United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

National Wood

in Transportation

Information Center

Research

Paper

FPL–RP–570

In cooperation

with the

United States

Department of

Transportation

Federal

Highway

Administration

Field Performance of

Timber Bridges

16. North Siwell Road

Stress-Laminated Bridge

James A. Kainz
Abstract

The North Siwell Road bridge was constructed during December 1994 in Hinds County, Mississippi. The bridge is a

single-span, stress-laminated T-beam structure that measures

9.1 m (30 ft) long and 8.7 m (28.5 ft) wide. Performance of

the bridge was monitored for 24 months, beginning at the

time of installation. Monitoring involved gathering and

evaluating data relative to the moisture content of the wood

components, force level of stressing bars, and 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.

Keywords: Wood, bridge, stress-laminated T, timber,

performance

Acknowledgments

We thank the following from the Hinds County Department

of Public Works for assistance with field data collection, load

tests, and visual inspection of the North Siwell Road bridge:

Wayne Williamson, Vonda Moore, and the bridge crew.

In addition, we express sincere appreciation to the following

from the USDA Forest Service, Forest Products Laboratory:

Mike Ritter, James Wacker, Paula Hilbrich Lee, and Lola

Hislop for assisting in the field evaluation, data analysis,

and report preparation; Vyto Malinauskas for fabricating

load cells; and the Information Services Team for assistance

in preparing this report.

March 1998

Kainz, James A. 1998. Field performance of timber bridges: 16. North

Siwell Road stress-laminated bridge. Res. Pap. FPL–RP–570. Madison, WI:

U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 17 p.

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

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

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

libraries in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the

University of Wisconsin.

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

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

service.

The United States Department of Agriculture (USDA) prohibits discrimination in 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

require alternative means for communication of program information

(braille, large print, audiotape, etc.) should contact the USDA’s TARGET

Center at (202) 720–2600 (voice and TDD). To file a complaint, write the

Secretary of Agriculture, U.S. Department of Agriculture, Washington,

DC 20250, or call 1–800–245–6340 (voice), or (202) 720–1127 (TDD).

USDA is an equal employment opportunity employer.

Contents

Page

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

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

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

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

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

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

Assembly............................................................4

Installation ..........................................................4

Cost ......................................................................5

Evaluation Methodology...............................................5

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

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

Load Test Behavior..................................................7

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

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

Predicted Deflection Analysis..................................9

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

Results and Discussion.................................................9

Moisture Content.....................................................9

Bar Force..............................................................10

Load Test Behavior................................................10

Load Test 1.......................................................10

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

Load Test Comparison ........................................12

Predicted Deflection Analysis................................12

Condition Assessment ............................................12

Bridge Geometry ................................................12

Wood Condition ................................................14

Preservative Treatment.........................................14

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

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

Conclusions .............................................................14

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

Appendix—Information Sheet......................................17
Field Performance of Timber Bridges

16. North Siwell Road Stress-Laminated Bridge

James A. Kainz, General Engineer

Forest Products Laboratory, Madison, Wisconsin

Introduction

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

Timber Bridge Initiative (TBI) (USDA 1995). As part of an

effort to revitalize rural America, the legislation established a

national program emphasizing wood as a structural material

for highway bridges. Recently, the emphasis of this program

has shifted to include all timber transportation structures,

such as sound barriers and bridges.

Responsibility for the development, implementation, and

administration of the TBI program was assigned to the

USDA Forest Service. The Forest Service established three

primary program areas under the TBI: demonstration structures, technology transfer, and research. As part of the demonstration bridge program, the National Wood in Transportation Information Center (NWITIC) in Morgantown, West

Virginia, awards annual grants for demonstration bridges on

a competitive basis. Funds are awarded for design and construction of demonstration timber bridges with innovative

designs and those that utilize locally available underutilized

wood species. The NWITIC also maintains a technology

transfer program to provide assistance and state-of-the-art

information related to all aspects of timber bridges.

Responsibility for the research portion of the TBI program

was assigned to the USDA Forest Service, Forest Products

Laboratory (FPL). The FPL has established a broad research

program to conduct a variety of timber bridge studies under

laboratory and field conditions, including a nationwide

bridge monitoring program. Through the bridge monitoring

program, FPL is able 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 timber bridge

design, fabrication, and construction.

This report is 16th in a series of reports that documents the

field performance of timber bridges. It describes the development, design, construction, and field performance of the

North Siwell Road bridge located in Hinds County, Mississippi. The structure is a 9.1-m- (30-ft-) long, double-lane,

stress-laminated T-beam bridge. Built in 1994, the North

Siwell Road bridge was funded jointly by a competitive

grant from the NWITIC and matching funds from Hinds

County, Mississippi. An information sheet on the characteristics of the North Siwell Road bridge is provided in the

Appendix.

Background

The North Siwell Road bridge is located in Hinds County,

approximately 7 miles southwest of Jackson, Mississippi

(Fig. 1). The bridge is owned by Hinds County and is

located on a secondary road that provides access to local

residences and schools. In 1994, the average daily traffic over

this section of the road was 6,520 vehicles. Estimates by

Hinds County Department of Public Works indicate that the

average daily traffic is expected to increase to approximately

9,300 vehicles by the year 2012. The majority of this traffic

consists of passenger vehicles and school buses.

The original North Siwell Road bridge was 7 m (23 ft) wide

and 6.1 m (20 ft) long and consisted of prefabricated concrete

deck panels on timber caps and piles. Based on an annual

inspection in 1992, it was determined that the wood components were deteriorating and the concrete deck was in need of

repair. Therefore, replacement of the bridge was deemed

necessary to provide adequate access for school traffic, emergency vehicles, and commuters.

In December 1992, the Hinds County Department of Public

Works submitted a proposal for partial funding of the North

Siwell Road bridge replacement as a timber bridge under the

NWITIC demonstration program. As part of this proposal, a

preliminary bridge design was developed for a timber stresslaminated T-beam bridge. Southern Pine was selected as the

primary material for the bridge because of economics and as a

locally grown species in Mississippi. After review by a

selection panel, funds were awarded and final design of the

replacement bridge was initiated. Hinds County had little

experience working with timber bridges; therefore, FPL was

contacted during the final design phase of the project to

evaluate the performance of the new North Siwell Road

timber bridge. As a result, personnel from FPL and Hinds

County Department of Public Works developed a performance monitoring plan that was initiated at installation.
2

Objective and Scope

The objective of this project was to evaluate the field performance of the North Siwell Road bridge for 24 months,

beginning at bridge installation. The scope of the project

included data collection and analysis related to the wood

moisture content, stressing bar force, bridge behavior under

static truck loading, and general structure performance. The

results of this project will be considered with the results of

similar monitoring projects to improve design and construction for future stress-laminated T-beam bridges.

Design, Construction, and Cost

Design and construction of the North Siwell Road bridge

were completed by several agencies and individuals. An

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

superstructure follows.

Design

A typical stress-laminated T-beam bridge consists of gluedlaminated timber (glulam) webs and sawn lumber flanges

that are connected by tensioned, high strength steel stressing

elements (Fig. 2). On the North Siwell Road bridge, an

American Association of State Highway and Transportation

Officials (AASHTO) approved method for designing a stresslaminated T-beam structure was not available, so design

criteria for the stress-laminated T-beam bridge was based on

guidelines developed from research conducted at West

Virginia University (Davalos and Salim 1993). All other

aspects of design including loading, wood treatment, and

wood strength were designed in accordance with the

AASHTO Standard Specifications for Highway Bridges

for two lanes of HS20-44 truck loading (AASHTO 1992).

Design and fabrication of the North Siwell Road bridge were

completed by contract. The design geometry provided for a

single span, simply supported structure, 9.1 m (30 ft) long

and 8.7 m (28.5 ft) wide (Fig. 3). The new bridge was designed to be 3.1 m (10 ft) longer than the original bridge,

because the increased span length allows additional hydraulic

flow capacity that is required during periods of high water.

The North Siwell Road bridge configuration consists of

Southern Pine glulam lumber webs and sawn lumber flanges.

The glulam webs were 171.5 mm (6-3/4 in.) wide and

593.7 mm (23-3/8 in.) deep. Glulam design was based on

material properties for combination 24F–V3 SP/SP

(AASHTO 1992). Tabulated design values were 16.5 MPa

(2,400 lb/in2

) for bending strength, 12.4 GPa

(1,800,000 lb/in2) for modulus of elasticity (MOE), 1.4 MPa

(200 lb/in2) for shear strength, and 3.9 MPa (560 lb/in2) for

compression strength perpendicular to grain. All design

values were adjusted by appropriate wet-use factors per

AASHTO requirements.

The sawn lumber flanges were constructed with standard

38- by 184-mm (nominal 2- by 8-in.) material that was

surfaced on four sides to provide uniform contact between

laminations. Butt joints were used in the flanges because

Figure 1—Location maps for the North Siwell Road

bridge.
3

material was not available in the full 9.1-m (30-ft) length.

The butt joints were specified at an interval of one butt joint

every four adjacent laminations, longitudinally spaced at

1.2 m (4 ft) (Fig. 4). The design values for the flange were

based on AASHTO specifications for Southern Pine lumber

visually graded No. 1 in accordance with Southern Pine

Inspection Bureau rules (SPIB 1993). The tabulated design

values were 10.3 MPa (1,500 lb/in2

) for bending, 11.7 GPa

(1,700,000 lb/in2) for MOE, 620 kPa (90 lb/in2) for shear,

and 3.9 MPa (565 lb/in2) compression perpendicular to

grain. All design values were adjusted by the appropriate

wet-use factors, and laminations were specified to be at or

below 19% moisture content prior to preservative treatment

and bridge installation.

The stressing system used ten 15.9-mm- (5/8-in.-) diameter

high strength steel bars that complied with the requirements

of ASTM A722 (ASTM 1988) and provided a minimum

ultimate tensile strength of 1.03 GPa (150,000 lb/in2). The

bars were inserted through oversized, predrilled holes located

at the center of the sawn lumber flange and 92 mm (3-5/8 in.)

from the top of the glulam webs (Fig. 3). The bars were

spaced 914 mm (36 in.) on center, starting 457 mm (18 in.)

from the ends of the bridge. Each bar required a tensile force

of 156 kN (35,000 lb) to provide 924 kPa (134 lb/in2) compression stress between the laminations. The value for interlaminar compression was based on the West Virginia

University design method in which the recommended design

interlaminar stress is 2.5 times the minimum interlaminar

stress of 365 kPa (53 lb/in2). The effect of this initial prestress is examined in detail in the following section on bar

force. The bar tension was transferred into the deck using a

discrete plate anchorage system, consisting of 184- by 394-

by 25.4-mm (7.25- by 15.5- by 1-in.) bearing plates and a

50.8- by 127- by 25.4-mm (2- by 5- by 1-in.) anchor plate

with a hexagonal nut (Fig. 5).

Design of the bridge rail and curb system was based on a

crash-tested system for longitudinal spike-laminated decks

(FHWA 1990). The bridge rail and curb consisted of a 267-

by 152-mm (10.6- by 6-in.) sawn lumber timber rail and a

140- by 292-mm (5.5- by 11.5-in.) sawn lumber curb with

140- by 292-mm (5.5- by 11.5-in.) sawn lumber scupper

blocks. The rail and curb were attached to six, 1,370- by

190- by 305-mm (54- by 7.5- by 12-in.) sawn lumber posts

per side.

For protection from deterioration, all steel components,

including stressing hardware, stressing bars, and anchorage

plates, were galvanized in accordance with AASHTO M232

(AASHTO 1992). All wood components were preservative

treated with creosote in accordance with American Wood

Preservers' Association standard C14 (AWPA 1990). No

asphalt wearing surface was specified for the bridge.

Figure 2—Typical configuration for a

stress-laminated T system.

Figure 3—Design configuration of the North Siwell

Road bridge.
4

Construction

Construction of the North Siwell Road bridge was completed

in late 1994. Construction extended over several months and

included assembly that was completed by the bridge manufacturer and installation that was completed by Hinds County

personnel.

Assembly

The assembly process began with manufacturing the glulam

beams and drying and surfacing of the sawn lumber flange

material. Following manufacturing and drying, all sawn

lumber and glulam members were prefabricated (cut and

drilled) and pressure treated with creosote preservative. After

preservative treatment, the treated wood was fabricated into

sections to facilitate shipping and installation. These sections were made by nailing flange pieces to the glulam webs

to form either an L or T module (Fig. 6). The L module was

used on the exterior of the bridge, and the T configuration

was used for the interior. After assembly, the L and T

sections were loaded on a flatbed truck and shipped to the

bridge site.

Installation

After demolition and removal of the existing bridge superstructure, a new substructure was installed, which consisted

of timber piling, wing walls, and caps. The new substructure

was installed behind the existing substructure to provide

additional erosion protection for the new substructure.

Approximately 0.5 to 0.7 m (2 to 3 ft) from the top, the

existing substructure was removed to provide clearance for

the new bridge.

On December 20, 1994, installation of the new superstructure

commenced by lifting one assembled L section into place on

the abutments with an overhead crane. After several T sections were in place next to the L section, the high strength

Figure 4—Butt-joint configuration used with Southern

Pine sawn lumber flange laminations on the North

Siwell Road bridge. A butt joint was placed transverse

to the span in every fourth lamination. Longitudinally,

butt joints in adjacent laminations were separated by

1.2 m (4 ft).

Figure 5—Discrete plate anchorage configuration

consisting of a bearing plate, anchor plate, and

hexagonal nut.

Figure 6—Assembled L and T sections used

for transportation to bridge site and ease of

installation.
5

steel bars were inserted through predrilled holes in the flange

and webs. All sections were placed in a similar manner until

the final L section was placed on the opposite end. After all

sections were placed and the stressing bars were fully inserted, the steel bearing and anchorage plates were installed.

A single hydraulic pump and jack was used to partially

tension the steel stressing bars to bring the T and L sections

into contact. After all sections were in contact, full design bar

force was then introduced into the bridge. The Hinds County

bridge crew began at one end and tensioned each bar along

the length of the bridge (Fig. 7). After all bars were fully

tensioned, the tensioning process was repeated to ensure that

the interlaminar compression level was uniform and at the

required design level. Seven days after this initial tensioning,

a second design tensioning was introduced into the steel

bars. The third tensioning was completed January 24, 1995,

approximately 5 weeks after the initial stressing.

After the second bar tensioning, the bridge was attached to

the substructure by connecting each glulam web to the timber cap with steel angles bolted to the webs and abutment

cap (Fig. 8). At this time, the timber curb and rail system

was also installed (Fig. 9). When the rail system was complete, a tack coat of asphalt and coarse rock was applied as a

wearing surface and the bridge was opened for traffic. The

bridge was posted with a reduced speed limit because the

approach roadway was very rough. The completed bridge is

shown in Figure 10.

Cost

The total cost of the North Siwell Road bridge superstructure

was $46,190, which included design, fabrication, materials,

and construction. To compare the cost of this bridge with

others, the total cost of the bridge is divided by the total

deck area to ascertain a cost per unit area. This equates to

approximately $582/m2 ($54/ft2) for the North Siwell Road

bridge.

Evaluation Methodology

To evaluate the structural and serviceability performance of

the North Siwell Road bridge, Hinds County personnel

contacted the FPL for assistance. Through a cooperative

agreement with Hinds County, FPL, and Federal Highway

Administration (FHWA), a 2-year bridge monitoring plan

was developed and implemented. The plan included performance monitoring of the deck moisture content, stressing bar

force, static load test behavior, and general bridge condition.

The evaluation methodology employed procedures and

equipment previously developed by FPL and used on other

similar structures (Ritter and others 1991).

Figure 7—Bar tensioning using a single hydraulic

pump and jack.

Figure 8—Bridge attachment to the sawn lumber

substructure with steel angle saddles.

Figure 9—Attachment of bridge curb and rail system

following the second bar tensioning.
6

Figure 10—Completed North Siwell Road bridge: (top) side view, (bottom) end view.
7

Moisture Content

To characterize changes in moisture content, an electricalresistance moisture meter was used to obtain wood moisture

content readings on a quarterly basis. Moisture meter measurements were taken by Hinds County personnel from the

bottom of glulam beams and the underside of the sawn lumber flanges and were assumed to be representative of the

overall moisture content of the bridge. Measurements were

obtained in accordance with ASTM D 4444–84 (ASTM

1992) by driving the insulated moisture pins into the

underside of the bridge at depths of 50 to 75 mm (2 to 3 in.),

recording the moisture content value from the unit, then

adjusting the moisture content value for temperature and

wood species.

Bar Force

To monitor stressing bar force, two calibrated load cells were

placed on the stressing bars just prior to the third stressing.

These cells were placed between the bearing and anchorage

plates to monitor the bar forces based on the strain variations

in the load cell. On a monthly basis, load cell measurements

were obtained by Hinds County personnel with a portable

strain indicator. The measurements were then converted to

force, based on laboratory load cell calibrations, to determine

the tensile force in the bar. At the conclusion of the monitoring period, the load cells were removed, adjusted for zero

balance shift, and re-calibrated in the laboratory. In addition,

hydraulic stressing equipment was used at the site visits to

verify bar force levels obtained from the load cells.

Load Test Behavior

To determine the live-load behavior of the bridge under

vehicle loading, two static load tests were conducted during

the monitoring period. The first load test was completed

immediately following the third stressing on January 24,

1995. The second load test was completed January 28, 1997,

2 years after the first load test.

The static load test consisted of positioning one or two fully

loaded trucks on the bridge, then measuring the resulting

deflections along the transverse centerline of the bridge.

Deflection measurements from an unloaded to loaded condition were obtained by hanging calibrated rules on the underside of the deck and reading values with a surveyor’s level.

The accuracy of this method for repetitive readings is

estimated to be ±1 mm (±0.04 in.).

For both load tests, the trucks were positioned for six transverse load positions: three centric and three eccentric

(Fig. 11). The first and second load positions placed one

vehicle on the bridge near the longitudinal centerline in

opposite lanes. The third load position placed both vehicles

on the bridge in the same positions as load positions 1 and

2. The fourth and fifth load positions placed one vehicle on

the bridge near each rail. The sixth load position placed both

vehicles on the bridge in the same positions as load positions 4 and 5 (Fig. 12).

Figure 11—Transverse load positions (looking north)

used for all static load tests on the North Siwell Road

bridge.
8

Figure 12—Transverse load test positions used for both load tests: (a) load position 1, 0.6 m upstream of longitudinal

center line; (b) load position 2, 0.6 m downstream of longitudinal center line; (c) load position 3, both trucks in same

locations as load positions 1 and 2; (d) load position 4, 1.5 m upstream of longitudinal center line;

(e) load position 5, 1.5 m downstream of longitudinal centerline; (f) load position 6, both trucks in same locations

as load positions 4 and 5.

Load Test 1

The test vehicles were fully loaded, three-axle dump trucks

with gross vehicle weights of 268.8 kN (60,440 lb) for truck

1A and 273.9 kN (61,570 lb) for truck 1B (Fig. 13). The

vehicles were positioned longitudinally with the transverse

centerline of the bridge, bisecting the tandem rear axles and

the front axles off the bridge. Both trucks faced north for all

load positions. Data points were positioned along the transverse centerline at each glulam web.
9

Load Test 2

The test vehicles were fully loaded, three-axle dump trucks

with gross vehicle weights of 233.2 kN (51,950 lb) for truck

2A and 233.5 kN (52,490 lb) for truck 2B (Fig. 14). The

vehicles were positioned longitudinally with the transverse

centerline of the bridge, bisecting the tandem rear axles and

the front axles off the bridge. Both trucks faced north for all

load positions. Data points were positioned along the transverse centerline at each glulam web.

Predicted Deflection Analysis

At the conclusion of load testing, predicted deflections were

calculated for AASHTO HS20–44 loading. The procedure

was based on the measured load test deflection and a ratio of

deflection coefficients (DCs) as determined through computer

analysis (Murphy 1994). The following relationship was

established to find the predicted deflection under HS20

loading:

 

∆ ∆ HS20 = 







 Load test 

DC

DC

HS20

Load test

where

∆HS20 is HS20 predicted deflection (mm),

∆Load test maximum measured load test deflection (mm),

DCHS20 HS20 deflection coefficient (kN⋅m4), and

DCLoad test load test vehicle deflection coefficient (kN⋅m4).

Condition Assessment

The general condition of the bridge was assessed at the time

of the two load tests. The assessments involved visual inspections, measurements, and photograph documentation of

the bridge. Items of specific interest included geometry,

wood condition, wearing surface, and stressing system.

Results and Discussion

Performance monitoring of the North Siwell Road bridge

extended for 24 months, beginning in January 1995. Results

and discussion of the performance data follow.

Moisture Content

The average trend in electrical resistance moisture content

readings is shown in Figure 15. As shown, there are slight

variations between the readings for the glulam webs and the

sawn lumber flanges. At the beginning of the monitoring

period, the glulam web moisture content was approximately

12.5%. During the monitoring period, the moisture content

exhibited seasonal changes of ±5%. At the end of monitoring, the moisture content was approximately 15%. The sawn

lumber flange moisture content was slightly higher at the

start of monitoring at approximately 17%. During the first

year of the monitoring, the sawn lumber flange moisture

content varied similarly at ±5%. Readings were not taken

during the second half of the monitoring period except during

the final quarter of 1996. Near the end of the monitoring

period, the sawn lumber flange moisture content was approximately 12.5%. Both the glulam webs and the sawn

lumber flange were installed at a moisture content less than

the recommended value of 19%.

Figure 13—Load test truck configurations and

axle loads for load test 1. All vehicle track

widths, measured center–center of the rear

tires, were 1.8 m (6 ft).

Figure 14—Load test truck configurations and

axle loads for load test 2. All vehicle track

widths, measured center–center of the rear

tires, were 1.8 m (6 ft).
10

Bar Force

The average bar force based on load cell readings is shown in

Figure 16. At the third stressing, all bars were tensioned to

142 kN (32,000 lb), which corresponds to a 848 kPa

(123 lb/in2) interlaminar stress. Two months later, the bar

force decreased to 98.2 kN (22,100 lb) or 579 kPa

(84 lb/in2). The rate of loss declined, and the bar force then

remained relatively stable for the remainder of the monitoring

period. At the conclusion of monitoring, the average bar force

was 87.6 kN (19,700 lb), which corresponds to a level of

490 kPa (71 lb/in2) interlaminar compression stress.

The observed bar force loss is most likely the result of

stress relaxation in the sawn lumber flange laminations.

Stress relaxation is a time-dependent phenomenon caused

by the long-term compressive force of the steel bars acting

on the wood microstructure. In the design of stress-laminated

bridges, it is assumed that the bridges will lose approximately 60% of the interlaminar compression over the life of

the structure as a result of stress relaxation. The stress relaxation behavior of this bridge was similar to previous monitored bridges (Wacker and Ritter 1995; Ritter and others

1995, 1996). However, the amount of stress relaxation in the

North Siwell Road bridge was affected by the wood moisture

content. Recent monitoring completed on a bridge installed

with a moisture content greater than 19% (Kainz and others

1996) showed that the bar force level decreased at a higher

rate than did the North Siwell Road bridge where the wood

moisture content was less than 19% at installation.

The amount of initial interlaminar compression stress can

also affect the bar force performance. In this case, 47% of

initial interlaminar compression stress was lost during the

monitoring period, which is similar to other bridges. The

initial interlaminar compression stress level was higher on

the North Siwell Road bridge than other typical stresslaminated bridges. Thus, the final interlaminar compression

stress level remained within acceptable levels.

Stress-laminated T-beam bridges can experience slip between

the web and flange when the interlaminar compression drops

below 345 kPa (50 lb/in2

). The final interlaminar compression stress of 490 kPa (71 lb/in2

) for the North Siwell Road

bridge is well above this level. The North Siwell Road

bridge has performed well during the 2 years of monitoring;

therefore, it is assumed that with standard periodic maintenance (i.e., bar force checks, routine inspections), the bar

force level should remain acceptable.

Load Test Behavior

In this section, results of the static load tests and analytical

assessment of the North Siwell Road bridge are presented.

For each load position, transverse deflection measurements

are given at the transverse mid-span as viewed from the east

end (looking west). No permanent residual deformation was

measured at the conclusion of load testing, and there was no

detectable movement at bridge supports. At the time of load

test 1, the interlaminar compression stress was approximately 841 kPa (122 lb/in2

). For load test 2, the interlaminar

compression stress was approximately 490 kPa (71 lb/in2).

Load Test 1

Transverse deflections for load test 1 with the locations and

magnitudes of the maximum measured deflections are shown

in Figure 17. The maximum measured deflection was 9 mm

(0.35 in.) for load position 1, 8.5 mm (0.34 in.) for load

position 2, and 11.5 mm (0.45 in.) for load position 3. For

the eccentric load positions, the maximum measured deflection was 8.5 mm (0.34 in.) for load positions 4 and 6 and

8.3 mm (0.33 in.) for load position 5. As shown in Figure 17, the maximum measured deflections occurred near the

wheel lines of the test vehicles. These maximum measured

deflections are symmetrical between similar load positions,

such as load positions 1 and 2 and load positions 4 and 5.

For the load positions with two vehicles, the deflection from

load position 3 was greater than all other measured deflections, and the smaller deflection from load position 6 was

similar to load positions 1, 2, 4, and 5. This was due to the

transverse stiffness of the deck that was created by the interlaminar compression stress.

Figure 15—Average trend in electrical

resistance moisture content readings.

Figure 16—Average bar force from load

cells installed after the third bar tensioning.
11

Figure 17—Transverse deflections measured at mid-span (looking north) for load test 1. Bridge cross-sections and

vehicle positions are presented to aid interpretation and are not to scale.
12

Assuming uniform material properties, symmetric loading,

and accurate deflection measurements, the summation of

bridge deflections from two single-truck load positions

should equal the deflections from the load position with both

trucks. Figure 18 displays the summation of load positions

1 and 2 overlaid on load position 3. As shown, there are

slight deflection variations near the longitudinal centerline,

but the deflections are essentially the same.

Load Test 2

Load test 2 transverse deflections are shown in Figure 19.

The maximum measured deflection was 7.4 mm (0.29 in.)

for load position 1, 7.3 mm (0.29 in.) for load position 2,

and 9.3 mm (0.37 in.) for load position 3. The maximum

measured deflections for load positions 4 through 6 were

8.3 mm (0.33 in.) for load position 4, 7.2 mm (0.28 in.) for

load position 5, and 7.6 mm (0.30 in.) for load position 6.

Observations from load test 1 were also made for load test 2.

The maximum measured deflections occurred near the wheel

lines of the test vehicles. These maximum measured deflections were symmetrical between similar load positions, such

as load positions 1 and 2 and load positions 4 and 5. For the

load positions with two vehicles, the deflection from load

position 3 was greater than all other measured deflections,

and the smaller deflection from load position 6 was similar

to load positions 1, 2, 4, and 5. Again, this was due to the

transverse stiffness of the deck that was created by the interlaminar compression stress.

As for load test 1, the summation of bridge deflections for

two single truck load positions should equal the deflections

from the load position with both trucks. Figure 20 displays

the summation of load positions 1 and 2 overlaid on load

position 3 for load test 2. Aside from slight deflection variations, the deflections are essentially the same.

Load Test Comparison

An examination of the results from load tests 1 and 2 revealed similar maximum deflection locations for each load

position (Fig. 21). As shown, the second load test exhibited

smaller deflections than the first load test. The decreased

deflections were due to 15% lighter loading on the second

load test. If the deflections for load test 2 were factored to

account for the 15% load reduction, they would appear very

similar to the deflections for load test 1.

On most stress-laminated bridges with butt joints, there is a

correlation between the transverse stiffness of the bridge and

the level of interlaminar compression stress. It has been

observed on several stress-laminated bridges that the transverse stiffness decreases when the level of interlaminar compression in the deck decreases (Wacker 1996). The change in

interlaminar compression seemed to have little effect on the

transverse stiffness of the North Siwell Road bridge.

Predicted Deflection Analysis

To compare the North Siwell Road bridge with other

bridges, a theoretical deflection based on a standard HS20–44

truck was determined. Using the method previously described, maximum deflections of 11 mm (0.43 in.) for load

test 1 and 10.4 mm (0.41 in.) for load test 2 were determined

for load position 3. These deflections correspond to L/809 for

load test 1 and L/849 for load test 2, which are significantly

less than the minimum design deflection criteria of L/500

and the actual design deflection of L/579.

Condition Assessment

Condition assessments of the North Siwell Road bridge

indicated that structural performance and serviceability were

good. Inspection results for specific items follow.

Bridge Geometry

Measurements obtained during site inspections reveal that

the bridge was slightly narrower at mid-span than at the

abutments. This behavior, commonly called “hour glassing”

is a result of reduction of lamination size caused by compressive deformation (creep) over time at mid-span and resistance

to movement provided by the attachment of the webs at the

abutments. Hour glassing does not affect the structural performance of the bridge but can have an adverse aesthetic

effect, if severe. The hour glassing on the North Siwell Road

bridge is minor and generally not noticeable.

Figure 18—Comparison of measured deflections for

load test 1, showing the actual deflection of load

position 3 and the sum of load positions 1 and 2

(looking north).
13

Figure 19—Transverse deflection measured at mid-span (looking north) for load test 2. Bridge cross-sections and

vehicle positions are presented to aid interpretation and are not to scale.
14

Wood Condition

Inspection of the wood components of the bridge showed no

sign of deterioration. Minor checking was evident on the end

grain of the rail posts exposed to rapid wet/dry cycles. This

checking could have been alleviated through the use of a

sealer on the post end grain.

Preservative Treatment

On the tension side of several webs, the preservative treatment appeared to be missing or spotty (Fig. 22). This may

be a result of banding blocks left in place during the preservative treatment process. The flange and web members exhibited no signs of excess preservative treatment and no leaching

was observed. However, excess preservative has migrated to

the surface of the Douglas Fir posts and rails.

Wearing Surface

The approach roadways were completed approximately

6 months after bridge installation by applying a new wearing

surface that was feathered over abutments but was not extended across the length of the bridge. Although there appears to be no cracks and degradation of the original tack coat

wearing surface, the surface allows water to penetrate the

flange and upper portions of the webs. During the final site

visit, water was observed flowing through the butt joints

while it was raining. The increased water in the laminations

could lead to premature deterioration in these areas.

Anchorage System

The stressing bar anchorage system has performed adequately. No measurable distortion in the bearing plates was

observed, and the exposed galvanized steel stressing bars,

hardware, and anchorage plates showed no visible sign of

corrosion or other deterioration. There was no apparent

crushing of the bearing plates into the Southern Pine glulam

exterior laminations.

Conclusions

After 24 months in service, the North Siwell Road bridge is

performing satisfactorily. Based on extensive bridge monitoring during this period, we make the following observations

and recommendations:

• The glulam and sawn lumber components of the North

Siwell Road bridge were initially installed at 12.5% and

17% moisture content, respectively. The average trend in

moisture content indicates that the glulam webs and sawn

lumber flanges have experienced little variation during the

monitoring period but had some fluctuation caused by

seasonal change.

Following the final design bar tensioning, the bridge lost

approximately 47% of the bar force introduced to end at

87.6 kN (19,700 lb), which corresponds to 490 kPa

(71 lb/in2) interlaminar compression stress. This decrease

in bar force was most likely attributable to transverse

stress relaxation in the wood laminations. The acceptable

bar force level at the end of monitoring is a result of retardation of the stress relaxation rate caused by the low moisture content in the wood laminations. The introduction of

a high initial bar force level that resulted in a high interlaminar compressive stress also contributed to the acceptable ending bar force performance of this bridge. To maintain this performance, bar force should be checked as part

of the routine inspection program.

• Load testing and analysis indicate that the North Siwell

Road bridge is performing in a linear elastic manner when

subjected to truck loading. The simulated HS20–44 truck

loading conditions produced maximum deflections of

11 mm (0.43 in.) for load test 1 and 10.4 mm (0.41 in.)

for load test 2. The similar deflections for load tests 1 and

2 correspond to approximately L/830 and are based on

center–center bearing lengths of the span. The deflections

are substantially smaller than the target design deflection

of L/500 for this span.

• Wood checking is evident in the exposed end grain of the

rail posts. It is likely this would not have occurred if a

sealer had been placed over the end grain at the time of

construction. A sealer could be applied at this time to

prevent additional damage in the post end grain.

• There are no indications of corrosion on the galvanized

steel stressing bars, hardware, or plates.

Figure 20—Comparison of measured deflections for load

test 2, showing the actual deflection of load position 3

and the sum of load positions 1 and 2 (looking north).
15

Figure 21—Comparison of load tests 1 and 2 at all load positions.
16

• The asphalt tack coat wearing surface provides little or no

moisture protection for the deck. During the second load

test, water was observed flowing through the butt joints.

Direct contact with water could increase the moisture content in these areas and could lead to premature deterioration. It is recommended that the bridge be resurfaced in

the near future to prevent moisture flow onto the timber

members.

• The preservative treatment of the bridge is adequate except

for several areas on the underside of the flange that appear

to be void of preservative treatment. The areas appear to

be on the corners of the T or L sections and are most

likely a result of banding. The small areas should pose no

threat to the preservative envelope as a result of their location on the underside of the bridge. Eventually, these areas

should be field treated with a preservative compound or a

coating.

References

AASHTO. 1992. Standard specifications for highway bridges

15th 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 A722–86. Philadelphia, PA: American Society for Testing and Materials. 3 p.

ASTM. 1992. Use and calibration of hand-held moisture meters;

ASTM D 4444–84. Philadelphia, PA: American Society for

Testing and Materials. 6 p.

AWPA. 1990. Standards. Woodstock, MD. American Wood

Preservers’ Association. 200 p.

Davalos, J.F.; Salim, H.A. 1993. Design of stress-laminated

T-system timber bridges. Morgantown, WV: U.S. Department of

Agriculture, Forest Service, Northeastern Area, Timber Bridge

Information Resource Center.

FHWA. 1990. Memorandum on crash tested bridge railings.

August 13, 1990. File Designation HNG–14. Washington, DC:

U.S. Department of Transportation, Federal Highway

Administration.

Kainz, James A.; Wacker, James P.; Nelson, Martin. 1996.

Field performance of timber bridges: 9. Big Erick’s stresslaminated deck bridge. Res. Pap. FPL–RP–552. Madison, WI:

U.S. Department of Agriculture, Forest Service, Forest Products

Laboratory. 23 p.

Murphy, J. 1994. PC bridge. Bridge analysis program. Version

2.60. Madison, WI.

Ritter, M.A.; E.A. Geske; W.J. McCutcheon;[and others].

1991. Methods for assessing the field performance of stresslaminated 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, M.A.; Wacker, J.P.; Tice, E.D. 1995. Field performance of

timber bridges: 2. Cooper Creek stress-laminated deck bridge.

Res. Pap. FPL–RP–536. Madison, WI: U.S. Department of

Agriculture, Forest Service, Forest Products Laboratory. 17 p.

Ritter, M.A.; Kainz, J.A.; Porter, G.J. 1996. Field performance

of timber bridges: 5. Little Salmon Creek stress-laminated deck

bridge. Res. Pap. FPL–RP–547. Madison, WI: U.S. Department

of Agriculture, Forest Service, Forest Products Laboratory. 15 p.

SPIB. 1993. Standard grading rules for Southern Pine lumber.

Pensacola, FL: Southern Pine Inspection Bureau.

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.

10 p.

Wacker, J.P.; Ritter, M.A. 1995. Field performance of timber

bridges: 4. Graves Crossing stress-laminated deck bridge. FPL–

RP–539. Madison, WI: U.S. Department of Agriculture, Forest

Service, Forest Products Laboratory. 17 p

Figure 22—Area with no preservative penetration,

probably a result of material banding during

treatment.
17

Appendix—Information Sheet

General

Name: North Siwell Road bridge

Location: Hinds County, Mississippi

Date of Construction: December 1994

Owner: Hinds County

Design Configuration

Number of Spans: 1

Structure Type: Stress-laminated T with glulam webs

and sawn lumber butt-jointed flanges

Butt-Joint Configuration: 1 in 4 transverse laminations

with joints in adjacent deck

flange laminations separated

1.2 m (4 ft) longitudinally

Total Length (out–out): 9.1 m (30 ft)

Skew: None

Span Lengths (center–center bearings): 8.5 m (28 ft)

Bearing Lengths: 305 mm (12 in.) full bearing,

no overhang

Width (out–out): 8.8 m (28.7 ft)

Width (curb–curb): 8.2 m (26.8 ft)

Number of Traffic Lanes: 2

Design Loading: AASHTO HS20–44

Wearing Surface Type: Asphalt tack coating; 25.4- to

50.8-mm (1- to 2-in.) thickness

Material and Configuration

Flange Laminations:

Species: Southern Pine sawn lumber

Size: standard 38 by 184 mm (nominal 2 by 8 in.)

Grade: No. 1

Moisture Condition: Approximately 17% at

installation

Webs:

Species: Southern Pine Glulam

Size (actual): 171.5 by 593.7 mm (6.75 by

23.375 in.)

Beam Designation: 24F–V3 SP/SP

Moisture Condition: Approximately 12.5% average

at installation

Rails:

Species: Douglas Fir sawn lumber

Size (actual): 152 by 269 mm (6 by 10.6 in.)

Posts:

Species: Douglas Fir sawn lumber

Size (actual): 191 by 305 mm (7.5 by 12 in.)

Curb and Scupper:

Species: Douglas Fir sawn lumber

Size (actual): 133 by 292 mm (5.25 by 11.5 in.)

Grade: No. 1

Preservative Treatment: Creosote

Stressing Bars:

Type: High strength steel thread bar with coarse righthand thread, conforming to ASTM A 722

Diameter: 15.9 mm (5/8 in.)

Number: 10

Design Force: 155.7 kN (35,000 lb)

Spacing: 914 mm (36 in.) center–center beginning

457 mm (18 in.) from ends of bridge

Anchorage Type and Configuration:

Discrete Plate: Anchor plate and nut: 51 by 127 by

25.4 mm (2 by 5 by 1 in.) plate and

flat hex nut

Bearing plate: 191 by 393.7 by 25.4 mm

(7.5 by 15.5 by 1 in.)