Timber Bridge 

Economics 

United States

Department of 

Agriculture 

Forest Service 

Forest 

Products 

Laboratory 

National Wood 

in Transportation 

Information 

Center 

Research 

Paper 

FPL−RP−593

 

In cooperation 

with the 

United States 

Department of 

Transportation 

Federal 

Highway 

Administration 
Abstract 

Interest in timber bridges has grown rapidly in recent years 

as a result of new technologies in design and construction as 

well as advances in material manufacturing and preservative 

treatments. Despite these advances, little is known about the 

initial and life-cycle costs of timber bridges relative to those 

of other construction materials. The objectives of this study 

were to evaluate the cost characteristics of timber bridges 

and to compare the initial cost of timber bridge superstructures with that of bridges constructed of steel, concrete, and 

prestressed concrete. For timber bridges, results show a 

relationship between cost per square foot and bridge length, 

load rating, and geographic location. In general, timber 

bridge superstructures tended to compete with steel and 

concrete bridge superstructures on an initial cost basis. However, the range in cost per square foot values for all bridges 

varied widely. This outcome was probably due to both the 

high variability in these data and the relatively small sample 

size of the data sets for steel and concrete. 

Keywords: Timber bridge, economics, superstructure, cost 

April 2001 

Forest Products Laboratory and Federal Highway Administration. 2001. 

Timber bridge economics. Res. Pap. FPL-RP-593. Madison, WI: 

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

40 p. 

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

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

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

of libraries in the United States and elsewhere. 

The Forest Products Laboratory is maintained in cooperation with the 

University of Wisconsin. 

The United States Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national 

origin, sex, religion, age, disability, political beliefs, sexual orientation, or 

marital or familial status. (Not all prohibited bases apply to all programs.) 

Persons with disabilities who require alternative means for communication 

of program information (Braille, large print, audiotape, etc.) should contact 

the USDA’s TARGET Center at (202) 720–2600 (voice and TDD). To file a 

complaint of discrimination, write USDA, Director, Office of Civil Rights, 

Room 326-W, Whitten Building, 1400 Independence Avenue, SW, Washington, DC 20250–9410, or call (202) 720–5964 (voice and TDD). USDA is 

an equal opportunity provider and employer. 

Acknowledgments 

This research was funded by the USDA Forest Service under 

Joint Venture Agreement FP–94–2288 and partially funded 

by the ISTEA Timber Bridge Research Program. Special 

recognition is extended to Glade M. Sowards, John Z. Wang, 

and Blair Orr, Michigan Technological University. We also 

thank Don Czarniewski, Julie Lyons, and Melissa Mielke, 

Michigan Technological University, for their assistance on 

this project. We express appreciation to the following individuals from the USDA Forest Service, Forest Products 

Laboratory: Kim Stanfill–McMillan for project guidance 

and assistance in developing the data questionnaire; Paula 

Hilbrich Lee for project guidance and assistance in preparing 

the report; Steve Verrill for reviewing the data analysis and 

providing valuable input pertaining to the statistical results; 

and the Information Services Team for assistance in preparing the report. 

Contents 

Page

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

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

Objectives and Scope............................................................ 3

Research Methodology ......................................................... 3

Data Collection ................................................................. 3

Data Analysis.................................................................... 6

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

Cost Characteristics for Timber Bridges........................... 8

Cost Characteristics for Matching Bridges.................... 10

Cost Trends for Timber Bridges ..................................... 12

Future Research .................................................................. 13

References .......................................................................... 13

Appendix A—Flowchart of Project Methodology ............. 14 

Appendix B—Survey of Bridge Superstructure Cost......... 15

Appendix C—Complete Data Set....................................... 18

Appendix D—Verification of Valid Data Set for Timber 

Bridges................................................................................ 23

Appendix E—Cost Analysis............................................... 28

Appendix F—Verification of Valid Data Set for 

Matching Bridges................................................................ 33
Timber Bridge Economics

Introduction 

Approximately 200,000 bridges throughout the United States 

are deficient; costs of replacement are estimated at 

$84 billion (Smith and Bush 1994). In the face of such staggering figures, it is obvious that a substantial need has developed for new economical bridges. In recent years, interest 

in timber bridges as one solution to the deteriorating infrastructure has been rapidly increasing. To a great extent, this 

rise in interest is due to new technologies in design and 

construction and advances in material manufacturing and 

preservative treatments. 

Throughout much of the 19th century, timber structures 

accounted for the majority of bridges and trestles in the 

United States. These structures were constructed of sawn 

lumber, and many lacked proper preservative treatments to 

protect against exposure to moisture and subsequent decay. 

In addition, older timber bridges were often crudely designed, with little or no input from engineers. It was not until 

1840 that a complete stress analysis of a timber bridge design was included with the bridge designer’s patent (Ritter 

1990). In the 20th century, timber bridges began to be replaced by steel. By 1910, steel competed with timber as a 

bridge construction material on a first-cost basis, and by 

1930, steel dominated the bridge market (Ritter 1990). 

The failure of older, primitive timber bridges and their eventual replacement first by steel and later by concrete is the 

likely source of a general perception held by some people 

that timber bridges are inferior in quality. Over time, however, the limitations of steel, concrete, and prestressed concrete have become apparent. These limitations range from 

susceptibility to corrosion to costly maintenance and 

replacement. 

In the mid-1940s, engineers began to reconsider timber for 

bridge construction. Later, the development of techniques 

such as glue- and stress-lamination demonstrated the 

strength of timber as a construction material, which promoted interest in timber bridge utilization (Ritter 1990). The 

rationale for this interest is threefold. First, timber may offer 

a low-cost alternative to other bridge construction materials 

such as steel, concrete, and prestressed concrete. Second, 

recent research indicates that timber bridges may be more 

durable than those constructed from other materials, particularly in cold climates where salts and other de-icing agents 

are frequently used. Third, it is hoped that the creation of a 

viable timber bridge market will encourage economic growth 

in rural areas with underutilized timber resources. 

As part of the renewed interest in timber bridges, Congress 

passed legislation known as the National Timber Bridge 

Initiative, now called the Wood in Transportation Program 

(WIT), which began receiving funding in Fiscal Year 1989. 

The program was established to help diversify local economies by “improving rural transportation networks, expanding the range of markets for wood products, and creating 

service industries for wood bridge construction” (USDA 

1994). Since its introduction, WIT has resulted in more than 

$17 million in congressional funding for bridge research, 

construction, and technology transfer (Smith and Bush 

1994). In addition to the WIT program, in 1991 Congress 

included provisions for timber bridge research and a demonstration program in the Federal Highway Administration 

Intermodal Surface Transportation Efficiency Act (ISTEA). 

The ISTEA provided $50 million for a 6-year program. As a 

result of such efforts, modern timber bridge designs, construction techniques, and preservative treatments have made 

it possible to improve the utilization of local wood species 

that were previously viewed as not marketable. 

Despite this renewed interest in timber bridges, little is 

known about their initial and life-cycle costs relative to those 

of bridges constructed from other materials. Such information is critical to convincing transportation agency officials 

that timber bridges are a viable alternative to bridges made 

of other materials. 

Background 

In recent years, several studies have been conducted on the 

subject of timber bridge economics. The stated impetus for 

the bulk of these projects is the need for cost-effective alternatives to traditional infrastructure components of the national highway system. Most of this research tends to 

(1) focus on bridge superstructure, (2) group all timber 

bridges together, (3) address initial costs as opposed to 

life-cycle costs, (4) be limited to a certain geographical 

area, (5) rely on estimated costs, and (6) lack statistically 

significant data. 

Most studies focus only on bridge superstructure, which 

includes the deck, beams, girders, wearing surface, and 

periphery such as guardrails. Researchers have found that 

substructure construction costs are more likely to vary with 

respect to the site as a result of differences in geological 

formations, soil types, and other site-specific characteristics 

that are difficult to quantify. For example, Behr and others

(1990) considered only superstructures in their cost comparison of several bridge designs in the New England area. 
2 

Similarly, Verna and others (1984) limited their treatment 

of bridge replacement costs to major superstructure 

components. 

Studies that compare timber bridges with bridges made from 

other materials tend to group all types of timber bridge 

designs together. However, timber bridges are not uniform. 

Timber deck bridges should be compared with prestressed 

concrete slab bridge designs and timber girders to steel 

girders, for example. 

Research efforts tend to focus on initial costs as opposed to 

life-cycle costs. Some studies rely on estimates of expected 

life as a rough indicator of life-cycle costs. There is some 

question as to the need for more than initial cost information, 

because such costs represent only a portion of bridge costs 

over time. According to Wolchuk (1988), the immense task 

of rebuilding the nation’s bridges “should call for planning 

based on sound economic principles, with due consideration 

of the total cost of structures over their entire projected 

service lives.” A review of maintenance and cost data by Hill 

and Shirole (1984) suggests that timber bridges with less 

than 30 years of service have few major problems. However, 

most researchers cite difficulty in obtaining accurate, complete maintenance and replacement cost figures as the main 

reason for omitting life-cycle cost analyses. 

Studies tend to be limited to a particular State or geographic 

region or are area specific. For example, because the Cost 

Comparison of Timber, Steel, and Prestressed Concrete 

Bridges by Behr and others (1990) applies to only potential 

bridge projects in New England, the results may not be 

entirely applicable in other regions. Verna and others (1984) 

and Hill and Shirole (1984) worked with similarly small 

geographic areas in Pennsylvania and Minnesota, respectively. Sarisley (1990) considered a single prototype stresslaminated timber bridge in Connecticut, thereby restricting 

the results to region and structure type. Such limitations 

create difficulties in applying the conclusions of past 

research to current bridge project proposals. 

Some research efforts rely on estimated compared with 

actual cost information. For example, in a study by Behr and 

others (1990), cost information was obtained by supplying 

participating contractors with a bridge design and asking 

each to provide an estimated bid. In another study (Verna 

and others 1984), bids were supplied for various deck replacement materials and designs. Despite its appeal from a 

data collection standpoint, this approach does not allow for 

potential cost overruns and may serve to skew cost comparison results. 

A final characteristic of past research is a lack of statistically 

significant databases necessary to make valid comparisons 

between the costs of various bridge projects. As Behr and 

others (1990) note with regard to their own study, this problem stems from sample sizes that are too small for the application of meaningful tests of statistical significance. 

Table 1 summarizes mean and median timber bridge costs 

per square foot from four major studies. (See Table 2 for 

metric conversion factors.) As Table 1 indicates, results of 

past studies are varied and, at times, contradictory. Three of 

these studies (Behr and others 1990, Verna and others 1984, 

and Hill and Shirole 1984) suggest that timber bridges are 

cost competitive with bridges composed of other materials 

for certain applications or within limited specifications, such 

as length and load rating. 

However, there appears to be no consistent pattern between 

studies as to the limits or characteristics of timber bridge 

feasibility. For example, Behr and others (1990) found a 

positive relationship between cost per square foot and span 

length, whereas Verna and others (1984) found an inverse 

correlation. Further clarification of timber bridge cost 

relationships is needed.

Table 1—Summary of timber bridge costs from 

four major studies 

Superstructure cost ($/ft2

) 

Study Mean Median 

Behr and others 1990 

20 ft — 46.12 

40 ft — 47.12 

60 ft — 57.87 

Hill and Shirole 1984a

 29.78 — 

Sarisley 1990b 

Single lane 68.00 — 

Two lane 52.00 — 

Verna and others 1984c

 

Case I 37.00 — 

Case II 57.00 — 

a

Calculated from weighted average of structure cost 

 per square foot from State and non-State routes in 

 Minnesota. Component costs are not indexed for 

 inflation. 

b

Single-lane cost is the actual cost from the bridge 

 project. Researchers used single-lane costs to 

 estimate two-lane costs. 

c

Represents costs from two bridge replacement 

 projects in western Pennsylvania. 

Table 2—SI conversion factors

Inch–pound unit 

Conversion 

factor SI unit 

inch (in.) 25.4 millimeters (mm) 

foot (ft) 0.3048 meter (m) 

square foot (ft2

) 0.093 square meter (m2) 
3 

Objectives and Scope 

The objectives of this study were to evaluate the cost characteristics of timber bridges and to compare the initial cost of 

timber bridge superstructures with that of bridges constructed of steel, concrete, and prestressed concrete. Where 

possible, the intent of this project was to overcome the limitations of past research efforts, as summarized in the Background. 

For this study, vehicular bridges, located throughout the 

United States, were load rated in accordance with American 

Association of State Highway and Transportation Officials 

(AASHTO) recommendations. This wide scope implies 

practical, generally applicable results. Cost data were taken 

from the June 1994 National Bridge Inventory (FHWA 

1994). Cost information was gathered from completed, nondemonstration bridge projects constructed during and after 

1980. Non-demonstration bridges are those bridges constructed outside WIT and/or ISTEA timber bridge demonstration programs. 

The study included both single- and multiple-span bridges. 

Because of the wide scope, some bridges of the same length 

have a different number of spans, thus different cost characteristics. Because of the potential variability of substructure 

costs with respect to site-specific conditions, such as differences in soil composition and terrain between bridge sites, 

the study focused primarily on superstructure costs. As a 

result, the bridge cost information pertains to superstructure 

cost per square foot, not total cost, unless otherwise stated. 

In addition, only initial cost was evaluated because there 

were inadequate data for a meaningful comparison of lifecycle costs. This is partially due to the fact that modern 

bridges have needed little repair. Most of the study bridges 

were constructed during or since 1980. Typically, long-term 

cost data are available only for bridges of antiquated design 

and construction. 

Research Methodology 

The research methodology consisted of data collection and 

data analysis. The section on data collection presents the 

methods used to obtain usable bridge cost information. The 

section on data analysis describes the analyses performed to 

interpret project data. 

Research efforts were continuous over the duration of the 

study. Data collection and analytical procedures often overlapped because of the limited availability of the timber 

bridge cost data and the limits placed on matching timber 

and nontimber bridges. It was possible to match the bridges 

only after the timber bridge data had been received and 

analyzed. The flowchart in Appendix A illustrates the project methodology in chronological order. Research stage 

numbers are provided for cross-referencing the flowchart 

with the body of this report. 

Data Collection 

Data collection procedures are described for timber and 

matched bridges. Limitations on the availability of data are 

also discussed. 

Timber Bridges 

The first step in the data collection process was to identify 

timber bridges throughout the United States that fit within 

the scope of the project. The June 1994 U.S. Department of 

Transportation Federal Highway Administration National 

Bridge Inventory (NBI) (FHWA 1994) was obtained to 

accomplish this task. This database contains structural and 

inspection statistics for 668,433 bridges and other highway 

structures across the United States that are more than 20 ft 

long. The NBI contains information for each bridge record, 

such as identification or structure number, location, ownership, length, width, number of lanes, and year built, as well 

as inspection information on structural condition; there are 

116 data fields. The NBI database is updated regularly 

(FHWA 1988). 

Thirty-six of the 116 NBI database fields were retained for 

this study (Table 3). Among these were the State, county, 

place, structure number, location, feature intersected, structure length, maximum span length, deck width, number of 

lanes, load rating, and year of bridge construction. 

The database was filtered to eliminate bridges that fell outside the scope of the project (see App. A, Stage 1). Records 

were maintained only for load-rated bridges constructed 

during or after 1980. This year was selected as being late 

enough to emphasize modern timber bridges, yet early 

Table 3—NBI database fields used for studya 

Database field Database field 

State code Design load (rating) 

State Highway Dept. Dist. Bridge status (open, 

County (Parish) code posted, closed) 

Place code Type of service 

Record type Structure type, main 

Route signing prefix Number of spans, main 

Level of service Maximum span length 

Route number Structure length 

Directional suffix Bridge roadway width 

Features intersected Deck width 

Facilities carried Deck condition 

Structure number Deck structure type 

Location Deck protection 

Milepoint Superstructure condition 

Maintenance responsibility Substructure condition 

Owner Operating rating 

Functional classification Wearing surface 

Year built Membrane type 

Lanes on or under 

a

Thirty-six of 116 NBI database fields were used. 
4 

enough to allow for a large data set. The database was also 

scanned to remove records representing pedestrian and 

railroad bridges. The resulting timber bridge database contained information for 1,604 highway bridges. Figure 1 

shows the geographic distribution of timber bridges meeting 

the project requirements. 

An owner-agency was identified for each bridge record 

using ownership information and State and county code 

numbers supplied in the NBI database (FHWA 1994), the 

Recording and Coding Guide for the Structure Inventory 

and Appraisal of the Nation’s Bridges (FHWA 1988), and 

the Codes for Named Populated Places, Primary County 

Divisions, and Other Locational Entities of the United States 

and Outlying Areas (National Bureau of Standards 1987). 

Addresses and contacts were found for each owner–agency 

and incorporated into the project database. This information 

was primarily requested from the Department of Transportation or other central transportation office in each State. In 

some cases, the owner–agency is the State transportation 

offices. In most instances, the owner–agency is a county; the 

address and contact of this type of owner–agency was obtained from the pertinent State transportation office. Similar 

information was obtained from the National Park Service 

and USDA Forest Service offices, and the appropriate 

agency contacts were linked with each bridge. 

The next step in the data collection process was to develop a 

questionnaire to obtain cost information from each timber 

bridge owner–agency. A background literature review was 

conducted to determine the best information to request, and 

a detailed 2-page survey was developed to obtain cost, bid, 

and contractor or supplier information (App. B). This 

questionnaire provided information to aid transportation 

officials in bridge identification. Owner–agency personnel 

were asked to review the information and report any 

discrepancies. 

Three cost figures were defined and solicited: (1) total superstructure cost, (2) total substructure cost, and (3) total 

bridge cost. Superstructure costs included materials, labor, 

and transportation expenses associated with the construction 

of all bridge components between abutments and above 

bents, including stringers, beams, deck, traffic railing, and 

wearing surface, and costs of protective membrane and 

excluding approach, approach railing, detour, and mobilization. Substructure costs were defined as materials, labor, and 

transportation expenses associated with the construction of 

all bridge components beneath the superstructure, including 

AK 28

WA 22

OR 34

CO 21

MN

246

WI

145

IA 78

NY

118

HI 0

NV 6

TX 8

NM 11

AZ 0

CA 18

UT 3

WY 3

MT 6

ID 13

KS 2

ND 5

SD 2

NE 9

IL 11

MO 6

AR 3 OK 22

LA

84

MS

88

IN

269

MI

104

KY 31

TN 23

WV

33 VA 2

OH 16

PA 16

SC

4

NC 0

GA

1

FL

9

AL

20

ME

2

MA 3

NH 8

VT 4

RI 1

CT 14

NJ 34

DE 16

MD 2

DC 0

Less than 10 10-20 Greater than 20

Figure 1—Geographic distribution of load-rated vehicular timber bridges constructed during or after 1980. 
5 

abutments and bents, and costs of excluding approach, approach railing, detour, and mobilization. Bridge costs included all materials, labor, and transportation expenses 

associated with the completion of the entire bridge project, 

with the exception of approach, approach railing, detour, and 

mobilization costs. 

Additional information was requested regarding factors that 

might skew cost figures, such as the inclusion of a given 

bridge in local, State, or Federal demonstration projects, 

such as the Timber Bridge Initiative, or volunteer/donated 

labor, materials, or services in bridge construction. In addition, a cost worksheet was provided to aid in superstructure 

cost tabulation; a copy of the final project cost worksheet 

was requested for verification purposes. Agency officials 

were also asked to list the number of bids placed for the 

bridge project in question. Finally, the name, address, and 

telephone number of the primary contractor and/or supplier 

of superstructure materials for the bridge in question were 

requested, to aid in verifying cost figures. The questionnaires 

were mailed to respective bridge owner–agencies across the 

United States. 

After the timber bridge questionnaires were received, any 

new bridge data obtained from owner–agencies were entered 

into the timber bridge database. Where discrepancies were 

noted by a transportation official regarding variables such as 

bridge length, year built, and location, corrections were 

made based on the comments written on the questionnaire. 

Other than those corrections, the timber bridge database was 

not altered. Those records containing superstructure cost 

were identified as valid or usable timber bridges. Throughout the remainder of this report, bridges for which usable 

superstructure cost information was available are referred to 

as “valid” bridges. All cost evaluations and comparisons for 

this study were based on valid bridges only. 

Matched Bridges 

To execute a useful cost comparison between timber and 

nontimber bridges, a matching scheme was developed for 

pairing each valid timber bridge with a steel, concrete, and 

prestressed concrete bridge possessing similar characteristics. Although exact matches of all characteristics were 

unlikely, it was expected that matches of quantitative characteristics, such as bridge width and length, would occur 

within an acceptable range. In the same manner, exact 

matches of discrete characteristics, such as load rating and 

location, were expected. 

Based on these assumptions, a bridge “match” existed for a 

given timber bridge if both bridges were located within the 

same State, were within 15% in structure length, had the 

same number of lanes, had the same load rating, and were 

built within 15 years of the year of construction of the timber 

bridge in question. This set of criteria is referred to as the 

primary matching scheme throughout the remainder of this 

report. The ranges were believed to be precise enough for 

comparison while broad enough to allow a suitable number 

of matches. 

Using the NBI, up to 10 matching bridges of each material 

type (steel, concrete, and prestressed concrete) were identified for each timber bridge. In cases where more than 

10 matching bridges of a particular material type were identified, only the 10 bridges closest in length to the original 

timber bridge were selected; 2,549 matching bridges were 

identified as valid timber bridges (App. A, Stage 2). 

The questionnaire distributed to timber bridge owner–

agencies was then distributed to owner–agencies of the 

matching steel, concrete, and prestressed concrete bridges 

(App. B). Bridges for which the completed questionnaires 

contained information on superstructure cost or a final contractor or bid worksheet by which such costs could be obtained were identified as valid or usable matched bridges. 

The study included both single- and multiple-span bridges. 

In addition, it was possible for two bridges of the same total 

length to have a different number of spans, thus different 

cost characteristics. For example, a 36-ft-long bridge 

consisting of two, 18-ft spans would have different 

superstructure design requirements than a bridge with a 

single 36-ft clear span. Structural members typically increase 

in size as span length increases to withstand applied bending 

and shear stresses and meet deflection criteria for a given 

design load (Ritter 1990). Thus, a relationship between cost 

and span length is likely because a greater volume of 

primary construction material is generally required as span 

length increases to meet design criteria. 

Because superstructure cost might be influenced by span 

length, bridges matched through the primary matching 

scheme were additionally screened to within 15% of maximum span length (secondary matching scheme). Bridges that 

did not meet the maximum span-length criterion were identified for further analysis (App. A, Stage 4). 

Project Data Limitations 

The availability of bridge cost information was limited for 

some portions of the Nation. For various reasons, some 

transportation agencies were unable or unwilling to provide 

cost information for timber and nontimber bridges under 

their jurisdiction. Many agencies responded by stating that 

they did not maintain the type of detailed information requested on the questionnaires. Some agencies were too busy 

or short-staffed and were unable to respond to the survey. 

Others had already developed an opinion regarding the use 

of timber bridges and were unwilling to respond. These data 

limitations precluded taking a smaller random sample, since 

such samples rely heavily upon high response rates. Instead, 

a large body of data was collected, where available, through 

an intensive survey. Reporting was self-selecting and not 

randomly drawn. 
6 

In an effort to establish a representative sample, multiple 

mailings were sent to bridge owner–agencies to increase 

response rates. In some cases, follow-up phone calls were 

made to obtain data and/or ensure accuracy of information. 

In addition, project response characteristics were compared 

or verified with those of the total populations of project 

bridges. This assessment is discussed further in the following section on data analysis. Considering these efforts and 

the broad scope of the study, it is likely that the results of 

this project are generally applicable to timber bridge 

construction. 

Data Analysis 

Data analysis procedures are outlined for verification of 

responses, timber bridge cost characteristics, and cost comparison. The section on verification of responses describes 

the steps taken to compare response data characteristics with 

those of the total population for both timber and nontimber 

bridges. The section on timber bridge cost characteristics 

details the manner in which timber bridge cost characteristics were evaluated. Finally, the section on cost comparison 

describes the procedures by which timber and nontimber 

costs were compared. 

Verification of Responses 

To ensure that a representative sample of the population was 

surveyed, valid bridges were compared with those from the 

total population (App. A, Stage 3). Specifically, the percentage of bridges from the valid data set possessing a given 

characteristic was evaluated against the percentage of 

bridges from the total population possessing that same characteristic. For example, the percentage of bridges with an 

HS20 load rating was compared with the percentage of the 

total population of bridges with this load rating. This analysis was completed for both timber and matching bridges for 

seven factors: construction type, structure length, deck 

width, number of lanes, load rating, year constructed, and 

geographic region. 

Timber Bridge Cost Characteristics 

The cost figures for each timber bridge were indexed for 

inflation based on the year of construction and the construction sector producer price index (PPI) from The Economic 

Report of the President: 1996 (Council of Economic Advisers 1996) (Table 4). The PPI is an indicator of the cost of a 

given set of goods at the point of the first significant commercial transaction and is appropriate for gauging changes in 

the construction market where raw materials and semifinished goods are utilized. Bridge costs were indexed 

around the base year 1982 by dividing cost figures by the 

PPI percentage for the year in which the bridge was constructed. The index year is a scalar, and the choice of year 

does not change the results. The year 1982 was selected 

because it is the baseline used in The Economic Report of 

 the President: 1996. By using that index year, it was not 

necessary to transform the indexes in this report. Prices were 

indexed to control for inflationary price changes not directly 

associated with events occurring in the bridge construction 

market. Any trend in bridge construction after indexing was 

likely the result of developments in the market. 

Timber bridge cost data were analyzed on a unit or per 

square foot basis for six factors: construction type, structure 

length, maximum span length, load rating, year of construction, and geographic region. Cost per square foot was compared with the continuous factors of structure length, maximum span length, and width. The data set was not random 

but self-selected by respondents. For this reason, no attempt 

was made to fit a regression line to these plots nor were 

statistical tests performed. Box plots were developed for 

comparisons between cost per square foot and categorical 

factors (construction type, load rating, year constructed, and 

region). 

Cost data were additionally analyzed across multiple factors 

or disaggregated. Such a data cross section allows for a clear 

understanding of the relationship between two variables 

while holding other factors constant. 

Because of the limited number of single-lane structures and 

in an effort to further disaggregate project data, only twolane structures were considered in this stage of analysis. 

Bridges were separated by construction type. Because of 

data limitations, only the two most common construction 

types were retained at this analysis stage: the slab and 

stringer/multi-beam. 

Data sets from both slab and stringer/multi-beam construction types were subdivided by region. The two largest regions, Midwest and Northeast, were used to create box plots 

showing cost per square foot by load rating. The process was 

Table 4—Construction sector producer 

price indexes (PPIs) used to adjust 

timber bridge costs for inflation 

Year bridge 

constructed PPI 

1980 91.3 

1981 97.9 

1982 100.0 

1983 102.8 

1984 105.6 

1985 107.3 

1986 108.1 

1987 109.8 

1988 116.1 

1989 121.3 

1990 122.9 

1991 124.5 

1992 126.5 
7 

then reversed. This time, the slab and stringer/multi-beam 

data sets were additionally subdivided by load rating and 

analyzed by region. The two largest load-rating data subsets, 

HS20 and HS20+Mod, were used to develop box plots 

showing cost per square foot by region. 

Plots of cost per square foot as a function of structure length 

were also developed. Such plots were created for HS20 and 

HS20+Mod bridges in each of two regions (Midwest and 

Northeast) for both slab and stringer/multi-beam construction types, for a total of eight plots. 

Cost Comparison 

A preliminary cost comparison was made based on all available data points for each bridge material. A box plot was 

developed using the complete valid data set for each material 

to compare median values and distribution characteristics 

based on the maximum number of observations available 

(App. A, Stage 5a). 

For a more informative comparison, valid bridges of each 

material type were linked to their respective timber bridge 

match (App. A, Stage 5a). The cost per square foot for each 

matching bridge was adjusted for inflation based on the 

construction sector PPI. As Figure 2 shows, mean cost per 

square foot (Nij) was calculated according to 

∑

=

=

k

t

ijt

ijt k

N

N

1

where 

Nij is the mean of nontimber bridge(s) of type j that match 

bridge i, 

i an index for the set of timber bridges, 

j the matching bridge type, where j = s, c, and p 

for steel, concrete, and prestressed concrete, 

respectively, 

k the number of bridges for matching bridge type j 

that match timber bridge i, and 

Nijt the adjusted cost for nontimber bridge t 

(t = 1, ..., k), of type j, that matches timber bridge i

in the case of multiple responses (questionnaires) for bridges 

of the same material type for a given timber bridge. As a 

result, only one cost per square foot value per material type 

was matched with that of each timber bridge (App. C). 

The matched bridges were divided into three groups of 

matched pairs: timber and steel, timber and concrete, and 

timber and prestressed concrete bridges. The size of each 

group depended on the number of timber bridge–nontimber 

bridge matches per material type. 

The individual differences between each timber bridge and 

its nontimber match (di) were calculated: 

dij = Ti − Nij, 

where Ti is timber bridge i adjusted superstructure cost per 

square foot. 

Note that for simplicity, dij = di and Nij = Ni throughout the 

remainder of this report. 

Individual differences (di) and median differences (md) were 

plotted in box plots. Positive differences (di) resulted when 

timber bridge cost per square foot values were greater than 

those for their nontimber matches, and negative differences 

(di) resulted from timber bridge cost per square foot values 

less than those for their nontimber matches. 

This process was repeated for the subset of bridges matched 

by the secondary matching scheme, which was governed 

by a stringent maximum span length criterion (App. A, 

Stage 5b). 

Figure 2—Calculation of mean cost per square foot for nontimber bridges. 
8 

Results and Discussion 

Characteristics of timber bridges in the valid data set are 

listed in Appendix C. To verify that these data were representative of the total population of timber bridges, bridges in 

the valid data and total population sets were compared by 

construction type, structure length, structure width, number 

of lanes, load rating, year constructed, and region (App. D). 

A trend was identified between the valid and total population 

sets for size-related data categories. The data on structure 

length, structure width, number of lanes, and load rating all 

demonstrate an underrepresentation of small bridges with 

low load ratings and a slight overrepresentation of larger 

bridges with higher load ratings. Furthermore, the figure of 

valid data set responses by region (App. D, Fig. 12) suggests 

an underrepresentation in 5 of the 10 regions, including the 

large Southeast region, and an overrepresentation in the 

Midwest and Northeast regions. However, given the wide 

scope of this project and the overall consistency between the 

valid and total population sets for the factors considered, it is 

likely that the findings of this report will be broadly applicable to timber bridge construction. 

Cost Characteristics for Timber 

Bridges 

The cost analysis of timber bridge superstructures is described in terms of cost characteristics by single factors and 

across multiple factors. Data on cost trends evaluated for 

construction type, structure length, maximum span length, 

load rating, year of construction, and geographic region are 

shown in Appendix E. Continuous factors are plotted against 

cost per square foot. Categorical factors are described by box 

plots. Data are also disaggregated to show trends across 

multiple factors. 

Single Factors 

Although measures of central tendency, such as median and 

mean, are useful in summarizing cost data, more complete 

understanding of bridge costs can be obtained by comparing 

costs against other factors, such as length and load rating. 

The sole use of measures of central tendency often hides 

important information about data. For example, costs could 

be greater for one construction type than for another. Considering median or mean timber bridge cost per square foot 

alone would not reveal such information. Therefore, it is 

useful to compare timber bridge cost per square foot with 

other factors. 

The highest median costs for T-beam, box beam multiple, 

and truss-through bridges were well above $50/ft2

 (Table 5). 

It is not known whether the unusually high cost of a singlelane frame timber bridge is representative of frame bridges 

because of the single observation in this category. In terms 

of structure length, cost was greater at both ends of the 

spectrum (between 20 and 50 ft long and greater than 150 ft 

long). In terms of bridge span length, costs were highly 

variable; the maximum span length of most bridges was less 

than 50 ft. 

No pattern emerged for the relationship between superstructure cost and bridge width. In some timber bridge designs, an 

increase in deck width not only requires an increase in material but also influences deck and/or floor beam thickness. In 

such cases, the volume of bridge material must increase as 

deck width increases to maintain a given load rating. In 

addition, some deck designs require transverse bracing, 

stiffener beams, or other components as width increases 

(Ritter 1990). Consequently, bridge width may influence 

superstructure cost even when cost is considered on a per 

square foot basis. Nevertheless, the effect of width on superstructure cost showed wide variation in our valid data set. 

It is likely that the cost effects of bridge width were obscured by other cost factors. 

Because bridges built to carry high loads may require more 

primary construction material per square foot of deck, it is 

likely that load rating is an important cost factor. All seven 

load classifications were present in the valid data set. However, few data were available for the H10, H15, HS15, and 

HS25 load ratings (Table 6). This shortage followed the 

general trend in the total population, with the majority of 

bridges falling in the H20, HS20, and HS20+Mod classifications. Cost tended to increase with load rating, except in the 

H10 and HS15 categories (Table 6). However, as Table 6 

indicates, the HS15 category was represented by only one 

data point. In addition, an unusually high value for one 

bridge in the H10 category dramatically influenced the mean 

cost for that load rating. This was an atypical one-lane frame 

timber bridge in Mississippi. The elimination of this outlying 

observation lowered the mean cost for the H10 category 

from $52.41 to $20.05. 

Table 5—Bridge cost by construction type 

Cost ($/ft2

) 

Construction type 

Observations 

(no.) Median Mean 

Slab 138 24.83 28.58 

Stringer/multi-beam 56 31.12 31.59 

Girder and floorbeam 

system 

1 45.25 45.25 

T-Beam 2 64.10 64.10 

Box beam or girder 

Multiple 7 56.00 57.80 

Single or spread 1 51.69 51.69 

Frame 1 149.50 149.50 

Truss, through 2 60.18 60.18 

Arch, deck 1 39.09 39.09 

Total 209 
9 

Median cost per square foot was consistent across construction years (Table 7). Because cost values were indexed for 

inflation, any observable cost trend over time was likely the 

result of developments in the timber bridge construction 

market or related markets. Thus, there appeared to be no 

identifiable relationship between cost per square foot and 

year of construction. 

Superstructure cost varied across regions and was particularly variable in the Southeast (Table 8). The mean cost for 

the Southeast was apparently skewed upward by the same 

unusually high cost per square foot noted for the H10 load 

rating. 

Multiple Factors 

Just as evaluating timber bridge cost data against other factors provided a clearer understanding of timber bridge cost 

characteristics, evaluating timber bridges for costs across 

multiple factors allowed for an even closer look at cost 

relationships. We compared costs of two types of two-lane 

timber bridges, slab and stringer/multi-beam. For each construction type, costs were compared for each region (Midwest and Northeast) by load rating and for each load rating 

by region; costs were also compared in relation to structure 

length (App. E). 

For both the Midwest and Northeast, costs were higher for 

HS20 bridges than for HS20+Mod bridges, the two load 

ratings most represented in these data. Costs were lower in 

the Midwest for both types of bridges. Costs were highest 

for short bridges, somewhat high for long bridges, and lowest for middle-length bridges. In both regions, cost values for 

the HS20 and HS20+ load ratings were widely variable 

across structure lengths. 

In the Midwest, the median cost of two-lane stringer/multibeam timber bridges increased as load rating increased. The 

same trend was evident in the Northeast, except for the 

HS15 category. When evaluated by region, cost of the HS20 

two-lane stringer/multi-beam timber bridges was lowest in 

the Midwest, higher in the Northwest, and highest in the 

Northeast. For the HS20+ bridges, which were evaluated in 

two regions, median costs were higher in the Midwest than 

in the Northeast but mean costs were reversed. 

Summary of Timber Bridge Costs 

In general, data received from owner–agencies were highly 

variable, making it difficult to identify relationships between 

factors. 

When costs were compared for one factor, we note the following: 

• Cost per square foot varied by construction type, with the 

lowest mean and median values reported for slab and 

stringer/multi-beam timber bridges, the most widely constructed types. 

• When plotted against structure length, cost per square foot 

exhibited a parabolic shape, with higher values for the 

shortest and longest bridge lengths. 

Table 6—Bridge cost by load rating 

Cost ($/ft2

) 

Load rating 

Observations 

(no.) Median Mean 

H10 4 27.70 52.40 

H15 7 7.87 14.75 

HS15 1 46.92 46.92 

H20 9 20.03 21.67 

HS20 154 25.48 30.34 

HS20 + Mod 37 35.56 40.44 

HS25 3 39.09 37.83 

Total 215 

Table 7—Bridge cost by year of construction 

Cost ($/ft2

) 

Year 

constructed 

Observations 

(no.) Median Mean 

1980 21 32.41 31.85 

1981 13 21.53 21.26 

1982 12 23.12 31.67 

1983 8 28.20 36.49 

1984 12 22.58 25.00 

1985 21 26.96 29.54 

1986 21 23.87 25.04 

1987 14 25.19 30.98 

1988 18 25.93 29.14 

1989 13 28.37 32.26 

1990 30 29.91 40.28 

1991 24 35.33 36.40 

1992 15 36.47 36.36 

Total 222 

Table 8—Superstructure cost by region 

Cost ($/ft2

) 

Region 

Observations 

(no.) Median Mean 

Central 5 36.98 34.69 

Intermountain 1 46.58 46.58 

Midwest 148 24.28 27.81 

Northeast 54 36.61 42.21 

Northern 2 28.79 28.79 

Northwest 7 28.76 30.06 

Southeast 5 4.80 37.00 

Total 222 
10 

• Neither maximum span length nor bridge width appeared 

to influence cost per square foot. 

• Year of construction had little effect on cost per square 

foot. 

• Cost per square foot increased with higher load ratings, 

with noted exceptions. 

• Midwest, Northern, and Northwest regions had lower 

median costs per square foot than those reported for Central and Northeast regions. Cost data varied widely for the 

Intermountain and Southeast regions. 

When costs were compared across multiple factors, we note 

the following: 

• Disaggregating superstructure cost information across 

multiple factors improved our understanding of cost relationships between factors. 

• Across multiple factors, timber superstructure cost per 

square foot was higher for the Northeast region than for 

the Midwest region. 

• Cost per square foot was higher for the HS20+Mod load 

rating than for the HS20 load rating across multiple factors. 

• A comparison of cost per square foot and structure length 

exhibited a parabolic shape for HS20 two-lane slab timber 

bridges in the Midwest region. 

• A comparison of cost per square foot and structure length 

displayed a parabolic shape for HS20 two-lane 

stringer/multi-beam bridges in the Midwest region. 

• Even with substantial disaggregation, all plots of cost per 

square foot compared with structure length showed high 

variability in these data. 

Cost Characteristics for 

Matching Bridges

As for timber bridges, the characteristics of valid matching 

steel, concrete, and prestressed concrete bridges were compared with those of the entire population of matching bridges 

to ensure that project data were broadly representative of the 

total population. Data from these comparisons are included 

in Appendix F. The comparisons revealed that the valid data 

set was generally representative of the total population for 

material type, construction type, width, number of lanes, 

load rating, and region. 

Median cost for the timber data set was less than that for the 

steel data set and greater than that for the concrete and 

prestressed concrete data sets (Table 9). The middle 50% of 

observations was more tightly distributed for prestressed 

concrete than for the other materials (Fig. 3). Several 

outlying data points were observed for both timber and 

prestressed concrete bridges. 

Cost Comparisons for Primary and 

Secondary Matching Schemes 

Additional cost comparisons were conducted separately on 

two data sets of matched bridges. The primary set included 

costs from bridges matched on the basis of structure length, 

number of lanes, load rating, year constructed, and region. 

The secondary set, a subset of the primary, included costs 

from bridges that were further screened based on the tighter 

maximum span length criterion. 

For data sets based on the primary matching scheme, the 

prestressed concrete set had the highest number of pairs (98), 

followed by steel (28) and concrete (20). Median and mean 

values are presented in Table 10. 

The individual differences (di = Ti − Ni) were used to develop the box plot shown in Figure 4. Negative differences 

resulted when the cost per square foot for a given timber 

bridge was lower than the mean cost per square foot of the 

matched nontimber bridge. Accordingly, positive differences 

Table 9—Cost comparison of complete 

timber and nontimber valid data sets 

Cost ($/ft2

) 

Data set 

Observa 

tions 

(no.) Median Mean 

Timber 222 26.40 31.84 

Steel 27 27.50 31.40 

Concrete 37 19.13 27.53 

Prestressed concrete 115 21.67 25.45 

Total 401 

Figure 3—Percentage of timber bridges from valid 

data set and total population by construction type. 
11 

resulted when the cost per square foot for a given timber 

bridge was greater than the mean cost per square foot of the 

matched nontimber bridge. In the resulting box plot, the 

median difference (md) was positive for each type of material. The middle 50% of observed differences was more 

closely distributed about the median difference for 

prestressed concrete than for steel or concrete. 

For 46.4% of timber–steel matches, timber was less costly 

than steel. Similarly, for 45% of timber–concrete matches, 

timber was less costly than concrete. In contrast, timber was 

less costly than prestressed concrete for only 33.7% of timber–prestressed concrete matches. 

The mean cost per square foot for steel, $30.07, was less 

than that for the matched timber set ($31.19) (Table 10). The 

mean cost per square foot for prestressed concrete, $24.37, 

was also lower than that for the matched timber set ($28.29). 

In contrast, the mean cost per square foot for concrete, 

$35.22, exceeded that for the matched timber set ($27.90). 

The preceding analysis was applied to the secondary data 

set, bridges meeting the maximum span length criterion. Of 

the matched sets meeting this criterion, the prestressed concrete set was largest (45 observations), followed by steel 

(16) and concrete (15). The resulting median and mean 

values are summarized in Table 10. 

As for the primary data set, individual differences 

(di = Ti − Ni) were used to develop the box plot for the secondary data set (Fig. 5). The median difference value (md) 

was positive for prestressed concrete and negative for steel 

and concrete. Again, the middle 50% of observed differences 

was more tightly distributed about the median for prestressed 

concrete than for steel or concrete. 

For 50% of timber–steel matches, timber was less costly than 

steel. Timber was less costly than concrete for 53.3% of 

timber–concrete matches. Again, timber compared less 

favorably to prestressed concrete. Timber was less costly 

than prestressed concrete for only 24.4% of timber–

prestressed concrete matches. 

Table 10—Cost comparison of primary and secondary data sets of matched timber and nontimber bridges 

Timber cost ($/ft2

) 

Nontimber cost 

($/ft2

) 

Difference in cost 

(di

 = Ti − Ni) 

Data set 

Observations 

(no.) Median Mean Median Mean Median Mean 

Primary 

Steel 28 25.80 31.19 27.05 30.07 1.38 1.12 

Concrete 20 28.65 27.90 23.49 35.22 3.01 −7.32 

Prestressed concrete 98 24.51 28.29 21.57 24.37 3.05 3.92 

Secondary 

Steel 16 26.90 32.81 34.88 33.78 −0.70 −0.96 

Concrete 15 28.94 27.48 23.58 31.62 −0.55 −4.13 

Prestressed concrete 45 24.66 27.37 17.21 21.34 4.94 6.03 

Figure 4—Percentage of timber bridges from valid data 

set and total population by structure length. Lower 

endpoint of each length interval is inclusive (closed); 

upper endpoint is noninclusive (open). 

Figure 5—Percentage of timber bridges from valid data 

set and total population by deck width. Lower endpoint 

of each width interval is inclusive (closed); upper 

endpoint is noninclusive (open). 
12 

Mean cost per square foot for steel bridges ($33.78) and 

concrete bridges ($31.62) was higher than that for the 

matched timber sets ($32.81 and $27.48, respectively) 

(Table 10). Mean cost per square foot for prestressed concrete bridges ($21.34) was less than that for the matched 

timber set ($27.37). 

Summary of Costs of Matched Bridges 

Results of cost comparison analyses varied depending on the 

matching scheme adopted. The comparison of complete data 

sets by material type indicted that median cost per square 

foot was lowest for concrete bridges, followed by 

prestressed, timber, and steel bridges. The middle 50% of 

these data were more tightly distributed for prestressed 

concrete bridges when compared with bridges of other 

materials. 

Results of comparisons based on primary and secondary 

matching schemes were as follows. 

Primary matching scheme 

• Median differences were positive for steel, concrete, and 

prestressed concrete bridges, indicating that the cost per 

square foot of timber bridges was greater than that of 

matched bridges at the median observation. 

• Mean cost per square foot of timber bridges was greater 

than that of steel or prestressed concrete bridges and less 

than that of concrete bridges. 

Secondary matching scheme 

• Median differences were positive for concrete and 

prestressed concrete bridges and negative for steel bridges. 

That is, the cost per square foot of timber bridges was 

greater than that of concrete and prestressed concrete 

bridges and less than that of steel bridges at the median 

observation. 

• Mean cost per square foot of timber bridges was less than 

that of steel or concrete bridges and greater than that of 

prestressed concrete. 

In general, timber bridge superstructures tended to compete 

with steel and prestressed concrete bridge superstructures on 

an initial cost basis. Mean and median differences for each 

material were typically close to one another. More importantly, the ranges of cost per square foot for these bridges 

were highly variable and tended to overlap. 

In contrast, cost per square foot values for prestressed concrete bridge superstructures tended to be more tightly distributed, with mean values consistently less than those for 

timber bridge superstructures. In addition, median differences for prestressed concrete were positive in both paired 

comparisons, suggesting that the cost per square foot of 

prestressed concrete bridge superstructures is less than that 

of timber bridge superstructures. 

Cost Trends for Timber Bridges 

The following text summarizes cost trends for timber bridges 

and provides possible explanations for data patterns. Possible causes for the variability in data are also discussed. 

Results are then summarized for the initial cost comparison 

of timber and nontimber bridges. 

Timber Bridge Cost Characteristics 

Results show a relationship between cost per square foot and 

bridge length, load rating, and geographic location. There 

appears to be a parabolic relationship between the cost per 

square foot of timber bridge superstructures and structure 

length, with higher costs for both the shortest and longest 

lengths. Cost per square foot also appears to increase with 

higher load rating. This result was expected because larger 

structural members are generally necessary to attain higher 

load ratings, other factors being constant (Ritter 1990). 

Finally, cost per square foot is higher for the Northeast 

region than for the Midwest. This observation is consistent 

with the findings of a 1994 Timber Bridge Information 

Resource Center report, in which average superstructure 

costs per square foot for the Wood in Transportation Program demonstration bridges were higher for the northeastern 

States of Pennsylvania and West Virginia than for all States 

in the Midwest and Northeast regions combined (USDA 

1994). 

The findings of the study reported here are limited by the 

wide variability in the available data and, in the case of load 

rating and region, by the limited number of data points for 

some categories. The high variability in the data may have 

been the result of unspecified cost factors, lack of standardization in timber bridge construction, or failure of some 

highway engineers to recognize the cost effectiveness of 

timber bridges. 

It is likely that cost components not addressed in this study 

play an important role in determining bridge costs. For 

example, there may be location-specific cost determinants 

relating to material and equipment transportation. The lack 

of standardization in timber bridge design and construction, 

which has resulted in ad hoc assembly practices by various 

transportation agencies, may have also contributed to data 

variability. If so, the implementation of standardization 

efforts by Lee and others (1995) will likely lead to a reduction in timber bridge costs. Finally, it is clear from the range 

of cost data that many transportation officials have been able 

to achieve low cost per square foot values using timber 

bridges, perhaps because they are familiar with costeffective timber bridge designs or experienced in timber 

bridge construction. In addition, timber bridges may have 

found their market niche in the form of small-crossing, rural, 

and, most important, nontraditional applications that 

encompass a wide-range of construction practices and design 

concepts unique to timber. The limited number of timber 
13 

bridges greater than 100 ft in length and the lack of bridges 

of more than two lanes in width support this rationale. 

Comparison of Timber and Nontimber Bridges 

In general, the data indicate that timber bridge superstructures tend to compete with steel and prestressed concrete 

bridge superstructures on an initial cost basis. Mean and 

median costs for each type of material are typically close to 

one another. However, the range in cost per square foot 

values for all bridges varies widely. This outcome was likely 

due to both the high variability in the data and the relatively 

small sample size of the data sets for steel and prestressed 

concrete. Again, variability in the data may have stemmed 

from the absence of standardization in past timber bridge 

construction. If so, current efforts in standardization may 

serve to clarify the results of future cost comparisons, reduce 

timber bridge construction costs, and increase timber bridge 

construction. 

In contrast, cost per square foot values for prestressed concrete bridge superstructures tend to be more tightly distributed, with mean values consistently lower than those for 

timber bridge superstructures. Median values for prestressed 

concrete bridges were positive in both paired comparisons, 

suggesting that prestressed concrete bridge superstructures 

cost less per square foot than do timber bridge superstructures. The higher cost of timber superstructures relative to 

that of prestressed concrete superstructures may be the result 

of underutilization of timber as a bridge material and subsequent lack of competition in the bridge market. Under this 

scenario, as more firms enter the timber bridge market, the 

cost of timber bridge superstructures should decrease, other 

factors remaining constant. 

Future Research 

As noted throughout this paper, the results of this study 

suggest a need for standardization in timber bridge construction and design. Only through such efforts will cost variability for timber bridges be minimized and knowledge about the 

true prospects for timber bridges be gained. 

Work should also be directed toward a thorough comprehension of bridge cost components. Specifically, the determination of bridge costs should take into consideration the role of 

proximity to prestressing and pressure-treating facilities in 

urban areas. The effect of transportation costs (to and from 

such facilities) on bridge construction needs to be assessed. 

Such efforts might include multiple regression analysis of 

bridge cost determinants. The effect of construction method 

on bridge costs also needs to be evaluated. 

The need for life-cycle cost analysis in timber bridge economics is widely recognized. Current data are prohibitively 

scarce; therefore, efforts should be directed towards more 

thorough recordkeeping at all government levels. Comparative studies need to match bridge materials. For example, 

timber slabs should be compared with prestressed concrete 

slabs and timber beams with steel beams. Finally, 

demonstration projects should be used to pave the way for 

research. 

References 

Behr, R.A.; Cundy, E.J; Goodspeed, C.H. 1990. Cost comparison 

of timber, steel, and prestressed concrete bridges. Journal of 

Structural Engineering. 116: 3448–3457. 

Council of Economic Advisers. 1996. Economic report of the 

President. Washington, DC: U.S. Government Printing Office. 

(February) 402 p. 

FHWA. 1988. Recording and coding guide for the structure inventory and appraisal of the nation’s bridges. Rep. FHWA–ED–89–

044. Washington, DC: Federal Highway Administration. 

(December). 

FHWA. 1994. National bridge inventory data as of June 1994. 

Washington, DC: Federal Highway Administration. 

Hill, J.J.; Shirole, A.M. 1984. Economic and performance considerations for short-span bridge replacement structures. Transportation Resource Record. 950: 33–38. 

Lee; P.D. Hilbrich; Ritter, M.A.; Triche, M. 1995. Standard 

plans for southern pine bridges. Gen. Tech. Rep. FPL–GTR–84. 

Madison, WI: U.S. Department of Agriculture, Forest Service, 

Forest Products Laboratory. 

National Bureau of Standards. 1987. Codes for named populated 

place, primary county divisions, and other locational entities of the 

United States and outlying areas. FIPS PUB 55–2. 29 p. 

Ritter, Michael A. 1990. Timber bridges: design, construction, 

inspection and maintenance. EM 770–8. Washington, DC: U.S. 

Department of Agriculture, Forest Service, Engineering Staff. 

944 p. 

Sarisley, E.F. 1990. Construction methods and costs of stresslaminated timber bridges. Journal of Construction Engineers and 

Management. 116: 432–447. 

Smith, R.L.; R.J. Bush. 1994. Marketing practices in the timber 

bridge industry. Forest Products Journal. 44: 27–33. 

USDA. 1993. Timber bridge initiative: superstructure costs report 

on project funded bridges 1989–1993. Morgantown, WV: U.S. 

Department of Agriculture, Forest Service, Northeastern Area, 

State and Private Forestry, Timber Bridge Information Resource 

Center. 

USDA. 1994. The timber bridge initiative, fiscal year 1994 status 

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

Service, State and Private Forestry, Northeastern Area. 14 p. 

Verna, J.R.; Graham, J.F., Jr.; Shannon, J.M.; Sanders, P.H.

1984. Timber bridges: benefits and costs. Journal of Structural 

Engineering. 110: 1563–1571. 

Wolchuk, R. 1988. The “first cost syndrome” in bridge rehabilitation. Civil Engineering. 58: 6. 

15 

Appendix B—Survey of Bridge Superstructure Cost 

A questionnaire on bridge superstructure cost was sent to timber, steel, concrete, and prestressed concrete bridge owner–

agencies throughout the United States. 

The example of a demonstration project given in item 3 (Timber Bridge Initiative) was not included in the questionnaire given 

to the steel and concrete bridge owner–agencies. Otherwise, the questionnaires were identical. 

Survey on the Cost of Timber Bridge Superstructures 

Bridge Description/Type of Bridge: 

Structure Number: 

Feature Intersected: 

Location: 

County: 

Year Built: 

Structure Length (in feet): 

1. Do the above characteristics accurately describe this bridge? If “no,” please specify: 

2. Please provide the following cost information for this bridge (Definitions of terms are listed below the table): 

TOTAL SUPERSTRUCTURE COST (in dollars):

TOTAL SUBSTRUCTURE COST (in dollars):

TOTAL BRIDGE COST (in dollars):

DEFINITIONS: 

Superstructure cost - includes materials, labor, and transportation expenses associated with the construction of all bridge components between abutments and above bents. Includes stringers, beams, deck, traffic railing, wearing surface, and protective membrane. Please 

exclude approach, approach railing, detour, and mobilization costs. 

Substructure cost - includes materials, labor, and transportation expenses associated with the construction of all bridge components below 

the superstructure. Includes abutments and bents. Please exclude approach, approach railing, detour, and mobilization costs. 

Bridge cost - includes all materials, labor, and transportation expenses associated with the completion of the entire bridge project, with the 

exception of approach, approach railing, detour, and mobilization costs. Includes superstructure and substructure costs. 
16 

3. Was this bridge part of a federal, state, or other demonstration project (e. g. Timber Bridge Initiative)? If “yes,” please specify:

4. Describe any volunteer or donated labor, materials, or services used on this bridge that might affect how its cost would compare 

with those of similar bridges (e. g. prison labor, donated timber, other): 

5. Who may we contact in the event that further information or clarification is required?: 

Name: 

Title: 

Address: 

City, State, Zip: 

Phone: 

Fax: 

6. If available, please provide the following cost information: 

Superstructure Cost (in dollars)

Structure/Structural Members 

Deck 

Traffic Railing 

Wearing Surface 

Fabrication 

Erection 

Labor (if not previously included) 

Miscellaneous 

 

TOTAL:
17 

7. If available, please include a copy of the final project cost worksheet.

8. How many companies placed a bid for the project in question?: _______________________________

9. If available, please provide the following contractor/supplier information: 

Primary contractor for bridge superstructure:

Firm: 

Contact: 

Address: 

City, State, Zip: 

Phone: 

Fax: 

Primary supplier of superstructure materials (if different from contractor): 

Firm: 

Contact: 

Address: 

City, State, Zip: 

Phone: 

Fax: 

Please return this form to: 

Glade Michael Sowards Phone: (906) 487-3598 

Michigan Technological University Fax: (906) 487-2915 

School of Forestry and Wood Products 

1400 Townsend Dr. 

Houghton, MI 49931-1295
18 

Appendix C—Complete Data Set 

The table in Appendix C shows NBI data and cost per square foot values for the timber bridge valid data obtained from owner–

agencies. Cost per square foot values for steel, concrete, and prestressed concrete matched bridges are also provided. 

 Adjusted cost ($/ft2

) 

NBI struct. no.a Construction type 

Struct. 

length 

(ft) 

Max. 

span 

length 

(ft) 

Deck 

width 

(ft) 

No. 

of 

lanes Load rating 

Year 

built State Timber Steel 

Concrete 

Prestressedconcrete 

012507 Stringer/multi-beam or girder 40 24 16 2 H10 1981 AL 4.79 

014472 Stringer/multi-beam or girder 72 72 20 1 H15 1988 AL 2.99 

014474 Stringer/multi-beam or girder 30 30 16 1 H15 1988 AL 3.59 

PUCO0.01-204BR Stringer/multi-beam or girder 32 31 30 2 HS20 1990 CO 31.68 39.46 

USFS15310-0.1 Stringer/multi-beam or girder 84 39 15 1 HS20 1985 CO 36.98 

067024300.6010A Girder & floorbeam system 58 54 32.3 2 HS20 1992 CO 45.25 25.15 

000000000103121 Stringer/multi-beam or girder 29 27 24 2 H20 1990 IA 31.33 

000000000142511 Stringer/multi-beam or girder 60 20 22 2 H15 1981 IA 8.28 

000000000183441 Stringer/multi-beam or girder 56 28 25 2 H15 1985 IA 6.99 

000000000213282 Stringer/multi-beam or girder 22 21 24 2 H15 1992 IA 7.94 

000000000214381 Stringer/multi-beam or girder 65 22 24 2 H20 1986 IA 4.90 

000000000217459 Stringer/multi-beam or girder 84 32 30 2 H20 1984 IA 19.06 24.01 

000000000245851 Other 32 32 24 2 HS20 1990 IA 13.14 7.61 27.32 

000000000305411 Stringer/multi-beam or girder 33 33 25.6 2 HS20 1990 IA 44.94 7.61 

000000000324401 Stringer/multi-beam or girder 80 39 24 2 HS20 1992 IA 21.56 

000000000324631 Other 26 25 24.5 2 HS20 1990 IA 17.44 

000000000324650 Stringer/multi-beam or girder 39 20 23.2 2 H20 1990 IA 17.68 2.70 

000000000325201 Other 52 25 22 2 HS20 1989 IA 18.17 20.65 

000000000325711 Other 40 40 24.8 2 HS20 1988 IA 17.37 18.62 

000000000326871 Other 40 20 23 2 HS20 1992 IA 15.80 

000089326229675 Stringer/multi-beam or girder 120 32 26 2 HS20 1992 IL 28.02 26.82 

0200030 Stringer/multi-beam or girder 201 40 32.1 2 HS20 1982 IN 27.28 16.65 

3200002 Slab 24 22 27.5 2 HS20 1988 IN 56.30 18.32 

3200112 Slab 26 24 27.5 2 HS20 1988 IN 63.41 21.76 

3200219 Slab 26 24 25.5 2 HS20 1987 IN 57.57 21.76 

3200220 Slab 26 24 25.6 2 HS20 1987 IN 60.91 21.76 

6000139 Stringer/multi-beam or girder 52 24 21.5 2 HS20 1985 IN 30.90 19.13 

8000004 Slab 32 30 32.3 2 HS20 1980 IN 34.05 

9000032 Slab 78 25 26 2 HS20+Mod 1980 IN 32.41 

9000042 Slab 54 26 25.5 2 HS20 1986 IN 26.87 19.13 

TWN719045100 Stringer/multi-beam or girder 59 20 24.8 2 H20 1986 MA 25.11 

200000D-0018010 Slab 36 36 28 2 HS20 1992 MD 36.47 

0332 Other 35 27 44 2 HS20 1992 ME 48.73 

10307H00020B010 Slab 44 22 30 2 HS20 1987 MI 24.84 25.25 

26306H00002B010 Slab 24 22 18 2 HS20+Mod 1983 MI 101.33 34.94 

34315H00005B010 Slab 72 24 32 2 HS20+Mod 1989 MI 34.21 26.54 

34315H00023B010 Slab 66 30 32 2 HS20+Mod 1985 MI 21.06 

37302H00002B020 Slab 30 28 20.3 2 HS20 1983 MI 38.34 22.77 

37302H00003B080 Stringer/multi-beam or girder 24 22 31.5 2 HS20 1980 MI 35.58 

41322H34031B010 Truss,thru 114 103 14 0 HS20+Mod 1980 MI 63.27 

42142021000B050 Arch,deck 152 152 40 2 HS25 1988 MI 39.09 
19 

 Adjusted cost ($/ft2

) 

NBI struct. no.a Construction type 

Struct. 

length 

(ft) 

Max. 

span 

length 

(ft) 

Deck 

width 

(ft) 

No. 

of 

lanes Load rating 

Year 

built State Timber Steel 

Concrete 

Prestressed 

concrete 

43200042000B070 Slab 72 24 35 2 HS20+Mod 1987 MI 26.74 26.54 

43200050000B020 Slab 55 24 32 2 HS20 1991 MI 33.68 

44304H00009B010 Slab 52 26 28 2 H10 1991 MI 31.44 

44308H00020B010 Slab 24 22 29.8 2 HS20+Mod 1985 MI 3.54 33.16 

44314H00020B010 Slab 128 32 34 2 HS20+Mod 1988 MI 26.64 22.12 

44316H00004B020 Slab 26 25 27.3 2 HS20+Mod 1984 MI 2.84 51.47 25.03 

49149022000B050 Slab 26 26 24 2 HS20 1990 MI 30.76 46.54 

58304A00021B020 Slab 26 24 29.3 2 HS20+Mod 1990 MI 29.98 51.47 27.31 

59309H00030B010 Other 27 26 22.5 2 HS20+Mod 1989 MI 28.94 40.54 25.03 

59310H00021B010 Other 24 24 23.5 2 HS20+Mod 1989 MI 58.21 29.59 

67310H00003B010 Slab 52 25 36 2 HS20+Mod 1985 MI 74.68 23.13 

67316H00001B010 Other 242 61 29 2 HS20+Mod 1982 MI 71.25 

74307H00002B010 Slab 52 24 34 2 HS20 1991 MI 37.16 198.33 

74307H00005B010 Slab 54 26 34 2 HS20 1991 MI 35.78 

77312H00012B010 Slab 36 35 36 2 HS20+Mod 1991 MI 37.21 110.65 

81307H00037B010 Slab 44 21 32.9 2 HS20+Mod 1980 MI 35.56 

81307H00040B010 Slab 70 26 30 2 HS20+Mod 1984 MI 26.15 26.96 

81319H00027B010 Slab 72 24 32 2 HS20+Mod 1989 MI 26.12 26.54 

83307H00010B010 Slab 22 22 30 2 HS20 1990 MI 46.85 

01513 Slab 96 32 34 2 HS20 1980 MN 22.61 

02534 Slab 72 24 34 2 HS20 1986 MN 22.95 21.90 

02535 Slab 60 20 46 2 HS20 1984 MN 22.39 19.29 

02554 Slab 62 22 34.6 2 HS20 1991 MN 35.77 21.09 

11514 Slab 62 26 34 2 HS20 1988 MN 17.48 17.99 

15509 Slab 90 30 34 2 HS20 1982 MN 16.71 21.94 

17524 Slab 96 32 30 2 HS20 1983 MN 23.64 

17526 Slab 84 32 30 2 HS20 1985 MN 17.61 23.14 

17527 Slab 116 32 30 2 HS20 1986 MN 16.85 12.04 18.77 

18511 Slab 60 24 32 2 HS20 1986 MN 20.06 17.99 

18512 Slab 93 30 37.3 2 HS20 1984 MN 20.05 21.94 

18513 Slab 93 30 34 2 HS25 1991 MN 25.22 19.24 

18514 Slab 78 26 34 2 HS20 1981 MN 4.94 19.99 

18519 Slab 62 26 26.3 2 HS20 1990 MN 18.07 21.09 

22552 Slab 95 31 30 2 HS20 1980 MN 32.34 27.58 

22555 Slab 62 26 30 2 HS20 1980 MN 25.42 14.89 

22557 Slab 72 24 30 2 HS20 1981 MN 24.18 20.67 

22562 Slab 96 32 30 2 HS20 1982 MN 25.70 

22564 Slab 78 26 30 2 HS20 1985 MN 24.27 15.13 

22566 Slab 77 25 30 2 HS20 1984 MN 24.36 16.84 

22567 Slab 62 26 30 2 HS20 1982 MN 23.10 16.35 

22569 Slab 84 32 30 2 HS20 1984 MN 22.77 

22570 Slab 70 26 30 2 HS20 1986 MN 21.94 22.51 

22571 Slab 84 32 30 2 HS20 1987 MN 22.93 

22573 Slab 89 29 30 2 HS20 1988 MN 28.12 

22574 Slab 72 24 30 2 HS20 1985 MN 25.78 21.90 

22575 Slab 89 29 30 2 HS20 1985 MN 29.00 16.31 
20 

 Adjusted cost ($/ft2

) 

NBI struct. no.a Construction type 

Struct. 

length 

(ft) 

Max. 

span 

length 

(ft) 

Deck 

width 

(ft) 

No. 

of 

lanes Load rating 

Year 

built State Timber Steel 

Concrete 

Prestressed 

concrete 

25530 Slab 62 26 34 2 HS20 1986 MN 15.53 17.99 

29512 Slab 72 24 34 2 HS20 1981 MN 17.55 20.67 

29516 Slab 38 36 30 2 HS20 1986 MN 37.37 

29518 Slab 38 36 34 2 HS20 1991 MN 43.58 

32535 Slab 67 31 38.3 2 HS20 1986 MN 24.62 

33516 Slab 77 25 38.3 2 HS20 1986 MN 17.73 16.84 

33521 Slab 124 32 38.2 2 HS20 1981 MN 23.65 22.21 12.04 21.38 

33523 Slab 86 28 30 2 HS20 1985 MN 24.40 

33524 Slab 62 26 30 2 HS20 1989 MN 18.76 19.25 

33526 Slab 94 31 30 2 HS20 1987 MN 24.73 

36518 Slab 103 34 26 2 HS20 1986 MN 23.87 16.27 

37530 Slab 95 31 30 2 HS20 1982 MN 23.14 

39509 Stringer/multi-beam or girder 102 34 30 2 HS20 1986 MN 25.07 16.56 

39516 Slab 78 26 30 2 HS20 1990 MN 23.58 15.97 

42530 Slab 96 32 30 2 HS20 1980 MN 22.60 22.07 

42534 Slab 90 30 30 2 HS20 1980 MN 22.57 21.94 

45533 Slab 132 32 26 2 HS20 1980 MN 26.00 22.21 15.57 

47522 Slab 78 26 34 2 HS20 1980 MN 9.44 15.15 

47523 Slab 78 26 30 2 HS20 1981 MN 34.51 16.84 

47524 Slab 160 32 38.2 2 HS20 1985 MN 29.19 

47526 Slab 160 32 34 2 HS20 1980 MN 38.83 19.29 

47529 Slab 96 32 30 2 HS20 1984 MN 44.62 

47530 Slab 65 21 34 2 HS20 1984 MN 43.15 22.89 

49527 Slab 108 28 30 2 HS20 1981 MN 24.66 27.05 20.02 

49531 Slab 128 32 34 2 HS20 1986 MN 18.38 22.65 

49532 Slab 112 37 34 2 HS20 1989 MN 28.37 27.05 12.04 20.96 

49533 Slab 90 30 34 2 HS20 1989 MN 19.51 

49534 Stringer/multi-beam or girder 150 50 34 2 HS20 1985 MN 25.00 21.76 20.17 

49536 Slab 94 31 30 2 HS20 1990 MN 29.50 

49537 Slab 88 32 38 2 HS20 1991 MN 23.94 

52509 Stringer/multi-beam or girder 49 48 34 2 HS20 1986 MN 24.30 

52510 Slab 96 32 30 2 HS20 1990 MN 29.59 

53530 Slab 95 31 34 2 HS20 1986 MN 24.12 

56527 Slab 58 22 34.6 2 HS20 1987 MN 36.58 19.29 

56528 Slab 78 26 42 2 HS20 1987 MN 16.15 15.13 

59516 Slab 104 26 30 2 HS20 1987 MN 15.92 27.05 17.74 

59520 Slab 68 32 30 2 HS20 1991 MN 21.77 22.97 

64538 Slab 90 30 30 2 HS20 1980 MN 26.14 21.94 

64539 Slab 104 26 30 2 HS20 1982 MN 19.56 27.05 16.27 

64540 Slab 90 30 30 2 HS20 1982 MN 22.05 21.94 

64544 Slab 144 36 30 2 HS20 1989 MN 27.13 17.21 

64546 Slab 78 26 30 2 HS20 1988 MN 20.07 15.13 

64547 Slab 68 32 30 2 HS20 1990 MN 23.85 22.97 

72529 Slab 96 32 38 2 HS20 1987 MN 19.86 

74533 Slab 78 26 32 2 HS20 1987 MN 20.64 15.13 

77514 Slab 62 26 28 2 HS20 1983 MN 19.30 17.99 
21 

 Adjusted cost ($/ft2

) 

NBI struct. no.a Construction type 

Struct. 

length 

(ft) 

Max. 

span 

length 

(ft) 

Deck 

width 

(ft) 

No. 

of 

lanes Load rating 

Year 

built State Timber Steel 

Concrete 

Prestressed 

concrete 

77515 Slab 66 30 46 2 HS20 1980 MN 21.59 

77516 Slab 32 30 34 2 HS20 1980 MN 23.65 

77519 Slab 76 25 34 2 HS20 1983 MN 20.86 20.67 

80518 Slab 128 32 30 2 HS20 1984 MN 19.38 22.21 22.61 

80521 Slab 78 26 30 2 HS20 1990 MN 17.54 15.97 

80522 Slab 79 31 30 2 HS20 1988 MN 18.97 15.13 

81524 Slab 92 32 34 2 HS20 1991 MN 20.62 

83519 Slab 96 32 30 2 HS20 1981 MN 21.53 22.07 

83520 Slab 95 31 30 2 HS20 1981 MN 21.77 21.94 

83522 Slab 90 30 30 2 HS20 1987 MN 25.53 16.31 

83523 Slab 120 30 31 2 HS20 1990 MN 19.66 12.04 14.99 

83529 Slab 128 32 30 2 HS20 1988 MN 22.08 19.08 

999901003600450 Frame 21 21 20 1 H10 1990 MS 149.50 

999915663400060 Stringer/multi-beam or girder 67 20 24 2 H10 1991 MS 23.91 

000000009121320 Other 88 31 30.3 2 HS20+Mod 1985 ND 21.30 

000000009133260 Stringer/multi-beam or girder 52 50 34.7 2 HS20 1991 ND 36.28 

C004000303 Stringer/multi-beam or girder 61 30 20.1 2 H15 1986 NE 7.87 

03E4900 Boxbeam or girders,multiple 45 21 29.1 2 HS20+Mod 1987 NJ 34.77 135.73 

10XXT82 Stringer/multi-beam or girder 55 51 27.6 2 HS20+Mod 1986 NJ 43.02 45.20 

10XXT83 Stringer/multi-beam or girder 54 49 27.3 2 HS20+Mod 1989 NJ 35.73 45.20 

1000A65 Slab 36 34 37.3 2 HS20 1981 NJ 55.91 

1000L61 Slab 29 27 25.8 2 HS20+Mod 1983 NJ 30.55 

1000095 Slab 25 23 27.3 2 HS20 1990 NJ 55.68 

1000124 Slab 30 27 27.7 2 HS20+Mod 1985 NJ 26.96 33.02 

1507020 Stringer/multi-beam or girder 101 25 35.5 2 HS20+Mod 1988 NJ 41.22 28.19 

1516003 Boxbeam or girders,multiple 104 26 29 2 HS20 1985 NJ 31.93 

1516004 Boxbeam or girders,multiple 200 48 29 2 HS20 1985 NJ 38.33 

1518014 Other 57 28 25 2 HS20 1980 NJ 34.74 

1530002 Other 28 25 31.3 2 HS20 1980 NJ 34.66 

1530003 Other 23 20 31.3 2 HS20 1980 NJ 36.75 

000000002208110 Stringer/multi-beam or girder 72 34 27.1 2 HS20 1991 NY 47.62 44.09 

000000002216750 Stringer/multi-beam or girder 30 27 26 2 HS20 1992 NY 27.80 39.81 

000000002218030 Slab 34 33 24.2 2 HS20 1992 NY 31.94 

000000002218630 Stringer/multi-beam or girder 48 46 25.1 2 Other 1986 NY 45.33 

000000003217560 Slab 24 23 29.4 2 Other 1986 NY 22.94 

000000003219250 Slab 82 30 32.2 2 HS20+Mod 1991 NY 38.74 

000000003219420 Stringer/multi-beam or girder 47 46 23.1 2 Other 1985 NY 59.38 

000000003307940 Stringer/multi-beam or girder 66 61 26 2 HS20 1992 NY 35.24 50.08 

000000003317930 Stringer/multi-beam or girder 60 59 20.5 2 HS15 1989 NY 46.92 

000000003332810 Slab 32 31 32.3 2 HS20 1990 NY 24.48 54.88 

000000003332950 Slab 29 28 30.4 2 HS20 1990 NY 25.60 39.81 

000000003333090 Slab 31 29 30.1 2 Other 1990 NY 25.75 

000000003333140 Slab 32 31 32.1 2 HS20 1988 NY 24.17 69.65 

000000003333210 Slab 30 28 32.1 2 HS20 1988 NY 25.22 39.81 

000000003333540 Slab 28 27 30.1 2 HS20 1991 NY 25.22 
22 

 Adjusted cost ($/ft2

) 

NBI struct. no.a Construction type 

Struct. 

length 

(ft) 

Max. 

span 

length 

(ft) 

Deck 

width 

(ft) 

No. 

of 

lanes Load rating 

Year 

built State Timber Steel 

Concrete 

Prestressed 

concrete 

000000003333680 Stringer/multi-beam or girder 50 49 33 2 HS20 1991 NY 44.00 20.91 

000000003333770 Stringer/multi-beam or girder 30 28 32.3 2 HS20 1991 NY 34.90 39.81 

000000003333910 Slab 30 29 30.2 2 Other 1991 NY 24.81 

000000003334220 Slab 28 25 30.2 2 Other 1990 NY 25.57 

000000003334450 Slab 36 34 32.2 2 Other 1989 NY 28.14 

000000003334900 Stringer/multi-beam or girder 39 37 26.1 2 HS20 1991 NY 28.22 55.47 36.28 

000000003334940 Stringer/multi-beam or girder 48 46 25.9 2 HS20 1990 NY 29.83 15.32 

000000003357610 Slab 23 20 29 2 HS20+Mod 1982 NY 51.65 

000000003359330 Slab 36 35 35 2 HS20 1985 NY 44.83 

0433780 Truss,thru 104 98 18.7 1 HS20 1986 OH 57.08 

4438507 Slab 30 28 22 2 HS20 1990 OH 42.38 22.81 5.81 29.23 

4458567 Stringer/multi-beam or girder 25 24 15 1 HS20 1991 OH 4.71 

6603165 Stringer/multi-beam or girder 128 31 30.2 2 HS20 1981 OH 19.55 27.62 19.23 

7630786 Stringer/multi-beam or girder 29 29 29.2 2 HS20+Mod 1980 OH 49.15 39.83 20.02 

8131422 Slab 62 26 26.3 2 HS20 1982 OH 20.68 27.50 

1419926200120 Stringer/multi-beam or girder 25 23 31 2 H20 1988 OR 28.76 

167213052600240 Stringer/multi-beam or girder 44 43 24 2 HS20+Mod 1991 PA 98.01 

327220040600560 Stringer/multi-beam or girder 57 54 26.1 1 HS20+Mod 1991 PA 52.39 

647205033200040 Stringer/multi-beam or girder 38 35 18.8 2 H20 1985 PA 34.85 42.71 

000000041095085 Boxbeam or girders,singleorspread 65 63 38 2 HS20+Mod 1992 SD 51.69 

017042A Slab 23 20 16.1 1 HS20 1987 UT 46.58 

079963000000000 Stringer/multi-beam or girder 82 77 27.4 2 HS20 1983 WA 32.08 21.86 

080131000000000 Stringer/multi-beam or girder 35 33 18.1 2 H20 1984 WA 20.03 

084354000000000 Stringer/multi-beam or girder 88 20 20 2 H20 1985 WA 13.29 

084429000000000 Stringer/multi-beam or girder 47 45 29.5 2 HS20 1983 WA 25.84 22.26 

085636000000000 Stringer/multi-beam or girder 58 58 28.9 2 HS25 1989 WA 49.18 

085890000000000 Stringer/multi-beam or girder 60 57 32 2 HS20 1992 WA 41.25 23.40 

B14007900000000 Stringer/multi-beam or girder 37 35 32 2 HS20+Mod 1980 WI 41.50 

B36012000000000 Slab 25 23 30 2 HS20+Mod 1982 WI 63.53 

P13010300000000 Stringer/multi-beam or girder 43 39 32 2 HS20+Mod 1992 WI 50.09 23.58 42.78 

P36008700000000 Slab 84 32 29.5 2 HS20+Mod 1981 WI 15.00 

P36016000000000 Slab 67 32 29 2 HS20+Mod 1982 WI 15.44 15.99 

P71006000000000 Slab 26 25 31.3 2 HS20+Mod 1984 WI 35.22 

00000000006A045 Stringer/multi-beam or girder 65 63 23.8 2 HS20 1992 WV 46.44 36.51 

00000000015A015 Boxbeam or girders,multiple 43 40 17.6 1 HS20 1990 WV 109.54 

00000000018A027 Boxbeam or girders,multiple 44 40 22.1 2 H15 1990 WV 65.60 

00000000020A126 Stringer/multi-beam or girder 75 72 17.3 1 HS20 1988 WV 61.07 39.96 

00000000027A035 Teebeam 43 40 18.1 2 HS20 1990 WV 71.09 

00000000027A036 Boxbeam or girders,multiple 44 40 18.1 2 HS20 1991 WV 68.58 

00000000030A044 Stringer/multi-beam or girder 52 50 14.8 1 HS20 1990 WV 49.54 

00000000043A027 Boxbeam or girders,multiple 40 37 18.3 1 HS20 1990 WV 56.02 29.95 

00000000044A100 Teebeam 54 49 23.8 2 HS20 1992 WV 57.11 

00000000051A074 Slab 44 21 25.8 2 HS20 1988 WV 27.92 

00000000052A048 Slab 25 21 19.6 1 HS20 1990 WV 52.11 21.98 
23 

Appendix D—Verification of 

Valid Data Set for Timber 

Bridges 

To verify the validity of the data set, timber bridge data were 

compared with data from the total bridge population by 

construction type, structure length, structure width, number 

of lanes, load rating, year constructed, and region (Figs. 6 

to 12). Responses were received for 556 of the original 

1,604 timber bridges surveyed; 223 of these responses 

included complete cost information. 

Information on construction type was available for 

1,554 bridges; 209 of these bridges were considered valid. 

Together, slab and stringer/multi-beam bridges constituted 

more than 90% of the valid data set and the total population 

of timber bridges. Slab and box beam or girder bridges were 

overrepresented in the valid data set, and stringer/multibeam bridges were underrepresented (Fig. 6). Other construction types constituted only a small fraction of the total 

population. 

Complete information on out-to-out structure length was 

provided for 1,591 bridges; of these, 223 bridges were considered valid. The percentage of bridges from the valid data 

set appeared to be about proportional to the percentage of 

bridges from the total population for each 10-ft interval 

(Fig. 7). The greatest discrepancy was the smaller percentage 

of valid bridges between 20 and 30 ft long compared with 

the percentage of bridges in that range from the total population. Bridges in this category made up nearly 30% of the 

total population, and less than 15% of the bridges in the 

valid data set fell into the same range. The underrepresentation of bridges shorter than 30 ft may stem from a lack of 

reliable recordkeeping for short, relatively inexpensive 

timber bridges. Questionnaire comments support this assumption, as do the trends in other bridge characteristics. 

Of the initial 1,604 timber bridges, complete information on 

width was provided for 1,570 bridges. All 223 bridges of the 

valid data set had usable information on width. Bridges 

ranged between 10 and 65 ft wide, although more than 90% 

were between 15 and 35 ft wide. Narrower bridges tended to 

be underrepresented in the valid data set. Bridges between 

10 and 30 ft wide were a smaller percentage of the valid data 

set compared with the total population of bridges (Fig. 8). 

Again, this may be the result of a deficiency in recordkeeping for smaller, less-sophisticated bridges. In contrast, wider 

bridges tended to be overrepresented. It is reasonable to 

assume that larger projects with higher total costs are better 

documented and that data for such projects are more readily 

available. 

Information on the number of lanes that cross each bridge 

was provided for 1,571 bridges; 222 bridges were considered valid. Only one- and two-lane bridges were present in 

the total population. The lane characteristics of the valid 

bridges appeared to follow those of the total population 

(Fig. 9). Any discrepancies followed the general trend described for bridge length and width, with an underrepresentation of the smaller, single-lane bridges and a slightly 

higher percentage of the larger, two-lane bridges in the valid 

data set. 

Information on load rating was missing for several bridges in 

the total population. Only 1,517 bridges were valid in regard 

to this characteristic. Seven load ratings were represented in 

the total data set: H10, H15, HS15, H20, HS20, HS20+Mod, 

and HS25. More than 85% of the total population was represented by H10, HS20, and HS20+Mod bridges. HS20 

bridges accounted for 58.7% of the total population. The 

trend towards underrepresentation of smaller bridges in the 

valid data set was as apparent for load rating as for other 

characteristics (Fig. 10). Only 1.9% of the valid data set 

consisted of H10 load-rated bridges, compared with 12.9% 

of the total population. The H15, HS15, and H20 categories 

were all slightly underrepresented in the valid data set. 

Conversely, HS20 and HS20+Mod bridges were overrepresented in the valid data set; HS25 bridges were almost 

evenly represented in the data sets. 

Information on year of construction was provided for 

1,590 bridges; of these, 222 bridges were valid. Bridge 

construction was underrepresented between 1981 and 1984, 

and overrepresented between 1990 and 1992 (Fig. 11). 

However, there was no identifiable trend between the valid 

data set and the total population associated with year of 

construction. No discrepancy was greater than 4%. 

The initial population of 1,604 timber bridges was spread 

across the country among 47 States. Regional boundaries 

were adopted from a USDA Timber Bridge Information 

Resource Center (TBIRC) report (USDA 1993). For clarity, 

the Northeastern TBIRC region was split into two new regions: Midwest and Northeast. Within the valid data set, the 

two largest regions were the Northeast (54 data points) and 

the Midwest (148 data points) (Fig. 12). No cost information 

was received from the Alaska, California, and Southwest 

regions. The remaining regions each had between one and 

seven valid bridges within their boundaries. The Intermountain and Southeast regions were underrepresented in the 

valid data set. Although valid data set shortfalls for most 

regions represented an actual difference of only one or two 

bridges, valid data set underrepresentation for the Southeast, 

a region that accounts for 18.4% of the total population set, 

represented an actual difference of 35 fewer bridges than 

expected. Finally, there was a slightly higher percentage of 

valid data set responses for the Midwest and Northeast 

regions. 
24 

Figure 6—Percentage of timber bridges from valid data set and total population by construction type. 

Figure 7—Percentage of timber bridges from the valid data set and total population by structure length. The 

lower endpoint of each length interval is inclusive (closed). The upper endpoint of each length interval is 

non-inclusive (open). 
25 

Figure 8—Percentage of timber bridges from the valid data set and total population by width. The lower endpoint 

of each length interval is inclusive (closed). The upper endpoint of each length interval is non-inclusive (open). 

Figure 9—Percentage of timber bridges from the valid 

data set and total population by number of lanes. 
26 

Figure 10—Percentage of timber bridges from the valid set and total population by load rating. 

Figure 11—Percentage of timber bridges from the valid set and total population by year constructed. 
27 

Figure 12—Percentage of timber bridges from the valid set and total population by region. 
28 

Appendix E—Cost Analysis 

The cost analysis of timber bridge superstructures is described in terms of cost characteristics by single factors and 

across multiple factors. Figure 13 shows a sample box plot. 

The boxes represent the middle 50% of data for each category, with the bottom of the box denoting the first quartile 

(Q1) of data and the top denoting the third quartile (Q3). The 

horizontal line within each box represents the median for a 

given category. Lines (whiskers) extend from each box to 

the lowest and highest value within the region whose lower 

limit is Q1 − 1.5(Q3 − Q1) and whose upper limit is 

Q3 + 1.5 (Q3 + Q1). Asterisks designate observations that 

fall outside this region (outliers). 

Figures 14 to 20 show cost trends for construction type, 

structure length, maximum span length, deck width, load 

rating, year of construction, and geographic region, respectively. Figures 21 to 35 show costs for two-lane timber 

bridges for two construction types (slab and stringer/multibeam) in two regions (Midwest and Northeast). Continuous 

factors are plotted against cost per square foot. Categorical 

factors are described using box plots. In addition, data are 

disaggregated to show trends across multiple factors for a 

detailed cost analysis. 

For comparisons across multiple factors, the Midwest data 

subset of two-lane slab timber bridges represented four loadrating categories: H10, HS20, HS20+Mod, and HS25 

(Fig. 21). The HS20 and HS20+Mod categories were represented by 96 and 18 observations, respectively. The H10 and 

HS25 categories were each represented by only one observation. Median and mean costs per square foot were lower for 

HS20 bridges relative to those for HS20+Mod bridges. Cost 

ranges for the middle 50% of the data set were narrow, with 

a spread of $8.98 for HS20 bridges and $13.15 for 

HS20+Mod bridges, compared with $14.53 and $24.91, 

respectively, for the same load-rating categories from the 

valid data set (Fig. 18). 

The Northeast data subset of two-lane slab timber bridges 

were represented by only HS20 and HS20+Mod load-rating 

categories (11 and 4 observations, respectively). The HS20 

bridges exhibited a lower median cost relative to that for 

HS20+Mod bridges (Fig. 22). In addition, mean cost was 

lower for HS20 bridges ($34.31) than for HS20+Mod 

bridges ($36.98). 

For HS20 bridges, the Midwest region was represented by 

96 observations and the Northeast region by 11 observations. 

For HS20+Mod bridges, the Midwest was represented by 18 

observations and the Northeast by 4 observations. For both 

the HS20 and HS20+Mod data subsets, mean and median 

costs per square foot were lower for the Midwest than for the 

Northeast (Figs. 23 and 24, respectively). 

A comparison of cost per square foot and structure length for 

HS20 slab timber bridges in the Midwest exhibited a parabolic trend similar to that for the timber bridge valid data set 

(Fig. 25). Despite the level of disaggregation, data were still 

widely variable across structure lengths. Cost values for 

HS20+Mod slab timber bridges from the Midwest were also 

widely variable across structure lengths (Fig. 26). The limited number of data points available made it difficult to 

assess any trend for this cross section. As previously indicated, data were limited for HS20 and HS20+Mod slab 

timber bridges in the Northeast region. For both HS20 and 

HS20+Mod bridges, plots of cost per square foot values of 

cost against structure length showed extreme variability 

(Figs. 27 and 28, respectively). 

The four load ratings available in the Midwest data subset of 

two-lane stringer/multi-beam timber bridges—H15, H20, 

HS20, and HS20+Mod—were represented by 3, 4, 10, and 

3 observations, respectively. Median cost per square foot 

increased as load rating increased (Fig. 29). Mean values 

followed this trend, ranging from $7.73 to $46.91/ft2

. Especially pronounced was the lower median cost per square foot 

of the HS20 bridges compared with that of the HS20+Mod 

bridges. The middle 50% of these data for each category also 

followed this trend. 

The four load ratings in the Northeast data subset of twolane stringer/multi-beam timber bridges were the same as 

those in the Midwest data subset. The HS15, H20, HS20, 

and HS20+Mod load ratings were represented by 1, 2, 8, and 

4 observations, respectively. Median and mean cost per 

square foot increased as load rating increased for all but the 

HS15 category (Fig. 30). As with the Midwest region, the 

middle 50% of the Northeast data for each category followed 

the general trend of higher cost for higher load ratings. 

The five regions (Central, Midwest, Northern, Northeast, 

Northwest) in the HS20 data subset of two-lane 

stringer/multi-beam timber bridges were represented by 1, 

10, 1, 8, and 3 observations, respectively (Fig. 31). Mean 

and median costs were lowest for the Midwest, higher for 

the Northwest, and highest for the Northeast. The middle 

50% of these data for each category followed this trend. 

The Midwest and Northeast regions in the HS20+Mod data 

subset of two-lane stringer/multi-beam timber bridges were 

represented by 3 and 4 observations, respectively. Median 

cost per square foot for the Midwest was greater than that for 

the Northeast (Fig. 32). However, the mean cost for the 

Midwest was lower than that for the Northeast ($46.91 

compared with $54.50). In addition, the range of the middle 

50% of these data for the Northeast region was broader and 

overlapped that of the Midwest region. 

Data on cost as a function of structure length for HS20 

stringer/multi-beam timber bridges in the Midwest showed a 

parabolic trend, with higher costs for shorter bridges and 
29 

lower costs for longer bridges (Fig. 33). Again, data were 

variable across structure lengths. Only three data points were 

available for HS20+Mod stringer/multi-beam bridges for the 

Midwest (Fig. 34). Plots of cost per square foot against 

structure length were also highly variable for HS20 bridges 

in the Northeast region (Fig. 35). 

Figure 13—Description of box plot. 

Figure 14—Cost per square foot by construction type: 

(1) slab, (2) stringer/multi-beam, (3) girder and floor 

beam, (4) T beam, (5) box beam, multiple, (6) box beam, 

single or spread, (7) frame, (8) truss, through, and 

(9) arch, deck. 

Figure 15—Cost per square foot by structure length. 

Figure 16—Cost per square foot by maximum 

span length. 

Figure 17—Cost per square foot by width. 
30 

Figure 18—Cost per square foot by load rating. 

Figure 19—Cost per square foot by construction year. 

Figure 20—Cost per square foot by region. NE is 

Northeast, MW Midwest, SE Southeast, N Northern, 

C Central, Int Intermountain, and NW Northwest. 

Figure 21—Cost per square foot by load rating for 

two-lane slab timber bridges located in the Midwest 

region. 

Figure 22—Cost per square foot by load rating for 

two-lane slab timber bridges located in the Northeast 

region. 

Figure 23—Cost per square foot by region for 

two-lane, slab, HS20 timber bridges. 
31 

Figure 24—Cost per square foot by region for 

two-lane, slab, HS20+Mod timber bridges. 

Figure 25—Cost per square foot by structure length 

for two-lane slab HS20 timber bridges located in the 

Midwest region. 

Figure 26—Cost per square foot by structure length 

for two-lane slab HS20+Mod timber bridges located 

in the Midwest. 

Figure 27—Cost per square foot by structure length 

for two-lane slab HS20 timber bridges located in 

the Northeast region. 

Figure 28—Cost per square foot by structure length 

for two-lane slab HS20+Mod timber bridges located 

in the Northeast. 

Figure 29—Cost per square foot by load rating for 

two-lane stringer/multi-beam, timber bridges located 

in the Midwest region. 
32 

Figure 30—Cost per square foot by load rating for 

two-lane, stringer/multi-beam, timber bridges located 

in the Northeast region. 

Figure 31—Cost per square foot by region for twolane, stringer/multi-beam, HS20 timber bridges. 

Figure 32—Cost per square foot by region for twolane, stringer/multi-beam, HS20+Mod timber bridges. 

Figure 33—Cost per square foot by structure length 

for two-lane, stringer/multi-beam, HS20 timber 

bridges located in the Midwest region. 

Figure 34—Cost per square foot by structure length 

for two-lane, stringer/multi-beam, HS20+Mod timber 

bridges located in the Midwest region. 

Figure 35—Cost per square foot by structure length 

for two-lane, stringer/multi-beam, HS20 timber 

bridges located in the Northeast region. 
33 

Appendix F—Verification of 

Valid Data Set for Matching 

Bridges 

Responses were received for 477 of the original 2,549 

matched bridges. Of these, only 190 questionnaires included 

complete information on cost. The low number of responses 

was obtained despite repeated mailings to transportation 

owner–agencies. 

Figure 36 shows the percentage of nontimber bridges in the 

valid data set and total population by material type. Despite 

the low response rate, characteristics of the valid data set 

matched those of the total population for material type, with 

slight deviations for concrete and prestressed concrete 

bridges. 

For each material type, the construction type distribution for 

the valid data set was similar to that of the total population 

(Figs. 37 to 39). Of interest was the shift in predominance of 

one construction type or another. For example, slab bridges 

accounted for a large portion of the concrete bridges, fewer 

of the prestressed concrete bridges, and none of the steel 

bridges, as might be expected. 

Structure length characteristics of the valid data set approximated those of the total population for steel, concrete, 

and prestressed concrete matched bridges for most categories 

(Figs. 40 to 42). Major exceptions were noted, however, in 

the 50- to 80-ft range for steel bridges, the 30- to 40-ft category for concrete bridges, and the 30- to 40-ft and 50- to 

60-ft categories for prestressed concrete. In these categories, 

the characteristics of the total population were either over- or 

underrepresented in the valid data set. 

With only a few exceptions, width characteristics for the 

valid data set matched those for the total population. Values 

for the valid data set matched those for the total population, 

with the exception of some overrepresentation in the valid 

data set of concrete bridges in the 25- to 35-ft range and in 

the valid data set of prestressed concrete bridges in the 35- to 

40-ft range (Figs. 43 to 45). 

The number of lanes of bridges in the valid data set followed 

that in the total population of steel, concrete, and prestressed 

concrete bridges, with the exception of a lack of representation of single-lane bridges in the valid data set for concrete 

and prestressed concrete bridges (Figs. 46 to 48). Note that 

single-lane bridges constituted less than 1% of the total 

population of concrete and prestressed concrete bridges and 

that similar underrepresentation was observed for timber 

bridges for this category. 

With few variations, the load rating trend of the valid data 

set followed that of the total population of steel, concrete, 

and prestressed concrete bridges. Values for the valid data 

set roughly matched those for the total population for each 

material type (Figs. 49 to 51). Exceptions include a lack of 

representation for H15 steel bridges and H10, H15, and H20 

concrete bridges. However, these categories accounted for 

only small portions of the total population of each material 

type. 

A comparison of steel, concrete, and prestressed concrete 

bridges by construction year yielded more pronounced differences between the valid data set and total population than 

were apparent for the other factors considered. Both underand overrepresentation occurred in the valid data set in 

various years (Figs. 52 to 54). The reason for such deviations is unknown. 

The valid data set matched the total population of steel, 

concrete, and prestressed concrete bridges for most regions 

(Figs. 55 to 57). Anomalies included a lack of representation 

in the valid data set for the Intermountain, Northern, and 

Southeast regions for all material types and for the Northwest region for steel bridges. Lack of representation in the 

valid data set occurred for categories that constituted only a 

small portion of the total population for each material type. 
34 

Figure 36—Percentage of nontimber bridges from the valid set and total population by material type. 

Figure 37—Percentage of steel bridges from the valid set and total population by construction type. 
35 

Figure 38—Percentage of concrete bridges from the valid set and total population by construction type. 

Figure 39—Percentage of prestressed concrete bridges from the valid set and total population by construction type. 
36 

Figure 40—Percentage of steel bridges from the valid set and total population by structure length. The lower 

end-point of each length interval is inclusive (closed). The upper endpoint of each length interval is 

non-inclusive (open). 

Figure 41—Percentage of concrete bridges from the valid set and total population by structure length. The 

lower endpoint of each length interval is inclusive (closed). The upper endpoint of each length interval is 

non-inclusive (open). 
37 

Figure 42—Percentage of prestressed concrete bridges from the valid set and total population by structure 

length. The lower endpoint of each length interval is inclusive (closed). The upper endpoint of each length 

interval is non-inclusive (open). 

Figure 43—Percentage of steel bridges from the valid set and total population by width. The lower endpoint of 

each width interval is inclusive (closed). The upper endpoint of each width interval is non-inclusive (open). 
38 

Figure 44—Percentage of concrete bridges from the valid set and total population by width. The lower endpoint 

of each width interval is inclusive (closed). The upper endpoint of each width interval is non-inclusive (open). 

Figure 45—Percentage of prestressed concrete bridges from the valid set and total population by width. The 

lower endpoint of each width interval is inclusive (closed). The upper endpoint of each width interval is 

non-inclusive (open). 
39 

Figure 46—Percentage of steel bridges from the 

valid set and total population by number of lanes. 

Figure 47—Percentage of concrete bridges from the valid 

set and total population by number of lanes. 

Figure 48—Percentage of prestressed concrete bridges 

from the valid set and total population by number of 

lanes. 

Figure 49—Percentage of steel bridges from the valid set 

and total population by load rating. 

Figure 50—Percentage of concrete bridges from the 

valid set and total population by load rating. 

Figure 51—Percentage of prestressed concrete bridges 

from the valid set and total population by load rating. 
40 

Figure 52—Percentage of steel bridges from the valid set 

and total population by year constructed. 

Figure 53—Percentage of concrete bridges from the 

valid set and total population by year constructed. 

Figure 54—Percentage of prestressed concrete bridges 

from the valid set and total population by year 

constructed. 

Figure 55—Percentage of steel bridges from the valid 

set and total population by region. 

Figure 56—Percentage of concrete bridges from the 

valid set and total population by region. 

Figure 57—Percentage of prestressed concrete bridges 

from the valid set and total population by region.