United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Timber Bridge

Information

Resource Center

Research

Paper

FPL–RP–550

Field Performance of

Timber Bridges

7. Connell Lake

Stress-Laminated Deck Bridge

Lola E. Hislop

Michael A. Ritter
Abstract

The Connell Lake bridge was constructed in early 1991 on

the Tongass National Forest, Alaska, as a demonstration

bridge under the Timber Bridge Initiative. The bridge is a

stress-laminated deck structure with an approximate 36-ft

length and 18-ft width and is the first known stress-laminated

timber bridge constructed in Alaska. Performance of the

bridge was monitored for 2-1/2 years, beginning at bridge

construction. Performance monitoring involved gathering and

evaluating data relative to the moisture content of the wood

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

under static load. In addition, comprehensive visual inspections were conducted to assess the overall condition of the

structure. Based on 2-1/2 years of field evaluations, the deck

is performing well with no structural deficiencies. However, a

slight sag has developed at midspan and several stressing bar

bearing plates have crushed into the outside deck laminations.

Keywords: Timber bridge, wood, stress laminated, gluedlaminated timber

Acknowledgments

We express sincere appreciation to John Weisz, Eleanor

Oman, and Carol Kaping–VanAart of the Ketchikan Area,

Tongass National Forest, for field data collection and monitoring assistance.

Thanks also to the following from the USDA Forest Service,

Forest Products Laboratory: Earl Geske and Robert Ross for

developing the instrumentation used for bridge evaluation

and the Publishing and Photography Teams for assistance in

preparing this report.

July 1996

Hislop, Lola E.; Ritter, Michael A. 1996. Field performance of timber

bridges: 7. Connell Lake stress-laminated deck bridge. Res. Pap.

FPL–RP–550. Madison, WI: U.S. Department of Agriculture, Forest

Service, Forest Products Laboratory. 13 p.

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Contents

Page

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

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

Objective and Scope.....................................................1

Design and Construction...............................................1

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

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

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

Moisture Content.....................................................4

Bar Force................................................................4

Behavior Under Static Load .......................................4

Static-Load Test...................................................6

Analytical Assessment...........................................6

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

Results and Discussion.................................................7

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

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

Behavior Under Static Load .......................................8

Static-Load Test...................................................8

Analytical Assessment...........................................8

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

Deck Camber .......................................................9

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

Wearing Surface..................................................10

Anchorage System ..............................................10

Conclusions .............................................................11

Literature Cited.........................................................12

Appendix—Information Sheet......................................13
Field Performance of Timber Bridges

7. Connell Lake Stress-Laminated Deck Bridge

Lola E. Hislop, 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 (TBI). One objective of this legislation was to provide for the construction of demonstration

timber bridges 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 become more aware of the attributes of wood as

a bridge material, and new, economical, structurally efficient

timber bridge systems should result. As a national wood

utilization research laboratory within the USDA Forest

Service, the Forest Products Laboratory (FPL) has taken a

lead role in assisting bridge owners in evaluating the field

performance of demonstration bridges, many of which use

design innovations that have not been previously evaluated.

This has involved the development and implementation of a

comprehensive national bridge monitoring program.

This report is seventh in a series that documents the field

performance of timber bridges built as a part of the TBI. This

report describes the design, construction, and field performance of the Connell Lake bridge located in the Ketchikan

Area, Tongass National Forest, near Ketchikan, Alaska. The

bridge, built in 1991, consists of a single-lane, single-span,

stress-laminated deck with a length of 36.1 ft and a width of

17.9 ft. (See Table 1 for metric conversion factors.) The

bridge design is unique in that it is the first known U.S.

application of a stress-laminated deck in Alaska. An information sheet on the Connell Lake bridge is provided in the

Appendix.

Background

The Connell Lake bridge is located approximately 5 miles

north of Ketchikan, Alaska, and is on an 18-ft-wide gravel

road that crosses a tributary of Ward Creek (Fig. 1). This

road provides access primarily for recreational and Forest

Service administrative traffic. The original structure was a

timber stringer and plank bridge, which inspections showed

as deteriorated and in need of replacement. A decision was

made by the Ketchikan Area personnel to replace the bridge

with a stress-laminated deck. The project was proposed and

Table 1—Factors for converting English units of

measurement to SI units

English unit

Conversion

factor SI unit

foot (ft) 0.3048 meter (m)

inch (in.) 25.4 millimeter (mm)

mile 1,609 meter (m)

pound (lb) 4.448 newton (N)

lb/in2

 (stress) 6,894 pascal (Pa)

lb/ft2 4.88 kilogram/meter2

 (kg/m2)

accepted as a demonstration bridge under the TBI. Subsequently, the FPL and Ketchikan Area entered into an agreement to monitor the performance of the bridge, thus obtaining information on the new, experimental bridge design and

the effects of the Alaskan environment on the experimental

bridge design.

Objective and Scope

The objective of this project was to evaluate the field performance of the Connell Lake bridge for a minimum of

2 years, beginning when construction was completed. The

project scope included data collection and analysis related to

the wood moisture content, stressing bar force, deflection

under static load, and general structural performance. The

results of this project, in conjunction with the results from

monitoring other bridges, will be used to formulate recommendations for the design and construction of similar bridges

in the future.

Design and Construction

Design of the Connell Lake bridge was completed by West

Virginia University (WVU) personnel and approved by the

Forest Service Alaska Regional Office. Construction was

completed by a local contractor. This paper presents an

overview of the design and construction of the bridge superstructure.
2

Design

The Connell Lake bridge deck was designed in accordance

with the WVU method that is based on criteria developed at

WVU (Davalos and others 1993). The bridge was designed

for the HS 20-44 vehicle as recommended by the American

 Association of State Highway and Transportation Officials

(AASHTO) Standard Specifications for Highway Bridges

(AASHTO 1989). The bridge configuration consisted of a

span length of 35.1-ft center–center of bearings, a 17.9-ft

width out–out, and a nominal 14-in. thickness (Fig. 2).

Douglas Fir–Larch, visually graded No. 1 Dense, nominal

Figure 1—Location of the Connell Lake bridge.

Figure 2—Design configuration of the Connell Lake

bridge.
3

4- by 14-in. laminations were selected as the deck material.

The allowable live-load deflection was limited to 1/360 of

the bridge span measured center–center of bearings, or

1.17 in.

The design included a butt-joint pattern developed at WVU.

Butt joints were located in every fifth lamination transversely, with a 2-ft longitudinal spacing between butt joints

in adjacent laminations (Fig. 3). Similar bridges are designed with butt-joint configurations based on butt joints in

every fourth lamination, with a 4-ft longitudinal spacing

between butt joints in adjacent laminations (Ontario Ministry of Transportation and Communication 1983).

Design of the stressing system was based on the WVU

method and provided an initial interlaminar compression of

approximately 83 lb/in2

. The design interlaminar compression was provided by 0.625-in.-diameter, high strength

stressing bars, which comply with the requirements of

ASTM A 722 (ASTM 1988) and provide a minimum ultimate tensile strength of 1.5 × 105

 lb/in2. Bars were spaced

25 in. on-center, beginning 16 in. from the bridge ends.

To achieve the 83-lb/in2 interlaminar compression, a bar

tension force of 28,000 lb was specified.

The stressing bar anchorage system was based on a relatively

new design developed by WVU. In the past, the bar anchorage for most stress-laminated bridges has been either the steel

channel or the discrete plate system (Ritter 1990). For the

Connell Lake bridge, a continuous 6- by 60- by 0.5-in.

bearing plate that extends over three stressing bars was used

for the interior bars, and a 6- by 10- by 0.5-in. discrete plate

was used for the two bars at the bridge ends (Fig. 4). In both

cases, a 3- by 3- by 0.75- in. anchor plate was placed between the bearing plate and the flat hex nut.

The deck was provided with a 12-in. curb and a bridge rail

system, meeting 50 percent of AASHTO static-load design

requirements, which is typical for Forest Service, single-lane

bridges (AASHTO 1989). The wearing surface consisted of

diagonally oriented 4- by 12-in. timber planks connected to

the deck with 8-in. double spiral spikes. Following fabrication, all wood components, with the exception of the

wearing surface, were specified to be pressure treated with

creosote-petroleum oil solution in accordance with American

Wood Preservers' Association (AWPA) standard C1 and C2

(AWPA 1991). To provide protection from deterioration, all

steel components were galvanized, including stressing bars,

hardware, bar anchorage plates, and nuts.

Construction

Bridge construction began February 12, 1991, with removal

of the existing abutments and placement of new, rock-filled,

welded wire basket abutments with 12-in. timber sills

(Fig. 5). In conjunction with the abutment construction,

scaffolding and falsework were erected. Banded bundles of

deck laminations were delivered to the bridge site and, upon

completion of the abutments, individual laminations were

placed on the falsework and the bars were inserted through

holes predrilled through the center of the laminations. This

construction technique was chosen as an alternative to installing prefabricated sections, because the weight of the

crane to place prefabricated sections exceeded the access

road weight restrictions. Individually placing laminations

was manageable due to the relatively small number of

Figure 3—Butt-joint configuration used for the Connell Lake bridge. A butt joint was placed transverse to the bridge

span in every fifth lamination. Longitudinally, butt joints in adjacent laminations were separated by 2 ft.

Figure 4—Details of the continuous bearing plate

bar anchorage system.
4

laminations necessary for the narrow deck width. After all

laminations were placed, the bars were tensioned to the

28,000-lb design force using a single hydraulic jack with a

built-in ratchet (Fig. 6).

Following the initial bar tensioning, the timber curb and rail

system and the timber plank wearing surface were installed.

Approximately 9 days after the initial tensioning, the bars

were retensioned to compensate for anticipated losses in the

bar force noted in previous research (Ritter 1990). Approximately 1 month after the second bar tensioning, the final bar

tensioning was completed. At that time, two stressing bars

were replaced because their length was inadequate for installation of the monitoring load cells. Construction of the Connell Lake bridge was completed in early 1991 (Fig. 7).

Evaluation Methodology

To evaluate the structural and serviceability performance of

the Connell Lake bridge, the Forest Service Alaska Regional

Office contacted FPL for assistance. Through mutual agreement, a monitoring plan was developed by FPL based on

cooperation with the Ketchikan Area, Tongass National

Forest. The plan called for monitoring several performance

indicators, including the deck moisture content, bar force,

deflection under static load, and condition of the structure for

a minimum of 2 years after the final stressing. At the initiation of monitoring, FPL representatives visited the bridge

site to install instrumentation and train Ketchikan Area

personnel in moisture content and bar force data collection

procedures. Load tests and condition assessments were conducted by FPL personnel with the assistance of Ketchikan

Area personnel. The evaluation methodology utilized procedures and equipment previously developed (Ritter and others

1991) and is discussed in the following sections.

Moisture Content

Moisture content measurements were taken with an electrical

resistance moisture meter with two insulated 3-in. pins in

accordance with ASTM D4444-84 procedures (ASTM

1990). Measurements were taken at probe penetrations of 1 to

3 in., at five locations along the deck’s underside, and were

assumed to be representative of the global bridge moisture

content (Fig. 8). The five measurements were averaged and,

when necessary, adjusted for temperature to determine the

actual moisture content value (Forintek 1984). Measurements

were obtained at installation and periodically thereafter.

Bar Force

Measurements of stressing bar force were obtained to monitor

changes and ensure that adequate bar force was maintained

during the monitoring period. Bar force was measured using

two calibrated load cells developed at FPL. The load cells

were installed between the bearing and anchor plates on the

third and ninth bars from the west abutment (Fig. 9). Load

cell measurements were collected with a portable strain

indicator by Ketchikan Area personnel. Strain units were

converted to bar force based on laboratory calibrations, and

the two bar forces were averaged to plot bar force change.

Readings were taken biweekly for the first 100 days and

periodically thereafter. Because equipment was not available

for removal of the load cells, recalibration for zero shift was

not performed.

Behavior Under Static Load

A static-load test was conducted at the conclusion of the

monitoring, 2-1/2 years after bridge installation, to determine

the bridge response to vehicle loading. The load test consisted of positioning a fully loaded, three-axle dump truck on

the bridge and measuring the resulting deflections at a series

of locations along a transverse cross section at midspan.

Deflection measurements were obtained prior to testing

(unloaded), after each placement of the test vehicle (loaded),

Figure 5—Welded wire basket abutments and

timber sills.

Figure 6—Hydraulic jack with built-in ratchet used

for tensioning bars.
5

Figure 7—Completed Connell Lake bridge.
6

and at the conclusion of testing (unloaded). Vehicle axle and

gross weights were measured on a local truck scale. In addition, an analytical assessment was conducted using the

theoretical bridge response developed by computer modeling.

Static-Load Test

The static-load test was performed August 25, 1993, using a

test vehicle (Fig. 10). At the time of the test, the bar force

was approximately 24,250 lb, which is equivalent to an

interlaminar compression of approximately 72 lb/in2

. The

vehicle was positioned longitudinally on the bridge facing

east, with the centroid of the vehicle at midspan. Transversely, three load cases were used (Fig. 11). A surveyor’s

level was used to read deflection values to the nearest

0.04 in. from calibrated rules suspended from the bridge

underside of the bridge. The accuracy of this method for

repetitive readings is estimated to be ±0.02 in.

Analytical Assessment

Previous research has shown that stress-laminated decks can

be accurately modeled as orthotropic plates (Oliva and others

1990). An orthotropic plate computer model developed at

FPL was used to analyze the load test results. Measured and

theoretical curves were compared considering the actual

bridge and test vehicle characteristics and the representative

analytical parameters of the computer model. The average

modulus of elasticity of the individual laminations was

assumed to be 1,900,000 lb/in2. Using the same analytical

parameters, the theoretical maximum live-load deflection for

AASHTO HS 20-44 vehicle loading was derived and compared with the design live-load deflection for the span length

center–center of bearings.

Condition Assessment

The general bridge condition was assessed on two occasions

during the monitoring period. The first assessment occurred

during construction when monitoring instrumentation was

installed. The second assessment occurred at the time of the

Figure 8—Electrical resistance moisture meter used

for measurements.

Figure 9—Load cell installed for measuring bar

tension force.

Figure 10—Load test vehicle configuration

and axle loads.

Figure 11—Transverse vehicle load cases as

viewed from the east, looking west. For all load

cases, the vehicle was facing east with the longitudinal vehicle centroid placed over the bridge

midspan. The transverse vehicle track width was

6 ft, measured center–center of the rear tires.
7

static-load test at the conclusion of the monitoring period.

These assessments involved visual inspections, measurements, and photographic documentation of the bridge’s

condition. Items of specific interest included condition of the

wood components, wearing surface, and stressing bar anchorage system. In addition, changes in deck camber were determined from relative elevation measurements at the deck ends

and midspan using a surveyor's rod and level.

Results and Discussion

The performance monitoring of the Connell Lake bridge

extended from April 1991 through August 1993. The following presents results of the performance data.

Moisture Content

The trend in the average measured moisture content of the

Connell Lake bridge is shown in Figure 12. As shown, the

laminations were initially installed at a moisture content

greater than 30 percent and remained at approximately the

same level for the entire monitoring period. Thus, the moisture content was above the fiber saturation point (FSP),

which is generally accepted as 30 percent but can vary from

25 to 30 percent (FPL 1987). Above the FSP, changes in

moisture content do not cause dimensional changes in wood.

However, dimensional changes do occur when the moisture

content drops below the FSP. For stress-laminated bridges,

changes in moisture content below the FSP are important

because moisture loss causes wood shrinkage and subsequent

bar force loss. Typically, the moisture content of a stresslaminated deck installed above the FSP gradually decreases

towards an equilibrium moisture content (EMC) for the site.

The EMC depends on several factors, including site exposure, temperature, and relative humidity, and EMC has

typically varied from 16 to 20 percent for other stresslaminated bridges (Ritter and others 1995). The Connell

Lake bridge has remained above the FSP, and changes in

moisture content during the monitoring period have been

insignificant. We expect that the eventual EMC of the bridge

may be as high as 25 to 28 percent and that changes from the

present moisture content toward the EMC will be very slow.

The basis for this conclusion centers on the site environmental conditions and the bridge wearing surface. The average rainfall and relative humidity are very high in the

Ketchikan area, and periods conducive to wood drying are

infrequent and of short duration. The bridge site is sheltered

by large trees and frequent cloud cover that retards evaporation caused by warm temperatures. In addition, the wood

plank surfacing on the bridge deck is not waterproof, which

leads to trapped and retained moisture.

Bar Force

Figure 13 shows the average bar force beginning at the time

of the final bar tensioning. Following the final bar tensioning, approximately 18 percent of the bar force was lost during

the first 100 days. After this, the force remained relatively

stable for the remainder of the monitoring period, with a

slight increase in force noted between day 250 and 350. At

the conclusion of the monitoring period, the average bar force

was 24,250 lb, which represents an interlaminar compression

of approximately 72 lb/in2.

The bar force loss is mainly attributable to stress relaxation

in the wood laminations, which is a slow deformation of the

wood cells caused by the compressive force exerted by the

stressing bars. Research has shown that the effects of stress

relaxation increase with moisture content (Youngs 1957).

Another possible contributing factor to bar force loss is

crushing of the bearing plates into the exterior laminations;

however, most of the crushing is believed to have occurred

prior to the final bar tensioning.

Based on observations during the monitoring, we do not

believe that retensioning of the bars will be required in the

foreseeable future. However, retensioning may be required at

a later date if stress relaxation and/or plate crushing continues

or if the lamination moisture content drops below the FSP

and the laminations begin to shrink.

Figure 12—Average moisture content changes in

the deck of the bridge.

Figure 13—Average bar tension force obtained from

load cells installed on two stressing bars.
8

Behavior Under Static Load

Results for the static-load test and analytical assessment of

the bridge are presented. Transverse deflection measurements

are given at the midspan from the east end looking west. No

permanent residual deformation was measured at the conclusion of the test. In addition, no movement at either of the

abutments was detected during the test.

Static-Load Test

Load test transverse deflection at midspan is shown in Figure 14, with maximum measured deflection locations and

magnitudes denoted by solid boxes. For each load case, the

deflection curves are typical of the linear elastic orthotropic

plate behavior of stress-laminated bridges (Ritter and others

1990). For all load cases, the deflection curves are relatively

smooth, with small differences between adjacent points in the

regions of maximum deflection. For load cases 1 and 3, a

single point of maximum deflection is not apparent as would

be expected. In these cases, it is likely that differences between the maximum deflection and adjacent points are within

the accuracy of the measurement method. Thus, the exact

location of maximum deflection cannot be conclusively

determined from these data.

We compared load cases 2 and 3 using a mirror image of

load case 2 (Fig. 15). Assuming uniform material properties,

symmetric loading, and accurate deflection measurements,

the deflection should be the same. As shown, there are differences up to 0.15 in. that are most pronounced at the bridge

edges. Given the locations of the deflection differences and

the relative orientation of the plots, it is likely that the truck

loading was slightly eccentric, which resulted in a greater

loading for one wheel line compared with the other.

Given the 2-ft longitudinal butt-joint spacing, the deflection

magnitude for the Connell Lake bridge was less than expected compared with similar bridges with a 4-ft longitudinal

butt-joint spacing. The reduced deflection is most likely due

to the diagonal timber plank wearing surface. We anticipate

that partial composite action was present between the wearing surface and the bridge deck, resulting in a slight increase

in the effective deck moment of inertia. This would decrease

the deflection magnitude in both the longitudinal and transverse directions.

Analytical Assessment

The measured load test deflections compared with theoretical

deflections are shown in Figure 16. The measured deflection

is generally less than the theoretical deflection, even though

the analysis considered approximately 50 percent of the

wearing surface in the deck thickness as a result of partial

composite action. Research has not been performed to assess

the effects of a diagonal timber plank wearing surface that

covers an entire stress-laminated deck. Although it is likely

that the moment of inertia is affected, the effect on other

analytical parameters is unknown.

Figure 14—Transverse deflection for load test,

measured at midspan. Bridge cross sections and

vehicle positions are shown to aid interpretation and

are not to scale.
9

Using the same analytical input parameters that were used for

the load test analysis, the theoretical bridge deflections under

AASHTO HS 20-44 truck loading are shown in Figure 17.

The maximum theoretical HS 20-44 deflection is 0.96 in. or

1/439 of the bridge center–center of bearings span length for

load case 1 and 1.19 in. or 1/354 for load cases 2 and 3.

Both are within or very close to the maximum design liveload deflection of 1/360 but do not meet the current

AASHTO recommendation of 1/500 (AASHTO 1989). It is

likely that the live-load deflection will increase over time as

the bar force declines and the partial composite action of the

wearing surface decreases as a result of traffic wear and the

deterioration of the attachment between the deck and the

timber planks.

Condition Assessment

Condition assessments of the Connell Lake bridge performed

at the conclusion of monitoring indicated that structural and

serviceability performance were acceptable, although several

serviceability deficiencies were noted. Inspection results for

specific items follow.

Deck Camber

At installation, the deck was approximately straight between

abutments. At the conclusion of monitoring, 2.5 in. of sag

was measured, mainly as a result of the butt-joint spacing

and partially as a result of creep. Sag has been more pronounced on stress-laminated decks designed with a 2-ft

longitudinal butt-joint spacing, regardless of the lamination

moisture content. Whereas, little or no sag has been

observed in stress-laminated decks with a 4-ft butt-joint

spacing when the moisture content of the laminations remained close to the FSP. Previous research has shown that

creep causes a loss of camber when the moisture content level

is high (Ritter and others 1990).

Wood Components

Inspection of the wood components of the bridge showed no

signs of deterioration, although minor checking was evident

on curb members. The checks did not appear to penetrate the

preservative treatment envelope of the members. Inspection

showed evidence of wood preservative loss with preservative

accumulations on the wood surface as well as on the stream

bottom (Fig. 18).

Figure 15—Comparison of load cases 2 and 3.

Figure 16—Theoretical deflection compared with

measured deflection under the actual applied load

using an orthotropic plate analysis.
10

Wearing Surface

Inspection of the timber plank wearing surface indicated

minor plank wear and checking. Aside from these items, the

wood planks were in good condition and showed no other

signs of distress. A 2-in. layer of soil has been tracked onto

the bridge deck and potholes have developed in the unpaved

approach roadway at both bridge ends (Fig. 19). The soil on

the bridge deck can trap moisture and inhibit drainage, which

may lead to premature deterioration. The potholes increase

vehicle impact to the bridge that can cause damage to the

preservative deck treatment and timber components.

Anchorage System

The exposed steel stressing bars and hardware showed no

visible signs of corrosion, except at the ends of some stressing bars. At bar ends, minor corrosion appeared where the

galvanized coating had been stripped from the bar, exposing

uncoated steel, as previously seen (Wacker and Ritter 1992).

The continuous bearing plate bar anchorage system is not

performing as designed. It is anticipated that the continuous

plate system was developed to alleviate the wood crushing

problem observed in some other bridges by providing a

larger bearing area than the discrete bearing plate system, but

Figure 17—Theoretical maximum deflection under

HS 20-44 load. Figure 18—Excessive preservative leaching from

laminations onto streambed.

Figure 19—Soil on deck surface should be removed,

and potholes along end of bridge should be filled.
11

at a lower cost than the channel bulkhead system (Ritter

1990). Inspection indicated that the portion of the bearing

plates near the bars were crushing approximately 0.5 in. into

the outside laminations, but no crushing was observed between the bars. This has caused a wave distortion of the

bearing plates (Fig. 20). Crushing may have occurred as a

result of insufficient bearing plate area for the strength of the

exterior lamination, insufficient plate thickness for uniform

force distribution, or reduced outer lamination strength because of the high moisture content level. It is probable that

the crushing occurred after the initial bar tensioning and is

not expected to continue unless the bars are retensioned.

Conclusions

After 2-1/2 years in service, the Connell Lake bridge is

exhibiting good performance and has met the expectations of

the bridge’s owner. Construction of the bridge has successfully demonstrated that stress-laminated timber decks are a

viable alternative for short-span structures in Alaska. In

general, the bridge is performing very well and only minor

serviceability deficiencies were noted during the monitoring

period. These deficiencies should not adversely affect structural performance and can be easily remedied in future

bridges. Although the bridge has been in place for only a

short time, we anticipate that the bridge will have a long,

useful life with minimal maintenance.

Based on the extensive monitoring conducted since bridge

fabrication, we note the following observations and recommendations:

• Moisture content level has remained above the FSP. It is

anticipated to remain high as a result of the high level of

rainfall and relative humidity, cool temperatures, and lack

of a waterproof surfacing. For this reason, minimal dimensional changes are expected. For future bridge designs, we

recommend that the moisture content of the laminations

be approximately 19 percent at installation.

• Stressing bar force has remained at an acceptable level

during the 2-1/2 years of monitoring, with less than an

18-percent average decrease in bar force. The loss is due to

stress relaxation of the wood and some crushing of the exterior laminations. We anticipate that restressing may

need to be performed during the life of the structure. This

is due to an expected interlaminar compression below the

recommended 40 lb/in2

 from continued stress relaxation at

the high moisture content level.

• Under static-load conditions, the measured deflection

curves represented linear elastic orthotropic plate behavior,

which is typical for stress-laminated decks. The maximum deflection was less than expected as a result of an

increase in the effective deck moment of inertia from the

partial composite action of the timber plank wearing surface. The theoretical maximum deflection of 0.96 in. for

the HS 20-44 vehicle is within the maximum design liveload deflection of 1/360 of the center–center of bearings

span length but is not within the current AASHTO recommendation of 1/500.

• A sag exists in the bridge’s deck and is likely due to the

butt-joint pattern and creep. This highlights the importance of a deck design that uses an alternative to the West

Virginia University butt-joint pattern, such as one butt

joint in every fourth transversely with

4-ft spacing between butt joints in adjacent laminations.

In addition, a deck design should include camber to counteract the effect of moisture content resulting in creep.

• Wood checking is evident on the curb system and timber

plank surfacing. Both are structurally sound with no penetration of the preservative treatment in the curb system.

• We anticipate that the 2-in. accumulation of soil on the

deck will result in deterioration because of trapped moisture and lack of drainage, and the potholes along the deck

ends will result in deck edge wear and impact loading.

The soil should be removed from the deck surface and the

potholes should be filled. Paving the approach roadways

would prevent pothole creation and their adverse effects.

• The use of the continuous bearing plate bar anchorage

system did not prevent crushing of the outside laminations. Further crushing may occur if the bars are retensioned.

• 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

nuts had been oversized to compensate for the thickness of

the galvanized coating or cold galvanizing was applied

after construction.

Figure 20—Crushing of outside laminations, causing

a wave distortion of the bearing plates.
12

Literature Cited

AASHTO. 1989. Standard specifications for highway

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

State Highway and Transportation Officials.

ASTM. 1988. Standard specification for uncoated,

high-strength steel bar for prestressing concrete; ASTM

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13

Appendix—Information Sheet

General

Name: Connell Lake Bridge

Location: Ketchikan, Alaska

Date of Construction: April 1991

Owner: USDA Forest Service, Tongass

National Forest, Ketchikan Area

Design Configuration

Structure Type: Stress-laminated deck

Butt-joint Frequency: 1 in 5; 2 ft longitudinal

Total Length (out–out): 36.1 ft

Skew: None

Number of Spans: 1

Span Length (center–center bearings): 35.1 ft (as-built)

Width (out–out): 17.9 ft (as-built)

Width (curb–curb): 15.9 ft (as-built)

Number of Traffic Lanes: 1

Design Loading: AASHTO HS 20–44

Wearing Surface Type: 4- by 12-in. timber planks

Material and Configuration

Timber:

Species: Douglas Fir–Larch

Size (nominal): 14 by 4 in.

Moisture Condition: Greater than 30 percent

at installation

Preservative Treatment: Creosote petroleum oil

solution

Stressing Bars:

Diameter: 5/8 in.

Number: 17

Design Force: 28,000 lb; 83 lb/in2 interlaminar

compression

Spacing: 25 in.

Type: Galvanized, high strength, steel threaded bar

with course right-hand thread, conforming

to ASTM A 722

Anchorage Type and Configuration:

Steel Plates: 1/2 by 6 by 60 in. bearing, galvanized

1/2 by 6 by 10 in. bearing at bridge

 ends, galvanized

3/4 by 3 by 3 in. anchor, galvanized