United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

National Wood

in Transportation

Information Center

Research

Paper

FPL–RP–560

In cooperation

with the

United States

Department of

Transportation

Federal

Highway

Administration

Field Performance of

Timber Bridges

12. Christian Hollow

Stress-Laminated

Box-Beam Bridge

James P. Wacker

Stephen C. Catherman

Richard G. Winnett
Abstract

In January 1992, the Christian Hollow bridge was constructed in Steuben County, New York. The bridge is a

single-span, stress-laminated box-beam superstructure that is

9.1 m long, 9.8 m wide, and 502 mm deep (30 ft long, 32 ft

wide, and 19-3/4 in. deep). The performance of the bridge

was continuously monitored for 28 months, beginning

shortly after installation. Performance monitoring involved

gathering and analyzing data relative to the wood moisture

content, force level in the stressing bars, vertical bridge

creep, and behavior under static load conditions. In addition,

comprehensive visual inspections were conducted to assess

the condition of the overall structure. Based on field evaluations, the bridge is performing well with no structural or

serviceability deficiencies.

Keywords: bridge, stress laminated, box beam,

field performance

Acknowledgments

We express sincere appreciation to the following individuals

who contributed to the success of this project: Sam Hadley,

Dave Burgess, Bryce Foster, and Jim Risley of the Steuben

County Department of Public Works for assistance with

conducting load tests and data collection; the staff of the

Sullivan Trail Resource Conservation and Development

Council; and Sheila Rimal Duwadi of the Federal Highway

Administration for assistance with administration of the

project.

Thanks are also due to the following individuals from the

USDA Forest Service, Forest Products Laboratory: James

Kainz for assistance with conducting field work, Mike Ritter,

Paula Hilbrich Lee, and Maureen Mathias (former student) for

assistance in data analysis and report preparation, and the

Information Services Team for assistance in preparing this

report.

January 1998

Wacker, James P.; Catherman, Stephen C.; Winnett, Richard G. 1998. Field

performance of timber bridges: 12. Christian Hollow stress-laminated boxbeam bridge. Res. Pap. FPL–RP–560. 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

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Contents

Page

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

Background and Development........................................1

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

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

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

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

Cost ......................................................................6

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

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

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

Vertical Creep..........................................................8

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

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

Load Test 2.........................................................8

Predicted Deflection Analysis..................................8

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

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

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

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

Vertical Creep........................................................10

Behavior Under Static Load .....................................11

Load Test 1.......................................................11

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

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

Predicted Response .............................................12

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

Bridge Geometry ................................................14

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

Wearing Surface..................................................15

Stressing Bar Anchorages.....................................15

Conclusions .............................................................16

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

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

12. Christian Hollow Stress-Laminated Box-Beam Bridge

James P. Wacker, General Engineer

Forest Products Laboratory, Madison, Wisconsin

Stephen C. Catherman, County Engineer

Steuben County Department of Public Works, Bath, New York

Richard G. Winnett, Director

Sullivan Trail Resource Conservation and Development Council, Bath, New York

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 program to provide effective

and efficient utilization of wood as a structural material for

highway bridges. Responsibility for the development, implementation, and administration of the TBI was assigned to

the USDA Forest Service. To implement a program, the

Forest Service established three primary emphasis areas:

demonstration bridges, technology transfer, and research.

Responsibility for the technology transfer and demonstration

bridge programs was assigned to the Forest Service National

Wood in Transportation Information Center (NWITIC)

(formerly the Timber Bridge Information Resource Center) in

Morgantown, West Virginia. Under the demonstration program, the NWITIC provides matching funds to local governments for constructing demonstration timber bridges that

encourage innovation through the use of new or previously

underutilized wood products, bridge designs, and design

applications.

Responsibility for the research portion of the TBI was assigned to the USDA Forest Service, Forest Products Laboratory (FPL), a national wood utilization research laboratory.

As part of this broad research program, FPL assumed a lead

role in assisting local governments in evaluating the field

performance of demonstration timber bridges, many of which

use design innovations or materials that have not been previously evaluated. Through such assistance, FPL is able to

collect, analyze, and distribute information on the field performance of timber bridges. This provides a basis for validating or revising design criteria and further improves the efficiency and economy in bridge design, fabrication, and

construction.

In addition to the TBI, Congress passed the Intermodal

Surface Transportation Efficiency Act (ISTEA) in 1991,

which included provisions for a timber bridge research

program aimed at improving the utilization of wood transportation structures. Responsibility for the development,

implementation, and administration of the ISTEA timber

bridge program was assigned to the Federal Highway Administration (FHWA) and included a demonstration timber

bridge, technology transfer, and research program. Because

many aspects of the FHWA program paralleled those underway at FPL, a joint effort was initiated to combine the respective research of the two agencies into a central research

program. As a result, the FPL and FHWA merged resources

to jointly develop and administer a national timber bridge

research program.

This report, twelfth in a series documenting the field performance of timber bridges, describes the design, construction, cost, and field performance of a 28-month monitoring

project of the Christian Hollow bridge, which is located in

Steuben County, New York. The bridge is a two-lane,

single-span, stress-laminated box-beam superstructure 9.1 m

long, 9.8 m wide, and 502 mm deep (30 ft long, 32 ft wide,

and 19-3/4 in. deep). Built in 1992, the Christian Hollow

bridge was constructed with matching funds through the

Forest Service NWITIC and monitored as a part of the

FPL/FHWA National bridge monitoring program. An information sheet on the specific characteristics of the Christian

Hollow bridge is given in the Appendix.

Background and Development

The Christian Hollow bridge is located in the town of

Greenwood, Steuben County, New York (Fig. 1). The

bridge is on Christian Hollow–West Union road (County

Highway 60) where it crosses Christian Hollow Creek,

approximately 1.2 km (0.75 miles) west of State Highway

248. The roadway over the bridge is a two-lane paved road;

traffic is mostly passenger vehicles at an average rate of 500

per day.
2

The original Christian Hollow bridge, constructed in the

1950s, consisted of steel stringers supporting a concrete slab

deck with tubular steel guardrails (Fig. 2). The girders and

deck were in poor condition, and the steel tubing guardrails

were inadequate and posed significant safety hazards to motorists. Because of an interest by the Steuben County staff to

use innovative timber bridge designs, a decision was made

to replace the existing structure with the newly developed

stress-laminated box-beam timber superstructure.

Through a cooperative effort between the Sullivan Trail

Resource, Conservation, and Development Council and the

Steuben County Department of Public Works, a proposal

was submitted to the USDA Forest Service for partial funding of the replacement structure as a demonstration bridge

under the TBI (USDA 1990). The project proposed a stresslaminated box-beam configuration utilizing local mixed

hardwood lumber and Southern Pine glued-laminated

(glulam) timber beams. In 1990, the project received funding

through the NWITIC, and plans for the design and construction of the Christian Hollow bridge were finalized. Subsequently, FPL was contacted to provide assistance in developing and implementing a field evaluation plan to monitor

bridge performance.

Objective and Scope

The objective of this project was to evaluate the field performance of the Christian Hollow bridge for 28 months after

installation. The scope of the project included data collection

and analysis related to wood moisture content, stressing bar

force, vertical bridge creep, static truck loading, and general

structure performance. The results of this project will be

considered with similar monitoring projects in an effort to

improve design and construction methods for future stresslaminated box-beam timber bridges.

Design, Construction, and Cost

Design and construction of the Christian Hollow bridge

involved mutual efforts from several agencies and individuals. An overview of the design, construction, and cost of the

project follows.

Figure 1—Location maps for the Christian Hollow

bridge.

Figure 2—Original bridge constructed in the 1950s.
3

Design

Design of the Christian Hollow bridge was completed by the

Steuben County staff with assistance from an engineering

consultant. The design features a stress-laminated box-beam

superstructure, a relatively new type of timber bridge, with

continuous glulam webs and sawn lumber flanges (Fig. 3).

For this bridge configuration, high strength steel bars are

inserted through prebored holes in the webs and flanges and

tensioned to provide sufficient friction between the individual

components to develop load transfer. Thus, the design is

predicated upon the components acting together as a single

unit.

With the exception of those features related to stress laminating, design of the Christian Hollow bridge conformed to the

American Association of State Highway and Transportation

Officials (AASHTO) Standard Specifications for Highway

Bridges (AASHTO 1989) for two lanes of HS20-44 loading.

At the time of design, an AASHTO-accepted design procedure for stress-laminated box-beam bridges was not available.

Therefore, specific design requirements for the stresslaminated box-beam were based on standard guidelines

developed from research conducted at West Virginia University (Lopez–Anido and GangaRao 1993).

The design geometry provided for a single-span superstructure, 9.1 m long, 9.8 m wide, and 502 mm deep (30 ft long,

32 ft wide, and 19-3/4 in. deep) (Fig. 4). Design calculations

were based on an 8.8-m (29-ft) span length (center-to-center

of bearings) and a 9.1-m (30-ft) clear roadway width. Design

of the glulam webs and the sawn lumber flanges was based

on requirements set by the American Forest and Paper Association (AFPA 1986, 1988) and the American Wood Preservers’ Association (AWPA 1989). Web members were

creosote-treated combination 24F-V3 Southern Pine glulam

beams and were 9.1 m long, 219 mm wide, and 502 mm

deep (30 ft long, 8-5/8 in. wide, and 19-3/4 in. deep). Sawn

lumber flange laminations were specified as standard 38- by

140- mm (nominal 2- by 6-in.) visually graded No. 2 mixed

hardwoods, pressure treated with creosote. Moisture content

of both the web and flange members was specified to be

19 percent (maximum) at installation. Because the flange

laminations were not available in lengths required to span

the entire bridge, the design included lamination butt joints

in a repetitive pattern (Fig. 5). Transversely, butt joints were

four laminations apart. Longitudinally, butt joints in adjacent laminations were 0.9 m (3 ft) apart.

For stress laminating, the design specified a 16-mm- (5/8-

in.-) diameter ASTM A722 high strength threaded steel bars

with an ultimate strength of 1,034 kPa (150 lb/in2

) (ASTM

1988). The bars were spaced at 0.9-m (3-ft) intervals along

the bridge except for the end bars. A design bar tension force

of 96 kN (21,600 lb) provided an interlaminar compressive

stress of 689 kPa (100 lb/in2). Bar anchorage was with a

discrete-plate anchorage system consisting of 152- by 305-

by 19-mm (6- by 12- by 3/4-in.) steel bearing plates, 51- by

127- by 25-mm (2- by 5- by 1-in.) steel anchor plates, and

hexagonal nuts. All components of the stressing system and

other steel hardware were galvanized for corrosion protection.

Design of the railing and curb was based on a crash-tested

railing developed for longitudinal spike-laminated timber

decks in accordance with AASHTO Performance Level 1

criteria (FHWA 1990). The bridge rail was specified as a

full-span glulam beam measuring 152 by 273 mm (6 by

10-3/4 in.) Rail posts were visually graded Dense Select

Structural Douglas Fir sawn lumber measuring standard

190 by 292 mm (nominal 8 by 12 in.) and were spaced

1.8 m (6 ft) on-center. The curbs were visually graded

No. 2 Douglas Fir sawn lumber measuring standard

139 by 292 mm (nominal 6 by 12 in.).

To compensate for dead load and creep deflection, a 25-mm

(1-in.) camber was specified for glulam beams. To protect the

bridge deck, a 76-mm- (3-in.-) thick asphalt wearing surface

and a waterproof geotextile membrane were specified.

Construction

Construction of the Christian Hollow bridge was completed

by the Steuben County bridge crew in 2 months, beginning

January 1992. During construction, a temporary crossing

consisting of corrugated steel culverts was installed just

upstream of the bridge site. Following demolition and removal of the existing bridge structure, new reinforced concrete abutments and wingwalls were constructed (Fig. 6). To

increase hydraulic flow capacity at the crossing, the span

distance between abutments was increased and the abutments

were elevated.

It took the Steuben County crew 1 week for in-place assembly of the stress-laminated box-beam superstructure. In-place

assembly is not a popular method of construction for stresslaminated box-beam bridges, because it is difficult to assemble the butt-jointed flanges in the field. Stress-laminated boxbeam bridges are typically prefabricated into partial width

bridge modules that are temporarily nailed together and

transported to the bridge site for placement with an overhead

crane. However, in-place assembly is advantageous when

low-cost labor is readily available.

Bridge components arrived at the bridge site on flatbed

trailers (Fig. 7) and were stockpiled adjacent to the crossing.

To begin the installation process, the glulam beams were

lifted with the backhoe and set to span the abutments at the

required center-to-center spacing (Fig. 8 top). After all glulam beams were in place, temporary support boards were

attached to the underside of the beams to facilitate the

Figure 3—Cross section of a typical stresslaminated box-beam bridge.
4

Figure 4—Design configuration of the Christian Hollow bridge.
5

Figure 8—On-site assembly of the superstructure

consisted of (top) placement of the glulam beam web

members with a backhoe and nylon straps, (middle)

placement of the lower flange lumber laminations by

hand with standard 38- by 89-mm boards providing

temporary support at the butt-joint locations, (bottom)

insertion of the solid-block diaphragms at the ends of the

bridge as the upper and lower flanges were completed.

Figure 5—Repetitive butt-joint pattern used for

the sawn lumber flange laminations.

Figure 6—Construction of reinforced concrete

abutments.

Figure 7—Delivery of timber components to

the bridge site with tractor trailers.
6

placement of the sawn lumber decking. The use of scaffolding

would have improved efficiency. This involves providing

temporary support for the bottom flange laminations and

creating a work platform. The sawn lumber decking was then

hand-placed at the flange locations, beginning at the bottom

layer (Fig. 8 middle). After the top and bottom flange layers

were placed, solid-block diaphragms were inserted between

the glulam beams at the bridge ends (Fig. 8 bottom).

Stressing bars were manually inserted through the webs and

flanges (Fig. 9). In some cases, the bars did not easily fit

through the prebored holes and a power hand-drill was

attached to the bars to turn them into place. After all bars

were threaded through the bridge, anchorage plates and nuts

were attached to the bar ends in preparation for the initial

stressing.

The bridge was stressed initially at installation and at 1 and

10 weeks thereafter. At the initial stressing, the bars were

individually tensioned, top to bottom, to the full design

force level in a sequential manner, beginning at one end of

the bridge. Subsequent stressings, completed at 1 and

10 weeks after installation, were performed to the full design

force level in the same manner.

The box-beam superstructure was anchored to the concrete

abutment with 152- by 152- by 5-mm (6- by 6- by 3/16-in.)

steel angle brackets attached to the underside of the beams

and the abutment wall. Because anchor holes were drilled at

installation and the superstructure became slightly narrower

after bar tensionings, hole alignment problems were encountered and only half the anchor bolts could be installed. These

alignment problems could have been avoided if hole drilling

had been delayed until after the second stressing, after which

little transverse movement occurred.

The curb and rail system was attached after 10 days, and the

asphalt wearing surface was applied 2 months after installation. The completed Christian Hollow bridge is shown in

Figure 10.

Cost

Cost for the design, fabrication, and construction of the

Christian Hollow bridge superstructure totaled $63,846.

Based on a total deck area of 89 m2 (960 ft2), the unit cost

was approximately $717/m2 ($67/ft2).

Evaluation Methodology

To evaluate the structural performance of the Christian Hollow bridge, the Steuben County Department of Public

Works contacted FPL for assistance. Through mutual agreement, a 28-month bridge monitoring plan was developed by

the FPL and implemented through a Cooperative Research

and Development Agreement with Steuben County. The

plan called for performance monitoring of the deck moisture

content, stressing bar force, vertical bridge creep, static load

behavior, and general bridge condition. The plan evaluation

methodology utilized procedures and equipment previously

developed (Ritter and others 1991) and used on similar

structures. The monitoring period was initiated at the final

design stressing, approximately 12 weeks after bridge

installation.

Moisture Content

Wood moisture content was measured with an electricalresistance moisture meter and 76-mm (3-in.) insulated probe

pins in accordance with ASTM D4444-84 (ASTM 1990).

Measurements were obtained from both the glulam webs and

sawn lumber flange members at several locations on the

underside of the superstructure by driving the pins to a depth

of 51 mm (2 in.). Moisture content measurements were

obtained by Steuben County personnel on approximately a

monthly basis. Meter readings were adjusted with appropriate temperature and wood species correction factors

(FORINTEK 1984).

Bar Force

Stressing bar force was measured with calibrated steel hollow-core load cells and a portable strain indicator. At the

initiation of monitoring, four load cells were installed at two

locations along the bridge by placing them on bars through

the top and bottom flanges. Load cell measurements were

obtained by Steuben County personnel on approximately a

Figure 9—Full-width stressing bars were inserted into

the upper and lower flanges by hand (top). When poor

hole alignment prevented insertion by hand, a portable

hand drill was used (bottom).
7

Figure 10—Completed Christian Hollow bridge: (top) side view, looking Northwest;

(bottom) end view, looking west.
8

monthly basis throughout the monitoring period. Load cell

measurements were converted from units of strain to force

based on laboratory calibrations. In addition, the accuracy of

the load cells was validated with laboratory re-calibrations

and hydraulic force measurements at the conclusion of

monitoring.

Vertical Creep

Vertical bridge creep was monitored by measuring the superstructure camber with a surveying level and calibrated rules.

Camber readings were taken at the initiation and conclusion

of the monitoring period. Superstructure camber was measured as the elevation difference between the underside of the

superstructure at the abutments and at the cross section of the

centerspan.

Behavior Under Static Load

To determine the response of the bridge to highway truck

loads, static load testing of the Christian Hollow bridge was

conducted at the beginning and end of the monitoring period.

In addition, predicted deflections were determined for each

load test based on static analysis for HS20-44 loading. Load

testing involved positioning fully loaded trucks on the

bridge span and measuring the resulting deflections at a

series of locations along the centerspan of the bridge and

cross sections of the abutments. A surveying level was used

to read deflection values from calibrated rules that were suspended from the glulam webs. Deflection measurements were

obtained prior to testing (unloaded), after placement of the

test trucks for each load case (loaded), and at the conclusion

of testing (unloaded). Truck positioning for each load case is

shown in Figure 11. For load cases 1 through 3, trucks were

positioned transversely 0.6 m (2 ft) from the roadway centerline. For load cases 4 through 6, the trucks were positioned

2.1 m (7 ft) from the centerline of the roadway. For all load

cases, the center of gravity of the truck was positioned at

centerspan to maximize deflections.

Load Test 1

Load test 1 was conducted March 17, 1992, with two dump

trucks (Fig. 11): truck A with a gross vehicle weight of

296 kN (66,500 lb) and truck B with a gross vehicle weight

of 278 kN (62,400 lb). Deflections were measured with a

precision of 1.5 mm (1/16 in.).

Load Test 2

Load test 2 was conducted June 28, 1994, with two dump

trucks (Fig. 11): truck A with a gross vehicle weight of

309 kN (69,300 lb) and truck B with a gross vehicle weight

of 292 kN (65,600 lb). Deflections were measured with a

precision of 1.0 mm (1/25 in.). Load cases 3 and 6 are

shown in Figure 12.

Predicted Deflection Analysis

At the conclusion of load testing, predicted deflections were

calculated based on actual load test conditions and AASHTO

HS20-44 loading. Design procedures and analytical models

for stress-laminated, box-beam bridges are currently under

development; therefore, a simplified procedure based on static

analysis and a linear extrapolation to AASHTO HS20-44

deflections was used. The procedure utilizes a deflection

coefficient (DC), a term used in calculating deflection independent of material properties, in the following relationship:

 

∆ ∆ HS Loadtest

HS20

Loadtest

DC

DC 20 = 



 



 (1)

where

 

∆HS20 = HS20 predicted deflection (mm);

 

∆Loadtest = maximum measured load test deflection (mm);

 

DCHS20 = HS20 deflection coefficient; and

 

DCHS20 = load test vehicle deflection coefficient.

Condition Assessment

The general condition of the Christian Hollow bridge was

assessed at initiation and conclusion of the monitoring

period. In addition, a subsequent assessment was performed

August 1995, 14 months after the conclusion of monitoring.

These assessments involved visual inspections, measurements, and photographic documentation of the condition of

the bridge. Items of specific interest included bridge

geometry, wood components, wearing surface, stressing bar

anchorages, and steel hardware.

Results and Discussion

Performance of the Christian Hollow bridge was continuously monitored for 28 months, beginning March 1992 and

ending June 1994. The following results are presented.

Moisture Content

The average trend in moisture content is presented in

Figure 13. At the initiation of monitoring, the average moisture content was approximately 20 percent for the glulam

webs and 26 percent for the sawn lumber flange laminations.

Minor fluctuations within the range of ±3 percent occurred

during the monitoring period, but the moisture content

remained relatively stable. At the conclusion of monitoring,

the moisture content of both the glulam beams and the sawn

lumber flange laminations had decreased slightly.

The average trend indicates that no significant changes in

moisture content occurred during the monitoring period. The

glulam web moisture content is near the anticipated equilibrium level for this site. Although installed slightly greater

than the specified maximum of 19 percent, the sawn lumber

moisture content did not decrease in service as typically

observed for most timber structures installed near fiber saturation. However, it is anticipated that the moisture content of

the sawn lumber will gradually decrease and equilibrate with

its surrounding environment during the next several years.
9

Bar Force

The average trend in stressing bar force is presented in

Figure 14. From the final design stressing in March 1992

through March 1993, the average bar force decreased from the

design level of 96 kN (21,600 lb) to approximately 58 kN

(13,000 lb), or approximately 414 kPa (60 lb/in2) interlaminar compression. During the subsequent 6 months ending

October 1993, the average bar force slightly increased but

then decreased to 49 kN (11,000 lb), or approximately

345 kPa (50 lb/in2) interlaminar compression. In October

1993, the bridge was restressed to the full design level of

96 kN (21,600 lb), because the potential for interlaminar slip

between the web and flange increases as the interlaminar

compression decreases below 345 kPa (50 lb/in2

). Between

October 1993 and March 1994, the bar force decreased to

67 kN (15,000 lb), or approximately 483 kPa (70 lb/in2)

interlaminar compression, and became somewhat stabilized

for the remainder of the monitoring period. Additional measurements taken in August 1995, 14 months after completion

of monitoring, indicated an average bar force of 58 kN

(13,000 lb), or approximately 414 kPa (60 lb/in2) interlaminar compression.

Figure 11—Truck configurations and transverse positioning used for static load testing. For all load cases,

the truck center of gravity was placed at the centerspan of the bridge. (GVW is gross vehicle weight.)
10

Bar forces remained greater than 48 kN (10,800 lb), or

345 kPa (50 lb/in2) interlaminar compressive stress,

throughout the monitoring period, although one restressing

was required in October 1993. The moisture content of the

web and flange laminations was essentially constant during

the monitoring period; therefore, lamination shrinkage resulting from decreasing moisture content did not substantially

affect bar force. A large majority of bar force loss can be

attributed to stress relaxation in the web and flange laminations. As observed in previous field evaluations (Ritter and

others 1995), bar force losses as a result of stress relaxation

may have been accelerated by the relatively high moisture

content (24 to 26 percent) of the sawn lumber flange laminations. Future inspections of the Christian Hollow bridge

should include hydraulic bar force measurements to ensure

that the forces remain greater than approximately 48 kN

(10,800 lb), or 345 kPa (50 lb/in2

) interlaminar compressive

stress.

Vertical Creep

Camber measurements are summarized in Table 1. As previously stated, the box-beam superstructure had a 25-mm

(1-in.) camber specified at installation. At the initiation of

monitoring, approximately 12 weeks after installation,

measurements indicated an average camber of 8.9 mm

(0.35 in.) in the superstructure. Therefore, an apparent decrease in camber of 16.8 mm (0.66 in.) occurred with the

initial dead loading at installation. At the conclusion of

monitoring, measurements indicated an average camber of

4.8 mm (0.19 in.) along both bridge edges. Therefore, average camber loss, or vertical creep, of the Christian Hollow

bridge totaled 4.1 mm (0.16 in.) during the monitoring

period.

Table 1—Summary of camber measurements

Longitudinal camber (mm (in.))

Centerspan

location

Initiation of

monitoring

Conclusion of

monitoring

Vertical creep

(mm (in.))

Upstream

 edge +9.1 (0.36) +4.8 (0.19) 4.3 (0.17)

Downstream

 edge +8.6 (0.34) +4.8 (0.19) 3.8 (0.15)

Average +8.9 (0.35) +4.8 (0.19) 4.1 (0.16)

Figure 12—Truck positions for (top) load case 3

and (bottom) load case 6.

Figure 13—Average trend in moisture content.

Figure 14—Average trend in stressing bar force.
11

Figure 15—Load test 1 transverse deflection measured at the centerspan of the bridge (looking east).

Cross sections of the bridge and vehicle positions are shown to aid interpretation and are not to scale.

Behavior Under Static Load

For each load case, transverse deflection measurements are

given at the bridge centerspan, as viewed from the west end

of the bridge (looking east). No permanent residual deflection

was measured between load cases or at the conclusion of load

testing. In addition, no measurable deflection was observed

at the bridge supports during load testing. At the time of

load tests 1 and 2, the interlaminar compressive stress was at

689 and 483 kPa (100 and 70 lb/in2), respectively.

Load Test 1

Transverse deflection for load test 1 is presented in

Figure 15. The maximum deflections for load cases 1 and 2
12

occurred under the inside truck wheel line and measured 6.4

and 5.3 mm (0.25 and 0.21 in.), respectively (Fig. 15a,b).

For load case 3, the maximum measured deflection of

7.4 mm (0.29 in.) was at the roadway centerline and was the

largest measured deflection of all load cases (Fig. 15c).

Maximum deflections for load cases 4 and 5 measured 6.4

and 4.8 mm (0.25 and 0.19 in.), respectively, and occurred

near the outside truck wheel line (Fig. 15d,e). The maximum deflection for load case 6 measured 6.4 mm (0.25 in.)

and was under the outside wheel line of truck A (Fig. 15f).

For both the centered and eccentric load cases, the deflected

shape of the cross section of the centerspan follows the symmetrical truck positions, with maximum measured deflections for the single truck load cases occurring at the same

relative positions for the two truck locations.

Assuming accurate load test results and linear-elastic behavior, the sum of the deflection resulting from individual truck

loads should equal the deflection from both trucks applied

simultaneously. Figure 16 shows the load test 1 comparison

between individual and simultaneous truck loading. As

shown in Figure 16, the two plots are nearly identical with

only minor variations, which are within the accuracy of the

measurements. From this information, it can be concluded

that bridge behavior was within the linear-elastic range.

Load Test 2

Transverse deflection for load test 2 is presented in

Figure 17. The maximum deflection of 6.4 mm (0.25 in.) for

load case 1 occurred near the outside wheel line (Fig. 17a),

and the maximum deflection of 5.8 mm (0.23 in.) for load

case 2 occurred near the inside truck wheel line (Fig. 17b).

For load case 3, the maximum deflection of 8.1 mm

(0.32 in.) was measured at the roadway centerline and was

again the largest deflection for all load cases (Fig. 17c).

Maximum deflections for load cases 4 and 5 occurred near the

outside truck wheel line and measured 7.4 and 6.6 mm

(0.29 and 0.26 in.), respectively (Fig. 17d,e). The maximum

deflection for load case 6 measured 7.4 mm (0.29 in.) and

was near the outside wheel line of truck A (Fig. 17f).

Figure 18 shows the load test 2 comparison between individual and simultaneous truck loading. As with load test 1,

the two plots are nearly identical, indicating that bridge

behavior was within the linear-elastic range.

Load Test Comparison

A comparison of measured deflections for both load tests is

presented in Figure 19 for load cases 3 and 6. The plots are

similar in shape, but load test 2 deflections are greater at

numerous data point locations. Different load testing conditions may have caused greater deflections for load test 2,

especially the differences in truck loads and interlaminar

compression. Load test 2 trucks were approximately 5 percent heavier than those for load test 1, which would tend to

cause slightly greater deflections for load test 2. In addition,

interlaminar compression was 207 kPa (30 lb/in2

) less at

load test 2 than at load test 1. A lower interlaminar compression of this magnitude has been shown to result in greater

relative deflections. The combined effect of these different

load test conditions resulted in the greater deflections

observed in load test 2.

Predicted Response

Table 2 summarizes the maximum measured deflections for

each load test and the predicted maximum deflections for

AASHTO HS20-44 truck loading. In both cases, the values

are based on the load case 3 vehicle positions where the

maximum load test deflections occurred. In addition to the

absolute deflections, Table 2 also presents the span/deflection

ratio as a function of the bridge span, L, measured center-tocenter of bearings.

For both load tests, the predicted HS20-44 deflection was

within the design limit of L/500. For load test 1, the predicted HS20-44 deflection was 7.1 mm (0.28 in.), which

resulted in a span/deflection ratio of L/1,242. For load test 2,

the predicted HS20-44 deflection was 7.9 mm (0.31 in.), 

Figure 16—Load test 1 comparisons (looking

east): (top) the sum of the measured deflections

from load cases 1 and 2 to the measured deflections for load case 3; (bottom) the sum of the

measured deflections from load cases 4 and 5

to the measured deflections for load case 6.
13

Figure 17—Load test 2 transverse deflection measured at the bridge centerspan (looking east). Cross sections of the

bridge and vehicle positions are shown to aid interpretation and are not to scale.
14

which resulted in a span/deflection ratio of L/1,123. A 30-

percent lower interlaminar compression for load test 2 compared with load test 1 caused an approximate 10-percent

greater predicted HS20-44 deflection. Thus, the stiffness of

the bridge may have decreased at the lower interlaminar

compression level for load test 2.

Condition Assessment

General condition assessments indicated that the structural

and serviceability aspects of the Christian Hollow bridge

were satisfactory. Results of specific areas follow.

Bridge Geometry

At the initiation of monitoring, a slight narrowing of the

bridge was noticed before applying the asphalt wearing

surface (Fig. 20). Width measurements indicated that the

cross section of the centerspan was 102 mm (4 in.) narrower

than the ends of the bridge. This was probably due to the

sequential bar tensioning with a single hydraulic jack. This

slight distortion has not increased in magnitude and should

not affect overall bridge performance.

Table 2—Summary of deflections from static load testing

Deflection (mm (in.))

Load test Measured

Predicted

HS20-44

HS20-44 span/

deflection ratio

1 7.4 (0.29) 7.1 (0.28) L/1,242

2 8.1 (0.32) 7.9 (0.31) L/1,123

Figure 18—Load test 2 comparisons (looking

east): (top) the sum of the measured deflections from load cases 1 and 2 to the measured

deflections for load case 3; (bottom); the sum of

the measured deflections from load cases 4 and

5 to the measured deflections for load case 6.

Figure 19—Comparison of measured deflections

for load tests 1 and 2 (looking east).
15

Wood Components

Visual inspection of the wood components indicated no

signs of deterioration or damage. All bolted connections

remained tight with no signs of wood member crushing

beneath the connectors. A cardboard “corner-protector” was

caught between laminations (Fig. 21) and caused a short

separation between adjacent laminations at the underside of

the bridge that potentially could have caused performance

problems. Fortunately, the effect of this small separation

between laminations on the performance of the bridge appears

to be insignificant.

Excess creosote preservative was bleeding from members

with a southern exposure and stained several steel bearing

plates (Fig. 22, top). In addition, the side and top surfaces of

the rail members had a visible layer of residue (Fig. 22,

bottom) that was leaching from the glulam members. Proper

treatment procedures, including a post-treatment vacuum or

steam process, could have alleviated this problem.

Wearing Surface

The asphalt wearing surface is in good condition, with minor

transverse reflective cracking visible near the supports of the

bridge. Reflective cracking near supports is typical of simplespan bridges and was expected. Minor longitudinal rutting,

approximately 6.4 mm (1/4 in.) deep, was evident at the

conclusion of monitoring, but it did not appear to collect

standing water because of the roadway grade. No longitudinal

asphalt cracking was observed.

Stressing Bar Anchorages

The steel bearing plate anchorage system is performing

satisfactorily. There are no signs of wood crushing beneath

the anchorage plates or corrosion on any of the steel components, including the exposed portion of the stressing bars.

Figure 20—Slight hourglass shape was noticeable

soon after initial stressing. Note stringline aligned

with web member in middle of photograph.

Figure 21—Cardboard corner strapping protector was

not removed at installation and caused a visible

separation between lower flange laminations.

Figure 22—Excess creosote preservative was

noticeable at several steel bearing plates (top) and

along the top surface of the glulam timber rail

(bottom).
16

Conclusions

Based on data collected during the 28 months of monitoring,

performance of the Christian Hollow bridge is satisfactory.

There are no structural or serviceability deficiencies that

would prevent the bridge from providing satisfactory performance in the future. Based on the monitoring results, the

following conclusions and observations are presented:

• The average trend in moisture content indicates that

both the glulam timber and sawn lumber components

behaved similarly with no significant changes in moisture content. At the conclusion of monitoring, the average moisture content was approximately 19 percent for

the glulam and 25 percent for the sawn lumber.

• The average trend in stressing bar force indicates that

sufficient bar force was maintained throughout the monitoring period. Bar force losses were primarily due to

stress relaxation of the glulam web and sawn lumber

flange laminations and warranted a bridge restressing after 1-1/2 years. Since that restressing, the bar forces leveled off at 67 kN (15,000 lb), or approximately 483 kPa

(70 lb/in2

) interlaminar compressive stress. Subsequent

measurements, taken 16 months after completion of the

monitoring, indicate that the bar forces were at 59 kN

(13,000 lb), or approximately 414 kPa (60 lb/in2) interlaminar compressive stress. Future inspections of the

Christian Hollow bridge should include bar force measurements to ensure that they remain greater than 48 kN

(10,800 lb).

• Vertical creep was small and approximately 5-mm

(0.19-in.) positive camber remained in the superstructure

at the conclusion of monitoring.

• Static load testing and analysis indicate that the Christian Hollow bridge is performing in a linear-elastic fashion. When the interlaminar compression decreased from

689 to 483 kPa (100 to 70 lb/in2), there was a

10-percent decrease in longitudinal bridge stiffness.

• Predicted maximum deflections for two lanes of

AASHTO HS20-44 loading were within the design

limit of L/500, where L is the span length measured

center-to-center of bearings. For load test 1 at 689 kPa

(100 lb/in2) interlaminar compression, the maximum

HS20-44 deflection is estimated to be 7.1 mm

(0.28 in.), or approximately 1/1,242 of the span length.

For load test 2 at 483 kPa (70 lb/in2) interlaminar compression, the maximum HS20-44 deflection is estimated

to be 7.9 mm (0.31 in.), or approximately 1/1,123 of

the span length.

• Visual inspections indicate no signs of deterioration of

the wood or steel components. Minor longitudinal rutting and reflective cracking were visible in the asphalt

wearing surface at the conclusion of monitoring.

References

AASHTO. 1989. Standard specifications for highway

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

State Highway and Transportation Officials.

AFPA. 1986. National design specification for wood construction. Washington, DC: American Forest and Paper

Association (formerly the National Forest Products Association). 87 p.

AFPA. 1988. Design values for wood construction, a supplement to the 1986 ed. of the national design specification.

Washington, DC: American Forest and Paper Association.

36 p.

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. 1989. Standards. Woodstock, MD: American

Wood-Preservers' Association. 200 p.

FHWA. 1990. Memorandum on crash tested bridge railings.

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

Federal Highway Administration.

FORINTEK. 1984. Moisture content correction tables for

the resistance-type moisture meter; SP511E. Ottawa,

Canada: FORINTEK Corp. 37 p.

Lopez–Anido, R.; GangaRao, H.V.S. 1993. Systematic

approach toward the design of stressed timber T- and boxbeam bridges. Report CFC–93–163. Morgantown, WV:

West Virginia University, College of Engineering,

Constructed Facilities Center.

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

2–5; London, England. London, England: Timber Research

and Development Association: 3.319–3.326. Vol. 3.

Ritter, M.A.; Wacker, J.P.; Duwadi, S.M. 1995. Field

performance of stress-laminated timber bridges on lowvolume 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. 1990. The timber bridge initiative, fiscal year 1990

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

Forest Service, State and Private Forestry, Northeastern

Area.
17

Appendix—Information Sheet

General

Name: Christian Hollow bridge

Location: Town of Greenwood, Steuben County,

 New York

Date of Construction: January 1992

Owner: Steuben County, New York

Design Configuration

Structure Type: Stress-laminated box-beam

Butt Joint Frequency: 1 in 4 laminations transversely,

0.9 m (3 ft) longitudinal spacing

Total Length (out-out): 9.14 m (30 ft), 9.02 m (29.6 ft)

as-built

Skew: 0 degrees

Number of Spans: 1

Span Length (center–center of bearings): 8.84 m (29 ft),

8.61 m

(28.25 ft),

as-built

Width (out–out): 9.75 m (32 ft), 9.91 m (32.5 ft),

as-built

Width (curb–curb): 9.14 m (30 ft), 9.30 m (30.5 ft),

as-built

Number of Traffic Lanes: 2

Design Loading: AASHTO HS20-44

Wearing Surface Type: Asphalt pavement, 51- to

76-mm (2- to 3-in.) thickness

Material and Configuration

Flange Laminations:

Species: Mixed hardwoods (sawn lumber)

Size : standard 38 by 140 mm (nominal 2 by 6 in.)

Grade: No. 2

Moisture Condition: approximately 26 percent at

initiation of monitoring

Web Laminations:

Species: Southern Pine (glulam)

Size (actual): 219 by 502 mm (8-5/8 by 19-3/4 in.)

Beam Designation: 24F-V3

Moisture Condition: approximately 20 percent at

initiation of monitoring

Preservative Treatment: Creosote

Stressing Bars:

Diameter: 16 mm (5/8 in.)

Number: 10 sets (through top and bottom flange)

Design Force: 96 kN (21,600 lb) interior; 104 and

88 kN (23,400 and 19,800 lb) two

exterior bars

Spacing (center–center): 0.91 m (3 ft) interior

1.1 m (3.5 ft) exterior

Type: High strength steel threadbar with course

right-hand thread, conforming to ASTM

A722

Bar Anchorage Type: Discrete steel plates

(with flat hex nut):

152 by 305 by 19 mm

(6 by 12 by 3/4 in.) bearing

51 by 127 by 25 mm

(2 by 5 by 1 in.) anchor

Rail and Curb System:

Design: Crash tested at Performance Level 1 on a

longitudinal spike-laminated deck

Species: Douglas-fir

Member Sizes: Rails: 140 by 235 mm (6 by

10-3/4 in.), 24F-V4, glulam

Posts: 184 by 286 mm (nominal 8

by 12 in.) Dense Select

Structural, sawn lumber

Curbs: 140 by 286 mm (nominal 6

by 12 in.) No. 1, sawn

lumber