deflection is approximately 7.6 mm

(0.30 in.) or 1/804 of the bridge span for load test 1, and

approximately 7.3 mm (0.29 in.) or 1/831 of the bridge span

for load test 2. The results indicate that the longitudinal

bridge stiffness was approximately the same for both load

tests.

Condition Assessment

Condition assessments of the Mohawk Canal bridge indicate

that structural and serviceability performance are acceptable.

Inspection results for specific items follow.

Bridge Geometry

Measurements taken during the monitoring period indicate

that the width of the bridge remained relatively stable,

narrowing approximately 20 mm (0.84 in.). The change in

width is most likely the result of stress relaxation in the

laminations, resulting from the applied compression.

Additional reductions in the width of the bridge are not

anticipated because the laminations are extremely dry.

Wood Condition

Inspection of the wood components of the bridge revealed

that the glulam beams appeared to be in good condition.

Checks were not present in the exterior deck laminations. No

evidence existed to indicate preservative loss, and preservative or solvent accumulations were not present on the wood

surface. However, inspection of the underside of the deck

revealed that small wood splinters with cross sections of

approximately 85 mm2

 (0.13 in2) were separating or splitting

longitudinally from the bottom lamination of the glulam

beams. This phenomenon did not occur in all glulam beams,

and typically only one separation or splinter occurred per

beam.

The glulam rail was in excellent condition, exhibiting no

checks or delaminations. Severe checks and splits were

observed in the railposts and spacer blocks (Fig. 23). A

review of photographs taken during construction revealed that

the posts were severely checked when installed. Such a

condition is not advisable because the splits penetrate the

preservative envelope and provide avenues for moisture

penetration, accumulation, and potential decay as well as

reduce the strength characteristics of the member. At this

time, the posts and spacer blocks exhibit no visible signs of

decay. Such deterioration is not anticipated in the near future

because of the dry local environment; however, these areas

should be closely monitored in future inspections of the

bridge.

Wearing Surface

Inspection of the wearing surface at the conclusion of monitoring revealed a slight deterioration of the asphalt. The top

surface had experienced some erosion, causing the wearing

surface to appear to be constructed of compacted gravel. In

addition, cracks were noted in the wearing surface at the end

of the bridge and approach road interface (Fig. 24). Both the

erosion and interface cracks are attributed to a deficiency in

the asphalt mix or application procedures. Aside from this

erosion, the surface of the bridge appeared to be in good

condition and exhibited no additional signs of cracking,

rutting, or other deterioration.

Bar Anchorage System

The galvanized stressing bar anchorage system is performing

as designed and shows no signs of distress. There is no

Figure 17—Transverse deflections for load test 1,

comparing the sum of measured deflections from load

positions 1 and 2 and load positions 4 and 5 with load

positions 3 and 6, respectively. Bridge cross-sections

and vehicle positions are shown to aid interpretation

and are not to scale.
12

Figure 18—Transverse deflections for load test 2, measured at the bridge midspan (looking east). Bridge cross-sections

and vehicle positions are shown to aid interpretation and are not to scale.
13

indication of the discrete plate anchorage crushing into the

exterior laminations and no measurable distortion of the

bearing plate. The steel plates and exposed steel stressing

bars and hardware exhibited no visible signs of corrosion or

other distress.

Conclusions

After approximately 2 years in service, the Mohawk Canal

bridge is exhibiting good performance, although minor

serviceability deficiencies were noted. Based on monitoring

conducted since construction of the bridge, the following

observations and recommendations are made:

• It is feasible to construct stress-laminated decks using

Douglas Fir glulam beams.

• On-site assembly of the deck is a viable method of bridge

construction, although some traffic disruption does occur.

• The average moisture content in the outer 25 to 51 mm

(1 to 2 in.) of the laminations of the Mohawk Canal

bridge remained relatively stable throughout the monitoring period at approximately 8%. Because the lamination

moisture content was low at the time of installation and

the local climate is extremely dry, the moisture content of

the deck underwent little change. The bridge is located in

a desert environment and the moisture content will likely

remain stable; therefore, dimensional stability problems as

a result of moisture content fluctuations and subsequent

bar force retention problems are not expected.

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

Mohawk Canal bridge decreased from 321.2 to 224.2 kN

(72,200 to 50,400 lb), which corresponds to a decrease in

interlaminar compression from 785 to 547 kPa (114 to

79 lb/in2

). Overall, the bars are maintaining a high level of

interlaminar compression. Slight changes in bar force may

be attributed to local changes in moisture content caused

by rainfall or evaporation of the canal.

• Measurable vertical creep did not occur during the

monitoring period.

• Load testing and analysis indicate that the Mohawk Canal

bridge exhibits linear elastic orthotropic plate behavior

when subjected to static truck loading. Comparisons of

the analytical load test results at different levels of interlaminar compression indicate that the longitudinal bridge

stiffness was approximately the same for both load tests.

The maximum predicted deflection as a result of

AASHTO HS20–44 static truck loading is estimated to

be 7.6 mm (0.30 in.) (L/804) for 785 kPa (114 lb/in2)

interlaminar compression, and 7.3 mm (0.29 in.) (L/831)

for 547-kPa (79-lb/in2) interlaminar compression.

• A minor reduction of 20 mm (0.84 in.) in bridge width

was noted at the conclusion of the monitoring period and

is probably the result of stress relaxation in the laminations. Additional reductions are not anticipated because

the laminations are extremely dry.

• Visual inspections of the bridge indicate that the performance of most wood components is satisfactory, although

severe checks and splits were evident in the railposts and

spacer blocks. Photographs taken during construction indicate that the railposts were severely split when installed.

Members exhibiting such severe conditions should not be

used in railing construction.

• The asphalt wearing surface exhibited deterioration, resulting in a gravel-like appearance. Cracks were visible at the

end of the bridge and approach road interface. These conditions are attributed to a deficiency in the asphalt mix or

application procedures.

• The exposed steel stressing bars and hardware show no

visible signs of corrosion or other distress. The discrete

plate bar anchorage is not distorted or crushing into the

lumber laminations.

Figure 19—Transverse deflections for load test 2,

comparing the sum of measured deflections from load

positions 1 and 2 and load positions 4 and 5 with load

positions 3 and 6, respectively. Bridge cross-sections

and vehicle positions are shown to aid interpretation

and are not to scale.
14

Figure 20—Comparison of the measured deflections for load test 1 with the theoretical deflections using

orthotropic plate analysis (looking east). Bridge cross-sections and vehicle positions are shown to aid

interpretation and are not to scale.
15

Figure 21—Comparison of the measured deflections for load test 2 with the theoretical deflections using

orthotropic plate analysis (looking east). Bridge cross-sections and vehicle positions are shown to aid

interpretation and are not to scale.
16

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. 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; SP511E. Ottawa,

Canada: FORINTEK Corporation. 37 p.

NFPA. 1991. Design values for wood construction. A supplement to the 1986 edition of the national design specification for wood construction. Washington, DC: National

Forest Products Association (currently AFPA). 51 p.

Ritter, Michael A. 1990. Timber bridges: design, construction, inspection and maintenance. EM 7700–8. Washington,

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

Engineering Staff. 944 p.

Ritter, Michael A.; Geske, Earl A.; Mason, Lola [and

others]. 1990. Performance of stress-laminated bridges.

Wood Design Focus. 1(3): 12–16.

Ritter, Michael A.; Geske, Earl A.; McCutcheon,

William J. [and others]. 1991. Methods for assessing the

field performance of stress-laminated timber bridges. In:

Marcroft, Julian, comp. Proceedings, 1991 International

timber engineering conference; 1991 September 2–5; London, United Kingdom. High Wycombe, United Kingdom:

Timber Research and Development Association: 319–326.

Vol. 3.

Ritter, Michael A.; Faller, Ronald K.; Lee, Paula D.

Hilbrich [and others]. 1995a. Plans for crash-tested bridge

railings for longitudinal wood decks. FPL–GTR–87.

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 27 p.

Ritter, Michael A.; Wacker, James P.; Duwadi, Sheila

Rimal. 1995b. Field performance of stress-laminated timber

bridges on low-volume roads. In: Proceedings, 6th International conference on low-volume roads; 1995 June 25–29;

Minneapolis, MN. Washington, DC: National Academy

Press: 347–356. Vol. 2.

S&PF. 1995. The timber bridge initiative, fiscal year 1995

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

Forest Service, State and Private Forestry, Northeastern

Area.

Figure 22—Maximum predicted deflection profiles

at the bridge midspan for AASHTO HS20–44 truck

loading for load positions 3 and 6 (looking east).

Bridge cross-sections and vehicle positions are

shown to aid interpretation and are not to scale.
17

Figure 23—Severe checks and splits in railposts and

spacer blocks noted during condition assessment.

Figure 24—Crack in asphalt at end of bridge and

approach road interface observed during condition

assessment.
18

Appendix—Information Sheet

General

Name: Mohawk Canal bridge

Location: County 5th Street, west of Roll, Arizona

Date of Construction: August 1994

Owner: Yuma County

Design Configuration

Structure Type: Stress-laminated deck

Butt Joint Frequency: None

Total Length (out–out): 6.4 m (21 ft)

Skew: 15 degrees

Number of Spans: 1

Span Length (center-to-center bearings): 6.1 m (20 ft)

Width (out–out): 10.2 m (33.9 ft)

Width (curb–curb): 9.6 m (31.9 ft)

Number of Traffic Lanes: 2

Design Loading: AASHTO HS20–44

Wearing Surface Type: Asphalt

Material and Configuration

Timber:

Species: Douglas-fir

Size (actual): 130 mm wide (5.13 in.)

298 mm deep (11.75 in.)

Grade: Glulam Combination 16F-V3

Moisture Content: 8% average 3 months after

installation at 25- to 51-mm

(1- to 2-in.) depth

Preservative Treatment: Pentachlorophenol with

heavy oil

Stressing Bars:

Type: High strength steel bar with left-hand

thread, conforming to ASTM A 722

Diameter: 25 mm (1 in.)

Number: 7 (3 transverse the full bridge width)

Design Force: 289 kN (65,000 lb)

Spacing: 1.4 m (54 in.) average center-to-center

Anchorage Type and Configuration:

Steel Plates: 343 by 305 by 32 mm (13-1/2

by 12 by 1-1/4 in.) bearing

178 by 102 by 25 mm (7 by 4

by 1 in.) anchor