United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL-RP-495

Stress-Laminated

Wood Bridge Decks

Experimental and

Analytical Evaluations

Michael G. Oliva

Al G. Dimakis

Michael A. Ritter

Roger L. Tuomi
Abstract Contents

Construction of stress-laminated timber bridge decks by

transverse prestressing of wood laminae is a relatively

new concept that originated in Ontario, Canada. This

report presents results of recent research completed

on solid wood stress-laminated decks. The primary

objective was to quantify the behavior of the bridge

decks when loaded with simulated truck loadings.

Identification of an accurate analytical technique for

predicting deck response and development of a new

simplified prestressing anchorage system were also

achieved. Our objectives were accomplished through

an experimental laboratory test program on fullsize bridges followed by computer-aided analytical

correlation studies. A summary of the project and

experimental and analytical results is presented with

conclusions and recommendations for tasks that should

be completed before stress-laminated decks are used in

bridge construction.

Keywords: Timber, bridges, stress-laminating, prestressing, bridge decks, orthotropic plates, creep.

March 1990

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

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

decks: experimental and analytical evaluations. Res.

Pap. FPL-RP-495. Madison, WI: U.S. Department of

Agriculture, Forest Service, Forest Products Laboratory.

24 p.

A limited number of free copies of this publication

are available to the public from the Forest Products

Laboratory, One Gifford Pinchot Drive, Madison, WI

53705-2398. Laboratory publications are sent to more than

1,000 libraries in the United States and elsewhere.

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

Background ................ 2

Objectives and Scope ............ 2

Experimental Procedures ........... 3

Deck Construction ............. 3

Deck Testing ................ 4

Material Properties ............ 4

Loading ................. 5

Prestress Distribution ........... 6

Data Collection ............... 6

Prestress Distribution ........... 6

Vertical Deflections ............ 6

Bridge Deck Testing Results .......... 6

Vertical Loading .............. 6

Prestress Distribution ............ 8

Loss of Prestress Over Time ......... 8

Analytical Procedures ............. 10

Vertical Loading .............. 10

Continuous Laminae ........... 11

Steel Channels .............. 13

Butt Joints ................ 13

Prestress Distribution ............ 13

Analysis of Results .............. 14

Moduli and Level of Prestress ........ 14

Loss of Stress Over Time .......... 15

Steel Channels ............... 15

Conclusions ................. 15

Analytical Capability ............ 15

Elastic Constants .............. 15

Loss of Stress Over Time .......... 17

Prestress Distribution ............ 17

Recommendations .............. 17

Literature Cited ............... 18

Appendix: Plate Tests ............. 19

The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin.
Stress-Laminated Wood Bridge Decks

Experimental and Analytical Evaluations

Michael G. Oliva, Associate Professor

Al G. Dimakis, graduate student1

Department of Civil and Environmental Engineering

University of Wisconsin, Madison, Wisconsin

Michael A. Ritter, Research General Engineer2

Roger L. Tuomi, Research General Engineer3

Forest Products Laboratory, Madison, Wisconsin

Introduction

The U.S. Secretary of Transportation has estimated

that 99,800 structurally deficient bridges exist in nonfederal-aid highway networks. Most of these bridges are

the responsibility of State or local governments or of

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

It is obvious that a substantial need has developed

for economical new bridges and for rehabilitation of

existing bridges. This report focuses upon evaluating

the behavior of timber bridges fabricated using a

new method of longitudinal deck assembly. This new

method of assembly, stress laminating, may provide an

economical method of building new timber decks as

well as rehabilitating existing nail-laminated lumber

bridge decks.

In addition to high maintenance costs, nail-laminated

lumber decks present a delamination problem. An

alternative to the existing method of nailing may be

found by stress-laminating the bridge deck. Deck

lumber in older bridges is laminated together by closely

spaced nails. When a wheel load is placed over a

lamina, the load is partially transferred into adjacent

laminae by the nails, acting as shear dowels.

1

 Now Research Engineer, Weyerhaeuser, Tacoma,

Washington

2

 Formerly Structural Engineer, National Forest

System, Division of Engineering, Washington, D.C.

3

 Retired.

Cyclic swelling due to moisture content variation and

movement from repeated loading, however, causes

the nails and laminae to loosen with time. Once the

nails loosen, the transverse transfer of wheel loads

decreases, and the deck no longer efficiently resists the

loads, resulting in increased flexibility and movement

of laminae. In this situation, maintenance becomes

expensive because the deck movement creates rapid

deterioration of the wearing surfaces.

Transverse prestressing or stress laminating the deck

enables shear to be transferred among laminae by a

means other than a mechanical fastener. The wheel

load is distributed among the laminae in this new system by friction and bending when the deck is stress

laminated together. The deck will be able to resist

transverse bending if the induced precompression is

high enough to counteract the transverse flexural tension stress. With this combination of load transfer and

resistance mechanisms, the stress-laminated deck outperforms a nail-laminated deck and avoids the deterioration problem. However, it is essential that sufficient

precompression be induced during construction and remain in the deck over its service life if the laminating

by stress is to be successful. Stress laminating is a relatively new system of bridge construction. Therefore,

methods need to be identified and developed for applying the prestress, for determining the necessary initial

levels of prestress to compensate for expected losses

over time, and for estimating the load-carrying capacity

of the bridges.
Background

The U.S. Transportation Secretary’s third annual report to Congress (1982) provided a definitive description of the deteriorated condition of our highway bridge

system. Two hundred forty thousand bridges were

listed as structurally deficient or functionally obsolete, representing an estimated replacement cost, of

$18.8 billion. McWilliams (1984) indicated that the

Federal Highway Administration found the rural collector system of roads to be in the poorest condition, with

90 percent of the listed bridge deficiencies.

The continued decline in the quality of the highway

system, and bridges in particular, is due to advancing

age, deferred maintenance, and heavier or more frequent truck loading. Nearly 140,000 of the off-system

bridges are more than 40 years old and were designed

for lower loads than are currently used in bridge design

or experienced by actual bridges.

The first use of stress laminating to rehabilitate and

upgrade the capacity of nail-laminated timber decks,

and later use for replacement bridge construction, occurred in Ontario, Canada. Research completed by the

Ontario Ministry of Transportation and Communication (MTC) in cooperation with Queen’s University

(Batchelor and others 1979, 1981, 1982; Taylor 1981)

showed that posttensioning considerably improved

the load distribution characteristics of decks. Based

on their findings, the Ontario Highway Bridge Design

Code (OHBDC) (OMTC 1983) now includes specifications for the design of stress-laminated wood deck systems that have steel channel bulkheads located along

the edge to distribute the stressing. The OHBDC design techniques vary significantly from present design

techniques in the American Association of State Highway and Transportation Officials Standard Specifications for Highway Bridges (AASHTO 1986). The load

distribution assumed for prestressed bridges in the

OHBDC is considerably higher than that currently

allowed in the AASHTO specification for wood deck

bridges with nailed connections. The AASHTO specifications do not have separate distribution factors for

prestressed bridges.

Although transverse prestressing is apparently being

used successfully in Ontario, there is a lack of information on the time-dependent losses of prestress, the

optimum distribution of prestress, the environmental

effects, and the actual load distribution characteristics.

Simple design guides are also needed. Additional research seemed necessary, particularly with large-scale

models, to identify more clearly the problems or benefits associated with the actual construction of this type

of system.

The construction and testing of the deck described

in this report, referred to as the FPL deck, followed

a research program conducted on another stresslaminated deck as part of the University of Wisconsin’s

research on solid-sawn stress-laminated wood decks.

The earlier deck, which will be referred to in this

report, was designated the Wheeler deck and was

assembled using 4- by 16-in. timber laminae with

a span length of 48 ft. A complete description of

construction and testing of that deck was reported by

Oliva and Dimakis (1986).

Objectives and Scope

The overall objective was to verify the desirable behavior of the bridge decks under simulated truck loadings that had been identified in earlier Ontario studies

and the general design method compiled in Ontario.

Secondary objectives included developing an analytical modeling technique for predicting the behavior of

stress-laminated decks and for identifying new stressing

and anchorage systems that could economically replace

methods required in the Ontario approach.

This report describes the extensive research that was

completed during a first-phase study on the performance of full-size solid-stressed deck systems. Numerous simulated truck load tests were conducted on

bridges, with and without steel channels placed at the

edges of the deck, to investigate load resistance and

distribution of prestress induced into the deck from

the tensioning rods and the possibility of replacing

the channels with alternative systems. The decks underwent various load tests in the University of Wisconsin Laboratory for Structures and Material Testing, and basic analytical correlation work was completed. In the remainder of this report, a brief summary of the project is followed by a detailed description

of the test program. Experimental and analytical results are presented, along with conclusions and recommendations for tasks that should be completed before

stress-laminated decks are commonly used in bridge

construction.

2
Experimental Procedures

Verification of the behavior reported in the Ontario

studies and the development of new, more economical

stressing methods could only be achieved through a

series of experimental tests. Analytical simulation

of the response of bridge decks could also be used

to efficiently examine how a wide range of design

modifications might improve bridge behavior. But

first, the analytical techniques were verified through

comparison with the experimental data.

Several bridges of various configurations were tested

in the laboratory to measure their overall vehicle-loadresisting capability with full-span wood laminae and

butt-jointed laminae. The impact of various stressing

arrangements, such as spacing of prestressing rods and

using steel channel sections or plates at rod anchorages,

was measured by comparing the bridge performance

when the arrangements were modified. Achieving the

necessary level of prestress, without using hydraulicjacking equipment, was also attempted in laboratory

studies by using specially threaded rods and tightening

anchor nuts to a specified torque.

Deck Construction

The stress-laminated bridge decks assembled in the

laboratory were of the configuration and components

as follows:

• The deck was 24 ft long (clear spans of 12 and

23.5 ft) and 8 ft 8 in. wide and made of 70 vertical

laminae nominal surfaced 2- by 12-in. Douglas-fir

No. 1 or Better

• A steel channel (C10 × 15) was used on both sides

of the deck when needed to perform certain tests in

combination with small bearing plates

• Prestressing rods were spaced at 2- and 4-ft intervals

• Rods were 3/4 in. diameter, 150 × 103 lb/in2 steel,

and 9- by 9- by 3/4-in. steel plates were used as

bearing plates when the steel channels were not

used.

The design of the experimental bridge was in general

accord with the OHBDC design specifications. In the

initial configuration, the deck consisted of 2- by 12-in.

by 24-ft full-span laminae. Subsequently, the laminae

were cut to lengths of 6, 10, 14, and 18 ft and laid out

such that butt joints in adjacent pieces occurred in a

precise sequence.

The deck was tested at two different span lengths, 12 ft

and 24 ft. Rods to prestress the deck were placed at

2-ft intervals initially and later at 4-ft intervals to study

the effect of rod spacing on the stiffness and prestress

distribution.

Construction of the deck was possible without the

assistance of heavy machinery. Several laminae were

transferred from the stockpile inside the laboratory

to their permanent position, and rods were inserted

through the predrilled holes. This procedure was

repeated until construction of the deck was completed.

The flexibility of the small prestressing rods (3/4-

in. diameter) and misalignment problems made it

necessary to assemble the deck laminae and insert the

rods simultaneously. When all the pieces were in place,

the system was prestressed.

The objective of developing a simple stressing method

was pursued by trying a manual torquing procedure as

a means of applying prestress by relating rod tension to

applied torque on an anchor nut. The special threaded

high strength rods, manufactured by Dywidag Inc.,

which were used on the Wheeler deck (Oliva and

Dimakis 1986), cannot be calibrated reliably due to the

long pitch of the thread. Rods used on this deck were

standard machine threaded with a very small pitch.

Torque calibration tests conducted using a torque

wrench prior to testing of the deck gave unreliable

results. An attempt to improve the results by using

Teflon4 washers was unsuccessful, thus the procedure

was abandoned. All subsequent stressings were done

using a hydraulic jack and pump.

The major problem encountered during construction

was inserting the rods through the predrilled holes.

Misalignment of the holes or warping of the wood can

delay construction considerably. The holes should

be drilled oversize to twice the bar diameter to make

insertion possible.

Illustrations of the construction of the FPL deck,

the Wheeler deck (Oliva and Dimakis 1986), and the

4

 The use of trade or firm names in this publication is

for reader information and does not imply endorsement

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

or service.

3
Figure 1—FPL deck and prestressing details.

(ML89 5804)

hydraulic jack used for stressing are shown in Figures 1

through 4.

Deck Testing

Two types of behavior were investigated during the

experimental phase of the research. First, tests were

conducted to obtain the load distribution behavior

of the deck under a simulated truck double-wheel

load applied at various locations. This information

was used to obtain the stiffness of the deck and to

verify the analytical capability of the model using

orthotropic plate theory. Second, tests to obtain the

relative distribution of induced compression stresses

in the wood created by prestressing were performed

to evaluate the effectiveness of the steel channels or

Figure 2—Assembling the FPL deck using

butt joints. (M89 0119)

Figure 3—The assembled FPL deck with

a reference frame mounted above and the

hydraulic actuator at midspan. (M89 0117)

alternative methods of anchorage in distributing the

stresses uniformly into the wood.

Material Properties

Young’s modulus of elasticity (MOE) in the direction

of the laminae is independent of the prestress level in

the deck (Batchelor and others 1981). Therefore, before building the entire deck, each lamina was tested to

obtain this material property by measuring its elastic

natural frequency of vibration in flexure. Additional

testing to determine the shear moduli of the wood deck

is described in the appendix.

4
Figure 4—Hydraulic jack used to prestress the

Wheeler deck. (M85 0256-15)

Loading

The stressed deck was loaded with a simulated doublewheel load applied at midspan. The load was applied

over an area of 19.5 in. transverse and 15 in. longitudinal. Three different types of loading were used (Fig. 5):

a point load, a two-point load applied symmetrically in

a lane, and a two-point load applied asymmetrically in

a lane. Truck load tests were conducted with clear span

lengths of 23.5 ft and 12 ft.

This testing procedure was followed while investigating

the deck under various rod-spacing arrangements and

deck configurations, as described in the following.

Rods spaced at 2-ft intervals—Tests with rods at 2-

ft intervals were conducted with full-span laminae for

seven levels of average prestress compression, 10, 20,

30, 40, 50, 75, and 100 lb/in2.

Rods spaced at 4-ft intervals—Upon the completion of

the testing on the deck with the 2-ft rod spacing, every

other rod was removed, and the deck was tested at five

different internal levels of prestress, 10, 20, 30, 40, and

50 lb/in2. The tensioning force in each rod had to be

doubled to obtain the same average level of prestress

as in the previous case. Each individual lamina was of

full-span length.

Deck with channels at its sides—A steel channel on

each side of the deck replaced the 9- by 9-in. steel

plates at anchorages, and tests were completed for only

one level of prestress, 50 lb/in2.

Figure 5—Three different methods of applying load used in the deck load tests. “Point

loads” were applied over a small wheel imprint area. (ML89 5806)

Deck with butt joints—One benefit of the stresslaminated deck is that shorter than full-length members

can be used, because the system depends primarily

on the friction among the laminae to distribute the

load. The laminae in the deck, which were originally

of full-span length, were cut in a predetermined pattern

to evaluate the effect of butt joints on the stiffness of

the deck. The cut laminae, in lengths of 6, 10, 14, and

18 ft, were laid out in such a manner that butt joints

in adjacent pieces occurred in this precise sequence:

(a) only one lamina in four was allowed to have a

butt joint within any 4-ft length parallel to the bridge

span; and (b) butt joints in adjacent laminae had to be

separated by at least 4 ft in the direction of the span.

Tests on the bridge with short laminae were conducted

for one level of prestress, 50 lb/in2

.

Deck with butt joints and steel channels at its sides—

Steel channels were again substituted for the 9-in.-

square anchorage plates, and the same tests were

carried out. The level of prestress in the deck was

50 lb/in2.

5
Prestress Distribution

The OHBDC requires steel channels along the edges

of stressed wood decks to distribute the prestress uniformly into the wood laminae. To evaluate the effect

of the steel channels, the deformations in a transverse

direction were measured within a region of the deck adjacent to a single rod as the prestress in the rod was

varied. The test was repeated three times: once with

the rods spaced at 2-ft intervals, then with rods spaced

at 4-ft intervals with only the 9- by 9-in. steel bearing

plates, and finally with the steel channels replacing the

plates and using 4-ft rod spacing.

Figure 6—Transverse section of the deck

showing the seven locations of vertical displacement measuring LVDTs, which were

reproduced at six stations along the length.

Data Collection (ML89 5809)

Two different data-measuring devices were used during various stages of the project to measure the behavior of the system. One device involved strain gauges

in load cells in a full-bridge arrangement, while the

other consisted of linear variable differential transformers (LVDTs). Strain indicator and switch and balance boxes were used to read the data from the load

cells, and data from the LVDTs were collected by a

computer-aided data acquistion system.

Prestress Distribution

A set of displacement-measuring LVDTs was attached

in a special pattern to the deck near a single rod to

measure transverse compression of the deck as the prestress in the single rod varied. One set of the LVDTs

was placed at intervals near the edge of the deck to

measure compression occurring in the first 17-1/2-in.

width of wood laminae. Two additional sets of LVDTs

placed over two subsequent 17-1/2-in. widths of laminae and ending at the center of the lane measured compression in the interior of the deck.

Vertical Deflections

Deflections were measured at six stations along the

length of the bridge and at seven places in the transverse direction (Fig. 6). The complete deflected shape

of the deck was determined from the 42 deflection

measurements taken from one-fourth of the deck.

LVDTs with a linear range of 1 in. were used as the

displacement-measuring devices. The LVDTs were attached to a separate reference frame above the deck,

which in turn was attached to the deck at the support

piers to measure deck displacement relative to its supported ends.

Bridge Deck Testing Results

Vertical Loading

Vertical deck loading, the first part of the testing, gave

the load distribution characteristics of the system. The

load was applied through a hydraulic actuator attached

to a heavy-duty load frame that was erected on the

laboratory’s tiedown slab.

The plate-like action of the deck with continuous

laminae, no steel channels, and a l-in. deflection at the

center of the deck is shown in Figure 7. The system

behaved in a linear elastic manner with an average

stiffness of 36 × 103 lb per inch of deflection at the

center. In this case, the rod spacing was 2 ft, and the

average prestress in the deck was 50 lb/in2

.

The effect of various levels of prestress in the deck on

the load distribution behavior from the first series of

tests is shown in Figure 8. The 0-lb/in2 level represents

the deck with loose dowel action. Even a small amount

of induced prestress greatly improves the load distribution characteristics of the deck, because interlaminar

friction provides for transfer of shear forces.

In the second series of tests, half the rods were removed

to increase the spacing of the rods to 4 ft. Then the

deck was load tested at five different levels of prestress.

At a prestress level of 50 lb/in2, the stiffness, defined

as the load per inch of deflection at center, was similar

to that of the deck with rods spaced at 2-ft intervals.

A plot of the stiffness and various levels of prestress

in the deck for two different rod spacings (Fig. 9)

suggests that there is no reduction in stiffness when the

6
Figure 7—Displacement profiles at various

longitudinal (a) and transverse (b) sections

with an applied displacement of 1 in. at the

center of the deck. The deck had continuous

laminae and no steel channels along the edges.

Stressing rods were spaced at 2-ft centers and

loaded to produce an average transverse stress

of 50 lb/in2

. In (a), the profiles from near

the deck edge are shown near the top and

those at the center on the bottom. In (b),

the profiles near the support are shown on

top and those at the center at the bottom.

(ML89 5808; ML89 5807)

same average prestress exists. With 4-ft rod spacing,

the system behaved in a linear elastic manner at any

constant. level of prestress.

The effect of the heavy steel channels on the overall

stiffness of the deck was noted during testing of the

Wheeler deck (Oliva and Dimakis 1986), which had

a small aspect ratio of 0.19. (Aspect ratio is defined

as the ratio of the width in plan to the span length of

the bridge.) The theoretical stiffness of that deck with

channels was 16.5 × 103

 lb per inch, which is 38 percent

Figure 8—Transverse displacement profiles

at the center of the deck for various levels of

prestress. (ML89 5810)

Figure 9—Comparison of deck stiffness

(measured in 103

 lb of load per inch of

deflection at the center) and the level of

uniform prestress in the deck for both 2- and

4-ft rod spacings. (ML89 5811)

greater than the 12 × 103 lb per inch predicted for the

deck without channels (Oliva and Dimakis 1986).

The stiffness effect of the channels in the FPL deck is

shown in Figure 10. The FPL deck, with an aspect

ratio of 0.36, had a measured stiffness of 47 × 103 lb

per inch with the channels on its sides, which is an

increase of 31 percent over the stiffness of the deck

without channels (36 × 103 lb per inch). As expected

from a comparison of the Wheeler and FPL decks, the

contribution in stiffness from the channels decreases as

the aspect ratio of the system increases.

7
Prestress Distribution

Figure 10—Comparison of measured transverse displacement profiles at midspan for the

FPL deck with and without steel channels.

(ML89 5812)

Figure 11—Comparison of transverse displacement profiles at center span for the deck with

and without butt joints. (ML89 5813)

Using pieces of laminae that are shorter than fullspan length is feasible, but a significant reduction in

stiffness takes place, as shown in Figure 11, which

compares the transverse displacement profiles for the

deck, with and without butt joints. The stiffness of

the deck with the butt joints was 29 × 103

 lb per

inch, and without the butt joints it was 36 × 103 lb

per inch (both without steel channels). The system

behaved linear elastically with a reduction in stiffness

due to joints of approximately 20 percent. Further

study of Figure 11 indicates that the decrease in the

overall stiffness is primarily due to the decrease in the

longitudinal stiffness of the deck, because the transverse

displacement profile maintains a similar shape.

Three tests were conducted to investigate the transverse deformation near a stressed bar. In each test, an

additional force was applied to one of the rods in increments of 2.7 × 103

 lb, up to 27 × 103 lb above the normal rod force, and the transverse displacements were

recorded for each increment of load. As described previously, LVDTs measured the compression of the wood

due to the prestress force; placement of the LVDTs is

shown in Figure 12. This placement assumes symmetry

with respect to the longitudinal centerline of the deck.

Figures 13 and 14 present data from the various prestress distribution tests previously described. The deformation is higher at the immediate region of the applied load than at a short distance to either side, as

expected, indicating that the prestress is effectively

transferred over a narrow width near the outside of the

deck. This phenomenon occurs regardless of the spacing of the rods or whether steel channels are included

in the bulkhead system. Figures 13a (rods at 2-ft intervals) and 13b (rods at 4-ft intervals) indicate that the

deformation becomes nearly uniform at location 3, less

than 3 ft from the edge of the deck. The total deformations in Figures 13a and 13b are not equal, because the

initial induced stress in the deck was different for 2-ft

and 4-ft spacings. The initial level of prestress in the

deck with rods spaced at 2-ft intervals was double that

of the deck with rods at 4-ft intervals.

A comparison of Figures 13b and 13c shows that steel

channels as bulkheads do not significantly affect the

distribution of prestress away from the edge of the

deck. Figure 14a compares compressive displacement

between the deck with and without channels, and

it can be seen that at locations near the edge of the

deck there are some differences. However, at location

3, toward the interior of the deck, the transverse

deformation and thus the prestress are essentially the

same (Fig. 14b).

Loss of Prestress Over Time

Relaxation tests were conducted by Oliva and Dimakis (1986). Figure 15a shows a typical short duration (210 h) creep curve obtained after the initial

stressing of the rods, to a force level in the range of

70 to 75 × 103 lb (90- to 98-lb/in2 stress in the deck).

8
Figure 12—Placement of LVDTs to measure transverse compression of the deck and to determine the distribution of prestressing

force in the deck. (ML89 5814)

This level of prestress is well below the allowable compressive transverse stress of 625 lb/in2

 in the wood.

Most of the loss in the stressed bars takes place immediately after stressing. The loss at the end of the 9-day

monitoring period was nearly 33 percent.

wood had been restressed to the 50-lb/in2 level several

times during the load tests and before the creep study

was started. The curve shows a more than 50 percent

loss of prestress. The deck was stored in a heated

laboratory; this unusually large loss was probably a

result of loss of moisture and shrinkage of the deck.

Figure 15b shows a typical long duration (9 months)

creep and relaxation curve based on a loss study

started after the deck had been stressed for approximately 5 months during the load-testing phase. The

9
Analytical Procedures

The analytical phase was conducted in stages. First, a

basic model was created to simulate the deck behavior

under gravity load. In this stage, the stiffening effect of

the steel channels was included when appropriate, and

discontinuity of laminae was considered by artificially

decreasing the longitudinal stiffness of the deck. Then,

another model was created to simulate the transverse

deformation in the deck, induced by the force from the

tensioning rods.

A commercial finite element analysis computer program

was selected to model the behavior of the wood deck.

A four-node shell element with orthotropic material

properties and six degrees of freedom per node was

selected to model the deck in its compressed stage,

because the material truly behaves in an orthotropic

manner. The channels on the sides of the deck were

modeled with beam elements having six degrees of

freedom per node. The relationships obtained from

the plate tests, described in the appendix, were used

in deriving the material properties, ET and GLT. EL

was obtained from the tests on the lumber and was

independent of the level of prestress in the deck but

was dependent on butt joints. In all analyses, the

Poisson’s ratio was taken as zero. The effect of this

assumption on the accuracy of the analytical results

is insignificant (Pama and Cusens 1968). Simulations,

unless otherwise noted, were conducted with a level

of prestress in the deck equal to an expected service

condition stress, 50 lb/in2.

Vertical Loading

Meshes of the computer models were assembled for

two different spans. The x and y coordinates of the

nodes were placed coincident with the locations of

the LVDTs that were used to measure displacements

during the experimental tests. Thus, the analytical

nodal displacements could be directly compared with

experimental results. Due to symmetry in loading

and boundary conditions, only one-quarter of the deck

was used in the analysis (except for the unsymmetric

load case where one-half of the deck was considered).

Several analyses were undertaken, and the results

were compared with the measured experimental deck

behavior.

10

Figure 13—Compressive displacements at

various locations with respect to the edge

of the deck (see Fig. 12). In (a), the transverse rods were spaced at 2 ft and in (b) at

4 ft. For both (a) and (b), there were no steel

channels at the edge. In (c), the transverse

rods were spaced at 4 ft, and load was applied through a steel channel at the edge.

Location 1 is from the edge to 17.5 in., location 2 is from 17.5 to 35 in., and location 3

is from 35 to 52.5 in. from the edge. The displacements are normalized to strain units.

(ML89 5815; ML89 5816; ML89 5805)
Figure 14—Comparison of the compressive

displacements in the deck caused by the

transverse stressing with and without the

steel channels as a bulkhead. For rods at

4-ft spacing, (a) shows a comparison of

displacements at location 1 (see Fig. 12)

and (b) shows a comparison at location 3.

(ML89 5817; ML89 5818)

Continuous Laminae

Figure 16 (deck with continuous members, without

channels, and rods spaced at 2-ft intervals) illustrates

the correlation between analytical and experimental

results and for center-point loading and two-point loading, applied symmetrically or asymmetrically. The span

length is 24 ft. Figure 17 shows the excellent agreement

among the longitudinal displacement profiles along the

center line. The elastic material properties ET and GL T

depend on the level of prestress in the deck and increase linearly with an increase in prestress. When the

appropriate values were used in the analysis (ET and

GLT obtained from the moduli compared to prestress

relations given in the appendix) for the same deck under a 100-lb/in2

 level of prestress, the results indicated

a good correlation between analytical and experimental

results (Fig. 18).

Figure 15—Stress loss in a prestressing rod

in a 9-day test (a) and a 9-month test (b) for

the Wheeler deck (Oliva and Dimakis 1986).

For the 9-day test, (a) represents the stress

loss following the initial stressing of the

rods to produce an average normal stress in

the wood of 90 to 98 lb/in2. Before the 9-

month test began (b), the wood had been

repeatedly stressed to 50 lb/in2 over a 5-

month period and was stressed to this level

at the start of the creep study. Stress ratio

is the stress at time T divided by the initial

stress. (ML89 5819; ML89 5820)

In a later test, both supports of the deck were moved

inward to reduce the span to 12 ft, and the deck was

tested under the same loads. Equally acceptable agreement between measured and predicted displacements

occurred with the shorter span.

11
Figure 16—Comparison of analytical results

and experimental transverse deck displacement profiles at midspan for the deck with

continuous laminae, without steel channels

and with rods on 2-ft centers. Deck span

was 24 ft, and the transverse stressing was

at 50 lb/in2. Comparisons are shown for

three loadings: (a) Point load at center at

midspan, (b) two point loads applied symmetrically at midspan, and (c) two point

loads applied unsymmetrically at midspan.

(ML89 5821; ML89 5822; ML89 5823)

Figure 17—Comparison of analytical results

and experimental longitudinal deck displacement profiles along the deck center line for

the deck with continuous laminae, without

steel channels, and with rods on 2-ft centers. Deck span was 24 ft, and the transverse stressing was at 50 lb/in2

. The comparison shown is for a point load at center at

midspan. (ML89 5824)

Figure 18—Comparison of analytical results and

experimental transverse deck displacement profiles at

midspan for the deck with continuous laminae, without

steel channels and with rods on 2-ft centers. Deck

span was 24 ft, and the transverse stressing was at

100 lb/in2

 (compared with Fig. 16 (a) at 50 lb/in2).

The comparisons shown are for a point load at center

at midspan. (ML89 5825)

12
Steel Channels

Another set of experiments involved deck testing with

the steel channels on the sides. The analytical models

had to be modified to include the stiffening effect of the

channels. The agreement between predicted and actual

displacements is shown in Figure 19 for center-point

loading with two different levels of prestress.

Butt Joints

The last tests were conducted on the deck with butt

joints. The experimental results clearly indicate that

the effective longitudinal stiffness is reduced when butt

joints are introduced into the system. A reduction in

stiffness was implemented in the plate analysis by decreasing the longitudinal modulus EL. Reducing stiffness by 25 percent (because one of any four laminae

may have a butt joint in any 4-ft length along the deck)

gave good agreement between analytical and experimental results. If any portion of a laminae 2 ft to either side of a butt joint was considered ineffective, the

longitudinal stiffness would theoretically be decreased

by 21 percent. Butt joints have little effect on the

transverse stiffness. The correlation of analytical and

experimental results is shown in Figure 20 when a longitudinal stiffness reduction of 21 percent due to butt

joints is used.

Finally, an analysis was performed on the deck with

butt joints, including the contribution in stiffness from

the steel channels. Again, good agreement between

analytical and experimental results was obtained.

Prestress Distribution

Plane stress-plane strain finite elements were used to

analytically model the deck and the anchorage plates,

and a beam element was used to model the steel channel team to try to simulate the compressive deformation of the deck caused by a prestress force. The 2 and

y coordinates of the nodes coincided with the location

of the LVDTs that were used to measure the transverse

in-plane displacements. The experimental data from

the Wheeler deck (Oliva and Dimakis 1986) were used

to verify the analytical model (Fig. 21).

Figure 19—Comparison of analytical results and experimental transverse deck displacement profiles at midspan for the deck

with continuous laminae, with steel channels, and with rods on 2-ft centers. Deck

span was 24 ft, and the transverse stressing

was at 50 lb/in2 for (a) and 100 lb/in2 for

(b). The comparisons shown are for a point

load at center at midspan. Compare with

Figures 16(a) and 18 for the deck having no

channels. (ML89 5826; ML89 5827)

plates were provided to distribute the rod-bearing pressure over a satisfactory wood area. An analysis was

done using 16- by 16- by 1-in. plates instead of the

steel channels on the Wheeler deck. Figure 22 indicates

that, although the deformation is larger near the edge

(Fig. 22a), at a distance into the interior of the deck

less than 3 ft from the edge (Fig. 22b) the deformations

using a channel or steel plate are essentially the same.

The displacement measurements taken near a prestressed rod indicate that the prestress is effectively

transferred into the wood over a narrow width near the

edge of the deck. This suggests that the steel channel

bulkheads would not be necessary if substantial steel

13
Figure 20—Comparison of analytical results

and experimental transverse deck displacement profiles at midspan for the deck with

butt joints, without steel channels, and with

rods on 2-ft centers. Deck span was 24 ft, and

the transverse stressing was at 50 lb/in2. The

comparison shown is for a point load at center

at midspan. (ML89 5828)

Analysis of Results

Moduli and Level of Prestress

The relationships obtained in Ontario between the

transverse and shear moduli and the level of prestress

are independent of species type. Twisting plate tests in

Ontario were conducted for Hem-Fir and red pine. Our

results from plate tests using Douglas-fir are somewhat

different. It is convenient to express the transverse and

shear moduli as a function of the longitudinal MOE.

A comparison among our study, the Ontario study,

and the relationships used in the OHBDC are shown

in Figures 23 and 24. A more conservative design is

expected by using the relationships obtained in our

study, because the lower transverse and shear moduli

result, in a narrower effective distribution width for

resistance of a wheel load.

Behavior of a 47-ft two-lane highway bridge was predicted using the stiffness values suggested by various

agencies. Figure 25 compares the displacement profiles across a transverse section. The higher transverse

stiffness values suggested by OHBDC and Queen’s University give, as expected, higher values for effective distribution width D. The D values obtained from the

various sources are as follows: 52 in. using AASHTO,

55 in. using the stiffness values recommended in the

study, 58 in. using the stiffness values recommended

Figure 21—Comparison between the analytical results and experimental compressive

displacements between the edge and center line of the deck (Wheeler deck) (Oliva

and Dimakis 1986) with steel channels.

(ML89 5829)

Figure 22—Comparisons of predicted compressive displacements in the deck using a

steel channel and a deck with just the 16-

by 16-in. steel plates. Displacements at

the edge of the deck are shown in (a) and

3 ft in from the edge of the deck in (b).

(ML89 5830; ML89 5831)

14
by Queen’s University, and 61 in. following OHBDC

recommendations.

Steel Channels

Loss of Stress Over Time

The relaxation and creep behavior of the system

should be well comprehended before construction of

the stressed deck. The OHBDC predicts a total loss of

stress of 50 percent over time, based on results obtained

from several tests conducted at Queen’s University

on small-scale models (Batchelor and others 1979,

1982). The Queen’s University results indicate that

changes in the relative humidity cause a change of

the same sign in the stress ratio (prestress at time T

divided by initial prestress) regardless of initial stress,

stage in the stressing sequence, or ratio of prestressing

element stiffness to stiffness of wood being compressed.

The magnitude of the eventual change in the stress

ratio depends upon the length of time the humidity

is maintained at the new level. The wood responds

more quickly to an increase in humidity than to a

decrease. Also, note that the decrease in stress ratio

for a given percentage decrease in humidity is larger

than the increase in stress ratio for the same increase in

humidity. This would indicate that a cycle in humidity

change would result in a net reduction in the stress

ratio. The 50 percent total loss assumption was verified

later by tests conducted on the Hebert Creek Bridge

(Taylor and others 1983).

However, recent unpublished information obtained from

Ontario suggests that an actual loss of 60 percent may

develop in some of the earlier bridges that used stress

laminating. A long-duration stress relaxation test conducted on the 47- by 9-ft Wheeler deck indicated a

loss of stress exceeding 50 percent. The test was conducted under controlled temperature but uncontrolled

humidity conditions and moisture contents. Comparison of results from laboratory tests conducted here

(9 months) and in Ontario (4 months) are shown in

Figure 26. (Additional data to complete the Queen’s

University plot were extrapolated from a log fit expression (Batchelor and others 1979).) A comparison of

long-duration creep results from a field test (Hebert

Creek Bridge) and a laboratory test (Wheeler deck)

is shown in Figure 27. The increased creep loss in the

Wheeler bridge test may have been a result of decreasing moisture in the wood and subsequent shrinkage.

A steel channel bulkhead to distribute the prestress

from the rod uniformly into the deck is required by

OHBDC. Extensive measurements of prestress distribution into the deck near a prestressing rod appear to indicate that the steel channel is unnecessary. We found

that the prestress distribution characteristics of both

bulkhead systems are essentially the same (Fig. 22b)

at a short distance from the edge. Removal of the steel

channels will effectively reduce construction complexity

and cost. When the aesthetic appearance of the sides

of the deck is important, use of the steel channel section may be preferred, because local crushing of wood

fibers and some splitting of the exterior laminae may

occur and be visible when only steel plates are used.

The same phenomena are likely to occur with channels

but are no longer visible.

Conclusions

Our conclusions are based on experimental testing and

subsequent analytical studies, conducted on prestressed

decks of different rod configurations, anchorage configurations, and span lengths constructed of two different

types of laminae.

Analytical Capability

A commercial finite element computer program with

orthotropic plate modeling capabilities can accurately

predict the behavior of stress-laminated bridge decks.

An orthotropic plate Fourier series algorithm was

successfully used to verify the finite element analytical

capability. The deformation responses obtained by both

analyses agreed closely. Both analytical techniques are

capable of providing satisfactory simulation of actual

stressed deck behavior as measured experimentally.

Elastic Constants

Longitudinal MOE can be obtained from static or

dynamic beam tests and should generally be near

published values for the wood species and grade, but

modification is necessary to account for loss of stiffness

when butt joints are used. The transverse and shear

moduli increase linearly with the level of prestress in

15
Figure 23—Ratio of ET /EL suggested by

various sources for use in estimating ET of

a prestressed deck. (ML89 5832)

Figure 24—Ratio of GL T /EL suggested by

various sources for use in estimating GLT of a

prestressed deck. (ML89 5833)

Figure 25—Predicted transverse deflection

profiles using deck stiffnesses suggested by

various sources. (ML89 5834)

Figure 26—Percentage of stress remaining

in the prestress rods after 9 months in the

Wheeler deck (Oliva and Dimakis 1986)

is compared with loss data from Queen’s

University and the expected long-term loss

suggested by the OHBDC. (ML89 5835)

Figure 27—Percentage of stress remaining in

rods (stress ratio) after long-term creep. The

Hebert Creek bridge losses are compared with

losses in the Wheeler deck. (ML89 5836)

the deck. Relationships developed in our research for

No. 1 Douglas-fir planks are ET = 149fp + 15,360

and GLT = 134fp + 16,600, where fp is the level

of prestress in the deck in pounds per square inch.

These relationships could hold true for other species

as well. Queen’s University concluded (Batchelor

and others 1981) from tests on three different species

that the variability of results from tests on the same

species was larger than from the various species. As

a result, the relationships of their moduli to level of

prestress are independent of species type. Elastic

moduli ET and GLT are usually expressed in terms

16
of the longitudinal MOE. Recommended ratios under

service conditions (i.e., level of prestress of 50 lb/in2

)

are ET = 0.0129EL and GLT = 0.0132EL. In designing

a prestressed deck, it is recommended that ET and

GLT be obtained first, using these relations. Then the

moduli and level of prestress relationships given in

this section should be used to solve for the constants

(15,360 and 16,600 for the Douglas-fir in this program)

by taking fp, = 50 lb/in2. The elastic constants ET

and GLT can then be obtained for the required level

of prestress. Note that these ratios vary significantly

from those recommended by Queen’s University or

the OHBDC, but they are more conservative. In the

analysis, the Poisson’s ratio can be taken as zero.

A limited parameter study, conducted by varying

the Poisson’s ratio, indicated that the effect of this

assumption on the accuracy of the analytical results

is insignificant.

Loss of Stress Over Time

A long-duration (9 months) stress relaxation test was

conducted under dry environmental conditions on the

Wheeler deck after multiple stressings of the rods. The

total loss of rod prestress exceeded 60 percent, which

exceeds that predicted by the OHBDC. The wood-tosteel stiffness ratios (ET Aw /EsAps, appendix) for the

decks tested were in the range of 0.8 to 1.2, which is

the practical range for systems of this type.

Ontario (Batchelor 1979; Taylor 1983) has completed

additional studies on prestress loss. Their results

indicate that changes in the relative humidity cause

a change of the same sign in the stress ratio (ratio of

prestress at time T to initial prestress). Also, note

that the decrease in stress ratio for a given percentage

decrease in humidity is larger than the increase in

stress ratio for the same increase in humidity. This

indicates that a cycle in humidity change would result

in a net reduction in the stress ratio. Cyclic humidity

changes might cause greater overall loss of stress than

expected at present.

Prestress Distribution

Assuming a 12-in. curb width, the minimum distance

between the edge of the deck and the centerline of

dual wheels is likely to be greater than 20 in. At

this distance from the edge, when plates of adequate

dimensions and stiffness are used for anchorage, the

prestress distribution from a stressed rod is nearly

uniform if rods are spaced less than 4 ft apart. A more

uniform distribution is obtained near the edge when

a heavy steel channel is used, as expected, but at 3 ft

from the edge, the distribution of prestress for either

the channel or the steelplate bulkhead systems is the

same. Stress distribution plots. obtained when plates

are used as the bulkhead system indicate that the stress

distribution from the stressed rod flows into the deck at

approximately a 45° angle from the rod centerline.

Recommendations

Tests conducted here and in Ontario have shown that

transverse prestressing considerably improves the

load distribution characteristics over those of a naillaminated lumber deck. The analytical capability of the

orthotropic plate theory has been verified extensively.

However, questions remain regarding design and use of

stressed deck bridges.

Tests on small-scale specimens conducted in Ontario

have shown that the wood responds quickly to a change

in relative humidity and that a cycle in humidity

change results in a net reduction in the stress ratio.

Difference in the scale between laboratory specimens

and actual structures influences the migration of

moisture and consequently the amount of prestress

loss that may occur due to moisture cycling. Specific

testing is needed to investigate cyclic moisture changes

that occur in a full-size creosote-treated wood deck and

their effect in changing the level of prestress.

Additional development work needs to be completed,

from the construction point of view, to allow the connection of multiple prebuilt panels with full prestressing during erection. A method was developed by Oliva

and Dimakis (1988) to connect two separate plant-built

panels and attain continuous prestressing across the

joint without needing any temporary supports or scaffolding. Similar construction methods should be developed for bridges with lane widths that require more

than two prebuilt panels.

Our study and the studies at Queen’s University in Ontario successfully used orthotropic plate modeling to

predict the behavior of stressed wood decks. However,

the relationships determined in the two research programs to define transverse stiffness and shear stiffness

17
moduli of the deck are substantially different. Additional parameter studies should be completed to identify the sensitivity of predicted results to those stiffness

moduli and to guide analysts in selecting the correct,

relationships.

Literature Cited

18
Appendix:

Plate Tests

Plate Tests to

Obtain Elastic Constants

A method proposed by Tsai (1965) was used to obtain

the elastic constants GLT and ET for various levels of

prestress. The method involved twisting tests on three

square plates of different laminae orientation. Data

from these tests were used to develop relationships

between the effective elastic moduli and the level of

prestress in the deck.

Young’s MOE

A dynamic MOE machine from the FPL was used

to obtain the longitudinal modulus of the individual

2 by 12 (nominal) lamina. Each beam was positioned

flatwise on two knife supports; cross-section dimensions were measured. Then, free vibration was imposed on the member, and a sensing device read the

natural period of the vibrating lamina. This information was used to obtain MOE, the effective material modulus parallel to grain. Each beam was tested

three times. The average MOE of the 78 pieces was

1.77 × 106

 lb/in2 with a coefficient of variation of

22.1 percent. This average was used in the analysis

stage of the study.

Twisting Plate Tests to

Obtain Elastic Constants

Elastic moduli ET and GLT (the transverse elastic

and shear moduli) are usually expressed in terms of

the longitudinal MOE. Typical relationships that are

used for Douglas-fir glulam decks or solid wood are

ET = 0.05EL and GLT = 0.078EL. These ratios, when

used in a previous study (Oliva and Dimakis 1986), did

not give good correlation between experimental and

predicted response. Ratios that provided the closest

analytic and experimental correlation were ET =

0.011EL, and GLT = 0.012EL for a level of prestress

in the deck equal to 50 lb/in2. Currently, the OHBDC

uses the ET = 0.05EL and GLT = 0.065EL in the design

of stressed wood decks. A study conducted at Queen’s

University indicates that these relationships are valid

for stressed decks made out of red pine under a high

level of prestress, 130 lb/in2

. This would indicate that

the elastic constants used by the OHBDC may not be

conservative, because the likely level of prestress in a

deck under service conditions may be around 50 lb/in2

due to long-term losses (with an initial minimum

stress of 100 lb/in2). Also, the same study gives ratios

even lower for the same high level of prestress for

other common species, such as Hem-Fir and white

pine. Separate tests were undertaken to determine

the stiffness moduli for the wood in the FPL deck

and compare them with the expected and measured

parallel-to-grain modulus.

The test method used in this study was proposed by

Tsai (1965) and consists of load application at the

corners of various plate configurations as diagrammed

in Figure 28.

Before testing, an analysis was performed to determine

the required dimensions of the plate. In the analysis,

the relationships of the Queen’s University study

were used, ET = 0.02EL and GLT = 0.03EL for a

low level of prestress. It was predicted that a plate

with dimensions 4 by 4 ft by 11.25 in. thick would

deflect 1 in. at the corner (0.25 in. at center), when

subjected to pure twisting action with a 10.5 × 103

 lb

load at the corner. The maximum principal stresses

were predicted as approximately 250 lb/in2, well

below the allowable stresses. Although a more flexible

plate would be preferable, space limitations made

it impossible. The accuracy of the LVDTs used to

measure the displacements was 0.001 in.

The laminae from the FPL deck were cut as needed

to fabricate the two plates. There were no butt joints

between laminae in the plate decks. The configurations

of the 0° and -45/+45° plates, and their dimensions

are shown in Figures 29 and 30, respectively. The

measuring devices consisted of an LVDT at the center

of the plate to measure the center line displacement

and a dial gauge at the corner to measure the corner

displacement. All displacements were measured with

respect to a rigid frame attached to the deck at the

supported corners of the plate. The plates rested on

bail-bearing supports at two corners and were pin

supported at the uplift corner (Fig. 31).

Each plate was tested under seven different levels of

prestress, 10, 20, 30, 40, 50, 75, and 100 lb/in2 of

average prestress between laminae in the plate. The

displacement at the center and the load were recorded

19
by a data acquisition system. The displacement at the

corner was read manually from the dial gauge. Load

cells were inserted directly under the supports and

under the load to measure reaction forces.

Each test consisted of 10 cycles of 0.05-in. displacement

increments at the corner of the deck for a particular

level of prestress. Figure 32 shows a typical load and

deflection at center plot, from tests conducted on the 0°

plate. Figures 33 and 34 represent typical stiffness plots

from the +45° and -45° plate tests, respectively. A

linear regression analysis was done on the data falling

in the linear elastic range to obtain the slope P/w for

each level of prestress. This information was used to

obtain the moduli ET and GLT as a function of the

level of prestress in the deck (Table 1). Finally, a linear

regression analysis was performed on these data to

come up with approximate relationships, which are

ET = 149fp

 + 15,360 and GLT = 134fp + 16,600.

These relationships, which hold true for the FPL deck

having a longitudinal MOE equal to 1.77 × 106 lb/in2,

are plotted in Figures 35 and 36 and are appropriate

when internal prestress is within a working range of 20

to 100 lb/in2.

Figure 28—Loading scheme for the plate twist tests to determine elastic

constants. (ML89 5837)

20
Figure 29—Plan diagram of the 0° plate. (ML89 5838)

21
Figure 30—Plan diagram of the +/ - 45° plate. (ML89 5839)

22
Figure 31—The 45° plate with corner loading

during testing. Note direction of laminae.

(ML89 0118)

Figure 32—Test results of corner load and

displacement at the center of the 0° plate.

(ML89 5840)

Figure 33—Result of loading the +45° plate,

displacement at the center and corner load.

(ML89 5841)

Figure 34—Result of loading the - 45° plate,

displacement at the center and corner load.

(ML89 5842)

Figure 35—The values of transverse stiffness

ET measured with different levels of prestress

in the plate. (ML89 5843)

Figure 36—Shearing stiffness GLT as measured with various prestress levels in the

plate. (ML89 5844)

23
Table 1—Results of twisting plate tests to obtain the elastic constants ET, GL T

0° plate tests 45° plate tests

Average Load deflection

Transverse

prestress Load deflection Shear modulus GL T

(lb/in) modulus ET

(lb/in2) (lb/in) (lb/in2

) +45 plate -45 plate (lb/in2

)

10 17,000 16,949 4,875 20,770 12,600

20 21,000 20,937 6,650 21,550 18,950

30 21,200 21,116 7,150 20,485 21,630

40 21,300 21,206 8,110 23,721 24,190

50 22,800 22,732 8,220 25,400 23,850

75 25,400 25,324 9,640 35,350 26,080

100 31,700 31,594 10,310 35,460 28,800

24

U. S. GOVERNMENT PRlNTlNG OFFICE: 1990/743-044/00017