United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

National Wood

in Transportation

Information Center

Research

Paper

FPL–RP–566

Field Performance of

Timber Bridges

15. Pueblo County, Colorado,

Stress-Laminated Deck Bridge

Lola E. Hislop
Abstract

The Pueblo County 204B bridge was constructed in March

1990 in Pueblo, Colorado, as a demonstration bridge under

the USDA Forest Service Timber Bridge Initiative. The

stress-laminated deck superstructure is approximately 10 m

long, 9 m wide, and 406 mm deep, with a skew of 10 degrees. Performance monitoring was conducted for 3 years,

beginning at installation, and involved gathering data on the

moisture content of the wood deck, the force level of the

stressing bars, the behavior of the bridge under static load

conditions, and the overall condition of the structure. In

addition, long-term performance data were gathered on the

force level of the stressing bars 6 years after installation.

Based on monitoring evaluations, the bridge is performing

well, with some crushing of the bearing plates into the outside laminations but no other structural or serviceability

deficiencies.

Keywords: Timber bridge, stress laminated, wood,

performance, Colorado

Acknowledgments

We express sincere appreciation to the following who

assisted in the structural monitoring of the Pueblo County

204B bridge: John Galusha and Keith Smith of the Pueblo

County (Colorado) Highway Department for their assistance

in obtaining field readings and conducting load tests; Earl

Geske of the USDA Forest Service, Forest Products Laboratory, Engineering Mechanics Laboratory, for developing the

load cells used for the bridge evaluation. We also thank the

Information Services Team of the Forest Products Laboratory

for their assistance in preparing this report.

January 1998

Hislop, Lola E. 1998. Field performance of timber bridges—Pueblo

County, Colorado, stress-laminated deck bridge. Res. Pap. FPL–RP–566.

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,

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University of Wisconsin.

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USDA is an equal employment opportunity employer.

Contents

 Page

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

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

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

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

Design ...................................................................1

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

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

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

Bar Force................................................................5

Behavior Under Static Load .......................................5

Static Load Testing...............................................7

Analytical Assessment...........................................7

Condition Assessment ..............................................7

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

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

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

Behavior Under Static Load .....................................10

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

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

Load Tests 1 and 2 Comparison............................12

Analytical Assessment.........................................12

Condition Assessment ............................................14

Deck Camber .....................................................14

Wood Components.............................................14

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

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

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

References................................................................18

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

15. Pueblo County, Colorado, Stress-Laminated Deck Bridge

Lola E. Hislop, Research General Engineer

Forest Products Laboratory, Madison, Wisconsin

Introduction

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

Timber Bridge Initiative (TBI). One purpose of the legislation was to provide partial funding towards 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.

Responsibility for the development, implementation, and

administration of the TBI program was assigned to the

USDA Forest Service. As a 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 employ design innovations that have not been previously evaluated. This report,

fifteenth in a series documenting field performance of timber

bridges, describes the development, design, construction,

and field performance of the Pueblo county bridge, which was

built as a part of the TBI.

The Pueblo County 204B bridge, built in 1990 and located

in Pueblo, Colorado, is a two-lane, single-span, stresslaminated deck with a length of 9.72 m, a width of 9.17 m,

a nominal depth of 406 mm, and a skew of 10 degrees (see

Table 1 for English conversion factors). An information sheet

on the Pueblo County 204B bridge is provided in the

Appendix.

Background

The Pueblo County 204B bridge is located on McCarthy

Boulevard in Pueblo, Colorado (Fig. 1). It crosses the

Bessemer irrigation ditch and is part of a two-lane, paved,

county road that provides residential access. Inspections of

the previous bridge indicated that the components were in

poor condition. In addition, the substandard guardrail system

and the narrow bridge width had become safety issues. Based

on inspection, Pueblo County officials determined that the

bridge should be replaced. A proposal was submitted to the

USDA Forest Service, requesting partial funding of the

bridge as a demonstration project under the TBI demonstration timber bridge program. The design was to incorporate

Ponderosa Pine lumber into a skewed, stress-laminated deck

configuration. In 1989, the project was approved and partial

funding was provided through the TBI.

Objective and Scope

The objective of this project was to evaluate the field performance of the Pueblo County 204B bridge for a minimum

of 2 years, beginning after bridge installation. The project

scope included data collection and analysis of the moisture

content of the wood, stressing bar tension force, bridge behavior under static truck loading, and general structural

performance. The results of this project will be used to formulate recommendations for the design and construction of

similar bridges in the future.

Design and Construction

The design, materials, and construction oversight of the

Pueblo County 204B bridge were provided by the contractor

supplying the timber bridge. Construction labor was provided by the Pueblo County Department of Public Works,

Road, and Bridge Division. An overview of the design and

construction of the bridge are presented.

Design

Although design information was not received from the

contractor, it is likely that the bridge design was based on

the Ontario Highway Bridge Design Code (OMTC 1983).

The bridge was designed for a HS 20–44 truck live load, as

Table 1—Factors for converting SI units of

measurement to English units

SI unit

Conversion

factor English unit

millimeter (mm) 0.0394 inch (in.)

meter (m) 3.281 foot (ft)

square meter (m2

) 11.11 square foot (ft2)

newton (N) 0.2248 pound (lb)

pascal (Pa) 0.000145 lb/in2 (stress)

meter (m) 0.000622 mile
2

recommended by the American Association of State Highway

and Transportation Officials (AASHTO) (AASHTO 1989).

The design geometry of the deck provided for a total 9.72-m

out–out length, a 9.17-m out–out width, and a skew of

10 degrees (Fig. 2). According to the proposal submitted to

the TBI, visually graded No. 1 Ponderosa Pine, 102- by

406-mm (actual size) solid-sawn laminations were specified

for the deck material. It was later discovered that visually

graded No. 1 & Better Douglas Fir–Larch laminations were

used.

The stress-laminated deck was designed with butt joints.

The butt joint configuration consisted of one butt joint in

every fourth lamination in the transverse direction, with a

1.22 m longitudinal separation between butt joints in adjacent laminations. To provide the design interlaminar compression of 717 kPa, 25-mm-diameter, high strength stressing bars were spaced 1,219 mm on center, beginning

610 mm from the end and tensioned to 355.84 kN. The bars

were specified to comply with the requirements of the American Society for Testing and Materials (ASTM) A 722

(ASTM 1988) and provide a minimum ultimate tensile

strength of 1.03 GPa. The bar anchorage system was designed using discrete steel plates, which consisted of

25- by 305- by 406-mm bearing plates with 25- by 102- by

152-mm anchor plates and flat hex nuts (Fig. 3). As a result

of the skew of the bridge, the length of the first bar at each

end was only half the width of the bridge with one bar end

anchored in the interior of the deck (Figs. 2, 4).

Figure 1—Location of Pueblo County 204B bridge.

Figure 2—Design geometry of the Pueblo County

204B bridge.
3

The deck was provided with a curb and guardrail system,

consisting of solid-sawn lumber curbs and posts and gluedlaminated (glulam) timber railing and approach rails. The

railing design was very similar to a design that was subsequently crash tested to the requirements for performance

level 1 as defined by AASHTO. A 51- to 76-mm asphalt

wearing surface was specified.

Following fabrication, all wood components were specified

to be pressure treated with a creosote–coal tar 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 hardware, bars, plates, and nuts.

Construction

Construction of the Pueblo County 204B bridge was completed in March 1990. The superstructure was delivered in

four preassembled sections of laminations, interconnected

with steel spikes and banded with steel straps (Fig. 5). The

delivered sections were placed on the abutments, one at a

time. Looking at the ends of each section, it became apparent

that many laminations were not cut to length for the skew.

Longitudinal alignment of each deck section was accomplished by a few laminations correctly cut to length (Fig. 6a)

or nailing very short pieces to the ends of the lamination

(Fig. 6b). It is unknown if this affected the bearing area

between the deck and the abutments.

After placement of a deck section, the bars were pushed

through the entire section and the steel straps were removed

prior to placing the next section. This proved to be a difficult

method of construction. The free ends of the full-length bars

needed to be suspended; otherwise, the bars deformed and

became difficult to push through the deck. In addition, it was

difficult to line up the holes in the laminations between

sections. Near the end of construction, a sledge hammer was

used to drive the bars through, with care taken not to damage

the ends of the bar (Fig. 7). Stressing began after all sections

were in place and was performed using an electric pump and

Figure 3—Details of the discrete plate bar

anchorage system.

Figure 4—Bar anchorage for interior bar ends

as a result of skew.

Figure 5—Bridge superstructure delivered in four

preconstructed sections banded with steel straps.
4

hydraulic jack with a built-in ratchet (Fig. 8). All bars were

tensioned to the full design force, sequentially, beginning at

one end of the deck. This sequential bar tensioning was

repeated once. The recommended number of repetitions

during each deck stressing is 4 to 6 to ensure that the full

design force is retained in all the bars. Figure 9 shows the

bridge after the initial deck stressing.

Following the initial stressing, the timber curb, guardrail,

and approach rail were constructed. On April 2, April 19,

and May 14, the deck of the bridge was restressed to compensate for losses in bar tension force (Ritter 1990), which

completed the construction of the deck. The asphalt wearing

surface was applied between the second and third stressings.

The completed bridge is shown in Figure 10.

Figure 6—Two options for obtaining longitudinal

alignment: (top) a few laminations correctly cut to

length or (bottom) nail-connected short pieces to

ends of laminations.

Figure 7—Stressing bars were hammered through

the final deck section.

Figure 8—Hydraulic jack with built-in ratchet (top)

and electric pump (bottom).
5

Evaluation Methodology

As a result of the unique characteristics of the bridge being

skewed and the original plan for the bridge to be constructed

from Ponderosa Pine, Pueblo County representatives contacted FPL for assistance in evaluating the structural performance of the Pueblo County 204B bridge. Through mutual

agreement, a bridge monitoring plan was developed by FPL

and implemented as a cooperative research effort with Pueblo

County. The plan called for performance monitoring of the

moisture content, stressing bar force, static load behavior,

and condition assessments of the structure for a minimum of

2 years following installation. At the initiation of field monitoring, FPL representatives visited the bridge site to install

instrumentation and train Pueblo County personnel in moisture content and bar force data collection procedures. Load

tests and condition assessments were conducted by FPL

personnel with assistance from Pueblo County personnel.

The evaluation methodology utilized procedures and equipment that were developed by Ritter and others (1991) and is

discussed in the following sections.

Moisture Content

Moisture content can significantly influence the performance

of stress-laminated decks. Changes in moisture content

between the fiber saturation point (FSP) of 25% to 30% and

the equilibrium moisture content of approximately 18%

cause dimensional changes in wood. A decrease in moisture

content causes shrinkage and a subsequent loss in bar force.

An increase in moisture content causes swelling and a

subsequent increase in bar force.

Moisture content was measured using an electrical-resistance

moisture meter with insulated 76-mm probe pins in accordance with ASTM D4444–84 (ASTM 1990). Measurements

were taken at five locations on the underside of the deck at

probe penetrations of 25 to 76 mm. Measurements were

taken once a week for the first 3 months, monthly for the

next 6 months, once every 2 months for the next year, and

once at the conclusion of the 3-year monitoring. Adjustments

for temperature and species were applied to meter values to

determine actual moisture content values (FORINTEK

1984).

Bar Force

Adequate live load distribution of the stress-laminated deck

design is dependent on interlaminar compression. Research

has shown that the interlaminar compression can decrease by

60% over the life of the structure as a result of stress relaxation of the wood. For this reason, design was based on an

initial interlaminar compression of 689.4 kPa, which is

anticipated to decrease to 275.8 kPa over the life of the structure. Factors other than stress relaxation, such as decreases in

moisture content and subsequent shrinkage of the laminations, can cause the interlaminar compression to decrease

below 275.8 kPa. Research has shown that vertical slip

between the laminations occurs below an interlaminar compression of approximately 172.4 kPa. Slip leads to a reduction in live load distribution. The Pueblo County bridge

was constructed with laminations having a high moisture

content; therefore, monitoring of the interlaminar compression was necessary to measure losses greater than 60% of

design and the subsequent need to schedule maintenance.

Interlaminar compression is directly related to bar force. To

measure bar force, load cells developed by FPL were placed

on the second and fifth bars from the south end and read with

a portable strain gage indicator (Fig. 11). Load cell measurements were collected in conjunction with moisture content measurements. Measurements were converted from strain

units to bar force based on laboratory calibrations. The accuracy of the load cells was validated with a hydraulic jack

during load tests and laboratory recalibrations after removal

at the end of the 3-year monitoring. Approximately 3 years

later, the bar force was measured again using a hydraulic jack

to obtain long-term performance information.

Behavior Under Static Load

Deflection of the stress-laminated deck under live load conditions is an easily measurable indicator of structural performance and design adequacy. Two static load tests were conducted on the same day—at the conclusion of the monitoring

period, approximately 35 months after installation. Each load

test was performed at a different bar force level to determine

the response of the bridge under static loading conditions for

varying interlaminar compression levels. The load tests

consisted of positioning fully loaded, three-axle dump trucks

on the bridge and measuring the resulting deflections at a

series of locations along a transverse cross section at midspan. For each load test, deflection measurements were obtained prior to testing (unloaded), after placement of the test

vehicle (loaded), halfway through the load testing (unloaded),

and at the conclusion of testing (unloaded). In addition, an

analytical assessment using computer modeling was completed to predict the response of the bridge.

Figure 9—Completed deck of the bridge after initial

stressing.
6

 Figure 10—Completed Pueblo County 204B bridge.
7

Static Load Testing

Two load tests were performed on February 17, 1993, with

two trucks (Fig. 12). For load test 1, the average bar force

was 117.87 kN, which is equivalent to an interlaminar compression of 241.3 kPa. For load test 2, the average bar force

was 359.40 kN, which is equivalent to an interlaminar compression of 723.9 kPa. For each load test, six load cases were

completed by placing the trucks in different positions transversely on the bridge (Fig. 13). Longitudinally, the rear

truck axles were centered over the skewed midspan cross

section. Figures 14 and 15 show load cases 3 and 6, respectively, from the north end facing south. For all load cases,

the front axles were off the bridge and the vehicles faced in

the proper direction of travel. A surveyor’s level was used to

read deflection values from calibrated rules suspended from

the underside of the bridge to the nearest 2 mm (Fig. 16).

Accuracy of the measurement method was estimated to be

±1 mm.

Analytical Assessment

Research has shown that stress-laminated decks can be accurately modeled as orthotropic plates (Oliva and others 1990).

At the completion of the load tests, the measured midspan

deflection curves were compared with theoretical deflection

curves for the actual truck loading, bridge configuration, and

materials. Theoretical curves were determined using an

orthotropic plate computer model developed at FPL and a

modulus of elasticity of 12.4 GPa. Providing that the measured and theoretical deflection curves were similar and the

analytical parameters in the computer model were reasonable,

the actual trucks were replaced with AASHTO HS 20–44

trucks to compute theoretical curves for the AASHTO design

live load. This resulted in a predicted maximum live load

deflection for the deck, which was then compared with design.

Condition Assessment

The general condition of the bridge was assessed on three

occasions during the monitoring period. The first occurred

during installation, the second occurred approximately

11 months after installation, and the third occurred at the

time of the load tests. These assessments involved visual

inspections, measurements, and photographic documentation

of the condition of the bridge. Items of specific interest included deck camber and the condition of the wood components, wearing surface, and stressing bar anchorage system.

Deck camber was measured using a stringline connected to

the timber caps at centerline and a calibrated rule suspended

at midspan.

Results and Discussion

The performance monitoring of the Pueblo County 204B

bridge extended from March 1990 through April 1993. In

addition, a final check of the bar force was performed in 1996.

Results of the performance monitoring data follow.

Figure 11—Measuring bar force data with a portable

strain gage indicator and load cell.

Figure 12—Load configuration axle loads of the test

trucks. The width of the transverse truck track was

1.83 m, measured center–center of the rear tires.
8

Moisture Content

The average moisture content in the deck of the bridge during the monitoring period is shown in Figure 17. The laminations were installed initially at an average moisture content

of 28%, near the FSP. After construction, moisture content

fluctuated and, in general, slowly decreased to approximately

25% during the first 2 years. Then, the moisture content

rapidly decreased to 16% during the next year. Typically,

moisture content fluctuates in the outer 25 to 76 mm of the

deck with a very slow downward trend.

For the Pueblo County 204B bridge, the reduction in moisture content was more rapid because the bridge is located in

an arid region of Colorado, which experiences very hot

summer temperatures. The asphalt protects the topside of the

deck from moisture penetration. The bridge is unprotected by

trees or buildings, thus it is fully exposed to a climate

(frequent sun, high wind, low relative humidity) that is

conducive to rapid moisture evaporation from the surface

areas of the deck. It is likely that the moisture content remained high during the first 2 years as a result of the migration of moisture from the interior to the underside of the

deck. The remaining rapid moisture reduction occurred as a

result of a reduction in the interior moisture content and a

subsequent slowing of moisture migration.

Based on the arid location, the equilibrium moisture content

of the deck is likely to be 13% to 15%. Although the outer

25 to 76 mm are near equilibrium, the interior portion is

probably greater. Globally within the deck, the moisture

content will continue to slowly decrease. Small fluctuations

will likely occur in the outer 25 to 76 mm throughout the

life of the structure with climatic changes. This will also

occur at the ends of the bridge as a result of the cracks in the

asphalt surface at the ends of the span, which allows moisture

migration from the topside.

Bar Force

The stressing bar force for the 3-year monitoring is shown in

Figure 18. In general, the bar force behaved similar to that of

other stress-laminated bridges (Ritter and others 1995). Each

restress was to a final level slightly less than the 355.84 kN

design force. During the first 4 months after the final bar

tensioning, the force rapidly decreased approximately 50% to

177.92 kN, where it remained level for 20 months. during

the next year, the force decreased to 117.87 kN.

The main causes of bar force loss were stress relaxation and

decreasing moisture content. Stress relaxation is the slow

compression of the wood cells. It is more pronounced as

moisture content increases above 20%. Because the moisture

content remained near the FSP for the first 2 years, stress

relaxation effected bar force changes whereas shrinkage caused

by loss of moisture had little effect. During the final year,

shrinkage had the greater effect with the decrease in moisture

content and subsequent dimensional changes in the laminations.

During the 3-year monitoring, the deck did not warrant

restressing until just prior to the end of the monitoring

period. Because the moisture content was anticipated to be

near equilibrium, it was assumed that restressing would not

be necessary for several years. After 3 more years, the bar

force had reduced by 50% on average (Table 2). This loss

occurred more quickly than anticipated and was probably due

to the global moisture content of the deck continuing to

decrease. This emphasizes the long-term effect of installing

laminations at a high moisture content level, especially for

decks installed in areas conducive to rapid drying.

Figure 13—Transverse load cases used for load testing.

For load cases 1 to 3, trucks were positioned to obtain

maximum deflection near the centerline of the deck.

For load cases 4 to 6, trucks were positioned to obtain

maximum deflection near the edges of the deck.
9

Figure 14—Truck positions for load case 3.

Figure 15—Truck positions for load case 6.
10

Figure 16—Calibrated scales suspended from the

underside of the deck to measure load deflection.

Figure 17—Average moisture content levels

in the deck.

Figure 18—Average bar force obtained from

load cells installed on two stressing bars.

Behavior Under Static Load

Results of the two static load tests and analytical assessments are presented. For each load case, transverse deflection

measurements are given at the skewed midspan, viewed

from the north end looking south. No permanent residual

deformation was measured at the conclusion of the testing.

In addition, no movement was measured at either of the

abutments.

Load Test 1

Transverse deflections for each load case are shown in

Figure 19. For load cases 4 and 6, there was no apparent

single point of maximum deflection. There may have been

differences between the points of maximum deflection within

each case, but the difference was indistinguishable as a result

of the measurement accuracy. In all cases, the deflection was

typical of the linear elastic orthotropic plate behavior of

stress-laminated bridges, and the location of the maximum

measured deflection corresponded to the location observed in

similar bridges (Ritter and others 1995).

Assuming that the vehicle positions were symmetrical, the

loading along the wheel lines of both trucks were identical,

and the material properties of the deck were uniform, several

comparisons can be made to validate the accuracy of the

measurements. The following comparisons concentrate on

the measured location and magnitude of the maximum deflection. Small differences in location and magnitude were due to

slight differences in the initial assumptions and the accuracy

of the measurement method.

Load cases 1 and 2 (Figs. 19a,b) and load cases 4 and 5

(Figs. 19d,e) should be mirror images of each other. For load

cases 1 and 2, the maximum deflection location was centered

under the truck and the magnitude was −10.3 and −9.5 mm,

respectively. For load cases 4 and 5, the general location was

under the outside wheel line and the magnitude was −9.5

and −10.7 mm, respectively. The sum of the deflections

under the single truck load cases should be similar to the

deflections under the two-truck load case. Comparisons of the

deflection curves of load cases 1 plus 2 to load case 3 and

load cases 4 plus 5 to load case 6 are shown in Figures 20

and 21. For load cases 1 plus 2 and 3, the maximum

deflection location was slightly to one side of the centerline,

Table 2—Bar force 6 years after bridge installation

Bar number Bar force (N) Loss (%)a

2 227,916 36

3 188,373 47

4 177,920 50

6 148,955 58

7 142,336 60

8 154,532 56

a

Design force of 355,840 N.
11

Figure 19—Transverse deflection for load test 1, measured at midspan. Solid box denotes maximum deflection.

Bridge cross sections and truck positions are shown to aid interpretation and are not to scale.
12

with a magnitude of −15.1 and −14.3 mm, respectively. For

load cases 4 plus 5 and 6, the location was under the outside

wheel line, with a magnitude of −10.3 and −10.7 mm,

respectively.

Load Test 2

Transverse deflections for each load case are shown in

Figure 22. As stated for load test 1, for some load cases,

there was no apparent single point of maximum deflection

and other points deflected the same magnitude. In all cases,

the deflection was typical of the linear elastic orthotropic

plate behavior of stress-laminated bridges and the location of

maximum measured deflection corresponded to the location

observed in similar bridges (Ritter and others 1995).

The same deflection comparisons that were made for load

test 1 were made for load test 2. For load cases 1 and 2

(Fig. 22a,b), the maximum deflection location was under the

truck and the magnitude was −7.1 and −8.7 mm, respectively. For load cases 4 and 5 (Fig. 22d,e), the location was

under the outside wheel line and the magnitude was −9.0

and −10.3 mm, respectively. Load cases 1 plus 2 compared

with load case 3 and load cases 4 plus 5 compared with load

case 6 are shown in Figures 23 and 24. For load cases 1 plus

2 and 3, the maximum deflection location was at the centerline with a magnitude of −13.3 and −12.7 mm, respectively.

For load cases 4 plus 5 and 6, the location was under the

outside wheel line with a magnitude of −10.4 and −9.9 mm,

respectively.

Load Tests 1 and 2 Comparison

A comparison of the maximum deflection magnitudes of the

load tests showed a decrease in the magnitude with an increase in interlaminar compression. The expected change in

load distribution with increased interlaminar compression

should be most pronounced for the truck positions of load

cases 1 to 3. The interlaminar compression was increased

67% from load test 1 (241.3 kPa) to load test 2 (723.9 kPa).

For load test 1, load cases 1 to 3 (Figs. 19a,c), the magnitudes were −10.3, −9.5, and −14.3 mm, respectively. For

load test 2, load cases 1 to 3 (Figs. 22a–c), the magnitudes

were −7.1, −8.7, and −12.7 mm, respectively. The decrease

in deflection between load tests 1 and 2 for load cases 1 to 3

were 31%, 8%, and 11%, respectively, with an average 17%

decrease in maximum deflection for the 67% increase in

interlaminar compression.

Analytical Assessment

Measured deflections were compared with theoretical deflections for all load cases in each load test (Figs. 25, 26).

The theoretical deflections were similar to the measured

deflections, especially for load test 2. In general, the measured deflections were slightly less than the theoretical. For

both load tests, the guardrail appeared to stiffen the edge

significantly, as seen in load cases 4 to 6.

Figure 27 shows the theoretical deflections for HS 20–44

loading for load cases 3 and 6 for each load test. For load

test 1, the predicted maximum deflection was −21.6 mm or

1/435 of the span length center–center of bearing and

−18.1 mm or 1/522 for load cases 3 and 6, respectively. For

load test 2, the predicted maximum deflection was −19 mm

or 1/497 and −16.1 mm or 1/585 for load cases 3 and 6,

respectively. For both load tests, the deflection was well

within the original design limit of 1/360. However, for both

load tests, load case 3 slightly exceeded the allowable live

load deflection of 1/500 recently established by AASHTO

(1991). Comparing load tests 1 and 2, the decrease in the

theoretical maximum deflection with the increase in interlaminar compression was 12% for load case 3. 

Figure 20—Load test 1: measured deflection of load

cases 1 plus 2 compared with load case 3.

Figure 21—Load test 1: measured deflection of load

cases 4 plus 5 compared with load case 6.
13

Figure 22—Transverse deflection for load test 2, measured at the midspan. Solid box denotes maximum deflection.
14

Condition Assessment

Condition assessments of the Pueblo County 204B bridge

indicated that structural and serviceability performance were

good. Inspection results for specific items follow.

Deck Camber

No camber was built into the deck during installation. After

placement of the guardrail system and asphalt wearing surface, measurements indicated a sag of 12.7 mm as a result of

the dead load. Measurements in vertical alignment at the end

of the monitoring period indicated no change. This is typical

of other stress-laminated decks that have shown little if any

loss in camber as a result of creep, even at higher moisture

content levels (Ritter and others 1995).

Wood Components

The outside deck laminations are generally in good condition. There has been some (up to 9.5 mm) plate crushing

into the outside lamination on the west side (Fig. 28). This

is probably due to undersized bearing plates, especially

considering the high moisture content of the laminations.

Checking was evident on the top surfaces of rail members

exposed to wet–dry cycles. Checking was most pronounced

in the end grain of the timber rail posts. In addition, the top

of the bridge rail showed minor checking, but the depth of

the checks did not appear to penetrate the preservative

treatment envelope of the member. Inspection showed an

excessive amount of preservative leaching on some exposed

surfaces.

Wearing Surface

Inspection of the asphalt wearing surface indicated 12.7-mmwide transverse cracks at the ends of the span (Fig. 29).

Otherwise, the wearing surface was in good condition with

no other signs of distress, such as cracking, rutting, or

shoving.

Bar Anchorage System

All bearing and anchor plates appeared to be in good condition, with no signs of bending or corrosion. The exposed

steel bars and hardware showed no visible signs of corrosion,

except at the ends of the bars where minor corrosion appeared

as a result of the galvanized coating having been stripped

from the stressing bars, thus exposing uncoated steel. This

occurred on only some of the bars, because the nuts were not

adequately oversized to compensate for galvanizing and were

forced into the bars during construction.

Conclusions

After the initial 3 years of FPL monitoring and a check of the

bar force after 6 years, the Pueblo County 204B bridge is

exhibiting good performance. Several minor deficiencies

include the crushing of the bearing plates into the outer

laminations and the slight sag of the deck. In addition, the

deflection is slightly greater than that allowed by current

AASHTO guidelines, although it is within the original

design allowable. This bridge was designed prior to the

current AASHTO guidelines being available. Design and

construction in accordance with the current guidelines will

alleviate these minor problems. Most concerns from the

condition assessment are minor and can be addressed as part

of routine maintenance of the bridge. For future bridges, the

most critical item is the moisture content of the timber

laminations. The moisture content at installation should be

near the anticipated equilibrium moisture content for the area.

Otherwise, a maintenance schedule of restressing two to three

times during the first 6 to 10 years will be necessary.

Figure 23—Load test 2: measured deflection of

load cases 1 plus 2 compared with load case 3.

Figure 24—Load test 2: measured deflection of

load cases 4 plus 5 compared with load case 6.
15

Figure 25—Comparison of theoretical and measured deflections at the midspan of the bridge for the test trucks,

load test 1, load cases 1 through 6.
16

Figure 26—Comparison of theoretical and measured deflections at the midspan of the bridge for the test trucks,

load test 2, load cases 1 through 6.
17

Figure 27—Theoretical deflections at the midspan of the bridge for two HS 20–44 trucks, load test 1,

load cases 3 and 6 and load test 2, load cases 3 and 6.

Figure 28—Crushing of the bearing plates into the

outside lamination, up to 9.5 mm.

Figure 29—Transverse crack (12.7 mm wide) in

asphalt at both span ends.
18

Based on the extensive monitoring conducted since bridge

construction, the following observations and recommendations are given:

• The average trend in moisture content of the deck indicates that the outer 25 to 76 mm have decreased to near

the anticipated equilibrium moisture content of approximately 13% to 15%. It is likely that the global moisture

content of the deck is continuing to slowly decrease.

Cyclic seasonal variations in moisture content are occurring and will continue to occur in the outer 25 to 76 mm

of the deck.

• Bar tension force decreased by 67% during the 3-year FPL

monitoring. This is attributable primarily to the large

decrease in moisture content as well as wood stress relaxation. At the end of the monitoring, the bars were retensioned and during the 3 years since that time, the force has

decreased by 50%. This is a slightly less loss compared

with the initial 3 years, showing a decrease in the rate of

moisture content loss and stress relaxation. Restressing of

the bridge should be performed within the next year and

will probably not be necessary again for several years.

• Load tests and analysis indicate that the bridge is performing as an orthotropic plate when subjected to static loading. The predicted maximum deflection caused by two

lanes of AASHTO HS 20–44 loading was estimated to be

21.6 mm or 1/435 of the center–center of bearings length

at 40% of the design bar force and 19 mm or 1/497 at

100% of the design bar force level. Both are within the

design allowable of 1/360 but do not meet current

AASHTO recommendations of 1/500.

• There is a 12.7-mm sag in the deck as a result of dead

load. No camber was built into the deck to offset the

effects of dead load.

• The bearing plates have crushed as much as 9.5 mm into

the outside laminations. This is probably due to the

plates not being adequately sized for the applied force and

the reduced strength of the wood as a result of the high

moisture content. The crushed areas of the outside laminations are susceptible to deterioration, and preservative

should be applied. Wood checking is evident in the

exposed end grain of the rail posts and other components.

This probably would not have occurred if a bituminous

sealer had been applied to the end grain at the time of

construction.

• The asphalt wearing surface is in good condition except

for transverse cracks at the ends of the bridge. These cracks

should be filled to prevent moisture migration to the ends

of the deck laminations and timber abutments.

• The ends of some stressing bars show signs of minor

corrosion at locations where the galvanizing was removed

during stressing nut placement. This would not have occurred if the nuts had been oversized to compensate for the

thickness of the galvanized coating or a cold galvanizing

compound had been applied to the stressing bars to

replace the removed coating.

References

AASHTO. 1989. Standard specifications for highway

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

State Highway and Transportation Officials.

AASHTO. 1991. Guide specifications for the design of

stress-laminated wood decks. Washington, DC: American

Association of State Highway and Transportation Officials.

ASTM. 1988. Standard specification for uncoated highstrength steel bar for prestressing concrete; ASTM A722–86.

Philadelphia, PA: American Society for Testing and

Materials. 3 p.

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

meters; ASTM D 4444–84. Philadelphia, PA: American

Society for Testing and Materials. 6 p.

AWPA. 1991. Standards. Woodstock, MD: American

Wood Preservers’ Association. 200 p.

FORINTEK. 1984. Moisture content correction tables for

the resistance-type moisture meter; SP511E. Ottawa, Canada: FORINTEK Corp. 37 p.

Oliva, M.G.; Dimaskis, A.G.; Ritter, M.A.; Tuomi, R.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.

OMTC. 1983. Ontario Highway Bridge Design Code, 2d

ed. Downsview, Ontario: Ontario Ministry of Transportation

and Communication.

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]. 1991.

Methods for assessing the field performance of stresslaminated timber bridges. In: Proceedings, 1991 International timber engineering conference; 1991 September;

London, England.

Ritter, Michael A.; Wacker, James P.; Tice, Everett 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.
19

Appendix—Information Sheet

General

Name: Pueblo County 204B Bridge

Location: Pueblo County, Colorado

Date of Construction: March 22, 1990

Owner: Pueblo County Highway Department

Design Configuration

Structure Type: Stress-laminated deck

Butt Joint Frequency: 1 in every 4 laminations

transversely, 1.22-m separation

in adjacent laminations

longitudinally

Total Length (out–out): 9.72 m

Skew: 10 degrees

Number of Spans: 1

Span Length (center–center bearings): 9.42 m

Width (out–out): 9.17 m perpendicular

Width (curb–curb): 8.56 m perpendicular

Number of Traffic Lanes: 2

Design Loading: HS 20–44

Wearing Surface Type: Asphalt pavement;

51 to 76 mm thick

Material and Configuration

Timber:

Species: Coastal Douglas Fir–Larch

Size (actual): 102 by 406 mm

Grade: No. 1 & Better visually graded

Moisture Condition: 22% to 28% at installation

Preservative Treatment: Creosote–coal tar solution

Stressing Rods:

Type: High strength steel thread bar with course

right-hand thread, conforming to

ASTM A722.

Diameter: 25 mm

Number: 9

Design Force: 355,840 N, interlaminar

compression of 717 kPa

Spacing: 1,219 mm

Anchorage Type and Configuration:

Discrete Steel Plates: 25 by 305 by 406 mm bearing

25 by 102 by 152 mm anchor

Railing and Curb: 152- by 305-mm glulam rail

152- by 305-mm solid-sawn posts

305- by 305-mm solid-sawn curb