Live Load Distribution 

on Longitudinal GluedLaminated Timber Deck 

Bridges

Final Report: Conclusions and 

Recommendations

Fouad Fanous

Jeremy May

Terry Wipf 

Michael Ritter

United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

General

Technical

Report

FPL–GTR–194
July 2010

Fanous, Fouad; May, Jeremy; Wipf, Terry; Ritter, Michael. 2010. Live load 

distribution on longitudinal glued-laminated timber deck bridges, final 

report: conclusions and recommendations. General Technical Report 

FPL-GTR-194. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 14 p.

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Abstract

Over the past few years the United States Department of 

Agriculture (USDA), Forest Products Laboratory (FPL), and 

the Federal Highway Administration (FHWA) have supported several research programs. This paper is a result of 

a study sponsored by FPL, with the objective of determining how truckloads are distributed to the deck panels of a 

longitudinal glued-laminated timber deck bridge. Currently, 

the American Association of State Highway and Transportation Officials (AASHTO) LRFD (load and resistance factor 

design) Bridge Design Specification provides live load distribution provisions for longitudinal glued-laminated timber 

deck-panel bridges.

The AASHTO LRFD live load distribution provisions for 

longitudinal glued-laminated timber deck bridges were 

based on the assumption that the bridge deck behaves as one 

slab and ignores the discontinuity of the bridge deck panels. 

This study investigated this assumption by using analytical 

models that validated field test data from several in-service 

bridges and data from a full-scale laboratory test bridge. The 

analytical models accounted for the effects of the interface 

between the deck panels as well as the effects of the 

transverse stiffener beams on the distribution of the liveload. The analytical live load distribution results above 

were compared with both the AASHTO LRFD and 

AASHTO Standard Specifications.

Keywords: timber, wood, bridges, girder, loads, load 

distribution

Acknowledgments

The authors of this report express our sincere appreciation to 

the engineers and staff from the USDA Forest Service Forest Products Laboratory (FPL).

SI conversion factors

Inch–pound unit Conversion factor SI unit

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

kip (1,000 lb) × 4448.2 N (newton)

psi (lbf/in2) × 6894.8 Pa (pascal)

ksi (kip/in2) × 6.894 Pa (pascal)

In this paper 1 billion = 109
Live Load Distribution on Longitudinal 

Glued-Laminated Timber Deck Bridges

Final Report: Conclusions and Recommendations

Fouad Fanous, Professor of Civil Engineering

Jeremy May, Graduate Research Assistant

Terry Wipf, Director, Bridge Engineering Center 

Iowa State University, Ames, Iowa

Michael Ritter, Assistant Director

Forest Products Laboratory, Madison, Wisconsin

Introduction

The Forest Products Laboratory (FPL) has sponsored several research projects involving timber bridges, specifically 

longitudinal glued-laminated timber deck bridges, over the 

past three decades. Iowa State University (ISU) has contributed to this research by field testing several in-service longitudinal glued-laminated timber deck bridges under static 

and dynamic loading conditions. The research at ISU included collection of field data, testing of a full-scale bridge 

in the laboratory, and using analytical models to study the 

structural performance of these bridges. Over the past several years, the volume of collected field data has increased. 

However, to the authors’ knowledge, these data have yet to 

confirm, or amend, the current bridge design provisions of 

the 2004 American Association of State and Highway Officials (AASHTO) load and resistance factor (LRFD) Bridge 

Design Specifications (AASHTO LRFD 2004). This paper 

focuses on these provisions, specifically the applicability of 

using the equivalent strip-width equations that are recommended to design slab bridges and this particular timber 

bridge type.

The 2004 AASHTO LRFD Specification recommends using 

the equivalent strip widths to design longitudinal gluedlaminated timber deck bridges. Equivalent strip widths 

represent the partial width of the deck over which designers can assume uniform stresses from the effect of the liveload. This assumption simplifies the design process of slab 

bridges.

Longitudinal glued-laminated timber deck bridges are panelized systems. A typical cross-section view of a longitudinal 

glued-laminated timber deck bridge is shown in Figure 1. 

The deck consists of glued-laminated timber panels placed 

beside each other and connected with stiffener beams from 

beneath the bridge deck, as shown in Figure 2. The standard 

connection of the stiffener beam to the deck panels consists 

of a through bolt and is located near the edge of each deck 

panel. Optional to the through bolt connection, an aluminum 

bracket may also be used to connect the stiffener beam to 

the deck panels. The stiffener beams are considered as the 

main load-transfer mechanism from one panel to another.

Figure 1. Cross section of a longitudinal deck bridge.

Figure 2. Plan view of a longitudinal deck bridge.
2

Objective and Scope

The overall objective of the study presented herein was to 

evaluate how an applied truck load is distributed among the 

deck panels of the longitudinal glued-laminated timber deck 

bridge system. This evaluation was attained by using test 

data from several in-service bridges, laboratory test bridges, 

and analytical results. These results were compared with the 

2004 AASHTO LRFD and 1996 AASHTO Standard Specification live load distribution provisions for longitudinal 

glued-laminated timber deck bridges.

The objectives listed above were accomplished by completing the following five tasks:

1. Review the AASHTO Bridge Design Specifications and 

the associated load distribution criteria for longitudinal 

glued-laminated timber deck bridges. This review included both the AASHTO LRFD and AASHTO Standard 

Specifications.

2. Develop detailed analytical finite-element models to 

evaluate the structural performance of the longitudinal 

glued-laminated timber deck bridges. These analytical 

models account for the orthotropic behavior of timber 

material, the interface between the deck panels, and the 

connection between the deck panel and the stiffener 

beams.

3. The finite-element results were validated by comparing 

the analytical results of the deck panel deflections and 

live load distribution values to the data attained from 

field tests of the in-service bridges conducted by ISU 

researchers.

4. Study the influence of other parameters such as the interface between the deck panels, stiffener beam spacing, 

and the stiffener beam size on the distribution of liveload.

5. If required, develop live load distribution formulas. 

These formulas should be based on simplified methods or 

parametric equations using variables that are known during preliminary design.

Background

Simple live load distribution equations have appeared in the 

AASHTO Standard Bridge Design Specifications for many 

years. However, the AASHTO LFRD Bridge Design Specification introduced major revisions to the live load distribution provisions for slab-type bridges. Longitudinal gluedlaminated timber deck panel bridges with spreader beams 

were included in these revisions.

The 1996 AASHTO Standard Specification (AASHTO 

1996) live load distribution factors for longitudinal gluedlaminated timber deck bridges were presented based on 

wheel loads, or half of the total axle load, carried by a single 

panel. The equations used for flexure design are listed in 

Table 1 for a panel under single or multiple truck loads. 

The AASHTO Standard Specification requires one stiffener 

beam to be placed at mid-span with all other stiffener beams 

placed at intervals of 10 ft or less. These stiffener beams are 

attached to the deck near the edges of the deck panels, typically with a bolted connection, and should have a stiffness 

of 80,000 kip/in2 (ksi) or greater (AASHTO 1996).

The 2004 AASHTO LRFD Bridge Design Specification 

(AASHTO LRFD 2004) provides equivalent strip-width 

equations for longitudinal glued-laminated timber deck 

bridges. The equivalent strip-width equations are based on 

lane loads, or full axle loads as shown in Table 2. These 

equations are also used to design reinforced concrete slab 

bridges and post-tensioned timber deck bridges. The 

AASHTO LRFD Specification requires one stiffener beam 

to be placed at intervals of 8 ft or less. The stiffener beam is 

connected with a through-bolt connection to the deck near 

the panel edges and should have a stiffness of 80,000 kip/in2

(ksi) or greater (AASHTO LRFD 2004).

Multiple presence factors are included in the AASHTO 

Standard and LRFD Specification equations that are listed in 

Table 1 and Table 2, respectively. These factors account for 

uncertainties associated with the number of loaded lanes and 

are shown in Table 3. For example, for bridges with multiple 

Table 1—1996 AASHTO Standard Specification, wheelload distribution factorsa

Design loading Equation for flexureb

One traffic lane Wp

4.25+ L

28

 or Wp

5.50

whichever is greater

Two traffic lanes Wp

3.75+ L

28 

 or Wp

5.00

whichever is greater

a

Source: AASHTO 3.25.3, 1996. AASHTO is the American Association 

of State Highway and Transportation Officials. 

b Wp is width of panel (ft) (3.5 ≤ Wp ≤ 4.5), 

and L, length of bridge, center of bearing to center of bearing (ft).

Table 2—2005 AASHTO LRFD Design Specification, 

equivalent strip width equationsa

Design loading Moment equationb

One traffic lane � � 1��� � ��� ���1���1� 

Two or more traffic 

lanes � � ���� � 1��� ���1���1� �

1����

�L

a

Source: AASHTO LRFD 4.6.2.3 (2004). LRFD is load and resistance 

factor design. 

b

E is equivalent width (in.), 

L1 modified span length taken to the lesser of the actual span or 

 60 ft), 

W1 modified width of the bridge taken to be equal to the lesser of 

 the actual width, or 60.0 for multi-lane loading, or 30.0 for 

 single-lane loading (ft), 

W physical edge-to-edge width of bridge (feet), and 

NL number of design lanes. 

General Technical Report FPL–GTR–194
3

design lanes, it is unlikely that three adjacent lanes will be 

loaded at the same time. Therefore, the design load is decreased. For the single design-lane condition, the 

AASHTO LRFD multiple presence factor is greater than 

one to account for an overload condition (AASHTO LRFD 

2004).

Literature Review

The 1996 AASHTO Standard Specification live load distribution provisions for longitudinal glued-laminated timber 

deck bridges, Table 1, were based on research performed by 

Sanders and others (1985). Sanders and others performed 

analytical studies to determine the load distribution characteristics of longitudinal glued-laminated timber deck 

bridges. The analytical models were created using SAP IV 

finite-element software (University of California, Berkeley 

1974). In their work, Sanders and others used plate elements 

to model the deck panels and beam elements to model the 

stiffener beam. These elements were connected using rigid 

links. With the finite-element model, parametric studies 

were performed on bridges with span lengths from 9 to 

33 ft, roadway widths from 16 to 40 ft, deck thickness from 

6.75 to 12.25 in., and various stiffener-beam arrangements. 

Additionally, the width of the deck panels was varied from 

42 to 54 in. (Sanders and others 1985).

Research of the longitudinal glued-laminated timber deck 

bridges was also conducted by Funke (1986). This research 

consisted of laboratory testing and analytical finite-element 

modeling using SAP IV finite-element software. The laboratory experiments were performed on full-scale bridges with 

a span length of 26 ft. Various stiffener beam, deck panel, 

and load-positioning arrangements were used in the laboratory testing. Laboratory results from this study verified the 

applicability of the live load distribution equations created 

by Sanders and others (1985). Favorable live load distribution behavior occurred when using at least three stiffener 

beams.

In the 1980s, the National Cooperative Highway Research 

Program (NCHRP) Project 12–26 (Zokaie and others 1991) 

developed live load distribution equations for slab bridges. 

The live load distribution equations documented in the 

NCHRP 12–26 report were the basis for the loaddistribution provisions presented in the 2004 AASHTO 

LRFD Design Specifications. To develop equations with 

a wide range of applicability, a large database of bridges 

with various parameters was selected. The database consisted of 130 reinforced concrete slab bridges. Longitudinal 

glued-laminated timber deck bridges were not considered in 

NCHRP Project 12–26 (Zokaie and others 1991).

Zokaie and others (1991) used grillage models to evaluate 

the 130 reinforced concrete slab bridges. From these results, 

the authors of NCHRP 12–26 developed relationships to 

calculate the equivalent strip-width equations provided in 

Table 2 using grillage models. The grillage mesh consists of 

longitudinal and transverse beam elements. Load distribution factors were determined for each of the longitudinal 

beam elements, similar to the method used for girder-slab 

bridges. Dividing the load distribution factor by the width 

of the deck represented by the longitudinal beam element 

in the grillage model produces a moment distribution factor per unit width. The load distribution design width, or 

equivalent strip width, is determined by taking the inverse 

of this factor. Simply, the equivalent strip-width values can 

be determined using Equation (1). This equation allows one 

to relate live load distribution factors to equivalent strip 

widths. Edge-stiffening effects from guardrails, or barriers, 

were not included in the analysis (Zokaie and others 1991).

 (1)

where

DFi is the lane load distribution factor of the ith 

 longitudinal beam,

Ei the equivalent strip width of the ith longitudinal 

beam (inches), and

WE the tributary width of longitudinal beam element.

Several analytical studies were performed on longitudinal glued-laminated timber deck bridges at ISU in recent 

years. Kurian (2001) conducted finite-element analyses to 

investigate the effects of several design parameters on the 

overall structural behavior of many in-service bridges. The 

parametric analyses performed by Kurian (2001) examined 

the effects of edge stiffening, boundary conditions, and the 

change in the timber modulus of elasticity. Kurian (2001) 

concluded that the modulus of elasticity of the deck material 

had a significant influence on bridge response when comparing the deflections attained from the analytical models with 

the field-test results. Kurian (2001) also noted that the influence of edge stiffening becomes insignificant to the panel 

deflections and stresses moving away from the exterior panels to the interior panels. Also in his study, Kurian (2001) 

focused only on deflection results and did not address load 

distribution.

Live Load Distribution on Longitudinal Glued-Laminated Timber Deck Bridges

Table 3—AASHTO multiple 

presence “m” factors

Number of 

loaded lanes 

Standard

specificationa

2005

LRFDb

1 1.0 1.2 

2 1.0 1.0 

3 0.9 0.85 

> 3 0.75 0.65 

a

Source: American Association of State 

Highway and Transportation Officials 

(AASHTO) 1996. 

b

Source: load and resistance factor design,

AASHTO LRFD 2004. 
4

Analysis of Longitudinal GluedLaminated Timber Deck Bridges

The results reported herein were attained from detailed 

finite-element analyses. These analyses were carried out 

using the ANSYS finite-element software (Canonsburg, 

Pennsylvania 2004). ANSYS is a general-purpose finiteelement program and was used to calculate deflections, 

stresses, and strains that are induced in several in-service 

longitudinal glued-laminated timber deck panel bridges under various truck loadings. To facilitate the construction of 

multiple finite-element models of various timber bridges, it 

was necessary to develop a preprocessor that simplifies the 

generation of such models. For this purpose, the ANSYS 

parametric design language (APDL) helped write the needed 

preprocessor. The preprocessor allows users with limited 

finite-element analysis knowledge to model longitudinal 

glued-laminated timber deck bridges. The preprocessor program accesses the information entered by the user to generate the finite-element model, as shown in Figure 3.

To execute the preprocessor, the user provides input parameters such as the span length, deck panel width, deck-panel 

thickness, material properties, the position and the magnitude of truckloads, and the bridge boundary conditions. In 

addition, the finite-element model constructed with the preprocessor allows the user to model the longitudinal gluedlaminated timber deck bridges as either one single-deck 

panel or with individual deck panels. The deck panels may 

act as one single panel due to swelling of the deck panels. 

When modeling the individual deck panels, the program 

allows the user to utilize interface elements between the 

deck panels using nonlinear spring elements. The nonlinear 

spring elements allow the user to adjust the interaction of 

the deck panels by defining different coefficient of friction 

values to model the normal and sliding forces acting between the panels.

The finite-element model used solid brick elements to model 

the timber deck panels as well as the stiffener beam. This 

element allows one to incorporate the orthotropic timber 

material properties in the longitudinal (L), radial (R), and 

tangential (T) directions. The longitudinal modulus of elasticity is typically known. The orthotropic timber properties, 

related to the longitudinal modulus of elasticity used for 

this report are provided in the FPL 1999 Wood Handbook. 

The Wood Handbook provides the 12 constants required to 

represent the orthotropic properties of timber. The selected 

timber species was Douglas-fir, which is a typical softwood 

species used for glued-laminated timber beams.

The stiffener beam interaction with the deck panels varies 

over the width of the bridge. For this purpose, compressiononly spring elements were used to idealize the interface 

between the panels and the stiffener beam. The stiffness 

of the spring element becomes zero when a gap exists between the deck panel and the stiffener beam. Additionally, 

tension-compression spring elements were used to model 

the through bolt, or aluminum bracket, connections that are 

required to connect the stiffener beam to the deck panels. 

The load displacement relationships of these connections, 

in tension, were determined from unpublished experimental 

test data provided by the Weyerhaeuser Company (Tacoma, 

Washington) (Hale 1978). The stiffness of the through bolt 

and aluminum bracket connections, when in compression, 

were assumed to be large and acted as a rigid connection. 

The tension-compression relationships of the aluminum 

bracket and through bolt connections are shown in Figure 4.

Analysis of In-Service Bridges

General

As previously mentioned, several in-service and laboratory 

longitudinal glued-laminated timber deck bridges were tested by ISU researchers. The collected data from these tests 

consisted of deflections that were recorded at the edges of 

each the deck panels. Longitudinally, these deflections were 

measured at, or near, the mid-span of each deck panel. The 

live load distribution factors of the in-service bridges, for Figure 3. Three-dimensional rendering of the finite-element 

model.

Figure 4. Load-deflection data used in the finite-element 

analysis, from Zokaie and others (1991).

General Technical Report FPL–GTR–194
5

each panel, were determined using Equation (2) (Hosteng 

2004). In the work presented herein, these in-service live 

load distribution results were compared with the AASHTO 

Standard and LRFD live load distribution provisions. Additionally, the in-service deflection and live load distribution 

results were compared with the values attained using the 

finite-element modeling described above.

 

(2)

where

Δi is average deck panel deflection,

DFi lane load distribution fraction of the ith panel,

ΣΔi sum of average panel displacement, and

n number of panels.

Angelica Bridge

Angelica Bridge in the Town of Angelica, New York, was 

tested by ISU researchers in 1996 and 2003 (Wipf and others 2004, page 10). The field test results presented herein 

were based on the 2003 results. This bridge has a span 

length of 21 ft by 4 in., a clear width of 28 ft by 3 in., and 

consists of seven glued-laminated deck panels. The deck 

panels have a width of 4 ft by 2 in. and a depth of 8.25 in. 

This bridge has two stiffener beams that are spaced at 7 ft 

by 6 in. (see Figure 5). The two stiffener beams are

6.875 in. wide and have a depth of 8.25 in. The stiffener 

beams were connected to the deck panels using through 

bolts. The asphalt-wearing surface on the deck panels was 

2.5 in. thick.

The worst-case deflections and live load distribution factors 

from the field-test results were obtained when the test vehicle is located near the guardrail (see Fig. 5). As the truck 

moved transversely toward the center of the bridge, the 

deflection and live load distribution values would decrease. 

The controlling deflection results were created from the load 

case shown in Figures 5a and 5b. The test vehicle configuration is shown in Figure 6.

The field-test deflection results from the load position above 

were compared with the results attained using the finiteelement analyses. Initially, the bridge was modeled with 

individual deck panels. However, this idealization resulted 

in larger overall deflections than those obtained from the 

field test. Notice from Figure 7 that the field-test results 

show minimal differential displacements between two adjacent deck panels. The maximum differential displacement 

between the panels is 0.037 in. Because of the small differential panel displacements, the bridge was then modeled 

as a single-deck panel. A combination of the swelling of the 

deck panels, close spacing of the stiffener beams, and the 

presence of the asphalt-wearing surface could be the reason 

the bridge behaves as a single panel.

Live Load Distribution on Longitudinal Glued-Laminated Timber Deck Bridges

Figure 5a. Controlling transverse load position for Angelica 

Bridge.

Figure 5b. Controlling load position for Angelica Bridge, 

plan view. (The front axle is outside the bridge.)

Figure 6. Angelica Bridge, test vehicle axle configuration.

–1.0

–0.9

–0.8

–0.7

–0.6

–0.5

–0.4

–0.3

–0.2

–0.1

0.0

0.1

0 50 100 150 200 250 300 350

Transverse distance (in.)

Field test FEM–Single deck panel

FEM–Single panel, as-built deck thickness

Deflection (in.)

Figure 7. Deflection results for Angelica Bridge. 
6

The effect of the asphalt-wearing surface was included in 

the analysis by assuming that the timber deck panel and 

the asphalt act compositely. Using strain compatibility, the 

modular ratio of the asphalt, and the timber deck panels, the 

thickness of the deck panels was increased by approximately 

10%. Similar adjustment was used when analyzing the other 

in-service bridges.

The finite-element results obtained from modeling the deck 

as a single panel are shown in Figure 7. As can be noticed, 

the finite-element deflection results compare well with the 

field test results when modeling the as-built deck thickness, 

or when accounting for the asphalt-wearing surface.

The live load distribution factor results for Angelica Bridge 

are shown in Figure 8. For comparison with the field test 

and finite-element results, the 2004 AASHTO LRFD equivalent strip values would need to be converted to live load 

distribution factors per panel. From Table 2, the equivalent 

strip-width equation for a longitudinal glued laminated timber deck bridge under a single truck load is

 (3)

Substituting the bridge length, L1, and width, W1, for Angelica Bridge into Equation (3), one will get the following 

equivalent strip-width value:

 

(4)

This equation includes the 1.2 multiple presence factor per 

the AASHTO LRFD Specification (AASHTO LRFD 2004). 

To remove the multiple presence factor, one must multiply 

the equivalent strip-width value from above by 1.2:

 (5)

Rearranging the equivalent strip-width and distribution 

factor relationship provided in Equation (1), provides Equation (6):

 (6)

where

DF is live load distribution factor converted from 

 AASHTO LRFD equivalent strip width,

Eadj equivalent strip width the multiple presence factor 

removed, and

WE tributary width longitudinal beam element, or 

width of the panel.

Using Equation (6), one can determine the AASHTO LRFD 

live load distribution factor for the width of the panel to be

(without 1.2 multiple presence factor)

Figure 8 and Table 4 summarize live load distribution results for the Angelica Bridge when subjected to the load 

case shown in Figure 5. The finite-element single panel live 

load distribution factor results compare well to the field-test 

results. Accounting for effects of the wearing surface had 

minimal influence on the finite-element live load distribution results. Both the finite-element and the field-test results 

exceed the limits set by the AASHTO LRFD Specification 

when the multiple presence factor is removed. However, 

with the inclusion of the single-lane multiple presence 

Figure 8. Live load distribution results for Angelica Bridge. 

General Technical Report FPL–GTR–194
7

factor, the AASTHO LRFD Specification does provide conservative results.

East Main Street Bridge

East Main Street Bridge located in the Town of Angelica, 

New York, was tested by ISU researchers in 1996 and 2003 

(Wipf and others 2004, page 9). The field-test results presented herein were based on the 2003 results. The bridge has 

a span length of 30 ft by 6 in., a clear width of 34 ft by 0 in., 

and consists of eight glued-laminated deck panels. The deck 

panels have a width of 4 ft by 5 in. and a depth of 14.25 in. 

This bridge has four stiffener beams, which are spaced at 

6 ft by 0 in. The stiffener beams are 6.875 in. wide and have 

a depth of 4.5 in. The stiffener beams were connected to the 

deck panels with through bolts. The asphalt-wearing surface 

is 3.0 in. thick. The worst-case deflections and live load 

distribution factors from the field tests were created from 

the load case shown in Figures 9a and 9b. The test vehicle 

configuration is the same as shown in Figure 6.

The deflection and live load distribution factors for East 

Main Street Bridge are shown in Figures 10 and 11, respectively. These results are based on the load condition 

shown in Figures 9a and 9b. Unlike the previous bridge, 

edge-stiffening effects were observed in the exterior panels. 

Further adjustments were made to the finite-element as-built 

deck thickness results, incorporating edge-stiffening effects. 

This was accomplished using the results published by Anil 

(Kurian 2001). The adjustment was made by reducing the 

deflections using the difference between the results obtained 

with and without the railing system as documented by Anil 

(Kurian 2001). Similar to the previous bridge, the AASHTO 

LRFD equivalent strip-width values, with and without the 

multiple presence factor, were converted to a distribution 

factor. The controlling exterior panel live load distribution 

results are provided in Table 5. In addition, the AASHTO 

Standard Specification live load distribution factors from 

Table 1 were included in the results for East Main Street 

Bridge.

Bolivar Bridge

Bolivar Bridge, located in the Town of Angelica, New York, 

was tested by ISU researchers in 1996 and 2003 (Wipf and 

others 2004, page 11). The field test results presented herein 

were based on the 2003 results. The bridge has a span length 

of 28 ft by 8 in., a clear width of 24 ft by 8 in., and consists 

of six glued-laminated deck panels. The deck panels have 

a width of 4 ft by 5 in. and a depth of 15.0 in. This bridge 

has three stiffener beams that are spaced at 7 ft by 6 in. The 

Live Load Distribution on Longitudinal Glued-Laminated Timber Deck Bridges

Table 4—Angelica Bridge, exterior panel 

live-load field test results 

Technique 

Live-load 

distribution 

factors 

FEMa—single deck panel 0.345 

AASHTOb Standard specification 0.416 

AASHTO LRFDc

 with mpfd 0.376 

AASHTO LRFD without mpf 0.313 

Adjusted mpf by 0.9 reduction 0.338 

a FEM is finite element model.

b AASHTO is American Association of State 

Highway and Transportation Officials, 

c LRFD is load and resistance factor design.

d Mpf is multiple presence factor.

Figure 9a. Controlling transverse load position for East 

Main Street Bridge.

Figure 9b. Controlling load position for East Main Street 

Bridge, plan view.

–0.7 –0.6 –0.5 –0.4 –0.3 –0.2 –0.1

0.0

0.1

0 50 100 150 200 250 300 350 400 450

Deflection (in.)

Transverse distance (in.)

Field test FEM–Single deck panel

FEM–Single deck panel, as-built deck thickness

FEM–Single panel, as-built deck and edge stiffness

Figure 10. Deflection results, East Main Street Bridge.
8

–0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

1 2 3 4 5 6 7 8

Lane load 

distribution factor

Panel 

Field test

FEM–Single deck panel 

FEM–Single deck panel, as-built deck thickness

FEM–Single deck panel, as-built deck thickness and edge stiffness

AASHTO LRFD w/out mpf

AASHTO LRFD with mpf

AASHTO standard specification

Figure 11. Live load distribution results for East Main Street Bridge. 

Table 5—East Main Street Bridge, exterior 

panel live-load field test results 

Technique 

Live-load 

distribution 

factors 

Field test 0.304 

FEM—single deck panela 0.301 

AASHTO Standard specificationb

 0.414 

AASHTO LRFD with mpfc

 0.329 

AASHTO LRFD without mpfd

 0.274 

a

FEM is finite-element model.

b AASHTO is American Association of State Highway 

and Transportation Officials. 

c

LRFD is load and resistance factor design.

d Mpf is multiple presence factor.

Figure 12a. Controlling transverse load position for 

Bolivar Bridge.

Figure 12b. Controlling load position for Bolivar Bridge, 

plan view.

General Technical Report FPL–GTR–194
9

Live Load Distribution on Longitudinal Glued-Laminated Timber Deck Bridges

two stiffener beams are 6.875 in. wide and have a depth 

of 4.5 in. The stiffener beams were connected to the deck 

panels with through bolts. The asphalt-wearing surface is 

2.5 in. thick. The effect of the wearing surface was included 

in the analysis, as explained above. The guard-railing system consisted of timber posts and a glued-laminated timber 

panel barrier; they were not explicitly included in the finiteelement model. The worst-case deflections and live load distribution factors from the field test results were created from 

the load case shown in Figures 12a and 12b. The test vehicle 

configuration is the same as shown in Figure 6.

The deflection and live load distribution factors for Bolivar 

Bridge are shown in Figures 13 and 14, respectively. These 

results are based on the load condition shown in Figures 12a 

and 12b. Edge-stiffening effects were observed in the 

exterior panels, and deflections were adjusted as described 

Figure 13. Deflection results for Bolivar Bridge.

Table 6—Bolivar Bridge, exterior panel 

live-load field test results 

Technique 

Live-load 

distribution 

factors 

Field test 0.312 

FEM—single deck panela 0.310 

AASHTO Standard specificationb

 0.411 

AASHTO LRFD with mpfc

 0.355 

AASHTO LRFD without mpfd

 0.296 

a

FEM is finite-element model.

b AASHTO is American Association of State Highway 

and Transportation Officials, 

c

LRFD is load and resistance factor design.

d Mpf is multiple presence factor.

Table 7—Scio Bridge, exterior panel liveload field test results 

Technique 

Live-load 

distribution 

factors 

Field test 0.366 

FEM—single deck panela 0.338 

AASHTO Standard specificationb

 0.447 

AASHTO LRFD with mpfc

 0.398 

AASHTO LRFD without mpfd

 0.331 

a

FEM is finite-element model.

b AASHTO is American Association of State Highway 

and Transportation Officials. 

c LRFD is load and resistance factor design.

d Mpf is multiple presence factor.

Figure 14. Live load distribution factor results for Bolivar Bridge. 
10

previously. As before, the AASHTO LRFD equivalent stripwidth values, with and without the multiple presence factor, were converted to distribution factors. The controlling 

exterior panel live load distribution results are provided in 

Table 6.

Scio Bridge

Scio Bridge in the Town of Angelica, New York, was tested 

by ISU researchers in 1996 and 2003 (Wipf and others 2004, 

page 12). The field test results presented herein were based 

on the 2003 results. The bridge has span length of 

20 ft by 8 in., a clear width of 30 ft by 0 in., and consists of 

six glued-laminated deck panels. The deck panels have a 

width of 4 ft by 4 in. and a depth of 9.0 in. This bridge has 

three stiffener beams that are spaced at 7 ft by 6 in. The two 

stiffener beams are 6.875 in. wide and have a depth of 4.5 

in. The stiffener beams were connected to the deck panels 

with the through bolt connection. The asphalt wearing surface is 6.0 in thick. The effect of the wearing surface was 

included in the analysis, as explained above. The guard railing system consisted of timber posts and a glued-laminated 

timber panel barrier. The worst-case deflections and live 

load distribution factors from the field test results were created from load case shown in Figure 15. The test vehicle 

configuration is the same as shown in Figure 6.

The deflection and live load distribution factors for Scio 

Bridge are shown in Figures 16 and 17, respectively. These 

results are based on the load condition shown in Figures 15a 

and 15b. Edge-stiffening effects were observed in the 

exterior panels and the deflections were adjusted as described previously. As before, the AASHTO LRFD equivalent strip-width values, with and without the multiple 

presence factor, were converted to distribution factors. The 

controlling exterior panel live load distribution results are 

provided in Table 7.

Analysis of the Laboratory Test 

Bridge

General

The full-scale laboratory bridge tested by Funke (1986) was 

also analyzed. This allowed studying the behavior of the 

longitudinal glued-laminated timber deck-panel bridge without the influence of swelling, the asphalt-wearing surface, 

and edge-stiffening effects from guardrails or barriers. The 

laboratory test bridge had a span length of 26 ft by 0 in. This 

bridge set-up consisted of six deck panels with one stiffener 

beam located at the mid-span of the bridge. The deck panels 

were 4 ft by 0 in. wide and had an average depth of 10.72 in. 

The stiffener beam had a depth of 4.5 in. and a width of 6.75 

in. The stiffener beam was connected to the deck panels 

with the through bolt connection described earlier. The load 

consists of a single HS20-44 design truck placed 30 in. from 

the edge of the deck as shown in Figure 18. Longitudinally, 

two axles were placed on the bridge. One axle was placed 

2 ft by 6 in. from the center line of the abutment and the 

other axle was placed 14 ft by 0 in. from the first.

The laboratory bridge (Funke 1986) was analyzed as having 

individual deck panels and as one single-deck panel. When 

modeling the bridge with the individual deck panels, the 

nonlinear spring elements connecting the deck panels were 

assigned negligible coefficient of friction and stiffness 

Figure 15a. Controlling transverse load position for Scio 

Bridge.

Figure 15b. Controlling load position for Scio Bridge, plan 

view.

Figure 16. Deflection results, Scio Bridge. 

General Technical Report FPL–GTR–194
11

Live Load Distribution on Longitudinal Glued-Laminated Timber Deck Bridges

values, allowing the deck panels to slide freely. Therefore, 

the stiffener beam was the only path to transfer the load 

from panel to panel. As mentioned above, the stiffener beam 

was connected to the deck panels with through bolts. Therefore, the compression-tension force versus displacement values for the through bolt connection, shown in Figure 4, were 

used by the preprocessor described above.

The displacement results obtained from the analytical model 

compared well to the displacement resulted from the laboratory test (see Fig. 19). The individual deck panel finite- element results were within a 2% difference of the laboratory 

displacement results.

The live load distribution factor results for the laboratory 

test of the bridge, finite-element analyses, and AASHTO 

LRFD and Standard Specifications are shown in Figure 20. 

One can observe that the controlling live load distribution 

factor is located at the exterior panel. The individual deck 

panel finite-element results are within a 2% difference of the 

laboratory live load distribution results. The controlling live 

load distribution factor when modeling the deck as a single 

panel compared well to the AASHTO LRFD live load distribution value with the multiple presence factor removed. 

However, when modeling the deck as individual panels, the 

finite-element and field test results compared well to the 

AASHTO Standard specification limit shown in Figure 20. 

A summary of the controlling live load distribution factors 

from the different analyses is provided in Table 8.

From the live load distribution factor results of the laboratory bridge, one can notice that the deck of the bridge does 

not behave as a single panel structure because of the large 

differential displacement between the deck panels. This 

was expected due to the large spacing between the stiffener 

beams, absence of a wearing surface, and small friction between the deck panels. Additional finite-element trials were 

later performed to investigate the effects of the stiffener 

Figure 17. Live load distribution factor results, Scio Bridge. 

Figure 18. Laboratory test bridge 1TE6-A.

Figure 19. Laboratory test bridge 1TE6-A, deflection. 
12

beam spacing, stiffener beam size, and influence of friction 

on the laboratory bridge above.

Table 8 shows that the live load distribution factor obtained 

from the laboratory test agrees with that obtained using the 

2004 AASHTO specification including the multiple presence factor. Also, the results in Table 8 illustrate that modeling the deck as individual panels resulted in higher distribution factors than those obtained from the test or the 2004 

AASHTO codes.

Effects of Stiffener Beam Properties and 

Spacing

Using the laboratory test bridge, a parametric study was 

conducted to investigate the influence of the stiffener beam 

properties and spacing on the live load distribution results. 

Using the load configuration shown in Figure 18, the controlling live load distribution values were determined for 

the exterior panel. These results are listed in Table 9. One 

can observe how the load is distributed from the exterior to 

the adjacent panels as the number of stiffeners is increased. 

However, increasing the number of stiffener beams alone 

does not provide a result that fully converges to the results 

obtained, assuming the deck panel acts as a single-deck 

panel. Therefore, for the single panel action to occur a combination of swelling and close-stiffener beam spacing must 

be present.

The influence of the swelling on the behavior of the bridge 

is difficult to quantify. As the bridge panels swell, additional 

load is transferred to adjacent panels through friction forces. 

Similar to Table 9, additional trials were performed modifying the interaction of the deck panels. When modeling the 

bridge with the individual deck panels, the nonlinear spring 

elements connecting the deck panels were assigned large 

Figure 20. Laboratory test bridge 1TE6-A, live load distribution results. 

Table 8—Laboratory Bridge, exterior panel live-load 

distribution results 

Technique 

Live-load 

distribution 

factor results 

Laboratory test 0.359 

FEM—individual deck panelsa 0.368 

FEM— single deck panel 0.311 

AASHTO Standard specificationb

 0.384 

AASHTO LRFD specification with mpfc

 0.355 

AASHTO LRFD specification without mpfd

 0.296 

a FEM is finite-element model.

b AASHTO is American Association of State Highway and 

Transportation Officials. 

c

LRFD is load and resistance factor design.

d Mpf is multiple presence factor.

Table 9—Stiffener beam parametric study

Parameter 

Lane-load 

distribution 

factor 

AASHTOa Standard specification 0.384 

AASHTO LRFDb without mpfc 0.296 

AASHTO LRFD with mpf 0.355 

No stiffener beam 0.500 

1 stiffener beam 0.368 

2 stiffener beams 0.360 

4 stiffener beams 0.351 

(2×) stiffener beam depth 0.356 

Single-deck panel 0.311 

a

AASHTO is is American Association of State Highway 

and Transportation Officials.

b LRFD is load and resistance factor design. c Mpf is multiple presence factor.

General Technical Report FPL–GTR–194
13

Live Load Distribution on Longitudinal Glued-Laminated Timber Deck Bridges

coefficient of friction and stiffness values. The controlling 

lane-load distribution results, for the controlling exterior 

deck panel, are shown in Table 10.

Comparing the results from Tables 9 and 10, one can observe the influence of the deck panel interaction with multiple stiffener-beam arrangements. Notice from Table 10 

the 7% difference between the single-deck panel results and 

results using four stiffener beams, including the deck panel 

interaction. In the author’s opinion, one could provide a 

transverse post-tensioning system to increase the deck panel 

interaction. This would aid in the distribution of load and 

assure the panelized system behaves similar to a single-deck 

panel structure.

Multiple Vehicle Loads

The above analyses focus on single-design truckloads. From 

these analyses, one can note that the in-service bridges 

perform similarly to a single-panel structure and compared 

reasonably well to the 2004 AASHTO LRFD live load 

distribution provisions. The AASHTO LRFD equivalent 

strip-width equations were further investigated by analyzing several bridges under multiple lane loads. These bridges 

were modeled for the finite-element analysis as a single 

or as individual deck panels. The effects from the asphaltwearing surface and edge-stiffening effects from guardrails 

will be neglected.

The first bridge analyzed with two vehicle loads is shown in 

Figure 21. The bridge has a span length of 26 ft by 0 in. 

and a clear width of 24 ft by 0 in., similar in dimensions to 

the laboratory test bridge. The deck panels were 4 ft by 

0 in. wide and had a depth of 10.72 in. Three stiffener beams 

were spaced at 6 ft by 6 in., each having a depth of 4.5 in. 

and a width of 6.75 in. As previously stated, the bridge was 

modeled as a single-deck panel, behaving similarly to the 

in-service bridges. The single-deck panel was divided into 

six sections, each having a tributary width of 4 ft by 0 in. 

The average stress and moment results for each of the six 

sections was used to determine the equivalent strip-width 

values, similar to a slab-girder bridge. The controlling 

beam-line moment of 275 ft/kips was due to the AASHTO 

LRFD tandem loading condition shown in Figure 2. The 

results are provided in Table 10.

Using the AASHTO 2004 LRFD code gives an equivalent 

strip-width value of 10.0 ft. This value agrees with the 

equivalent strip width calculated using the induced stresses 

in panel 6 (see Table 11).

The second bridge analyzed with two vehicle loads was the 

previously described East Main Street Bridge. As stated before, the bridge was modeled as a single-deck panel. Edgestiffening effects were neglected, modeling the clear width 

of the bridge. The single-deck panel was divided into eight 

sections, the inner sections had a tributary width of 4 ft by 

6 in. and the two outer sections had a tributary width of 

3 ft by 5 in. The average stress and moment results for each 

of the eight sections were used to determine an equivalent 

strip-width value. The controlling beam-line moment of 

331 ft/kips was due to the AASHTO LRFD tandem loading 

condition shown in Figure 22.

Using the AASHTO 2004 LRFD code gives an equivalent 

strip-width value of 10.6 ft. This value agrees with the 

equivalent strip width calculated using the induced stresses 

in panel 8 (see Table 12).

Conclusions

This research involved the evaluation of the existing live 

load distribution equations for longitudinal glued-laminated 

Table 10—Stiffener beam parametric study 

including deck panel interaction

Parameter 

Lane-load 

distribution 

factor 

AASHTO Standard specificationa 0.384 

AASHTO LRFD without mpfb,c 0.296 

AASHTO LRFD with mpf 0.355 

No stiffener beam 0.408 

1 stiffener beam 0.355 

2 stiffener beams 0.341 

4 stiffener beams 0.331 

(2×) stiffener beam depth 0.341 

Single deck panel 0.311 

a

AASHTO is American Association of State Highway 

and Transportation Officials. 

b LRFD is load and resistance factor design. c Mpf is multiple presence factor.

Figure 21. AASHTO LRFD tandem truck loading.

Table 11—Multiple lane load results 

Panel 

number

Stress 

psi

Moment

ft./kips 

Equivalent

width

E (ft.) 

1 706.61 54.13 20.32 

2 1,002.98 76.84 14.32 

3 1,095.64 83.94 13.10 

4 1,285.36 98.47 11.17 

5 1,283.70 98.35 11.18 

6 1,367.50 104.77 10.50 

Sum 6,741.79 516.50 – 
14

General Technical Report FPL–GTR–194

timber deck bridges provided in the 2004 AASHTO LRFD 

Bridge Design Specification. This was accomplished by using analytical finite-element models, which were validated 

with test data from in-service and laboratory bridges. The 

test data consisted of deflections and live load distribution 

factors for each panel.

The analyses of the four in-service bridges illustrated that 

the decks of these bridges behaved as a single panel. The 

single-deck panel behavior of the in-service bridges could 

result from the effect of the stiffener beams and the swelling 

of the deck panels. Based on the analytical and in-service 

bridge results, the 2004 AASHTO LRFD live load distribution provisions for longitudinal glued-laminated timber 

bridges are acceptable. This was observed for both the 

single and multiple lane loading conditions.

Recommendations

Based on the analytical finite-element results and the comparison of the results above, the following can be recommended:

1. The AASHTO LRFD (2004) equivalent strip-width 

equations assume that the panelized structure behaves 

as a single-panel bridge. This assumption appears to be 

valid based on the performance of the in-service bridges. 

To assure that the panelized structure performs as a 

single panel, additional research should be performed on 

the panel-to-panel connections.

Figure 22. AASHTO LRFD (American Association of State 

Highway and Transportation Officials, load and resistance factor design) tandem truck loading, East Main 

Street Bridge.

Table 12—Multiple lane load results, East 

Main Street Bridge 

Panel 

number

Stress 

psi (lb/in2

)

Moment

(ft/kips) 

Equivalent

width

E (ft) 

1 108.48 12.40 93.68 

2 159.42 23.43 63.74 

3 271.77 39.95 37.39 

4 521.12 76.60 19.50 

5 703.69 103.44 14.44 

6 877.06 128.93 11.59 

7 907.26 133.37 11.20 

8 993.96 113.64 10.22 

Sum 4,542.76 631.77 – 

2. For newly constructed longitudinal glued-laminated 

timber deck bridges, their behavior will be similar to the 

laboratory bridge analyzed in this report. If the panel-topanel connection is not investigated, as recommended 

above, one should consider using the AASHTO Standard 

Specification load distribution factors for design.

3. The effects of edge stiffening were observed at the inservice bridges. However, further study of the curb and 

guardrail should be conducted to aid in better understanding the edge-stiffening effects.

References

AASHTO. 1996. Standard specifications for highway bridges, Sixteenth Edition. Washington, DC: American Association of State Highway and Transportation Officials. 

AASHTO LRFD. 2004. Bridge design specifications-with 

interim. Washington, DC: American Association of State 

Highway and Transportation Officials.

ANSYS. 1992. User’s manual for revision 5.0, Procedures. 

Canonsburg, PA: Swanson Analysis Systems, Inc. 

FPL. 1999. Wood handbook: wood as an engineering material. Madison, WI: Forest Products Laboratory, U.S. Department of Agriculture, Forest Service. FPL–GTR–113. 

Funke, R.W. 1986. Behavior of longitudinal glued-laminated timber deck bridges. Ames, IA: Iowa State University. 

M.S. thesis.

Hale, C.Y. 1978. Stiffened longitudinal decked bridge—

evaluation of stiffener hardware. Report No. 045-16093. 

Tacoma, WA: Weyerhaeuser Co. 

Hosteng, T.K. 2004. Live load deflection criteria for gluedlaminated structures. Ames, IA: Iowa State University. M.S. 

thesis.

Kurian, A.V. 2001. Finite-element analysis of longitudinal 

glued-laminated timber deck and glued-laminated timber 

girder bridges. Ames, IA: Iowa State University. M.S. thesis.

Sanders, W.W.; Klaiber, F.W.; Wipf, T.J. 1985. Load distribution in glued-laminated longitudinal timber deck highway 

bridges. Report No. ERI-85441. Ames, IA: Iowa State 

University.

Wipf, T.J.; Phares, B.M.; Bigelow, J.; Wood, D.L.; Ritter, 

M.A. 2004. Live load deflection of timber bridges, Final 

report: conclusions and recommendations. Ames: IA: Iowa 

State University, Bridge Engineering Center. 

Zokaie, T.; Osterkamp, T.A.; Imbsen, R.A. 1991. Distribution of Wheel Loads on Highway Bridges. Washington, 

DC.: NCHRP Research Results Digest 187. TRB, National 

Research Council.