United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Timber Bridge

Information

Resource Center

Research

Paper

FPL–RP–555

Field Performance of

Timber Bridges

10. Sanborn Brook

Stress-Laminated Deck Bridge

Paula D. Hilbrich Lee

Michael A. Ritter

James P. Wacker
Abstract

The Sanborn Brook bridge was constructed in August 1991,

10 miles northeast of Concord, New Hampshire, as part of

the demonstration timber bridge program of the USDA Forest Service. The bridge is a simple-span, double-lane, stresslaminated deck superstructure constructed from Southern

Pine lumber and is approximately 25 ft long and 28 ft wide

with a skew of 14 degrees. The performance of the bridge was

monitored continuously for approximately 2 years, beginning

shortly after installation. Performance monitoring involved

collecting and evaluating data pertaining to the moisture

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

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

static-load conditions. In addition, comprehensive visual

inspections were conducted to assess the overall condition of

the structure. Based on field evaluations, the bridge is performing well, with no structural or serviceability deficiencies.

Keywords: Timber, bridge, wood, stress laminated

December 1996

Lee, Paula D. Hilbrich; Ritter, Michael A.; Wacker, James P. 1996. Field

performance of timber bridges: 10. Sanborn Brook stress-laminated deck

bridge. Res. Pap. FPL–RP–555. Madison, WI: U.S. Department of

Agriculture, Forest Service, Forest Products Laboratory. 19 p.

A limited number of free copies of this publication are available to the

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

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

1,000 libraries in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the

University of Wisconsin.

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.

Persons with disabilities who require alternative means of communication

of program information (braille, large print, audiotape, etc.) should contact

the USDA Office of Communications at (202) 720–2791. To file a complaint, write the Secretary of Agriculture, U.S. Department of Agriculture,

Washington, DC 20250, or call (800) 245–6340 (voice), or (202) 720–1127

(TTD). USDA is an equal employment opportunity employer.

 Acknowledgments

We express sincere appreciation to the following individuals

from the New Hampshire Department of Transportation, Bureau of Materials and Research, who assisted in the structural

monitoring of the Sanborn Brook bridge: Mike Juranty and

Joe Constant for their assistance in obtaining field readings

and conducting load tests and James Magee and

Alan Lugg for their assistance in obtaining field readings.

Thanks also to the following from the USDA Forest Service,

Forest Products Laboratory: Earl Geske and Vyto

Malinauskas for developing and manufacturing load cell instrumentation and the Information Services Team for assistance in preparing this report.

Contents

Page

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

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

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

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

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

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

Cost ......................................................................3

Evaluation Methodology...............................................3

Moisture Content.....................................................3

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

Vertical Creep..........................................................4

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

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

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

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

Bar Force................................................................9

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

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

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

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

References................................................................17

Appendix—Information Sheet......................................19
Field Performance of Timber Bridges

10. Sanborn Brook Stress-Laminated Deck Bridge

Paula D. Hilbrich Lee, General Engineer

Michael A. Ritter, Research Engineer

James P. Wacker, General Engineer

Forest Products Laboratory, Madison, Wisconsin

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

promote effective utilization of wood as a structural material

for highway bridges. Responsibility for the development,

implementation, and administration of the timber bridge

program was assigned to the USDA Forest Service. A key

element of the TBI is a demonstration bridge program that

provides matching funds to local governments for the construction of selected demonstration bridges (USDA 1995).

A primary objective of the demonstration bridge program is

to encourage innovation through the use of new or previously

underutilized wood products, bridge designs, and design

applications. In so doing, bridge designers and users become

more aware of the attributes of wood as a bridge material, and

new, economical, structurally efficient timber bridge systems

should result. In addition, it is contemplated that timber use

in bridges will be expanded to include several abundant but

underutilized wood species.

As the national wood utilization research laboratory within

the USDA Forest Service, the Forest Products Laboratory

(FPL) has taken a lead role in assisting local governments in

evaluating the field performance of demonstration bridges,

many of which use design innovations that have never been

evaluated. This has involved the development and implementation of a comprehensive national bridge monitoring

program. The objectives of the monitoring program are to

collect, analyze, and distribute information on the field performance of timber bridges to provide a basis for validating

or revising design criteria and further improving efficiency

and economy of bridge design, fabrication, and construction.

This report, tenth in a series documenting field performance

of timber bridges, describes the development, design, construction, and field performance of the Sanborn Brook bridge,

which is located 10 miles northeast of Concord, New Hampshire. This bridge, built in 1991, is a double-lane, simplespan, stress-laminated deck that is approximately 25 ft long

and 28 ft-wide, with a skew of 14 degrees. (See Table 1 for

metric conversion factors.) An information sheet on the

Sanborn Brook bridge is provided in the Appendix.

Background

The Sanborn Brook bridge is located approximately

10 miles northeast of Concord, New Hampshire (Fig. 1). It

is on Kellys Corner Road, a double-lane paved road that

crosses Sanborn Brook and provides a link for local traffic

between Pleasant Street and Route 28. The estimated average

daily traffic over this section of road is 350 vehicles per day.

The original Sanborn Brook bridge was constructed in 1930

with a superstructure consisting of a concrete deck supported

by steel I-beam stringers. The original bridge was 26 ft long

and 25.9 ft wide. Inspections conducted in the 1980s by the

New Hampshire Department of Transportation (DOT) indicated the bridge was structurally deficient. It was apparent

that major rehabilitation or replacement of the structure

would be required in the near future.

Subsequent to the bridge inspection, New Hampshire DOT

officials determined that the Sanborn Brook bridge would be

replaced. A project proposal was submitted to the USDA

Forest Service for funding the Sanborn Brook bridge

replacement as a demonstration bridge under the TBI. The

proposed replacement bridge consisted of a simple-span,

sawn-lumber, stress-laminated deck system constructed

with a species local to New Hampshire. In 1991, the project

was approved and funds were provided to the State of

New Hampshire through the USDA Forest Service,

Table 1—Factors for converting English units of measurement to SI units

English unit Conversion factor SI unit

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

square foot (ft2

) 0.09 square meter (m2)

pound (lb) 4.448 newton (N)

lb/in2

 (stress) 6,894 pascal (Pa)

mile 0.0016 meter (m)
2

Timber Bridge Information Resource Center in Morgantown,

West Virginia. Because the stress-laminated deck was a relatively new system, FPL was contacted by New Hampshire

DOT to provide technical advice and monitor the field performance of the bridge.

Objective and Scope

The objective of this project was to evaluate the field performance of the Sanborn Brook bridge for approximately

2 years, beginning shortly after bridge installation. The project scope included data collection and analysis related to the

deck moisture content, stressing bar force, vertical creep,

bridge behavior under static load, and general structural performance. The results of this project will be evaluated with

similar monitoring projects in an effort to formulate recommendations for design and construction of future stresslaminated bridges.

Design, Construction,

and Cost

The design of the Sanborn Brook bridge was completed by

New Hampshire DOT with assistance from FPL. Construction was completed by contract. An overview of the design,

construction process, and cost of the bridge superstructure

follows.

Design

Design of the Sanborn Brook bridge was completed by the

engineering staff of New Hampshire DOT. Because the bridge

was designed before a nationally recognized design procedure

was available for stress-laminated bridges, criteria relating to

stress laminating was based on work completed at the University of Wisconsin and FPL (Ritter 1990).

The bridge was designed for American Association of State

Highway and Transportation Officials (AASHTO) HS 25–44

loading (AASHTO 1983), resulting in a deck geometry 25 ft

long and 28 ft wide, with a deck thickness of 14 in. and a

skew of 14 degrees (Fig. 2). It was the original intent of New

Hampshire DOT to utilize a local species, such as red maple,

Hem-Fir, or Eastern Spruce, as the lamination material.

However, these local species were unavailable in the size

necessary for the project; therefore, Southern Pine Grade No.

2 and better was substituted as the lamination material. Butt

joints in the deck laminations were placed in every fourth

lamination transversely with a 4-ft longitudinal spacing between butt joints in adjacent laminations (Fig. 3). The

bridge rail system was designed to meet AASHTO staticload requirements and consisted of a W-beam mounted on

timber posts. All lumber components were treated with pentachlorophenol in heavy oil in accordance with American

Wood Preservers' Association Standard C14 (AWPA 1990).

The stressing system for the Sanborn Brook bridge was designed to provide a uniform interlaminar compressive stress

of 100 lb/in2

, which corresponds to a design bar force of

55,000 lb. High strength 1-in.-diameter stressing bars, complying with the requirements of ASTM A722 (ASTM

1988), were spaced 39.5 in. on-center. The anchorage system

was composed of discrete steel plates and is illustrated in

Figure 4. To provide protection from deterioration, all steel

components were galvanized and an asphalt wearing surface

was specified.

Figure 1—Location of the Sanborn Brook bridge.
3

Construction

Construction of the Sanborn Brook bridge was completed by

contract in the Fall of 1991. Following work on the approach

roadway and construction of toewalls and mortar rubble masonry wingwalls, construction of the bridge superstructure

commenced August 22 and was completed September 17.

The wearing surface was applied October 15, and the bridge

railing, curb, and approach railing were completed

October 17.

Construction of the Sanborn Brook bridge deck began with

the arrival of the Southern Pine laminations at the bridge

site. The laminations, each measuring 1.5 in. wide by 14 in.

deep, were transported to the site by truck in banded bundles. Upon delivery to the bridge site, it was discovered that

the bundled laminations had been predrilled for the wrong

skew angle. The contractor redrilled the holes for the stressing bars so that the holes aligned properly, but a preservative

was not applied to the newly exposed wood surfaces.

After all laminations were placed on the abutments, steel

stressing bars were inserted through holes in the laminations,

bearing plates and anchor plates were installed, and nuts were

hand tightened. Because of the difficulties encountered in

properly aligning the lamination holes, positive camber in

the deck could not be achieved. The stressing bars were tensioned with a single hydraulic jack to the design force of

55,000 lb. Approximately 1 and 8 weeks after the initial

tensioning, the bars were retensioned to the design force a

second and third time to compensate for losses in bar force

(Ritter 1990). Approximately 2 weeks prior to the third

retensioning, the asphalt wearing surface was applied and the

bridge railing, curb, and approach railing were completed

(Fig. 5).

The as-built configuration of the Sanborn Brook bridge varied slightly from the design configurations in Figure 2. After

the final stressing, the average out–out bridge width measured 27.7 ft, and the bridge length measured 24.9 ft. The

bridge span, measured center–center of bearings, was

24.1 ft. The completed bridge is shown in Figure 6.

Cost

The total contract cost for materials, fabrication, and construction of the Sanborn Brook bridge superstructure, including the bridge railing and curb, was approximately $50,400.

Based on a total deck area of 690 ft2, the cost was approximately $73/ft2

. These costs do not include the superstructure

design or wearing surface that were completed by the New

Hampshire DOT.

Evaluation Methodology

To evaluate the structural performance of the Sanborn Brook

bridge, the New Hampshire DOT contacted FPL for assistance. Through mutual agreement, a bridge monitoring plan

was developed by FPL and implemented as a cooperative

effort with the New Hampshire DOT. The plan called for

performance monitoring of the deck moisture content, force in

stressing bars, vertical creep, load test behavior, and condition assessments of the structure for approximately 2 years.

The evaluation methodology utilized procedures and equipment previously developed by FPL (Ritter and others 1991).

Moisture Content

Global changes in the moisture content of stress-laminated

timber decks can significantly affect the performance of the

structure. If moisture is lost, the deck can shrink, resulting in

a decrease in stressing bar force. Conversely, if moisture is

gained, swelling of the timber can occur and cause an increase in stressing bar force. Changes in moisture content can

also affect the deck stiffness, vertical creep, and transverse

stress relaxation.

Figure 2—Design configuration of the Sanborn Brook

bridge.
4

The deck moisture content of the Sanborn Brook bridge was

measured using an electrical-resistance moisture meter with

3-in. insulated probe pins in accordance with ASTM

D4444–84 (ASTM 1990) (Fig. 7). Measurements were obtained by the New Hampshire DOT on a monthly basis by

driving the pins into the underside of deck at depths of 2 to

3 in., recording the moisture content values, and adjusting

the values for temperature and wood species (Forintek 1984).

Bar Force

For stress-laminated bridges to perform properly, adequate

bar force levels must be maintained in order to achieve an

acceptable level of interlaminar compression. To monitor bar

force, load cells developed by FPL were installed on the first

and third stressing bars from the west abutment, along the

upstream bridge edge (Fig. 8). Load cell measurements were

obtained by the New Hampshire DOT, using a portable

strain indicator (Fig. 9). Strain from the indicator was

converted to units of bar tensile force by applying a laboratory conversion factor to the strain indicator reading. Bar

force measurements were taken on a weekly basis for 2

months following load cell installation and monthly thereafter. At the end of the monitoring period, the load cells were

unloaded and checked for zero balance shift, and the measurements were adjusted accordingly.

Vertical Creep

As a structural material, wood can deform permanently, or

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

an important consideration, because excessive creep can result in a sag of the superstructure (Ritter and others 1990).

Camber of the Sanborn Brook bridge was measured at each

load test. At the time of load test 1, measurements were obtained by suspending displacement rules from the underside

of the deck at the abutments and midspan and reading values

Figure 3—Butt joint configuration used for the Sanborn Brook bridge. Butt joints were placed

transverse to the bridge span in every fourth lamination. Longitudinally, butt joints in adjacent

laminations were separated by 4 ft.

Figure 4—Discrete plate anchorage system. Figure 5—Completed Sanborn Brook bridge rail.
5

Figure 6—Completed Sanborn Brook bridge.
6

with a surveyor's level. At the time of load test 2, camber

was measured by attaching a stringline to the bearings to

create a horizontal benchmark and with a calibrated rule,

measuring the deck elevation at midspan with respect to the

benchmark.

Load Test Behavior

Static-load testing of stress-laminated bridges is an important

part of a comprehensive bridge monitoring program. The

information obtained from these tests is used to refine and

improve design procedures and evaluate the effects of design

variables on bridge performance. To determine the load test

behavior of the Sanborn Brook bridge, static-load tests were

conducted twice during the monitoring period. In each test,

fully loaded trucks were positioned on the bridge deck and

the resulting deflections were measured at a series of locations along the bridge midspan. Measurements of bridge

deflections 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). In addition, analytical

assessments were conducted to determine the theoretical

bridge response.

Load Test 1

The first load test occurred October 31, 1991, immediately

following the third retensioning of the bars, approximately

2 months after bridge installation. The interlaminar compression at the time of the test was approximately 96 lb/in2

. Two

test vehicles were employed: truck A with a gross vehicle

weight of 31,040 lb and truck B with a gross vehicle weight

of 38,780 lb (Fig. 10). Transversely, six load positions were

used (Fig. 11). Longitudinally, both trucks were positioned

on the bridge with the rear axle of the vehicle centered at the

skewed midspan (Fig. 12). Measurements of bridge deflections were obtained by suspending calibrated rules from the

underside of the deck and reading values to the nearest

0.06 in. with a surveyor’s level (Fig. 13). The accuracy of

these measurements is estimated to be ±0.03 in.

Figure 7—Measuring moisture content with an

electrical-resistance moisture meter.

Figure 8—Load cells positioned on stressing bars to

monitor bar force.

Figure 9—Obtaining a load cell reading with a

portable strain indicator.

Figure 10—Load test 1 vehicle configuration and axle

loads. (Lighter axle is vehicle front.) The transverse

vehicle track width, measured center–center of the rear

tires, is 6 ft.
7

Load Test 2

The second load test was conducted June 23, 1993,

approximately 21 months after bridge installation. At the

time of the test, the interlaminar compression was 39 lb/in2.

Two test vehicles were used: truck A with a gross vehicle

weight of 30,340 lb and truck B with a gross vehicle weight

of 38,540 lb (Fig. 14). The six transverse load positions

(Fig. 11, 15) and measurement method were identical to

those employed during load test 1. Values were read to the

nearest 0.04 in., and the measurement accuracy is estimated

to be ±0.02 in. Longitudinally, the vehicles were positioned

with the rear axle centered along the nonskewed midspan of

the bridge, perpendicular to the roadway (Fig. 16).

Figure 11—Transverse load positions used for both

load tests (looking east). Trucks A and B were in the

north and south lanes, respectively.

Figure 12—Typical configuration of longitudinal

vehicle placement for load test 1. For all positions,

the rear axle of each truck was centered at the

skewed midspan.

Figure 13—Load tests 1 and 2 (load test 2 shown)

deflection measurements were obtained by reading

values from calibrated rules suspended from the

underside of the deck with a surveyor’s level.
8

Analytical Evaluation

Previous research has shown that stress-laminated decks can

be accurately modeled as orthotropic plates (Oliva and others

1990). To further analyze the theoretical behavior of the Sanborn Brook bridge, an orthotropic plate computer model,

currently being developed at FPL, was used to analyze the

load test results and predict the bridge deflection for

AASHTO HS 25–44 loading. A modulus of elasticity

(MOE) value of 1,400,000 lb/in2

 was used for modeling

based on established design values (NFPA 1991).

Condition Assessment

The general condition of the bridge was assessed on two

different occasions during the monitoring period. The assessments occurred at the time of the load tests, which corresponded with the beginning and approximate ending of the

monitoring period. The assessments involved visual inspections, measurements, and photographic documentation.

Items of specific interest included the geometry of the bridge

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

wearing surface, stressing bars, and anchorage systems.

Results and Discussion

Performance monitoring of the Sanborn Brook bridge covered

approximately 20 months, from October 31, 1991, to

June 23, 1993. Results of the performance data follow.

Moisture Content

The average lamination moisture content of the bridge was

approximately 23 percent at the beginning and ending of the

monitoring. Throughout the 20 months, the moisture con

tent remained relatively stable, although there were

fluctuations of 4 to 5 percent in the measurement zone as a

result of seasonal climatic changes. The moisture content at

the interior of the laminations is presumed greater than the

values obtained in the measurement zone because of the

slower moisture migration through the lamination depth.

The average moisture content of the deck is expected to

slowly decline (Ritter and others 1995) and eventually stabilize at an equilibrium value of 18 to 20 percent (McCutcheon

and others 1986), although short-term seasonal changes will

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

deck.

Figure 14—Load test 2 vehicle configurations and axle

loads. (Lighter axle is vehicle front.) The transverse

vehicle track width for both vehicles, measured

center–center of the rear tires, is 6 ft.

Figure 15—Transverse load positions used for load

test 2. From top to bottom: load position 3, load

position 6, load position 6 (side view) are shown.
9

Bar Force

The average trend in bar force, beginning at the time of the

third bar retensioning, is shown in Figure 17. The third retensioning, which occurred in October 1991, was to the design force of 55,000 lb. Immediately following the retensioning procedure, the force began a rapid decline that continued

about 3 months. The time-related rate of bar force loss then

decreased and remained approximately linear for the remainder of the monitoring period. In June 1993, at the conclusion

of monitoring, the bar force had decreased to approximately

21,300 lb, 39 percent of the design force.

The bar force loss is attributed to stress relaxation of the

lumber laminations caused by the applied compressive

force and enhanced by the high moisture content. Although

the bar force decreased approximately 60 percent during the

monitoring period, it did not fall below acceptable levels.

Because the decline in bar force was expected to continue as a

result of further stress relaxation and an eventual decrease in

the moisture content, the bridge was restressed at the conclusion of the monitoring period to the full design force.

Vertical Creep

The laminations for the Sanborn Brook bridge were installed

with no camber. At the time of the first load test, approximately 2 months after bridge installation, measurements

verified that the bridge was approximately level between

abutments. No noticeable sag was measured at the conclusion of the monitoring period, thereby documenting the absence of vertical creep.

Load Test Behavior

Results of both static-load tests as well the theoretical bridge

response under load test and AASHTO HS 25–44 loading

are presented in this section. For each case, transverse deflections are shown at the bridge midspan, as viewed from the

west end looking east. For each load test, no permanent residual deformation was measured at the conclusion of the

testing. In addition, there was no detectable movement at

either of the abutments.

Load Test 1

Transverse deflections for load test 1 with the locations and

magnitudes of the maximum measured deflections are shown

in Figure 18. For each load position, the deflections are typical of the orthotropic plate behavior of stress-laminated

bridges (Ritter and others 1990). The maximum deflection

resulting from load position 6 occurred beneath the outside

wheel line of the heavier truck (Fig. 18f). Load position 3

caused an absolute maximum measured deflection of 0.38 in.

at the bridge centerline (Fig. 18c). Except for load position

5, the load positions involving one truck resulted in a

maximum deflection under the outside truck wheel line

(Fig. 18a,b,d). For load position 5, the maximum measured

deflection occurred at three locations, one of which was under

the outside wheel line (Fig. 18e). Although it is likely that

minor differences existed between these three points, it is not

possible to make a more accurate assessment, given the relatively small magnitude of deflections and the accuracy of the

measurement method.

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 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. The plots

are similar with minor variations that are within the accuracy

of the measurement methods, indicating that the bridge behavior under the applied loads is within the linear elastic

range.

Figure 16—Typical configuration of longitudinal vehicle

placement for load test 2. For all positions, rear axles

were placed at the nonskewed midspan.

Figure 17—Average trend in bar force.
10

Figure 18—Transverse deflection for load test 1 (looking east), measured at the midspan of the bridge. Bridge crosssections and vehicle positions are shown to aid interpretation and are not

to scale.
11

Load Test 2

Transverse deflections for load test 2 are shown in Figure 20.

As with load test 1, the plots are indicative of orthotropic

plate behavior (Ritter and others 1990). For load position 6,

the maximum deflection occurred beneath the outside wheel

line of the heavier truck (Fig. 20f). The absolute maximum

measured deflection of 0.39 in. occurred at the data point

immediately to the right of the bridge centerline during load

position 3. For load positions 1 and 2, the maximum deflections occurred between the truck wheel lines and under the

outside truck wheel line, respectively (Fig. 20a,b). The

maximum deflection for load position 4 occurred under the

outside wheel line, and the maximum deflection for load

position 5 occurred at the three locations indicated

(Fig. 20d,e). A single maximum point cannot be identified

for load position 5 for the same reasons as indicated in load

test 1.

As illustrated with the measured deflections of load test 1,

the summation of deflections resulting from two separately

applied truck loads should equal the deflection of both trucks

applied simultaneously, if uniform material properties, proper

vehicle placement, and accurate deflection measurements are

assumed. This is illustrated in Figure 21, 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 plots

are virtually identical, with slight variations well within the

accuracy of the measurement methods, indicating that the

bridge behavior 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 22 and 23.

As shown, the theoretical bridge deflection, based on orthotropic bridge behavior, is very close to that measured. Employing the analytical parameters used to determine the theoretical bridge response for each load test, the theoretical

deflection for AASHTO HS 25–44 loading is shown in

Figure 24. Based on this analysis, the theoretical maximum

AASHTO HS 25–44 static deflection occurs at centerline

when two HS 25–44 trucks are placed centrically on the

bridge (load position 3), with the rear axles centered about

the skewed midspan (Fig. 24c). The resulting deflection is

0.68 in. or approximately 1/425 of the bridge span for load

test 1, and 0.74 in., approximately 1/391 of the bridge span

for load test 2.

Assuming constant bridge properties, the same theoretical

bridge deflection would be expected for both load tests, because the same AASHTO HS 25–44 loading is applied in

each case. However, for stress-laminated bridges with butt

joints, it is known that a decrease in interlaminar compression results in a decrease in longitudinal bridge stiffness

(Oliva and others 1990). The interlaminar compression decreased from 96 lb/in2

 at the time of load test 1 to 39 lb/in2

at the time of load test 2, resulting in an approximate 9-

percent decrease in longitudinal bridge stiffness. This change

in stiffness resulted in a slightly greater deflection for load

test 2 compared with that for load test 1.

Condition Assessment

Condition assessments of the Sanborn Brook bridge indicate

that structural and serviceability performance are acceptable.

Inspection results for specific items follow.

Bridge Geometry

At the time of the second load test, a variation in bridge

width was noted along the edge of the deck. A comparison of

width measurements taken at the beginning and ending of the

monitoring period indicates a slight narrowing of the out–out

deck width (Table 2). The deformation is probably the result

of transverse stress relaxation in the laminations, enhanced

by the high moisture content, and further supports the conclusion that the majority of bar force loss is attributable to

stress relaxation in the laminations.

Figure 19—Transverse deflections for load test 1 (looking

east), measured at the midspan of the bridge, comparing

the sum of measured deflections from load positions 1

and 2 and load positions 4 and 5 to load positions 3 and

6, respectively. Bridge cross-sections and vehicle

positions are shown to aid interpretation and are not to

scale.
12

Figure 20—Transverse deflections for load test 2 (looking east), measured at the midspan of the bridge. Bridge crosssections and vehicle positions are shown to aid interpretation and are not to scale.
13

Wood Condition

Inspection of the wood components of the bridge showed no

signs of deterioration, although minor checking was evident

in timber members exposed to wet–dry cycles. The top endgrain surfaces of the timber railposts exhibited minor checking. This probably could have been prevented if a bituminous end grain sealer had been applied at the time of construction. In addition, minor checks appeared along the sides

of exposed exterior deck laminations (Fig. 25). There was no

evidence of wood preservative loss and no preservative or

solvent accumulations on the wood surface.

Wearing Surface

Inspection of the wearing surface indicated proper performance

at the time of the second load test. Although minor tire

imprints from the static trucks were observed during the test,

the impressions probably resulted from a deficiency in the

asphalt mix, water-proofing membrane, or application procedures. Aside from this, the asphalt was in good condition

and showed no signs of cracking, deterioration, or other distress.

Anchorage System

The stressing bar anchorage system performed as designed

with no signs of distress. There was no indication of the

discrete plate anchorage crushing into the outside laminations and no measurable distortion of the bearing plate.

However, the exterior laminations, when viewed from the

underside of the deck, appeared distorted and wavy, curving

away from the deck between the discrete anchorages

(Fig. 26). This distortion does not appear to have affected the

performance of the deck and is not typical of stress-laminated

bridges. The exposed steel stressing bars and hardware

showed no visible signs of corrosion.

Conclusions

After approximately 2 years in service, the Sanborn Brook

bridge is performing well and is expected to provide many

years of acceptable service. Based on the monitoring conducted since October 31, 1991, we make the following observations and recommendations:

• It is practical and economical to construct stress-laminated

decks using Southern Pine lumber laminations.

• When prefabricating a skewed stress-laminated bridge

with butt joints, attention to lamination layout and hole

placement is critical in order to avoid last minute, on-site

field drilling that penetrates the preservative envelope and

exposes untreated material to the elements. All field cuts

and bores should be treated with a preservative.

• The moisture content of the deck at the time of installation was approximately 23 percent and remained relatively

stable throughout the monitoring period, although moisture content fluctuated 4 to 5 percent in the measurement

zone as a result of seasonal climatic changes. This somewhat high moisture content level has not had any adverse

affects on the structural integrity of the Sanborn Brook

bridge, although it may have contributed to narrowing of

the bridge width. The global moisture content is expected

to decline towards equilibrium, affecting the dimensional

stability of the deck and resulting in additional bar force

losses.

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

Sanborn Brook bridge decreased from the design force of

55,000 lb to approximately 21,300 lb (39 lb/in2

 interlaminar compression). The decline in bar force is attributed to stress relaxation of the lumber laminations, which

was enhanced by the high moisture content. Every

2 years, future bridge inspections should verify bar forces

to ensure adequate interlaminar compression. Bars should

be retensioned as required.

Figure 21—Transverse deflections for load test 2

(looking east), measured at the midspan of the bridge,

comparing the sum of measured deflections from load

positions 1 and 2 and load positions 4 and 5 to load

positions 3 and 6, respectively. Bridge cross-sections

and vehicle positions are shown to aid interpretation and

are not to scale.
14

Figure 22—Comparison of the measured deflections for load test 1 (looking east) with the theoretical bridge response.

Bridge cross-sections and vehicle positions are shown to aid interpretation and are not to scale.
15

Figure 23—Comparison of the measured deflections for load test 2 (looking east) with the theoretical bridge response.

Bridge cross-sections and vehicle positions are shown to aid interpretation and are not to scale.
16

• The laminations were installed without camber. The vertical creep of the deck of the bridge was negligible with no

noticeable sag at the conclusion of the monitoring period.

• Load testing and analysis indicate that the Sanborn Brook

bridge is performing as a linear elastic orthotropic plate

when subjected to static truck loading. Based on an analytical comparison of load test results at different levels of

interlaminar compression, the longitudinal bridge stiffness

decreased approximately 9 percent when the interlaminar

compression decreased from 96 to 39 lb/in2

. The maximum theoretical deflection as a result of two lanes of

AASHTO HS 25–44 loading is estimated to be 0.68 in.

(L/425) at 96 lb/in2 interlaminar compression and 0.74 in.

(L/391) at 39 lb/in2 interlaminar compression.

• A reduction in bridge width was noted at the conclusion

of the monitoring period. The deformation is likely the

result of stress relaxation in the laminations enhanced by

the high moisture content.

• Visual inspections of the bridge indicate that performance

of wood components is satisfactory. Minor checks and

splits were noted in some wood members. This could be

avoided by installing the laminations at a lower moisture

content level and applying a bituminous sealer to any exposed end grain.

• The wearing surface appears to be in good condition and

shows no signs of distress.

• The exposed steel stressing bars and discrete anchorage

plates showed no visible signs of corrosion or other distress. The plates were not distorted and crushing of the

exterior lumber laminations was negligible, although the

exterior laminations did appear distorted and wavy along

the length of the bridge.

Figure 24—Maximum theoretical midspan deflection profile for AASHTO HS 25–44 truck loading

(looking east). Load positions 1 and 4 are mirror images of load positions 2 and 5, respectively.

Bridge cross-sections and vehicle positions are shown to aid interpretation and are not to scale.
17

References

AASHTO. 1983. Standard specifications for highway

bridges 13th ed. Washington, DC: American Association of

State Highway and Transportation Officials.

ASTM. 1988. Standard specification for uncoated highstrength steel bar for prestressing concrete; ASTM A 722–

86. Philadelphia, PA. American Society for Testing and

Materials. 3 p.

ASTM. 1990. Use and calibration of hand-held moisture

meters; ASTM D4444–84. 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 Corp. 37 p.

AWPA. 1990. Standards. Woodstock, MD: American

Wood Preservers' Association. 200 p.

McCutcheon, W.J.; Gutkowski, R.M.; Moody, R.C.

1986. Performance and rehabilitation of timber bridges.

Trans. Res. Rec. 1053. Washington, DC: National Academy

of Sciences, National Research Council, Transportation Research Board: 65–69.

National Forest Products Association. 1991. Design values for wood construction. A supplement to the 1991 edition

National Design Specification for Wood Construction.

Washington, DC: National Forest Products Association.

Figure 25—A check in an exterior lamination noted

during condition assessment.

Table 2—Reduction in bridge width during the monitoring period

Measured bridge width

(in.) Reduction

October 31,

1991

June 23,

1993

in bridge

width (in.)

East abutment 334.2 334.0 0.2

Midspan 331.8 330.2 1.6

West abutment 334.8 333.8 1.0

Figure 26—Distorted exterior lumber lamination

noted during condition assessment at the time of

load test 2.
18

Oliva, Michael G.; Dimakis, Al G.; Ritter, Michael A.;

Tuomi, Roger L. 1990. Stress-laminated wood bridge

decks: experimental and analytical evaluations. Res. Pap.

FPL–RP–495. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 24 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, M.A.; Geske, E.A.; Mason, L. [and others]. 1990.

Performance of stress-laminated bridges. Wood Design Focus. 1(3): 12–16.

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;

London, England.

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

Rimal. 1995. 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.

USDA. 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.
19

 Appendix—Information Sheet

General

Name: Sanborn Brook bridge

Location: Kellys Corner Road,

10 miles northeast of Concord, New Hampshire

Date of Construction: August 1991

Owner: State of New Hampshire

Design Configuration

Structure Type: Stress-laminated deck with butt joints

Butt Joint Frequency: 1 in 4 laminations transverse with

joints in adjacent laminations

separated 4 ft longitudinally

Total Length (out–out): 25 ft

Skew: 14 degrees

Number of Spans: 1

Span Length (center–center bearings): 24 ft

Width (out–out): 28 ft

Width (curb–curb): 26 ft

Number of Traffic Lanes: 2

Design Loading: HS 25–44

Wearing Surface Type: Asphalt

Material and Configuration

Timber:

Species: Southern Pine

Size (actual): 1.5 in. to 2 in. wide; 14 in. deep

Grade: No. 2 and better visually graded

Moisture Condition: Approximately 23 percent at

installation

Preservative Treatment: Pentachlorophenol in heavy oil

Stressing Bars:

Type: High strength steel threaded bar with coarse

right-hand thread, conforming to ASTM A 722

Diameter: 1 in.

Number: 6 transversing entire deck width, 4 partially

imbedded within deck

Design Force: 55,000 lb

Spacing: 39.5 in. average center–center

Anchorage Type and Configuration:

Steel Plates: 12 by 14 by 0.75 in. bearing

 4 by 6.5 by 1.25 in. anchor