Guidelines to Restoring 

Structural Integrity of Covered 

Bridge Members

Ronald W. Anthony

United States Department of Agriculture

Forest Service

Forest Products Laboratory

General Technical Report

FPL–GTR–252

January 2018

In cooperation with the 

United States Department of Transportation

Federal Highway Administration

A FHWA-NHCBP (National Historic Covered 

Bridge Preservation) publication
Abstract

These guidelines are designed for decision makers 

(selectmen, county commissioners, city planners, 

preservation officers, etc.) that have responsibility for 

repairing and maintaining existing covered bridges to help 

them understand what goes into making effective decisions 

about how, and when, to repair a covered bridge. The 

purpose of these guidelines is to present the steps necessary 

for decision makers to identify effective rehabilitation 

techniques for restoring the structural integrity of covered 

bridge members. The intent is to retain the maximum 

amount of historic fabric while ensuring public safety and 

minimizing future maintenance requirements. To make 

informed repair decisions about existing covered bridges, it 

is important to (1) start with a basic understanding of the 

type of bridge, (2) know the current condition, (3) be 

informed of what to consider when conducting an 

engineering analysis, and (4) be aware of various repair 

options that can meet the long-term goals for the bridge.

Only after the decision maker knows the type of bridge, its 

condition, and to what loads it is subjected can a repair 

strategy be developed. How to support the bridge during the 

repair phase, what repairs are appropriate, and how to 

maintain the bridge after repairs are implemented are all 

critical to ensuring a long service life for the covered bridge.

Keywords: covered bridges, historic, guidelines, repair, 

restore, structural integrity

January 2018

Anthony, Ronald W. 2018. Guidelines to restoring structural integrity of 

covered bridge members. FPL–GTR–252. Madison, WI: U.S. Department 

of Agriculture, Forest Service, Forest Products Laboratory. 56 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 53726-2398. This publication is also available online 

at www.fpl.fs.fed.us. 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 use of trade or firm names in this publication is for reader information 

and does not imply endorsement by the United States Department of 

Agriculture (USDA) of any product or service.

English unit Conversion 

factor

SI unit

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

pound (lb), mass 0.45359 kilogram (kg)

ton 0.90718 tonne (t) (×103 kg)

In accordance with Federal civil rights law and U.S. Department of Agriculture (USDA) civil rights regulations and policies, the USDA, its Agencies, 

offices, and employees, and institutions participating in or administering USDA programs are prohibited from discriminating based on race, color, national 

origin, religion, sex, gender identity (including gender expression), sexual orientation, disability, age, marital status, family/parental status, income derived 

from a public assistance program, political beliefs, or reprisal or retaliation for prior civil rights activity, in any program or activity conducted or funded by 

USDA (not all bases apply to all programs). Remedies and complaint filing deadlines vary by program or incident.

Persons with disabilities who require alternative means of communication for program information (e.g., Braille, large print, audiotape, American Sign 

Language, etc.) should contact the responsible Agency or USDA’s TARGET Center at (202) 720–2600 (voice and TTY) or contact USDA through the 

Federal Relay Service at (800) 877–8339. Additionally, program information may be made available in languages other than English.

To file a program discrimination complaint, complete the USDA Program Discrimination Complaint Form, AD-3027, found online 

at http://www.ascr.usda/gov/complaint_filing_cust.html and at any USDA office or write a letter addressed to USDA and provide in the letter all of the 

information requested in the form. To request a copy of the complaint form, call (866) 632–9992. Submit your completed form or letter to USDA by: (1) 

mail: U.S. Department of Agriculture, Office of the Assistant Secretary for Civil Rights, 1400 Independence Avenue, SW, Washington, D.C. 20250–9410; 

(2) fax: (202) 690–7442; or (3) email: program.intake@usda.gov.

USDA is an equal opportunity provider, employer, and lender.

This study is part of the Research, Technology, and 

Education portion of the National Historic Covered 

Bridge Preservation (NHCBP) Program administered by 

the Federal Highway Administration. The NHCBP 

program includes preservation, rehabilitation, and 

restoration of covered bridges that are listed or are 

eligible for listing on the National Register of Historic 

Places; research for better means of restoring and 

protecting these bridges; development of educational 

aids; and technology transfer to disseminate information 

on covered bridges in order to preserve the Nation's 

cultural heritage.

This study is conducted under a joint agreement 

between the Federal Highway Administration–Turner 

Fairbank Highway Research Center and the Forest 

Service–Forest Products Laboratory.

Federal Highway Administration Program Manager—

Sheila Rimal Duwadi, P.E.

Forest Products Laboratory Program Manager–

Michael A. Ritter, P.E.
Contents

Executive Summary............................................................2

Acknowledgments ..............................................................4

Chaper 1: Introduction........................................................5

Purpose of the Guidelines ...............................................5

Need for Repairs to Covered Bridges..............................5

Truss Types.....................................................................5

The Secretary of the Interior’s Standards for the 

Treatment of Historic Properties..............................6

Where do You Start?.......................................................8

References.......................................................................8

Chapter 2: Condition Assessment.......................................9

Mechanisms of Deterioration..........................................9

What to Look For..........................................................11

Where to Look...............................................................12

What Tools to Use.........................................................12

Challenges when Assessing Condition..........................16

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

Chapter 3: Engineering Analysis......................................21

Construction Details......................................................22

Timber Joinery and Connections...................................23

Considerations for Engineering Analysis of Timber 

Joinery and Connections........................................24

Considerations for Engineering Analysis of a 

Burr Arch ...............................................................25

Chapter 4: Supporting the Bridge for Repairs ..................28

Cribbing, Shoring, Staging, and Rigging ......................28

Other Considerations.....................................................31

References.....................................................................32

Chapter 5: Repairs............................................................ 34

Rehabilitating the Bridge Using Traditional Timber 

Repairs................................................................... 34

Strength Enhancement .................................................. 38

Visual Grading to Determine Strength of 

Bridge Members.................................................... 38

Increasing Load Capacity by Augmenting or 

Reinforcing Existing Members.............................. 40

Increasing Load Capacity Using Laminated Timber .... 41

Increasing Load Capacity by Reinforcing with FiberReinforced Polymers ............................................. 42

Increasing Load Capacity through Post-Tensioning ..... 43

Increasing Clearance..................................................... 44

Reducing Risk caused by Overload .............................. 44

Increasing Durability .................................................... 45

References .................................................................... 46

Chapter 6: Maintenance.................................................... 47

Cleaning........................................................................ 47

Vegetation and Moisture Management......................... 49

Siding and Roofing ....................................................... 49

Fire Protection .............................................................. 51

Remedial Treatments to Retard Decay and 

Insect Attack.......................................................... 52

Connections and Fasteners............................................ 53

References .................................................................... 54

Chapter 7: Bibliography ................................................... 55

Guidelines to Restoring Structural Integrity 

of Covered Bridge Members

Ronald W. Anthony, President and Wood Scientist

Anthony & Associates, Inc., Fort Collins, Colorado
General Technical Report FPL–GTR–252

2

Executive Summary

A covered bridge is a unique structure. Repairing and 

restoring a covered bridge properly is the primary means of 

keeping the bridge in service and preserving the craft that 

went into building the bridge for future generations to 

appreciate. It is an investment that requires decision makers 

and contractors to understand the importance of condition 

assessment and engineering analysis in selecting costeffective shoring and repair strategies.

Giving strong consideration to repair and/or rehabilitation of 

original bridge components is a cornerstone of the National 

Historic Covered Bridge Preservation Program. A covered 

bridge is a deck for carrying pedestrian and vehicular loads, 

supported by trusses and protected by the nonstructural roof 

and cladding. Retaining the maximum amount of historic 

fabric while ensuring public safety and minimizing future 

maintenance requirements are key to ensuring that a covered 

bridge will be there for future generations to appreciate 

and enjoy.

These guidelines are designed for decision makers 

(selectmen, county commissioners, city planners, 

preservation officers, etc.) that have responsibility for 

maintaining an existing covered bridge to help them

understand what goes into making effective decisions about 

how, and when, to repair a covered bridge. These guidelines 

present the steps necessary for decision makers to identify 

effective rehabilitation techniques for restoring the structural 

integrity of covered bridge members. To make informed 

repair decisions, it is important to (1) start with a basic 

understanding of the type of bridge, (2) know the current 

condition, (3) know what to consider when conducting an 

engineering analysis, (4) have a basic understanding of how 

to support the bridge during repairs, and (5) be aware of 

various repair options that can meet the long-term goals for 

the bridge.

Prior to conducting any repairs, the historic status of a 

bridge should be determined and the impacts of the repairs 

on that status evaluated. But regardless of any formally 

recognized historic status, covered bridges should be 

recognized as places of regional, cultural, and personal 

significance. As such, every effort should be made to adhere 

to The Secretary of the Interior’s Standards for the 

Treatment of Historic Properties, which require that the 

historic character of a property be retained and preserved 

and prohibit the replacement of intact or repairable historic 

materials. This includes the character-defining features of a 

structure, such as the trusses.

Bridge stewards, engineers, and contractors need to know 

when deterioration has occurred and if the current strength 

of the structural members is adequate for implementing a 

repair strategy for the bridge. A condition assessment is a 

key step to acquire that information. Is deterioration present 

and, if so, how extensive is it and why did it develop? Are 

there failed timber members or connections? These are basic 

questions that need to be asked when establishing the 

priorities for the condition assessment. It is essential to 

remember that the purpose of the condition assessment is to 

provide data that can be used to answer questions about the 

areas needing repair because of deteriorated conditions or 

inadequate capacity of the bridge.

In addition to conducting a condition assessment, 

understanding the structural behavior of the various 

components of the bridge is essential to making effective 

repair recommendations. Understanding the structural 

behavior is achieved when structural engineers conduct an 

engineering analysis of the entire structure — the trusses, 

deck, roof, abutments, and piers. A covered bridge performs 

differently from other structures, and without proper 

understanding of how a bridge behaves, incorrect design and 

implementation of repairs can be detrimental to the longterm performance of the bridge.

For some bridge repairs, such as those focused on roof 

covering, siding, or decking, repair work can often be 

conducted without unloading the trusses or lifting the 

bridge. Additionally, limited structural repairs can 

sometimes be undertaken without shoring or extensive 

rigging on truss types that have many redundant members.

Before more extensive repairs can be implemented, 

however, support of the bridge must be considered. Prior to 

implementing repairs, properly supporting the bridge, when 

necessary, is accomplished through cribbing, shoring, 

staging, and rigging to prevent unnecessary damage to the 

bridge during the repair phase.

The first question that should be asked when someone 

suggests that repairs are needed is if the bridge currently 

provides the required level of safety for its intended use. It 

may be obvious — a bridge with reverse camber (the bridge 

is sagging) probably warrants some attention if it is intended 

to carry vehicular traffic. If it is just a matter of running a 

computer model and concluding that the bridge is 

inadequate (and therefore needs reinforcement or

replacement), further consideration is warranted. What is the 

intended use? What did the condition assessment reveal? If 

the bridge shows no signs of distress, it is possible that

repairs may not be necessary.

Based on the condition assessment and engineering analysis, 

an informed decision about repairs can be made. Perhaps the 

wood siding or timbers are only weathered, and 

maintenance is all that is required. Perhaps there is isolated 

deterioration that should be addressed, but the bridge is able 

to carry the required loads. If deterioration is more 

widespread or members have failed, then multiple repairs 

may be necessary. If there are areas of deterioration severe 
Guidelines to Restoring Structural Integrity of Covered Bridge Members

3

enough to require repair or in-kind replacement of a 

member, the deteriorated material should be replaced with 

the same species and the original material should be 

matched in composition, design, color, and texture.

Repairing timbers using traditional timber framing 

techniques and joinery is critical for maintaining the historic 

character of the bridge. An experienced craftsperson that 

understands timber joinery can produce tight structural 

joints that will work and last for decades.

Strength enhancement is considered when load upgrades are 

needed. Load upgrades are required when the bridge cannot 

support anticipated loads based on engineering analysis. In 

cases like these, bridges need to be strengthened. There are 

several means to increase load capacity. One approach that 

is often overlooked but can account for slight increases in 

allowable design values is visual grading of the timber used 

in the bridge.

Strengthening methods include incorporating mechanically 

laminated decking or glued-laminated timber beams and 

deck panels. Post-tensioning, which involves adding steel 

cables or rods to carry some of the load that the bottom 

(tension) chord may not be able to carry because of its 

species, size, or grade, is a viable option for bridges that 

carry heavy loads. Fully threaded structural screws may 

allow for repairing damaged connections or members 

without replacement of the existing materials. There are also 

means to increase clearance and durability to extend the life 

of a bridge. A bridge that is properly repaired can provide 

decades of reliable performance before significant repairs 

are needed again.

One of the comments made about covered bridges is "This 

bridge is 150 years old. It had really high quality timber 

with no knots in it. I can't get that quality of material today."

The reality is, you can. There are less than a thousand 

covered bridges remaining in the United States. They are not 

all being repaired at the same time, and on any given bridge,

a finite volume of timber is needed to do the repairs.

Through good specifications, the proper quality of material 

can be acquired. There may be more expense, but the cost of 

the material is more than offset by the longevity of the 

repair.

Although these guidelines focus on considerations for 

making more effective repair decisions, covered bridges 

often suffer from a lack of regular maintenance that can lead 

to deterioration and failure of wood roofing and siding that 

protect the structural timbers and trusses and, in extreme 

circumstances, to failure of critical structural members.

Routine inspection and maintenance can significantly extend 

the service life of a covered bridge and may require only a 

day each year to conduct a thorough visual inspection and 

limited probing to identify potential problem areas. Cleaning 

and maintaining the painted surfaces are essential 

maintenance tasks. Decreasing the likelihood of wood decay 

or insect attack through the use of remedial wood 

preservative treatments can be very cost effective, and 

installing fire protection monitoring, alarm, or sprinkler

systems can prevent the loss of a bridge from careless acts 

of vandalism, contributing to the likelihood that the bridge 

will last for many decades.
General Technical Report FPL–GTR–252

4

Acknowledgments

The author thanks the following individuals and 

organizations that contributed to this report. Many were 

instrumental in coordinating site visits and/or reviewing 

drafts of this report. Without their cooperation and 

assistance, this work would not have been possible.

Deb Anthony, Anthony & Associates, Inc., Fort Collins, 

Colorado

Doug Porter, Porter & Associates, Burlington, Vermont 

(Contributed to “Cribbing, Shoring, Staging, and Rigging” 

in Chapter 4)

David P. Fischetti and Joyce Fischetti, DCF Engineering, 

Cary, North Carolina

Arnold and Meg Graton, Arnold M. Graton Associates, 

Holderness, New Hampshire (Contributed to “Cribbing, 

Shoring, Staging, and Rigging” in Chapter 4)

Jan Lewandoski, Restoration and Traditional Building, 

Greensboro Bend, Vermont (Contributed to “Cribbing, 

Shoring, Staging, and Rigging” in Chapter 4)

John Treybal, Sellers Treybal Structural Engineers of 

Williston, Vermont (formerly of Engineering Ventures, Inc. 

of Burlington, Vermont) (Contributed “Considerations for

Engineering Analysis of Timber Joinery and Connections” 

in Chapter 3)

Katie Hill, The Structures Studio, North Ferrisburgh 

Vermont (formerly of Engineering Ventures, Inc. of 

Burlington, Vermont) (Contributed “Considerations for

Engineering Analysis of a Burr Arch” in Chapter 3)

Dick Schmidt, Fire Tower Engineered Timber, Laramie, 

Wyoming (Contributed “Timber Joinery and Connections” 

in Chapter 3)

Greg Ausland, Ausland Group (formerly of OBEC 

Consulting Engineers), Eugene, Oregon

Tony LaMorticella, Ausland Group (formerly of OBEC 

Consulting Engineers), Eugene, Oregon(Contributed to 

“Increasing Load Capacity through Post-Tensioning” in 

Chapter 5)

John W. Smolen, Smolen Engineering Ltd., Jefferson, Ohio

Jim Barker, VS Engineering (formerly of J.A. Barker 

Engineering), Bloomington, Indiana

Christopher H. Marston, Historic American Engineering 

Record, Washington, DC

Ben Brungraber, Fire Tower Engineered Timber, 

Providence, Rhode Island

Mike Cotroneo, Mike Cotroneo Construction, Morrisville, 

Vermont

Paul Ide, Ide Construction, Danville, Vermont

Tom Vitanza, National Park Service, Washington, DC

Dario Gasparini, Case Western Reserve University, 

Cleveland, Ohio

Bill Caswell, National Society for the Preservation of 

Covered Bridges, Inc., Hillsboro, New Hampshire

Rudy and Laura Christian, Christian & Son, Inc., Burbank, 

Ohio

Kimberly Dugan (formerly of Anthony & Associates, Inc.)

Stan Lebow, USDA Forest Service, Forest Products 

Laboratory, Madison, Wisconsin

Karen Martinson, USDA Forest Service, Forest Products 

Laboratory, Madison, Wisconsin

Mike Ritter, USDA Forest Service, Forest Products 

Laboratory, Madison, Wisconsin

Sheila Rimal Duwadi, Federal Highway Administration, 

Turner Fairbank Highway Research Center, McLean, 

Virginia
Guidelines to Restoring Structural Integrity of Covered Bridge Members

5

Chapter 1: Introduction

Purpose of the Guidelines

These guidelines are designed for decision makers 

(selectmen, county commissioners, city planners, 

preservation officers, etc.) that have responsibility for 

maintaining an existing covered bridge to help them 

understand what goes into making effective decisions about 

how, and when, to repair a covered bridge.

The purpose of these guidelines is to present the steps 

necessary for decision makers to identify effective 

rehabilitation techniques for restoring the structural integrity 

of covered bridge members. The intent is to retain the 

maximum amount of historic fabric while ensuring public 

safety and minimizing future maintenance requirements. To 

make informed repair decisions about existing covered 

bridges, it is important to (1) start with a basic 

understanding of the type of bridge, (2) know the current 

condition of the bridge components, (3) have knowledge of 

what to consider when conducting an engineering analysis, 

and (4) be aware of various repair options that can meet the 

long-term goals for the bridge.

Only after the decision maker knows the type of bridge, its 

condition, and what loads it must carry can a repair strategy 

be developed. How to support the bridge during the repair 

phase, what repairs are appropriate, and how to maintain the 

bridge after the repairs are all critical to ensuring a long

service life for the bridge.

Need for Repairs to Covered Bridges

Giving strong consideration to repair and rehabilitation of 

original bridge components is a cornerstone of the National 

Historic Covered Bridge Preservation Program. Maintaining 

the structural and architectural fabric of these American 

bridges is critical to historic preservation goals. However, in 

assessing the condition of these structures, unnecessarily

conservative decisions are usually made when replacing 

members because there is uncertainty about the long-term 

effectiveness of repair techniques. Decision makers, 

structural engineers, and contractors need an understanding 

of bridge conditions and bridge performance to establish 

sound approaches for implementing effective and durable 

repairs and to support their replacement and repair 

decisions. Effective and durable repairs begin with knowing 

the current condition of the bridge components and the 

desired use of the bridge, which are often dictated by 

engineering requirements. The foundation for effective 

repairs is a proper condition assessment and engineering 

analysis.

A covered bridge is a unique structure in that it is built 

primarily to be supported by the trusses. It has a deck for 

carrying traffic loads and pedestrians. There are 

nonstructural components, including the roof and cladding.

The roof and cladding protect the structural members and

consist primarily of the trusses and deck framing.

Covered bridge members are sometimes compromised by 

decay, insect attack, or a range of nonbiologic factors.

Although publications are available describing methods of 

inspecting covered bridges for degradation, little 

information is available to guide decisions after a problem is 

detected. Additionally, assuming the cause of degradation is 

correctly identified, a decision still must be made as to 

whether the member can continue to function without 

corrective action or how it can be repaired, reinforced, or 

replaced while retaining the maximum amount of historic 

fabric. Proper repairs can extend the service life of a 

covered bridge for decades (Fig. 1.1).

There is a need to describe and evaluate repair techniques 

that are intended to extend the service life of bridge 

components. Rehabilitation techniques explored in this 

context include improved traditional timber framing or 

timber joinery (Fig. 1.2); use of fully threaded structural 

fasteners; post-tensioning of bottom chords (by adding steel 

cables or rods); epoxy injections or fiber-reinforced plastic 

(FRP) plates to reinforce top chords and floor beams; and 

replacement of members with glued-laminated timber 

(glulam), engineered wood products, or pressure-treated 

lumber. In some situations, the in-place application of 

preservative treatments may be used to decrease the 

likelihood of future decay or insect attack.

Truss Types

Any discussion of covered bridge repairs must be based on 

knowledge of the various trusses that are used in the 

bridges. It is not the purpose of this report to describe each 

truss type; these are discussed in numerous other 

publications (see References section and the Bibliography).

Figure 1.1—Replaced diagonal and knee brace that were 

deteriorated (Taftsville Covered Bridge, 1836, Windsor 

County, Vermont).
General Technical Report FPL–GTR–252

6

However, repair options depend on the type of truss; 

therefore, diagrams of typical truss types are given in 

Figures 1.3 and 1.4.

Some of the most common types of trusses used for covered 

bridges in the United States are the multiple kingpost, Town 

lattice, Burr arch, and Howe types (Figs. 1.5 to 1.8).

The Secretary of the Interior’s Standards for 

the Treatment of Historic Properties

The majority of covered bridges are more than 50 years old

and are eligible for inclusion in the National Register of 

Historic Places. Prior to conducting any repairs, the historic 

status of a bridge should be determined and the impacts of 

Figure 1.2—A tapered splice common to traditional timber 

framing ready to receive a wedge or shear key (Used with 

permission from DCF Engineering, Inc.).

Figure 1.3—Truss types (Used with permission from World 

Guide to Covered Bridges, 2009 edition). The types are 

identified below (dashed lines typically denote iron rods):

1. Left, kingpost, right queenpost.

2. Multiple kingpost. With odd number of panels, the 

center panel is open or has crossed braces as 

shown by dashed lines.

3. Town lattice. Disposition illustrated on 1820 patent 

drawing shown (only two chords indicated 

thereupon; one at the top and one at the bottom of 

the truss). For railroads and many highways, two 

secondary chords were used for additional strength.

4. Burr arch. A multiple kingpost truss with one or two 

arches added on inside and outside. Ends of the 

arch extend below the lower chord and rest on the 

abutments.

5. Arch (tied (shown) or two hinged (not shown)). The 

latter is where the ends of the arch are seated on the 

faces of the abutments. The figure shows various 

arrangements of suspension: verticals, diagonals, 

and crossed X-bracing.

6. Long. With three wood diagonals and double timber 

posts in each panel.

7. Paddleford. Ends of counterbraces cross both the 

kingposts and the chords. An inside arch is often 

added.

8. Howe, usual type. Three wood diagonals and two or 

three iron rod verticals in each panel.

9. Howe, single type.

10. Howe, western (Oregon?) type. Center panel 

sometimes open.

11. Haupt, 1839 patent. One remaining example is the 

Bunker Hill Covered Bridge in Catawba County, 

North Carolina.

12. Warren. Single system is in solid lines, double 

system with added timbers indicated by dashed 

lines.

Figure 1.4—Truss types continued (Used with 

permission from World Guide to Covered Bridges, 2009 

edition). The types are identified below (dashed lines 

typically denote iron rods):

13. Pratt, revised design. Teco-Pratt designs usually 

have triple timbers. The California design has 

wooden posts and two iron rods as diagonals, 

crossed in center panel.

14. Childs, 1846 patent. Diagonals are mortised to 

chords.

15. Brown, 1857 patent. Diagonals are mortised to 

chords.

16. Smith, type 2, 1869 patent. Type 3, no patent, 

reinforced as indicated by dashed center panel 

timbers.

17. Smith, type 4 improved, no patent.

18. Partridge, 1872 patent. Note addition of metal 

footplates. The seven surviving examples are 

modified designs with reinforcing rods and 

additional timber diagonals.

19. Post, 1863 patent. Iron rods indicated by dashed 

lines. The only surviving example is the Bell’s 

Ford Bridge (1869), Seymour (Jackson County), 

Indiana.

20. McCallum, 1867 patent. Posts are flared slightly. 

One example survives, the Powercourt Bridge 

(1861) Huntington County, Quebec, Canada.

21. Suspension of Bowstring. Two examples, both in 

Ohio.

22. No Name Truss (neither a Haupt nor a modified 

Burr). Two examples are Sayers Bridge (1839), 

Orange County, Vermont, and Bath Village 

Bridge (1833) in Grafton County, New Hampshire.
Guidelines to Restoring Structural Integrity of Covered Bridge Members

7

the repairs on that status evaluated. But regardless of any 

formal recognized historic status, covered bridges should be 

recognized as places of regional, cultural, and personal 

significance. As such, every effort should be made to adhere 

to The Secretary of the Interior’s Standards for the 

Treatment of Historic Properties, which require that the 

historic character of a property be retained and preserved 

and prohibit the replacement of intact or repairable historic 

materials. This includes the character-defining features of a 

structure, such as the trusses.

All covered bridge work, including repairs, preservative 

treatments, and the application of protective coatings, 

should be compatible both physically and visually with the 

structure and the site as a whole and should be documented 

and identifiable (upon close inspection) for future research 

and preservation efforts. Distinctive materials, features, and 

construction techniques or craftsmanship of the bridge 

should also be preserved, which may limit some treatment 

options. If chemical or physical treatments are necessary, 

those treatments should be applied using the gentlest 

methods possible. If repairs to wooden elements or timber 

members are necessary, the existing condition should be 

evaluated to determine the appropriate level of intervention 

needed. If there are areas of deterioration severe enough to 

require repair or replacement of an element, the deteriorated 

material should be replaced with the same wood species and 

match the original material in composition, design, color, 

and texture. Additional information on The Secretary of the 

Interior’s Standards for the Treatment of Historic Properties

can be found at the National Park Service’s Technical 

Preservation Services’ website (http://www.nps.gov/

tps/standards).

Figure 1.5—Example of a multiple kingpost truss (BlowMe-Down Covered Bridge, 1877, Sullivan County, New 

Hampshire).

Figure 1.6—Example of a Town lattice truss (North 

Hartland Twin Bridge, 2001, Windsor County, Vermont). 

There is a twin bridge at this river crossing (the Willard 

Covered Bridge) that dates from 1871.

Figure 1.7—Example of a Burr arch truss (Durgin 

Covered Bridge, 1869, Carroll County, New Hampshire).

Figure 1.8—Example of a Howe truss (Shoreham Bridge,

1897, Addison County, Vermont).
General Technical Report FPL–GTR–252

8

Where Do You Start?

There are several books that the layperson may find helpful 

in the early stages of bridge research. A series of books by 

Richard Sanders Allen, including Covered Bridges of the 

Middle West and Covered Bridges of the South, identifies

locations, truss types, construction dates, brief histories, and 

general information about covered bridges. But these books 

do not tell decision makers anything about how to conduct a 

condition assessment, how to do a structural analysis, or 

how to support, repair, or maintain a covered bridge. Yet, 

these are probably the most common references people have 

readily available to them unless they research the technical 

literature to understand the issues with covered bridges in

greater depth.

A deeper look into covered bridges reveals more technical 

publications, such as Timber Bridges: Design, Construction, 

Inspection, and Maintenance from the U.S. Department of 

Agriculture. This publication is not specific to covered 

bridges, but it does contain a wealth of information about 

material properties. Wood properties are among the most 

important topics to consider because timber bridges are 

made of different wood species, perhaps eastern white pine, 

hemlock, or spruce in the east and Douglas-fir in the west.

For those working on covered bridges, it is critical to have a 

basic understanding of the material properties of these 

different species, and that information is available in 

technical publications on wood properties.

Additional research into the literature on covered bridges 

will uncover documents such as Phillip C. Pierce’s Covered 

Bridge Manual or Covered Bridges and the Birth of 

American Engineering edited by Justine Christianson and 

Christopher H. Marston, both from the Federal Highway 

Administration. Similar to the USDA timber bridge 

publication mentioned, these references have a wealth of 

information and are dedicated to covered bridges. They 

discuss the history, inspection, assessment, engineering, and 

repair of covered bridges. A decision maker consulting any 

of these references would find enough information to make 

an informed decision when hiring an engineer or a 

contractor to repair a covered bridge.

That being said, these publications cover each topic in 

considerably more detail than most decision makers have 

time to digest. The role of these guidelines is to condense 

much of the technical information into a format that 

identifies the key considerations for establishing a costeffective repair strategy for a covered bridge.

References

Christianson, J.; Marston, C.H., executive eds. 2015. 

Covered bridges and the birth of American engineering. 

Historic American Engineering Record, Washington, DC: 

National Park Service.

Pierce, P.C.; Brungraber, R.L.; Lichtenstein, A.; Sabol, S.; 

Morell, J.J.; Lebow, S.T. 2005. Covered bridge manual, 

U.S. Department of Transportation, Federal Highway 

Administration, Publication No. FHWA-HRT-04-098.

McLean, Virginia: Federal Highway Administration.

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

inspection, and maintenance. Publication No. EM 7700-8. 

Washington, DC: U.S. Department of Agriculture, Forest 

Service.

Weeks, K.D.; Grimmer, A.E. 1995. The Secretary of the 

Interior's standards for the treatment of historic properties, 

with guidelines for preserving, rehabilitating, restoring & 

reconstructing historic buildings. Washington, DC: U.S. 

Department of the Interior, National Park Service, Cultural 

Resource Stewardship and Partnerships, Heritage 

Preservation Services.

Wright, D.W.; Caswell, W.S., Jr.; Brock, D. 2009. World 

guide to covered bridges. Concord, New Hampshire: The 

National Society for the Preservation of Covered Bridges, 

Inc.
Guidelines to Restoring Structural Integrity of Covered Bridge Members

9

Chapter 2: Condition Assessment

Bridge stewards, engineers, and contractors need to know 

when deterioration has occurred and if the current strength 

of the structural members (even when they are oversized 

and provided more strength than required for the original 

use) is adequate for implementing a repair strategy for the 

bridge. A condition assessment is a first step to acquire that 

information.

Wood decay, insect attack, fire, mechanical damage, and 

vandalism are typical mechanisms of deterioration for 

covered bridge members. The presence of any of these 

factors is a primary reason for initiating a condition 

assessment to determine if repairs are warranted. The role of 

moisture in wood decay and insect attack are critical to the 

decision-making process and are subsequently described 

because they are among the most common reasons that 

bridge members are replaced or repaired. Fire is not unique 

to covered bridges and is discussed in a separate publication 

on fire prevention and alarms for covered bridges, 

Evaluating Fire-Damaged Components of Historic Covered 

Bridges (Kukay and others 2016). That publication 

discusses the effects of fire on bridge timbers and how to 

make informed decisions about when to repair or replace

damaged members. Mechanical damage (primarily vehicle 

impact or overloading) and vandalism (primarily graffiti) 

are also not discussed in these guidelines.

In addition to wood deterioration, a condition assessment 

may be warranted because of a change in use of the bridge.

Strength requirements may change to carry heavier (or 

lighter) traffic loads. Proposed changes in load requirements 

or previous alterations may have unintended consequences 

(for example, by compromising the ability of the wood to 

dry quickly or overloading the structural members).

Mechanisms of Deterioration

Wood performs well in covered bridges when it is kept dry 

and protected from the deleterious effects of prolonged 

contact with moisture and biological deterioration. The open 

construction typical of covered bridges, which provides air 

flow through much of the structure, makes it possible for the 

wood to dry quickly if it gets wet. However, sometimes 

moisture is trapped, leading to wood decay or insect attack.

Prolonged exposure to moisture can produce undesirable 

conditions and long-term maintenance issues for wood in 

covered bridges, including moisture stains, peeling paint, 

checking, splitting, and warping of the roofing and siding.

Stains can be the result of a single wetting or of periodic 

wetting and drying. For example, a roof leak that was 

repaired many years ago may have resulted in a stain that 

has not affected the wood in any substantive way. Such 

stains are of no consequence structurally and can be ignored, 

unless aesthetics warrant a repair.

In other cases, stains may be the result of periodic leaks in 

roofs or walls, which may lead to more serious problems, 

such as decay or warping of the protective wood covering, 

or worse, decay of the trusses and other structural members.

Therefore, it is important to determine if a stain is the result 

of an isolated historical event or the result of active leaks 

and ongoing moisture intrusion (Fig. 2.1). Decay and insect 

attack, subsequently discussed in more detail, are also 

significant problems associated with periodic leaks or 

moisture intrusion. Measurements of moisture content can 

identify wood that has moisture levels favorable for the 

growth of wood-decay fungi. When feasible, moisture 

measurements should be taken during the season when 

elevated moisture content is sufficient to support active 

wood decay.

Most covered bridges were built well over 100 years ago 

using large-dimension green (wet) timber. The timber was at 

a moisture content at which it had not yet dried, or at least 

not dried adequately, when the bridge was built. With time, 

seasoning checks may have developed in the timbers.

Bridge timbers will typically have checks on one or two 

faces, which were caused by differential shrinkage of the 

timber and are part of the natural process as the wood dries 

(Fig. 2.2). A check is a separation of wood fibers in a piece 

of lumber, post, or timber, typically along the length of the 

piece, that results from the wood drying after processing or 

installation in a bridge.

Checks are not a defect and do not decrease the structural 

performance of a bridge member, except in rare occasions 

where two checks on opposite faces join to form a through 

split. However, if the timber has split through the entire 

thickness, a more detailed investigation is necessary to 

determine if the split is a failure resulting from overload or 

mechanical damage or if it is associated with shrinkage 

around connections. Also, differential shrinkage in mortiseand-tenon joints can result in failure of the joint that is 

restricted by the treenail (wooden peg).

Figure 2.1—Moisture stain with no wood decay present 

(Unity Bridge, 1936, Lane County, Oregon).
General Technical Report FPL–GTR–252

10

Weathering of wood results from cyclic wetting and drying 

of the wood, exposure to ultraviolet light, and erosion by 

wind-blown debris, a process similar to sandblasting. Unlike 

decay or insect attack, weathering is typically not a 

significant factor in the failure of wood components and the 

collapse of a structure. Weathering will change the 

appearance of wood, but the process is so slow that failure 

of components caused by decay generally occurs long 

before weathering becomes a major factor in the failure.

Weathering seldom damages the wood enough to require 

replacement, with the exception of shingles and cladding.

Weathered wood is often considered aesthetically pleasing 

(Fig. 2.3).

Biological deterioration is generally caused by fungal or 

insect attack, as discussed in the Wood Handbook (FPL 

2010). Bacteria can degrade wood but are generally not a 

concern with covered bridges because decay fungi and 

insects tend to impact material properties more rapidly than 

bacteria. Thus, the focus of a condition assessment is 

typically identifying the presence and extent of wood decay

or insect activity.

The most prevalent means of biological deterioration is 

wood-decay fungi, which can ultimately lead to the inability 

of structural members in a covered bridge to carry the 

required loads. Moisture absorption through end grain, 

checks, or holes in large timbers provides a highly favorable 

environment for decay fungi to attack the heartwood at the 

center of a large timber (Fig. 2.4). The heartwood (the inner 

growth rings of the tree) typically has more decay resistance 

than the sapwood (the outer growth rings of the tree).

However, even the heartwood of naturally durable species 

such as chestnut will decay when exposed to enough 

moisture. Deterioration is a particular concern where wood 

is in contact with the ground or with other materials, such as

porous stone abutments, that may allow moisture to be 

absorbed into the wood.

In addition to decay, fungi associated with wood also 

include mildew and stain fungi that propagate from spores 

present in the air. Mildew grows on the surface of wood or 

the surface of paint and does not affect the strength of the 

wood. Stain fungi (not to be confused with moisture stains) 

penetrate the surface of the wood but do not decrease its 

strength. Decay fungi, however, break down wood 

components with time. All types of decay fungi — brown 

rot, white rot, soft rot, and dry rot (which is often mistaken 

for all decayed wood that is no longer wet) — affect the 

ability of wood to perform its intended function. Although 

identifying the specific fungus during wood inspection is not 

important, identifying the location and extent of 

deterioration caused by decay fungi is essential.

Generally, if the moisture content of the wood is less than 

20%, fungi are unable to grow. Areas with moisture 

contents between 20% and 30% can support the growth of 

fungi, but the moisture may not be sufficient to support 

long-term active decay. Moisture contents between 30% and 

40% are highly favorable for active fungal growth and are 

Figure 2.2—Drying check in a truss diagonal (Cataract 

Falls Covered Bridge, 1876, Owen County, Indiana).

Figure 2.3—Weathered wood resulting in a desirable 

aesthetic effect. The siding does not require repair or 

replacement because of weathering (Lincoln Gap 

Covered Bridge, 1879, Washington County, Vermont).

Figure 2.4—Decay of a timber sill in a bridge pier (Stewart 

Bridge, 1930, Lane County, Oregon).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

11

often an indication of advanced decay, with symptoms that 

may include internal voids and surface deterioration. Insects 

generally require that moisture be greater than 10% for them

to be active and cause deterioration in the wood. Moisture is 

the controlling factor for active decay in covered bridges.

The early stage of decay, known as incipient decay, is 

characterized by discoloration and results in an initial loss of 

integrity of the wood. No voids are present. Probing with an 

awl or a screwdriver may reveal that the surface of the wood 

is soft or punky. As the decay progresses, the wood cellular 

integrity deteriorates until small voids develop. This stage is 

termed intermediate decay. The small voids continue to 

extend primarily along the wood grain, where it is easier for 

moisture to move through the wood, but voids can also 

extend across the grain. Larger voids develop where the 

decay originated, and the boundaries of the decay continue 

to extend, decreasing the integrity of the wood and 

compromising its structural capacity. At this stage, termed 

advanced decay, simple probing with an awl or a 

screwdriver may not detect the hidden deterioration in 

internal voids. An increment borer or a portable hand drill 

may be used to examine wood removed from the interior of 

larger timbers. However, more advanced techniques, such as 

resistance drilling, are able to quantify the extent of 

deterioration rather than simply identify its presence.

Termites and wood-boring insects decrease the size of the 

wood member cross section by either digesting or tunneling 

through the wood. Subterranean and drywood termites 

digest the wood as they move below the surface of the 

wood. Termites can often be detected through the presence 

of mud tubes on the exterior of either the structure or the 

individual wood members. The tubes allow the termites to 

maintain a favorable moisture environment as they move 

towards a new food source. Wood-boring beetles create 

holes that are packed with frass (the byproduct of the 

tunneling process). Carpenter ants and bees leave large 

clean tunnels in affected wood. Evidence of insect activity is 

often found in piles of wood substance below affected 

timbers or on the ground (Fig. 2.5).

With decay, there is a definite progression from sound wood 

to punky wood to a total loss of wood fiber, that is, a void.

Unlike decay, insect damage tends to have an abrupt 

transition between affected and unaffected areas of the 

wood. Wood that has not been penetrated by insects retains 

its structural integrity, although there can be a loss of cross 

section. The loss of cross section directly relates to the loadcarrying capacity of the member by decreasing the volume 

of wood available to carry loads. The decrease in cross 

section, if known, can be taken into account by engineers to 

determine if the affected member can still carry the required 

loads or if it needs to be reinforced or replaced. For both 

types of deterioration, moisture is generally required, and 

the result is a loss of integrity of the wood member, as well 

as a loss of cross section.

An additional type of wood deterioration that can occur with 

covered bridges members is called fuzzy wood and is 

caused by the chemical and physical action of salt breaking 

down wood fibers (Fig. 2.6). The crystallization of salt in 

between wood fibers can produce strands of fibers or, if 

severe enough, a mat of fibers on the surface. Salt damage is 

typically caused by the de-icing salts used on roadways. In 

salt environments, fuzzy wood is typically limited to the 

surface of the timber.

What to Look For

It is not the intent of these guidelines to provide details of 

how to conduct a wood inspection. Publications listed in the

reference sections and the Bibliography provide that 

information. However, in the context of understanding how 

a condition assessment helps to make informed repair 

Figure 2.5—Evidence of insect attack found below bridge 

timbers (Belknap Covered Bridge, 1966, Lane County, 

Oregon).

Figure 2.6—A joist showing fuzzy wood. This joist 

probably does not need to be replaced. An engineering 

analysis determined that it should be able to carry the 

original loads because only the very perimeter of the 

member was deteriorated (Blair Bridge, 1829, 1869, 

Grafton County, New Hampshire).
General Technical Report FPL–GTR–252

12

decisions, a summary of basic wood inspection questions 

and techniques is included here.

Determining the condition of wood components is the most 

common reason for conducting an inspection. Given the 

mechanisms of deterioration previously described, what is 

important to look for when conducting a condition 

assessment? Although not all inclusive, the following list 

identifies the primary conditions worth noting that may 

impact a decision regarding the need for a covered bridge 

repair:

• Moisture stains (Fig. 2.1)

• Presence of wood decay (Fig. 2.4)

• Insect activity (Fig. 2.5)

• Fuzzy wood (Fig. 2.6)

• Crushing of timbers (Fig. 2.7)

• Moss or lichens growing on wood components

• Peeling or flaking paint

• Fire damage (Fig. 2.8)

• Mechanical damage from vehicle impact (Figs. 2.9

and 2.10)

• Failed or damaged members (Fig. 2.11)

• Missing, loose, or displaced members (Fig. 2.12)

• Loose, missing, corroded, or broken connections

(Figs. 2.13 to 2.17)

• Reverse camber (sagging) of the bridge (Fig. 2.18)

Where to Look

When conducting a condition assessment, one needs to 

know where to look. Knowing what areas of a covered 

bridge to inspect and what tools to use depends on the goal 

of the inspection. Where are the typical problems, and how 

do we go about looking for them? The inspection should 

begin with looking for problems where they are most likely 

to occur in a covered bridge. An inspection should focus on 

likely problem areas, such as the following:

• Wood with moisture stains, visible decay, or insect 

damage

• Wood in contact with the ground

• Notched or drilled timber connections where 

moisture can accumulate

• Truss members

• Deck joists, stringers, and girders (Fig. 2.19)

• Openings (for example, portals, frames, and 

windows; Fig. 2.20)

• Material interfaces (for example, bolster beams on 

masonry; Figs. 2.21 and 2.22)

• Exterior woodwork, including cladding and 

shingles (Fig. 2.23)

• Areas where debris can build up (for example, 

guardrails and bridge approaches; Figs. 2.24 and 

2.25)

• Areas of the bridge that have been altered (for 

example, previous repairs; Fig. 2.26)

What Tools to Use

There are three “tools” for basic wood inspection: visual 

inspection, a probe, and a moisture meter. An individual

experienced in wood inspection may also use a hammer for 

sounding or a portable hand drill to gain information about 

the relative condition of the wood, although neither of these 

methods allows for quantifying the extent of deterioration.

They are best suited for identifying locations that warrant 

further investigation. A probe with a somewhat dull tip will 

not allow for quantifying the extent of deterioration in larger

Figure 2.7—Crushed timbers at lower chord (West Engle 

Mill Road Covered Bridge, 1877, Greene County, Ohio).

Figure 2.8—Fire damage of a floor beam (Blair Bridge, 1829, 

1869, Grafton County, New Hampshire).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

13

Figure 2.9—Damage at portal from vehicular impact 

(Dunbar Bridge, 1880, Putnam County, Indiana).

Figure 2.10—Damaged knee brace caused by vehicular 

impact (Lincoln Gap Covered Bridge, 1879, Washington 

County, Vermont).

Figure 2.11—Compression diagonal notch in timber 

sheared off in a multiple kingpost truss (Blow-Me-Down 

Covered Bridge, 1877, Sullivan County, New 

Hampshire).

Figure 2.12—Displaced top chord members (Dunbar 

Bridge, 1880, Putnam County, Indiana).

Figure 2.13—Split at bolted connection in a Burr arch 

truss (Cornstalk Covered Bridge, 1917, Putnam County, 

Indiana).

Figure 2.14—Loose and missing connections in the 

arch of a Burr arch truss (Oakalla Covered Bridge,

1898, Putnam County, Indiana).
General Technical Report FPL–GTR–252

14

Figure 2.15—The steel rods, washers, and nuts have 

surface rust in this Howe truss. This is typically not 

problematic, but if the connections are loose, that 

may indicate more serious problems with the bridge 

(Shoreham Bridge, 1897, Addison County, Vermont).

Figure 2.16—The bolts on the bottom chord of this 

Burr arch truss have extensive corrosion. This may 

be superficial, but the efflorescence of salt on the 

concrete bolster beam below the bottom chord 

indicates an environment where de-icing salts are 

used and where corrosion of the fasteners may be a 

significant problem (Waitsfield Village Bridge (or Big 

Eddy Bridge), 1833, Washington County, Vermont).

Figure 2.17—Close up of a severely corroded bolt 

(Blair Bridge, 1829, 1869, Grafton County, New 

Hampshire).

Figure 2.18—Sagging (reverse camber) (Shoreham 

Bridge, 1897, Addison County, Vermont).

Figure 2.19—Bottom chord of the truss with deck 

joists above. Gaps can develop in the splices, 

weakening the bottom chord, especially towards 

midspan (Shoreham Bridge, 1897, Addison County, 

Vermont).

Figure 2.20—Portal opening and window (Lincoln Gap 

Covered Bridge, 1879, Washington County, Vermont).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

15

Figure 2.21—Material interface at the abutment 

(Cornish-Windsor Covered Bridge, 1866, Sullivan 

County, New Hampshire, and Windsor County, 

Vermont).

Figure 2.22—Material interface between an arch and 

abutment (Durgin Covered Bridge, 1869, Carrol 

County, New Hampshire).

Figure 2.23—Exterior cladding and roof over bearing 

beam (Blow-Me-Down Covered Bridge, 1877, Sullivan 

County, New Hampshire).

Figure 2.24—Location along a bottom chord where 

debris can build up between the diagonals and the 

vertical post (Pine Bluff Covered Bridge, 1915, Putnam 

County, Indiana).

Figure 2.25—Debris build up at bridge approach 

(Oakalla Covered Bridge, 1898, Putnam County, 

Indiana).

Figure 2.26—Previous repair to a truss post (Baker’s 

Camp Covered Bridge, 1901, Putnam County, Indiana).
General Technical Report FPL–GTR–252

16

members, but it easily detects areas of surface deterioration 

and should be included in any wood inspection tool kit.

A visual inspection allows for identifying components that 

are missing, broken, or in an advanced state of deterioration. 

Missing components are those that have been removed or 

have fallen away, frequently because of extensive 

deterioration. If missing components were intended to 

provide structural support or protection from the elements 

(for example, to prevent moisture intrusion), their 

replacement may be essential to prevent long-term damage 

to the bridge structure. A small mirror with a telescoping 

handle and a flashlight are useful when inspecting relatively 

inaccessible areas.

Visual inspection also allows for the detection of past or 

current moisture problems as evidenced by moisture stains 

on the exposed surface of the wood.

Further, visual inspection enables detection of external

wood-decay fungi or insect activity as determined by the

presence of decay fruiting bodies, fungal growth, insect bore 

holes, mud tubes, or wood removed by wood-destroying

insects (frass).

Internal decay and insect damage are often difficult to detect 

because of the lack of evidence on the exposed surface of 

the wood. Probing the wood with an awl enables rapid 

detection of voids in the wood just below the surface that 

may or may not be visible. It can also indicate the 

approximate depth of the deterioration. Visual inspection 

and probing provide a rapid means of identifying areas that 

may need further investigation or repair.

Moisture measurements can be taken to identify potential 

sources of moisture intrusion that could result in premature 

deterioration of the wood and to determine if further 

investigation of potential areas of decay is warranted.

Moisture meters are extremely useful for identifying the 

current moisture content of wood and can identify problem 

areas that are not easily seen with the unaided eye. Moisture 

content measurements identify wood with moisture levels 

that are favorable for the growth of wood-decay fungi.

Specific moisture content ranges that indicate areas of 

concern were previously discussed in the section 

“Mechanisms of Deterioration”.

In addition to the three basic tools (visual inspection, a 

probe, and a moisture meter), hand drilling with smalldiameter drill bits can be used not only to test the relative 

difficulty of drilling into wood (resistance) but also to 

observe the color and integrity of the wood chips extracted 

by the drill bit. Solid wood chips of light color are indicative 

of sound wood, whereas dark material with a consistency 

more like sawdust is indicative of decayed wood.

Nondestructive testing (NDT) equipment can give much 

more information about wood condition, but the use of such 

tools is often reserved for situations in which a basic 

inspection cannot sufficiently answer the questions of the 

bridgewright, engineer, or owner.

The most useful NDT methods for assessing covered 

bridges are the following:

• Moisture meters are extremely useful for 

identifying the current moisture content of wood 

and can identify problem areas that are not easily 

seen with the unaided eye.

• Resistance drilling using a calibrated electronic 

drill is useful to quantify the loss of material 

caused by decay or insect damage.

• Stress-wave analysis is useful to locate advanced

decay.

• Digital radioscopy is useful to view hidden 

conditions and construction.

• Visual grading is useful for determining the 

strength of wood members in situ (discussed in 

Chapter 5).

Resistance drilling using a calibrated electronic drill 

provides quantified information about the internal, hidden

condition of the wood members in a covered bridge (Fig.

2.27). The term resistance drilling has been used to describe 

any drilling technique intended to measure the ease of drill 

penetration into wood. Portable drills with standard bits 

have been used not only to test the relative difficulty of 

drilling into wood (resistance) but also to observe the color 

and integrity of the wood chips extracted by the drill bit.

However, this type of drilling cannot quantify the loss of 

material.

Challenges When Assessing Condition

One of the most common issues when assessing a covered 

bridge is difficult access. Some members may have easy 

access on only one side (Figs. 2.28 and 2.29), some 

Figure 2.27—Resistance drilling of a Howe truss diagonal 

(Belknap Covered Bridge, 1966, Lane County, Oregon).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

17

members may be wholly or partially covered by other 

members (Fig. 2.30), and some members have excellent 

accessibility (Fig. 2.31). Other members may have no easy 

access although they are visible from a distance (Fig. 2.32).

The assessment may be limited to visual inspection, perhaps 

using a mirror or videoscope, or limited to touch to sense 

loose or missing connections or pockets of deterioration.

On the bridge shown in Figure 2.30, the guardrail is on 

shims to allow debris to flow underneath the guard rail and 

off of the deck. The bottom chord of the truss is only 

accessible from below. However, because of the elevated 

guardrail, this bridge is more conducive to “natural” 

cleaning that decreases maintenance requirements.

Contrasted with the hidden bottom chord shown in Figure

2.30, the bottom chord of the truss shown in Figure 2.31 

provides nearly ideal access for inspecting the timber. The 

timbers are visible, and accessible for probing or conducting 

resistance drilling. It is easy to see if debris is trapped.

Although each bridge and bridge type present their own 

challenges for assessing condition and identifying 

appropriate repairs, there are generalities that can be made 

regarding common deficiencies for various bridge truss

types. Table 2.1 summarizes common deficiencies for 

common truss types.

It is essential to remember that the purpose of the inspection 

is to provide data that can be used to answer questions 

raised by the architect, engineer, or owner about the 

condition of the wood in a covered bridge. If the wood has 

moisture stains, are the stains recent, as indicated by high 

moisture content readings, or is the wood sufficiently dry

that the stain probably occurred long ago? If decay is 

present, is it active, as indicated by a moisture content 

reading greater than 20%, or is the decay fungus dormant?

Were splits caused by normal drying checks, or are they an 

indication of failure of that component? If so, was the 

failure caused by loads exceeding the capacity with time, or 

could it have been caused by a one-time occurrence? The 

Figure 2.28—The siding makes inspection of the treenail 

connections located on the outside of this Town lattice 

truss difficult (Cornish-Windsor Covered Bridge, 1866, 

Sullivan County, New Hampshire, and Windsor County, 

Vermont).

Figure 2.29—Top chord of a Town lattice truss that is 

difficult to inspect on the top and outside surfaces 

because of the roof pitch (North Hartland Twin Bridge,

2001, Windsor County, Vermont). There is a twin bridge 

at this river crossing (the Willard Covered Bridge) that 

dates from 1871.

Figure 2.30—Limited access to bottom chord of the truss 

for conducting a condition assessment (Willard Covered 

Bridge, 1871, Windsor County, Vermont).

Figure 2.31—Excellent access to bottom chord of the truss 

for conducting a condition assessment (Belknap Covered 

Bridge, 1966, Lane County, Oregon).
General Technical Report FPL–GTR–252

18

inspector should ask these types of questions. Sound 

technical data about the current condition of the wood are 

necessary for effective repair and replacement decisions to 

be made. Such data are the result of a thorough wood 

inspection.

References

Kasal, B.; Tannert, T., eds. 2010. In situ assessment of 

structural timber. State of the art report of the FILEM 

technical committee 215-AST. Dordrecht, Heidelberg, 

London, New York: Springer.

Kukay, B.; Todd, C.; Jahn, T.; Sannon, J.; Dunlap, L.; 

White, R.; Dietenberger, M. 2016. Evaluating fire-damaged 

components of historic covered bridges. Gen. Tech. Rep.

FPL–GTR–243. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory.

34 p.

Pellerin, R.F.; Ross, R.J., eds. 2002. Nondestructive 

evaluation of wood. Madison, WI: Forest Products Society.

Ross, R.J.; Brashaw, B.K.; Wang, X.; White, R.H.; Pellerin, 

R.F. 2004. Wood and timber condition assessment manual.

Madison, WI: Forest Products Society.

FPL. 2010. Wood handbook: wood as an engineering 

material. Gen. Tech. Rep. FPL–GTR–190. Madison, WI: 

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

Products Laboratory.

Figure 2.32—Girders and stringers that are visible from 

below but are difficult to access because of the height 

above the river (Lincoln Gap Bridge, 1879, Washington 

County, Vermont).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

19

Table 2.1—Common deficiencies found by bridge type

Truss type Deficiencies

Kingpost 1. Loose diagonals (caused by moving loads) or braces lacking wedges for adjustment. Moving 

loads cause flexure in the chords. Without counter braces, diagonals become alternately 

loose and then tight fitting as chords flex and recover. With time, diagonals may fall out of 

position entirely.

2. Undersized or overly wide spacing of vertical members near abutments or piers, sometimes 

signaled by crushing of diagonals near abutments, open joints nearer the center of a span, 

and pronounced deflection of chords in the abutment bays. Strains in the diagonals and ties 

(vertical members) are greatest near the abutments, although bridge builders did not always 

adjust panel length and member section to reflect this.

3. Decay in open joinery of the top and/or bottom chords. Both are places where water is apt to 

get in — at the top primarily as the result of roof leaks and at the bottom as the result of 

leaks in the siding, water blowing off the river, and water brought onto the roadway by 

vehicles. Inspection of all four sides of either chord is rarely possible; therefore, decay 

around joinery often reaches an advanced state before becoming visible.

Town lattice 1. Loss of camber (sag) caused by inadequate stiffness of the truss. This can be the result of 

having too few pinned connections (that is, having too few lattice members and/or 

insufficient depth of the truss so that there are too few lattice intersections), lattice members 

installed at an angle that results in ineffective diagonals, or defects in the pinned connections 

(too few pegs or splitting of lattice members around pegs because the pins were installed too 

near the edges of lattice members). Builders occasionally included additional chords 

(crossing the second intersections of ties and diagonals), but because they are located nearer 

the neutral axis, these are less efficient than chords at top and bottom.

2. Warping of the trusses. This condition may be especially recognizable as bowing or 

buckling of the top chord and is caused by insufficient thickness of the chord material and/or 

insufficient cross bracing. This condition is frequently found in conjunction with loss of 

camber. Historically, bridge builders sometimes addressed this issue by framing double 

lattice trusses, trusses with two sets of ties and diagonals sandwiched between three sets of 

chords. This arrangement stiffened the truss against warping but also doubled the timber 

devoted to ties and diagonals while only increasing the cross section of the chords by 50%.

3. Decay at chord–lattice and lattice–lattice connections. Because of the tight laps at 

intersections, these areas are slow to dry and prone to decay.

4. Compression failure in lattice members located near abutments. This is because diagonals 

and ties are shortest where stresses are highest. Builders frequently avoided the problem by 

extending the length of abutments by a distance roughly equal to the depth (height) of the 

truss. In historic bridges, where extending the trusses is no longer an option, trusses may be 

strengthened by the addition of arch braces (with straining beams) or arches. These can also 

be used to address insufficient section in the chords.

5. Decay of pegs in lattice–bottom chord connections caused by the high level of exposure to 

the elements.

Burr arch 1. Loose diagonals (caused by moving loads) or diagonals lacking wedges for adjustment. 

Moving loads cause flexure in the chords. Without counter braces, diagonals become 

alternately loose and then tight again as the chords flex and recover. With time, diagonals 

may fall out of position entirely unless connections are configured to prevent this.

2. Trusses with inadequate stiffness. Where moving loads are heavy, there should be counter 

diagonals or at least ties parallel to the diagonals to stiffen the trusses (this is also true of 

kingpost trusses). Counter diagonals fitted with wedges (or threaded connections) allow for 

preloading, decreasing flexure from moving loads.

3. Insufficient depth of the arch to resist vertical loads.

4. Decay in open joinery of the top or bottom chords. Both are places where water is apt to get 

in, at the top primarily as the result of roof leaks and at the bottom as the result of leaks in 

the siding, water blowing off the river, and water brought onto the roadway by vehicles. 

Inspection of all four sides of either chord is rarely possible; therefore, decay around joinery 

often reaches an advanced state before becoming visible.

5. Differential settlement of arch and truss, resulting in bolted connections between the two 

being overstressed.
General Technical Report FPL–GTR–252

20

Table 2.1—Common deficiencies found by bridge type—continued

Truss type Deficiencies

Howe 1. Loose counter diagonals or counter diagonals lacking wedges for preloading. Moving loads 

cause flexure in the chords. Without counter diagonals, preloaded against the maximum 

moving load, the bridge will be prone to excessive vibration as the chords flex and recover. 

Counter diagonals fitted with wedges (or threaded connections) allow for preloading, 

decreasing flexure from moving loads.

Long 1. Loose counter diagonals or counter diagonals lacking wedges for preloading. Moving loads 

cause flexure in the chords. Without counter diagonals preloaded against the maximum 

moving load, the bridge will be prone to excessive vibration as the chords flex and recover. 

Counter diagonals fitted with wedges (or threaded connections) allow for preloading, 

decreasing flexure from moving loads.

2. Loss of relish in tie–chord connections. The truss relies on vertical wooden ties to connect 

the chords. Decay or weathering of the ends of the ties can result in failed connections.

All bridges 1. Decay where bottom chords contact abutments and absence of bolster beams or bedding 

timbers, which are frequently made of denser, naturally decay-resistant wood and support 

the bottom chords off the surface of the abutment.

2. Loss of camber in chords, accompanied by buckling of top chords. This can be the result of 

loss of connection capacity, insufficient depth of the truss, or failing/open splices in the 

bottom chords in tension.

3. Damage by cars, trucks, or farm machinery especially at wind braces, siding at openings,

abutments, and truss members (where there is no guardrail).

4. Damage to the upstream side by floating debris (may be accompanied by displacement of 

lower chords on piers or abutments).

5. Alteration or removal of wind bracing to accommodate larger vehicles. This can result in 

buckling of the top chords and/or inclination of the trusses toward the leeward side.
Guidelines to Restoring Structural Integrity of Covered Bridge Members

21

Chapter 3: Engineering Analysis

In addition to conducting a condition assessment, 

understanding the structural behavior of the various 

components of the bridge is essential to making effective 

repair recommendations. Understanding the structural 

behavior is achieved when structural engineers conduct an 

engineering analysis of the entire structure — the trusses, 

deck, roof, abutments, and piers. This chapter is intended to 

provide the bridge steward or decision maker with a sense of 

the language and process for properly analyzing a covered 

bridge, which should contribute to repairs being based on 

sound technical data and analyses.

A covered bridge behaves as a system different from other 

structures. Without proper understanding of how a bridge 

works, the process of designing and implementing repairs 

can be detrimental to the long-term performance of the 

bridge. These guidelines are not intended to serve as a 

primer on the engineering analysis of covered bridges, but a 

few points for consideration and some examples are 

provided to illustrate some of the subtleties that are often 

overlooked when analyzing a covered bridge.

Once condition assessment information is available, an 

engineering or structural analysis can be conducted.

Structures are analyzed by engineers for their behavior 

under various loading conditions. This is typically 

accomplished through either structural analysis software or 

conducting historical analysis. There are several 

publications and books that discuss design and engineering 

analysis of timber bridges in general and covered bridges in 

particular.

Structural engineers that are covered bridge aficionados 

appreciate the idiosyncrasies and evolution of covered 

bridge design. The various truss designs shown in Chapter 1 

evolved from improvements made by those that designed 

bridges. There are excellent sources of information, listed in 

the Bibliography chapter, to gain an understanding of the 

original design intent and the structural performance of early 

timber bridges, including covered bridges. Steven H. Long, 

Squire Whipple, Ithiel Town, and Herman Haupt all 

produced early treatises on timber trusses. More recent 

publications serve as useful primers on how a covered 

bridge works from an engineering perspective, including 

publications by the Federal Highway Administration, Dario 

Gasparini (and others), Justine Christianson and Christopher 

Marston, Jan Lewandoski, David C. Fischetti, and Phillip 

Pierce.

Most analyses of covered bridges are done using standard 

structural analysis software. However, covered bridges are 

somewhat unique and typically do not appear to perform 

well when analyzed using standard structural analysis 

programs that may have been developed for concrete, steel, 

or modern timber construction. There are several reasons for 

this, a key one being that many covered bridges rely on 

traditional carpentry joints that are not considered in most 

structural analysis programs. Covered bridges rely on subtle 

construction details (such as modeling traditional timber 

joinery) that are not well suited to most computer modeling 

software. Additionally, a lack of knowledge about the wood 

species and structural grade of the timber often results in 

underestimation of a bridge that may have been performing 

adequately for decades. Thus, the model inputs for timberjoinery connection details and material properties for 

analyzing a covered bridge are often unknown, and these 

inputs may be quite variable.

For new wood construction, structural engineers rely on 

design values referenced in building codes to determine 

acceptable species, size, and grade for a particular load 

condition. For existing covered bridges, engineers often rely 

on current codes and standards to determine adequacy of the 

wood members. However, current standards are generally 

based on lower quality material than what may be found in 

many historic covered bridges. Because many older bridges 

were constructed before building codes or design values for 

wood products were established (and thus before grade 

stamps were used), engineers inexperienced with historic 

structures or materials are often in a quandary when 

determining what design values are appropriate. Frequently,

a species and grade are assumed, leading to wood members 

being declared structurally deficient. The result is often an 

overly conservative estimate of design values and 

unnecessary replacement, repair, and retrofit decisions, with 

the associated unnecessary project costs.

Even for a relatively simple structure, such as a multiple 

kingpost truss bridge (Fig. 3.1), the results from most 

commercially available computer modeling software

typically show that the bridge is structurally inadequate, 

when in fact, it has been performing quite well for some

time. There is a certain irony in watching a 30-ton truck 

cross a bridge with a posted 3-ton limit without producing a 

visible deflection. The bridge is performing well. What is 

Figure 3.1—Multiple kingpost truss (Blow-Me-Down 

Covered Bridge, 1877, Sullivan County, New Hampshire).
General Technical Report FPL–GTR–252

22

needed is an understanding of why it is performing well.

Understanding how a bridge is performing is a goal of the 

condition assessment and the engineering analysis.

Figure 3.1 shows that how and where the truss members are 

connected can dramatically impact the structural 

performance of the truss and bridge, and accurately 

modeling that performance in a computer model is often 

very difficult to do. To illustrate some of the peculiarities of 

covered bridge structural behavior, examples of issues to 

consider when analyzing simple joints or the composite 

action of a Burr arch truss are presented later in this chapter.

Construction Details

It is important to understand that there are very few 

unnecessary components in a covered bridge. If there is 

something inserted into the structure, no matter how small, 

there is probably a reason it is there. An example of this is 

the small check brace shown in Figure 3.2. The truss was 

constructed with the diagonal web member connecting to 

the vertical below the panel point. The intent of the design 

was that the vertical post be axially loaded and the diagonal 

web member connects to the post below the top chord to 

simplify the joinery where the vertical post meets the top

chord. The diagonal web member induces a bending 

moment in the vertical post at the connection (remember 

that the post is designed to carry axial loads, not bending 

loads). To resist the bending moment and keep it from 

pushing the vertical post out of plane, the bridge builders 

inserted a short check brace. Some contractors or structural 

engineers may not realize the importance of this small 

detail. If the check brace is removed during repairs, a 

bending moment is introduced in the vertical post that it was 

never intended to carry, possibly resulting in unintended 

consequences.

The repair detail shown in Figure 3.3a and b is a round shear 

pin, designed to transfer shear loads between adjacent 

members of a Town lattice truss. To the untrained 

individual, it may look like a plug with no structural role.

The shim in Figure 3.4 was inserted during previous repairs 

to ensure that the compression load in the diagonal was 

transferred adequately. Sometimes these small construction 

details seem like an afterthought, and people want to pull 

them out (sometimes as a souvenir). If a shim needs to be 

removed during repairs because of deterioration or damage, 

a new one should be installed using one made of the same 

wood species. If it is not replaced, leaving a gap, the 

Figure 3.2—Check brace (Pulp Mill Covered Bridge, 1820, 

Addison County, Vermont).

Figure 3.3—Round shear pin between diagonal truss 

members (a) with close up (b) (Cornish-Windsor Covered 

Bridge, 1866, Sullivan County, New Hampshire, and 

Windsor County, Vermont).

Figure 3.4—Shim in a compression diagonal of a Town 

lattice truss (Cornish-Windsor Covered Bridge, 1866, 

Sullivan County, New Hampshire, and Windsor County, 

Vermont). The shim should not be removed.
Guidelines to Restoring Structural Integrity of Covered Bridge Members

23

diagonal will no longer have full contact in bearing and the 

compression load will not be transferred as originally 

intended.

Timber Joinery and Connections

Structural analysis of a traditional covered bridge will 

provide information on the internal forces in the framing 

members and estimates of the deformations (deflections) 

that the bridge will experience under various loads, such as 

those caused by structure self-weight, vehicular traffic, 

pedestrians, snow, and wind. The analysis models also 

report the forces acting at connections (also referred to as 

joints) between members. Resolution of the member forces 

at a connection, such that they are effectively transferred 

through the connection, is critical to the performance and 

long-term durability of the structure. Hence, special 

consideration must be given to connection design and also 

to repair of distressed or failed connections. The critical role 

of connections is easily overlooked relative to the overall 

configuration and condition of the structure, probably

because the load-transfer mechanisms in a traditional timber 

connection are typically not accurately characterized by the 

structural analysis model. Thus, professionals with 

experience in covered bridge evaluation and repair must be 

engaged early in the project to perform the required 

assessment, analysis, design, and repair.

Traditional joinery used in historic structures, including 

mortise and tenon joints, notched heel joints in trusses, scarf 

joints, and others have performed well in hundreds of 

bridges for centuries and are effective solutions for repair 

situations. An uninformed rush to use nontraditional 

methods, such as steel side plates, gussets, weldments, etc., 

demonstrates a lack of understanding on the part of the 

engineer or contractor and can actually compromise the 

integrity of the repair. Rather, an understanding of the loadtransfer mechanisms in traditional joinery and a sensitivity 

to the properties of timber used in these structures can yield 

a repair that possesses structural integrity along with the 

aesthetic qualities of the original construction.

For example, consider the roof-truss heel joint shown in 

Figure 3.5 (this type of joint can be found elsewhere in 

covered bridges but is often used in the roof trusses). Under 

normal circumstances, the rafter load thrusts into the notch, 

producing a tension force in the tie. The post supports the 

truss by applying compression to the bottom of the tie at the 

joint. Aside from selection of the members themselves, 

designers of such a connection must consider at least the 

following load-transfer mechanisms:

1. The tension force in the horizontal tie is resisted 

across the net cross-sectional area of the tie at the 

root of the notch, where stress concentrations act.

2. Thrust in the rafter is transferred into the tie 

without crushing wood fibers at an angle to the 

grain of either the rafter or the tie.

3. The post provides vertical support to the truss 

without crushing the wood fibers of the tie 

perpendicular to the grain.

4. The horizontal component of the rafter thrust must 

be carried into the tie without causing shear failure 

of the tie. Stress concentrations act at the root of 

the notch as well.

In addition to understanding the basic load-transfer 

mechanisms in traditional joinery, professionals engaged in 

repair of covered bridges must understand the properties of 

wood sufficiently to make wise design decisions. Some 

factors that a design professional should allow for when 

designing the repair of a connection include but are not 

limited to the following:

• Repair in-kind, that is, match the wood species, 

structural grade, surface texture, and moisture 

content of the original members with the 

replacements. Timbers salvaged from other 

structures (mill buildings, mines, trestles, etc.) 

might be preferred to freshly cut and dried stock.

• Consider partial replacement of decayed or 

damaged members by removing only the decayed 

or damaged portions of a member and scarfing in a 

replacement section.

• Design joinery that effectively sheds water or can 

be protected from rain and runoff. Notches, 

pockets, or other depressions that can hold water 

must be protected by siding, flashing, or other 

means.

• Avoid using steel side plates or knife plates. They 

constrain the connected timber from its natural 

tendency to shrink and swell with changes in 

moisture content that occur naturally throughout 

the year. They also hold moisture against the 

Figure 3.5—Roof-truss heel joint (Used with permission 

from Dick Schmidt, Fire Tower Engineered Timber).
General Technical Report FPL–GTR–252

24

timber, preventing it from drying (which can lead 

to decay), and possibly prevent detection of decay

during maintenance inspections.

• If necessary, connect steel rods (which can be 

effective tension members) to the timber members 

by fully penetrating the member so that load is 

transferred by direct bearing on the opposite side of 

the member using a nut and washer.

• Minimize eccentricity in joints, that is, the 

centerlines of all members that frame into a joint 

should meet at a common working point. For 

example, the heel joint of the roof truss in 

Figure 3.6 should have its support post at position 

A rather than at position B. An exception to this 

approach is to use a check brace as was previously 

discussed if the eccentricity cannot be avoided.

• Attempt to transfer load through contact by woodon-wood bearing surfaces rather than through 

lateral load in bolts. For example, case C in 

Figure 3.7 shows that a keyed scarf joint with a 

table is preferred to case D with the simple half-lap 

joint. The “table” is the offset portions between the 

two pieces of timber that bear against the shear 

key, which is indicated by the X in Figure 3.7C.

To illustrate the unique structural performance of covered 

bridges and the issues to consider when developing a 

structural model, two examples are given below that 

represent typical questions and considerations that should be 

asked by the structural engineering firm as part of their 

analysis. The first example addresses general considerations

in the analysis of timber joinery, and the second example 

poses questions about the composite behavior of a more 

complex Burr arch truss.

Considerations for Engineering Analysis of

Timber Joinery and Connections

As was previously mentioned, joint design and structural 

evaluation begins with an analysis of the structural system 

to determine member forces acting on the joints. This 

analysis includes a number of considerations that can have a 

significant impact on the results. Looking a bit closer at a 

particular joint or connection, the following questions might

be asked:

• Is the joint pinned, fixed (rigid), or somewhere in 

between?

• Are there loading conditions that cause the reversal 

of member forces?

• When there is more than one possible load path 

through the structure, how much load is resisted by 

each path?

Often member forces are bounded by analyzing multiple 

conditions and assumptions and then considering the 

maximum of each. These forces and stresses can be 

compared with field observations in an attempt to more 

closely reflect past performance. Components of the 

structural system that are working well and others that show 

signs of distress should be identified because this 

information can help guide the analysis to some degree. The 

result of this process is a set of member forces that can be 

used in the analysis of the joints.

At first glance, joint analysis can appear quite simple. Freebody diagrams of the joint are created for each controlling 

load combination, including all of the member forces. Each 

of these forces must be transferred through the joint using a 

calculated structural approach. Usually, bearing or fasteners 

are used to resist the loads.

Figure 3.6—Heel joint of a roof truss (Used with 

permission from Dick Schmidt, Fire Tower Engineered 

Timber).

Figure 3.7—Wood joints using bolts (Used with 

permission from Dick Schmidt, Fire Tower Engineered 

Timber).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

25

There are a number of subtle yet important considerations 

that can add to the complexity of joint analysis. For joints 

that involve a mixture of existing timbers and new timbers, 

it is often important that new timbers are dried to a moisture 

content that closely matches the existing timbers. If not, 

there will be shrinkage of new timbers that could affect the 

structural capacity of the joint:

• Shrinkage rates vary with orientation to the growth 

rings. Typically, shrinkage in the direction 

tangential to the rings is greater than radial 

shrinkage, and longitudinal shrinkage is negligible.

This variability can affect bearing areas.

• Bearing lengths can be affected as a supporting 

member shrinks away from the supported member.

• Bearing areas can be affected by changes in the 

angles of bearing surfaces as new members come 

to equilibrium.

• The capacity of connections involving fasteners 

can be drastically affected by moisture content. The 

National Design Specification for Wood 

Construction includes strength reduction factors to 

account for the use of unseasoned timbers and for 

timbers used in high moisture conditions. For 

instance, when multiple rows of bolts are used to 

connect steel plates to timbers with a moisture 

content greater than 19%, the capacity of each bolt 

can be decreased to 40% of that with dry timbers

(less than 19% moisture content). This is caused by

the steel plates, in conjunction with the bolts, 

restraining the natural shrinkage of the wood, 

causing splits at the bolts.

Other considerations:

• Ideally, the structural capacity of a joint will be 

governed by a nonbrittle failure mode. It is 

preferable for joint capacities to be limited by 

bending or bearing rather than tension or relish 

(material between the bolt and the end of the 

timber) strengths. This helps to ensure that there 

will be a visual indication of joint overstress before 

complete failure occurs.

• Friction is typically neglected; therefore, bearing 

surfaces can only resist components of loads that 

are perpendicular to the surface.

• In mortise and tenon joints, how much tenon 

bearing should be included? This can be affected 

by swelling and shrinking and by the tightness of 

fit of the mortise with respect to the tenon.

• How will member rotation affect joints with longer 

bearing lengths?

• What are the tolerances and how will they affect 

bearing areas?

• Consideration needs to be given for reduced 

member strength at connections because of 

mortises, housings, etc.

• It is generally not recommended to combine 

multiple load resistance methods (peg shear with 

bearing for instance) because of differing 

stiffnesses.

• Is the joint configured in an eccentric way, and has 

this been accounted for?

Considerations for Engineering Analysis of a 

Burr Arch

A Burr arch is a combination of a truss, typically a multiple 

kingpost truss, and an arch (Fig. 3.8). The truss and arch are 

intended to work together to share the loads acting on the 

bridge. To assess the structural adequacy of the existing 

members and their connections, one must first establish how 

much of the applied load is resisted by the truss and how

much is resisted by the arch.

This question is more nuanced than it may initially appear 

and has been a topic of spirited debate among bridge 

builders, bridge engineers, and bridge lovers since the time 

the first Burr arch was constructed. Contemporary structural 

analysis software can quickly do the calculations, but the 

analysis results are only as good as the assumptions made.

There are so many variables at play in a Burr arch that it is 

not feasible to model the system with enough accuracy to 

provide a definitive answer about load sharing between truss 

and arch and the resulting forces in each member and 

connection. The engineer must use a healthy dose of 

engineering judgment to decide how a particular Burr arch 

bridge is behaving based on field observations, to decide 

how best to model it within the constraints of the available 

analysis software and project budget, and then to attempt to 

bracket that solution to account for the various uncertainties 

that were not explicitly included in the analysis model.

What follows is a discussion of some of the engineering 

issues and uncertainties to be considered when embarking 

on an analysis of a Burr arch.

Figure 3.8—A Burr arch, constructed using a multiple 

kingpost truss and an arch (Image from Theodore Burr’s 

1817 patent).
General Technical Report FPL–GTR–252

26

1. How does the geometry of arch versus truss affect the 

distribution of load between the two?

When more than one load path is available, load will be 

distributed based on the relative stiffness of the competing 

load paths. The arch and the truss are two different load 

paths with different behaviors. An arch is very stiff under 

uniform loading — much stiffer than a truss of comparable 

span and depth; therefore, there is a tendency for more of 

the dead load to be resisted by the arch. The truss is stiffer 

and thus more effective for unbalanced (off-center) live 

loads because an arch with an unbalanced load tends to 

sway to the side in-plane of the arch. Connecting the arch to 

the truss to prevent this side sway stiffens the arch and 

allows it to carry more of the live load.

Structural analysis software does a reasonably good job of 

resolving the load distribution issues that are based purely 

on the geometry of arch and truss. However, there are many 

other factors that are difficult to quantify or model that 

affect the relative stiffness of arch versus truss and thus alter 

the resulting member design forces.

2. How does the nature of the connections affect load 

sharing between arch and truss?

The notched timber joinery used between truss members 

relies on side-grain or angle-to-grain bearing, which 

compresses more when loaded than the end-grain bearing at 

the ends of the arch segments. In addition, it is more 

difficult to cut the truss joinery to fit with perfect bearing on 

all surfaces, leaving more potential gaps in the truss joints 

than in the arch connections. When the Burr arch is initially 

loaded, the truss connections will have more "give" than the 

arch connections as everything compresses into tight 

bearing. Because tightening of joints is not considered in the 

computer analysis model, the model will overestimate the 

stiffness of the truss and underestimate the forces in 

the arch.

3. How is the end of the arch supported?

The most common detail is for the arch to pass by the truss 

bottom chord to bear directly against the abutment, as 

shown in Figure 3.8. The arch is only effective if the 

abutments prevent spreading at the arch supports. Poor 

quality construction at the abutments, particularly if the arch 

was a later addition and there is little behind the abutment 

wall to resist the thrust, can lead to abutment spread, 

decreasing the effectiveness of the arch and increasing the 

load carried by the truss.

Arches that project below the deck to bear against the 

abutment are prone to decay where in contact with the 

abutment; significant rot can cause the arch to settle 

downward with respect to the truss supports, completely 

changing the distribution of forces between arch and truss 

and potentially damaging members and connections.

Some Burr arches use a tied-arch detail instead (Fig. 3.9). In 

this case, the bottom end of the arch is notched directly into 

the truss bottom chord near the end of the chord. This has 

the advantage of better protecting the end of the arch from 

the elements and eliminating the possibility of differential 

settlement between arch and truss supports. However, the 

thrusting force of the arch is now resisted by the truss 

bottom chord rather than the abutments, significantly 

increasing the tension force on the bottom chord splices —

the connection that typically limits the overall load capacity 

of the bridge. The arch-to-bottom-chord connection is prone 

to shear failure because the notch is close to the end of the 

chord; failure of this connection can produce distress in the 

truss as it results in complete loss of the arch action.

4. Are the arches connected to the trusses? If so, how rigid is 

the connection?

Most of the previous discussion has assumed that the arches 

are connected to the truss in a manner that prevents relative 

horizontal or vertical displacement between arch and truss at 

their connection point (rigid or fixed connection). Reality is 

more complicated.

In some cases, there are no connections between arch and 

truss; the arch is free to move in-plane under unbalanced 

loads and there is no way to share vertical loads between 

arch and truss. Thus, load will stay where it is applied, either 

to the truss or to the arch. Of more concern in these cases is 

the possibility that the arch will buckle out-of-plane, which 

has been observed in cases where an arch was added after 

the original construction to help support the floor but was 

not tied back to the truss.

Details vary for those cases in which the arch is connected 

to the trusses. A single iron through-bolt is a common detail, 

typically grossly undersized by today's design standards and 

often decreased in cross section because of corrosion

(Fig. 3.10). If the bolt is undersized, does the engineer want 

to rely on that existing bolted connection as a load-transfer 

Figure 3.9—A Burr arch using a tied-arch detail (Quinlan 

Covered Bridge, 1849, Chittendon County, Vermont. Used 

with permission from Katherine Hill PE, The Structures 

Studio – Structural Engineers).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

27

mechanism? Some of these bolts show visible signs of 

distress when removed, including bending of the bolt and 

crushing of the surrounding wood; such a bolt cannot be 

assumed to provide a rigid connection from arch to truss. In 

theory, one could treat the bolt as a spring and continue on 

with the computer analysis; in reality, it is difficult to 

estimate the spring stiffness of the bolt with sufficient 

accuracy to justify the increased time and complexity to 

include this spring behavior in the computer model.

Other Burr arches have the arch notched slightly into the 

truss verticals. This increases the reliability of the 

connection compared with a bolt alone, but gaps will 

develop with time as the arch shrinks; therefore, this is not a 

truly rigid or fixed connection either.

5. Does the construction sequence affect member forces?

Distribution of dead loads between the arch and the truss 

can be significantly altered by the chosen repair sequence. If 

the whole system is shored until arch and truss are 

completed and connected together, gravity is effectively 

"turned on" upon removal of the shoring and dead loads will 

be shared between truss and arch based on relative stiffness.

However, if the arch is completed first and used to support 

the bridge while repairing the truss, then the arch will be 

carrying 100% of the dead load upon completion of the truss 

(and vice versa).

6. How does the passing of time affect member forces?

As previously discussed, wood shrinks as it dries, and it can 

take years for a large green timber to dry to the point of

equilibrium with ambient moisture conditions. There is 

greater shrinkage across the grain (that is, cross section)

than along the length of a timber. This means that shrinkage 

introduces larger gaps at the truss joints (perpendicular or 

angle-to-grain bearing) than at the arch joints (parallel-tograin bearing). The increased play caused by the gaps that 

develop in the truss joints decreases the stiffness of the truss 

compared with the arch, causing a portion of the dead load 

in the truss to migrate into the arch.

Computer analysis of a Burr arch provides the engineer with 

a starting point for understanding the behavior of the system 

and estimating design forces for members and connections, 

but it is only a starting point. Engineering judgment is 

needed to account for numerous other factors that can affect 

the distribution of forces within the system.

From the discussions of traditional timber joinery, from 

analyzing a simple joint, and from considerations for 

analyzing a more complex Burr arch, it is apparent that 

covered bridges can have complex behavior that is difficult 

to model as is done with modern buildings. Having an 

engineer that understands the idiosyncrasies of covered 

bridges will allow for more efficient analysis and effective 

repair design.

Figure 3.10—A Burr arch bolted to the truss vertical 

(Quinlan Covered Bridge, 1849, Chittendon County, 

Vermont). The bolt is a 1- by 1-in. wrought-iron bolt 

penetrating 19 in. of wood, turned down to a 1-in.

diameter at the threads (Used with permission from

Katherine Hill PE, The Structures Studio – Structural 

Engineers).
General Technical Report FPL–GTR–252

28

Chapter 4: Supporting the Bridge 

for Repairs

For some bridge repairs, such as those focused on roof 

covering, siding, and decking, repair work can often be 

conducted without unloading the trusses or lifting the 

bridge. Additionally, limited structural repairs can 

sometimes be undertaken without shoring or extensive 

rigging on truss types that have many redundant members. 

For example, for a Town lattice truss, it may be possible to 

replace single lattice members because there are multiple 

lattice members present. Also, it may be possible to replace

arch laminae across the depth of an arch when most of the 

laminae are intact.

Before more extensive repairs can be implemented, 

however, support of the bridge must be considered. When 

necessary, properly supporting the bridge prior to 

implementing repairs is accomplished through cribbing, 

shoring, staging, and rigging. Although these terms for 

supporting the bridge during repairs are sometimes used 

interchangeably, the following definitions point out the 

differences:

• Cribbing normally refers to a temporary support 

structure installed underneath the bridge to support 

a small area.

• Shoring refers to the process of temporarily 

providing support for the entire bridge during 

repairs.

• Staging refers to temporary structures for either 

access and/or support.

• Rigging refers to a system of ropes, cables chains, 

steel beams, or other supports used for temporarily 

supporting or moving materials, sections of the 

bridge, or the bridge as a unit.

In addition to these support mechanisms, moving the bridge 

off of its supports is sometimes done to facilitate repairs but 

this relies on adequate rigging, shoring, and cribbing during 

or after moving the bridge to prevent further damage and

maintain (or recover) camber.

Cribbing, Shoring, Staging, and Rigging

For extensive truss repairs (such as replacing primary 

members), recovering lost camber, or installing new bed 

timbers, it is usually necessary to lift and support portions of 

the bridge while repairs are underway. Lifting and shoring 

are seldom focused on a single point along the length of a 

bridge. Instead, it is almost always necessary to lift and 

provide support at several points along the trusses, cribbing,

or shimming at tie (column) locations to return lost camber 

to the trusses. Shoring and rigging strategies can generally 

be characterized as follows, although each bridge repair is 

unique and has specific conditions or situations that must be 

considered individually:

• Support from the riverbed. Timber piles can be 

driven into muddy river bottoms and configured to 

make bents for shoring. If the riverbed is rocky or 

sandy enough to support shoring without driving 

piles, timber cribs can be placed at jacking points. 

These can be supported on sand bags or makeshift 

raft foundations of large (3000- to 5000-lb) 

concrete blocks. These blocks are set into the river 

using a crane. Once in the water, they can be 

maneuvered into position from the bridge and 

leveled into foundations for timber cribs. Similarly, 

sand bags can be piled to provide a base for timber 

cribs; this is especially useful when working over 

uneven topography, which might be encountered 

along a riverbank, for example. It is often 

necessary to install coffers on the downstream side 

of these shores to prevent scour.

• Shoring from the abutments. The bridge abutments 

can provide useful foundations for shoring a bridge 

for repairs. Trusses or I-beams can be rolled into 

position across the bridge deck, cribbed over the 

abutments, and used to support needle beams for 

jacking and shoring the top chords of a bridge. If 

the abutments are wide enough, trusses or I-beams 

can be placed on either side of the bridge to support 

steel or timber needle beams for jacks and cribbing. 

When repairing a single truss, the shoring system 

can combine a beam placed in the bridge with one 

to the outside of the truss to be repaired.

• Suspension or cable-stay support. These rigging 

systems use cables supported on temporary pylons 

(or frames) to support a bridge while under 

construction. In cable-stay systems, stay cables 

fasten directly to (or over) the pylons (resulting in 

fan-shaped arrangements of the cables), whereas

suspension systems use vertical suspenders 

connected to parabolic suspension cables. Cable 

shoring systems include proprietary systems (such 

as those by Dywidag Systems International USA, 

Inc., Bolingbrook, IL) but can also include custombuilt systems with cables anchored to deadmen on 

either shore and supported on temporary timber 

towers. Related to these cable systems, a crane 

equipped with slings and spreaders can sometimes 

be used to lift and support discrete portions of a 

bridge while repairs are made, saving the expense 

of systems designed to support whole bridges.

• Shoring from existing supplementary support.

Many covered bridges have already been altered to 

include supplementary support of the roadway, 

with I-beams or glulams supported on the 
Guidelines to Restoring Structural Integrity of Covered Bridge Members

29

abutments and carrying the loads associated with 

the bridge deck. If supplementary members have 

sufficient capacity, they may be used to support 

crib towers erected on the roadway (inside the 

bridge) for shoring the top chords and making 

repairs to the trusses.

• Removing the bridge from the river. Smaller 

bridges of limited span can sometimes be removed 

from the river by crane (the size of the load will be 

a function of the capacity of the crane and the 

length and inclination of the boom) and placed on 

cribs or rails set up on the riverbank for this 

purpose. Once on temporary support, bottom 

chords can be shimmed to re-establish camber prior 

to making repairs.

Cribbing is one way to support a bridge for repair. In 

general, structural repairs should not be made when a bridge 

is under load. It is preferable to take the load off of members 

to be repaired, make the repairs, and then reapply the weight 

of the bridge. Cribbing allows you to transfer the weight of 

the bridge off the members to be repaired. Cribbing can be 

placed in the river if there is bedrock to serve as a 

foundation. If not, sandbags can serve as a base to support 

the cribbing timbers. Jacks are placed on top of the cribbing 

to raise the bridge members to remove the load, allowing

repairs to be made. Cribbing is an art that is generally 

practiced by experienced bridgewrights. One of the early 

bridgewrights, Milton Graton, repaired many covered 

bridges and developed cribbing as part of the repair process 

that is widely used today (Fig. 4.1).

Knowing where to apply the cribbing below the bridge is 

critical. In some cases, cribbing and shoring are rather 

elaborate and need to be engineered because of the 

significance of the repairs needed to the bottom chord. For 

the bridge shown in Figure 4.2, the support was needed at 

the abutment to repair the bedding timbers. Additionally, 

cribbing and shoring were installed further from the 

abutment to support the heavy-timber lattice truss railroad 

bridge because of the need to repair the damaged timber that 

extends further from the abutment (Fig. 4.3). The cribbing 

and shoring allowed for replacing failed lattice members and 

adding supplementary braces to address displacement of the 

bottom chords just beyond the abutments (Fig. 4.4).

Shoring to support the bridge may require supports placed in 

the river. Although careful planning is critical, it is possible 

to place staging on a frozen river provided that the loads are 

not too heavy and the work can be completed before a thaw.

More frequently, temporary foundations of large concrete 

blocks or sandbags can be leveled on the riverbed to support 

timber cribs (Fig. 4.5). Structural staging can be used for 

Figure 4.1—Cribbing installed to allow for repair of the 

deteriorated bottom chord in 1966 (Bartlett Bridge, ca. 

1870, Carroll County, New Hampshire. Used with 

permission from Clifford-Nicol, Inc., Plymouth, 

New Hampshire).

Figure 4.2—Cribbing positioned to allow for repairs to the 

bottom chord and bedding timbers at the abutment (Fisher 

Railroad Bridge, 1908, Lamoille County, Vermont. Used 

with permission from Jan Lewandoski).

Figure 4.3—Cribbing under the Fisher Railroad Bridge 

(1908, Lamoille County, Vermont. Used with permission 

from Jan Lewandoski).
General Technical Report FPL–GTR–252

30

shoring and may double as a work platform. The staging 

comes with jack heads that will carry lintels of wood 

or steel.

For shoring truss chords and decks, timbers, steel beams, or 

wood I joists suspended below the bridge deck often make 

up part of the shoring (Figs. 4.6 to 4.10). The shoring 

provides a walkway beside the bridge deck and can be used 

to support members that are to be repaired. Jacks can be 

placed on the shores when it is necessary to remove loads 

from members that require repair. As with cribbing, setting 

the staging is an art. Although the repairs are the final 

product, the logistics of repair implementation — setting the 

cribbing, shoring, and staging — are essential to 

implementing effective repairs and are frequently more 

challenging than the repair carpentry.

Rigging makes use of cables, chains, or other means to lift 

or support heavy timbers, staging, shoring, assemblies (for 

example, trusses), or the entire bridge. In the context of 

covered bridges, rigging typically involves the use of cables 

for lifting or providing temporary support (Figs. 4.11 and 

4.12).

Figure 4.4—Repairs being made to the failed lattice 

members and bottom chords after placement of cribbing 

and shoring (Fisher Railroad Bridge, 1908, Lamoille 

County, Vermont. Used with permission from Jan 

Lewandoski).

Figure 4.5—Shoring of the Israel River Bridge in 

progress. Note the staging hung from the cribs (Israel 

River Bridge, 1862, Coos County, New Hampshire. Used 

with permission from Jan Lewandoski).

Figure 4.6—Steel I-beams supported on timber cribs laid 

on the abutments. Timber shores were placed across the 

steel beams at each column, and the top chord was 

jacked. Note the suspended staging for accessing the 

bottom chords and deck frame (Best’s Covered Bridge,

1899, Windsor County, Vermont. Used with permission 

from Jan Lewandoski).

Figure 4.7—Temporary steel trusses installed 

above the deck for shoring this Town lattice 

truss bridge. After rolling the trusses through 

the bridge, they were cribbed over the 

abutments and used to support the top chords 

at several points along their length 

(Dummerston Covered Bridge, 1872, Windham 

County, Vermont. Used with permission from 

Jan Lewandoski).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

31

The Cornish–Windsor Bridge is the longest two-span 

covered bridge in North America. The main support 

structures are Town lattice trusses, visible after the siding 

was removed (Fig. 4.13). A proprietary cable-stay system 

(Dywidag Systems International USA, Inc.) was used to 

shore the bridge for repairs. Primary cables were rigged on 

temporary steel frames, with cable stays supporting steel 

needle beams inserted below the top chords at regular 

intervals. With this system, it was possible to restore camber 

to the spans.

Modular support trusses are available to slide into the 

opening of a covered bridge, connect to the timber trusses, 

and remove loads from the wood trusses so that repairs can 

be made. These modular trusses essentially provide jacking 

points along the entire length of the bridge, so that the 

bridge can be fully supported to recover camber and make 

repairs. In this way, modular support trusses can expedite 

repairs and decrease the need for other means of structural 

support. The trusses are typically panel systems that are 

extended through the bridge from abutment to abutment.

The weight of the multispan truss is not insignificant and 

requires care in installation to prevent damage to the timber 

bridge that may already be in fragile condition (Figs. 4.14 

and 4.15). Monitoring movement at key locations, such as 

piers, may be desirable. Although accelerometers, lasers, or 

other sensors can be used to monitor movement or vibration, 

sometimes watching for vibrations in a cup of water 

strategically placed can be as useful as more sophisticated 

sensors (Fig. 4.16).

Other Considerations

Moving the bridge is accomplished either by setting rails 

and towing the bridge, raising and lifting the bridge off of its 

Figure 4.8—Supplementary beams installed to support 

the roadway in an earlier repair were used to shore 

one of the queenpost trusses for repair. Note the 

repair at the bottom chord and replacement bed timber 

and arch brace (Lumber Mill Covered Bridge, ca. 1890, 

Lamoille County, Vermont. Used with permission from

Jan Lewandoski).

Figure 4.9—Staging and shoring installed prior to 

beginning structural repairs (Blair Bridge, 1829, 1969, 

Grafton County, New Hampshire). The siding has been 

removed to facilitate repairs to the trusses, and the 

staging serves as a walkway to safely access the bridge.

Figure 4.10—Structural wood I-joists used as shoring 

beneath the truss chords and deck (Blair Bridge, 1829, 

1969, Grafton County, New Hampshire).

Figure 4.11—Rigging using a crane, equipped with slings 

and spreaders to lift and support the portion at the 

abutment. Additional shoring of the Big Eddy included a 

timber crib near the middle of the river to decrease the 

effective span by approximately 40% (Waitsfield Village 

Bridge (or Big Eddy Bridge), 1833, Washington County, 

Vermont. Used with permission from Jan Lewandoski).
General Technical Report FPL–GTR–252

32

supports, or lifting it with a crane. In the temporary location,

it can be repaired without working over the river (Fig. 4.17).

If it is deemed necessary to remove the bridge from the river 

to most effectively achieve the needed repairs, the crane 

operator must coordinate with the structural engineer to 

identify locations for attaching cables and rigging. The 

structural engineer will need to provide a rigging plan 

specifying the lifting points, cable sizes, and connection 

details. This information, much of which can be derived 

from the engineer’s structural model, is critical to avoid 

damage to the bridge during lifting.

Cofferdams are useful when repairs to piers and abutments 

are necessary. The cofferdam serves as a temporary 

enclosure around a pier or adjacent to an abutment to allow 

for the water at the base of the pier to be drained (Fig. 4.18).

Once watertight, the enclosure creates a dry area for the 

repair work to proceed.

Repairing timber bridges can require custom sawmilling to 

obtain timbers in lengths not readily available or when 

transportation logistics may be prohibitive. The spruce log 

shown in Figure 4.19 was sawn into a 60-ft-long,

12- by 12-in. timber for replacing the decayed bottom chord 

of a small queenpost truss bridge.

When a bridge is over a deep ravine, moving the bridge or 

installing shoring may be challenging or infeasible because 

of logistics, cost, or environmental concerns. In these 

situations, it may be possible to install additional structure 

and avoid disassembly and repair procedures that require 

shoring. For example, on the bridge in Figure 4.20, many of 

the connections on the lower ends of the kingposts and 

diagonals had failed because of inadequate end distance in 

the connections. Shear blocks were installed to help transfer 

load between the diagonal and the vertical posts (Fig. 4.21) 

in areas of the timbers unaffected by deterioration.

In summary, cribbing, shoring, staging, rigging, and perhaps 

moving the bridge can all be part of the logistics necessary 

to implement effective repairs to covered bridges and 

provide the desired performance for decades to come.

References

Graton, M.S. 1978. The last of the covered bridge builders. 

Plymouth, New Hampshire: Clifford-Nicol, Inc.

Figure 4.12—Using a crane to rig cable stays for 

supporting a small section of the bridge while repairs 

were made (Waitsfield Village Bridge (or Big Eddy 

Bridge), 1833, Washington County, Vermont. Used with 

permission from Jan Lewandoski).

Figure 4.13—Rigging (Cornish-Windsor Covered 

Bridge, 1866, Sullivan County, New Hampshire, and 

Windsor County, Vermont. Used with permission 

from Jan Lewandoski and D. Huston).

Figure 4.14—Acrow multispan panel truss being inserted 

(Blair Bridge, 1829, 1869, Grafton County, New 

Hampshire. Used with permission from Katherine Hill).

Figure 4.15—View from inside the bridge of the Acrow 

multispan panel truss as it is being inserted into the bridge 

(Blair Bridge, 1829, 1869, Grafton County, New Hampshire. 

Used with permission from Katherine Hill).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

33

Figure 4.16—Monitoring pier movement using a paper 

cup filled with water to watch for vibrations (Blair 

Bridge, 1829, 1869, Grafton County, New Hampshire).

Figure 4.17—Bridge removed from the river for repairs. 

This small Burr arch truss bridge has been removed 

from the river (by crane) and temporarily shored on Ibeams for repair. Because repairs included the 

addition of lamina to the arch and replacement of 

decayed bottom chord members to increase cross 

section of the chords, it was more efficient and costeffective to move the bridge to implement the repairs 

(Bowers Covered Bridge, 1919, Windsor County, 

Vermont. Used with permission from Jan Lewandoski).

Figure 4.18—Constructing a cofferdam to allow for work 

on the pier (Blair Bridge, 1829, 1869, Grafton County, 

New Hampshire).

Figure 4.19—Felling a locally available tree to meet 

size requirements and transportation limitations 

(Lincoln Gap Covered Bridge, 1879, Washington 

County, Vermont. Used with permission from Jan 

Lewandoski).

Figure 4.20—A deep ravine that limits access to the 

bridge from below (Blow-Me-Down Covered Bridge,

1877, Sullivan County, New Hampshire).

Figure 4.21—Shear blocks to transfer load between 

diagonal and vertical members that have deteriorated 

connections that could not practically be repaired 

from below (Blow-Me-Down Covered Bridge, 1877, 

Sullivan County, New Hampshire).
General Technical Report FPL–GTR–252

34

Chapter 5: Repairs

The first question that should be asked when someone 

suggests that repairs are needed is whether the bridge 

currently provides the required level of safety for its 

intended use. It may be obvious. A bridge with reverse 

camber (in other words, it is sagging) probably warrants 

some attention if it is intended to carry vehicular traffic. If it 

is just a matter of running a computer model and concluding 

that the bridge is inadequate (and therefore needs 

reinforcement or replacement), further consideration is 

warranted. Things to consider are the intended use and what 

the condition assessment revealed. If the bridge shows no 

signs of distress, it is possible that repairs may not be 

necessary.

The purpose of this chapter is not to provide details on all 

possible repairs for covered bridges but to provide decision 

makers with a sense of the options available to them when 

repairs are necessary. The Last of the Covered Bridge 

Builders (Graton 1978) has a wealth of information from a 

bridgewright’s perspective about the idiosyncrasies of how 

covered bridges perform and how to repair them. Depending 

on the goals for the bridge, repairs tend to come under one 

of the following categories:

• Rehabilitating the bridge

• Strength enhancement

• Increasing clearance

• Increasing durability

As indicated in The Secretary of the Interior’s Standards for 

the Treatment of Historic Properties, distinctive materials, 

features, and construction techniques or craftsmanship 

should be preserved whenever possible, which may limit 

some repairs. If repairs are necessary, they should be 

applied using the gentlest methods possible. As discussed in 

Chapter 2, prior to determining if repairs are necessary, the 

existing condition should be evaluated to determine the 

appropriate level of intervention needed.

Rehabilitating the Bridge Using Traditional 

Timber Repairs

Based on the condition assessment and engineering analysis, 

an informed decision about repairs can be made. Perhaps the 

wood siding or timbers are only weathered, and 

maintenance is all that is required. Perhaps there is isolated 

deterioration that should be addressed, but the bridge is able 

to carry the required loads. If deterioration is more 

widespread or members have failed, then multiple repairs 

may be necessary. If there are areas of deterioration severe 

enough to require repair or in-kind replacement of a 

member, the deteriorated material should be replaced with 

the same species and should match the original material in 

composition, design, color, and texture. Repairing timbers 

using traditional timber framing techniques and joinery are 

well suited to maintaining the historic character of the 

bridge.

Figure 5.1 shows the exposed bottom chord of a truss. It 

looks a bit weathered, and there are small seasoning checks 

on the outer face. Seasoning checks are a natural 

characteristic of wood that develop as the wood dries and 

shrinks until it comes into equilibrium with environmental 

conditions of humidity and temperature. The short seasoning 

checks on this timber are approximately parallel to the long 

edge of the timber. That indicates that the timber has very 

little slope of grain and is a high-grade timber for structural 

purposes, at least based on the outer face. Looking for 

checks or large knots on the other faces is the means of 

assessing the grade, which is discussed later in this chapter.

For this timber, no repairs are needed.

If deterioration caused by wood decay or insect attack has 

decreased the cross section of the timber to the point at 

which it is no longer able to perform its intended function, 

replacement is warranted. Figure 5.2 shows a bedding 

timber, also called a bolster beam. The bedding timber 

distributes the load of the bottom chord at the abutment to 

prevent overstress that could result in crushing. For timber 

that is in direct contact with other materials, such as 

Figure 5.1—Weathered bottom chord in good condition 

(Blair Bridge, 1829, 1869, Grafton County, 

New Hampshire).

Figure 5.2—Bedding timber supporting the bottom chord 

of the truss. Note spacer blocks to provide separation 

that limits moisture absorption from the concrete into the 

bedding timber, thereby extending the service life of the 

bedding timber (Cornwall-Salisbury Covered Bridge,

1865, Addison County, Vermont).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

35

concrete or masonry, moisture can migrate from the 

concrete or masonry into the bottom face of the timber.

After prolonged contact with moisture, the timber can 

provide a favorable environment for wood decay. The 

bottom chord of a truss is a difficult member to replace 

because of numerous connections. The bedding timber 

carries the load from a quarter of the bridge into the 

abutment (because there is one at the end of each bottom 

chord). If a bedding timber deteriorates, replacing it is easier 

and less costly that repairing the bottom chord. The bedding 

timber is there to protect the bottom chord of the truss from 

deterioration. In essence, it can be considered sacrificial.

The bedding timber in Figure 5.2 has wood spacer blocks 

under it to create a barrier between it and the concrete.

Wood dutchman repairs (Fig. 5.3) can be let into 

deteriorated portions of bridge members in which decay is 

limited in extent. Dutchman inserts will improve the bearing 

area of deteriorated members, improve the performance of 

connections, and fill recesses and cavities that might 

otherwise collect water. Using wood to make the repairs has

the advantage of introducing repair materials that have 

physical and mechanical properties that are similar to the 

original material while maintaining aesthetic continuity.

Traditional scarf joints, such as those shown in Figure 5.4, 

can be used to splice new timber into deteriorated members. 

These repairs are not invisible, but the visual impacts are not 

obtrusive or unattractive, and the joinery is interesting to 

look at. Limited replacement of decayed portions of 

members using traditional joinery has the advantages of 

preserving the salvageable portions of historic fabric as well 

as the historic craft tradition that produced the original 

construction.

Although many repairs can be done in situ, damaged timbers 

are often temporarily removed from the bridge for repair.

The tenon in the post shown in Figure 5.5 had deteriorated,

but the rest of the post was intact. Splicing in a new tenon 

allows for restoring the strength of the joint while keeping 

much of the historic fabric.

When making a traditional timber joinery repair, it is critical 

that the moisture content of the replacement timber be in a 

similar range to the original member. If a green timber, one 

that has a high moisture content, is used for the repair, the 

green replacement timber will shrink after the wood dries.

The joint will not have the tightness between the wood 

surfaces in the repair to ensure a good connection. The 

repair should be done at essentially the same moisture 

content. With a moisture meter, the moisture contents of the 

original and replacement timbers can be measured. Because 

shrinkage properties vary between species and orientation of 

the timber, repairing in-kind with the same species, moisture 

content, and wood quality will ensure a long-lasting repair.

Figure 5.3—Typical dutchman repair. The decayed 

portion of the existing member is removed, and a new 

piece is cut to fit tightly into the void. The repair may be 

secured with mechanical fasteners and/or an adhesive 

(Used with permission from Keri Stevenson).

Figure 5.4—Two forms of a scarfed splice: half-lap with 

nose or bladed scarf (top) and stop splayed (bottom) 

(Used with permission from Keri Stevenson).

Figure 5.5—Tenon repair on the bottom of a post. The 

tenon was spliced into the deteriorated area, saving the 

rest of the post.
General Technical Report FPL–GTR–252

36

If structural timbers are thoroughly deteriorated, the best 

way to recover adequate capacity is by replacing the 

member in-kind. When the structural members of a bridge

are replaced, material should be used that matches the 

original in species and is of at least equal quality. In some 

cases, higher quality material (species or grade) may be 

warranted to decrease the likelihood of needing to do a 

similar repair in the near future. Higher quality materials 

will have greater structural capacity and greater durability; 

therefore, the investment in making the replacement will 

yield better performance and long-lasting results.

Although using the same species, moisture content, and 

wood quality are key to a successful structural repair, there 

are historical reasons for repair or replacing nonstructural 

material in-kind as well. Siding, roofing, or decking (which 

is structural) that is visible to the public may have 

characteristics that, if not replicated, may be obvious 

replacement material. Features such as saw marks can be 

specified on repair material to ensure that the textural 

characteristics of the historic material are not lost (Fig. 5.6).

Recent repairs where the wood looks new compared with

original fabric do not need to be artificially weathered to 

match the appearance (Fig. 5.7). Replacement material, 

structural or not, will weather with time and develop a 

similar patina to the original material.

Partial replacement using traditional timber joinery (or 

mechanical fasteners, although they should be used 

sparingly) allows for retaining the sound wood while 

limiting the amount of cribbing, shoring, or staging needed 

to implement the repair. The overuse of bolts and steel side 

plates in connections is detrimental to the aesthetics of the 

historic covered bridge, but more importantly, the steel 

allows for moisture to be trapped between the wood and the 

steel, possibly leading to decay of the wood or corrosion of 

the steel at the interface, which is a location at which the 

condition is difficult to observe during inspection and 

maintenance of the bridge. This is not to say that steel 

should not be used in repairs, just that it should be used 

sparingly and with consideration of unintended 

consequences.

Figure 5.8 shows a post and bottom chord that have been 

repaired using the same wood species and timber joinery.

The damaged portions of the post and bottom chord, 

including the connections, were cut away and the 

replacement timber was spliced to the sound wood of the 

original members. When the same wood species, grade, and 

quality are used at the same moisture content, an effective 

in-kind repair is likely to be a long-term repair.

An experienced craftsperson that understands timber joinery 

can produce tight structural joints that will work and last for 

decades. That is what is needed with a spliced repair. A 

loose joint will not only fail to correctly transfer the loads 

across the joint but will allow for moisture intrusion into the 

joint that can lead to early failure caused by wood decay, 

thereby requiring another repair well before it should be 

needed. The splices in Figure 5.8 in the bottom chord of the 

truss eliminated the need to replace the entire bottom chord.

A number of other splices are visible in the post and the 

diagonal, all using traditional timber joinery.

Figure 5.9 is an example of an arch that had some 

deterioration in laminae that made up the arch, probably 

caused by decay that developed with time. Rather than 

replacing the entire arch, it was possible to open the arch 

Figure 5.6—Circular-saw marks of decking boards (North 

Hartland Twin Bridge, 2001, Windsor County, Vermont).

Figure 5.7—Replacement of a bottom chord timber

(Parvin Bridge, 1921, Lane County, Oregon).

Figure 5.8—Bottom chord and post repaired using 

spliced timber (Bunker Hill Covered Bridge, 1895, 

Catawba County, North Carolina. Used with 

permission from DCF Engineering).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

37

and replace the individual laminations. That is usually the 

best means of doing an arch repair. It is typically not 

necessary to replace the entire end of the arch unless it is in 

poor condition (Fig. 5.10). If not properly integrated with 

the rest of the arch, replacing a segment of the entire cross 

section of an arch can introduce stresses into the 

connections between the original and replacement segments 

that alter the behavior of the arch. As was discussed in 

Chapter 3, the distribution of loads between the arch and the 

associated truss is complicated and any effort to repair the 

arch should strive to maintain the balance of load 

distribution. If individual laminations are in need of 

replacement or repair, replacing the individual laminations 

is possible using an experienced craftsperson.

Understanding the structural behavior of a bridge, in part 

through proper engineering analysis, is key to implementing 

effective repairs. When a repair is not as simple as a partial 

or full replacement, innovative solutions to challenging 

repair situations may be necessary. The bridge shown in 

Figure 5.11 had failed corbels at the bottom of the truss 

posts. The wood joinery that connected the vertical posts to 

the diagonals and the bottom chord of the truss had decayed 

and needed to be repaired. The bridge crosses a deep ravine,

making the use of cribbing and shoring to support the bridge 

during repairs costly and challenging. The craftsperson used 

a sistered diagonal in which the short replacement timber 

bears in a new notch in the post (Fig. 5.12). The repaired 

diagonal has a shear key between the members to 

adequately transfer the loads. Although not the most elegant 

or aesthetic treatment, the repair is functional, it leaves the 

historic structural system in place, and it addresses site 

conditions, which are beyond the control of the engineer and 

the bridgewright or contractor.

Many contractors believe that obtaining the quality of wood 

and timber to match that of the piece being repaired is 

impossible. However, the reality is that timber of the 

original quality still available. There are less than 700

historic covered bridges remaining in the United States.

They are not all being repaired at the same time, and on any 

given bridge, a finite volume of timber is needed to do the 

repairs. Through good specifications, the proper quality of

material can be acquired (Figs. 5.13 and 5.14). There may 

be more expense, but the cost of the material is more than 

offset by the longevity of the repair.

Figure 5.9—Replaced laminations in a double Burr arch 

truss (Pulp Mill Covered Bridge, 1820, Addison County, 

Vermont).

Figure 5.10—Blending of replacement laminations near 

the end of an arch with original laminations that are still in 

good condition (Mechanicsville Covered Bridge, 1867, 

Ashtabula County, Ohio).

Figure 5.11—Deep ravine, which made use of shoring 

cost-prohibitive (Blow-Me-Down Covered Bridge, 1877, 

Sullivan County, New Hampshire).

Figure 5.12—Sistered diagonal repair, using shear keys to 

transfer load (Blow-Me-Down Covered Bridge, 1877, 

Sullivan County, New Hampshire).
General Technical Report FPL–GTR–252

38

Sometimes sourcing the material requires being a bit 

creative. Many bridges are in rural areas with forest or 

woodlots nearby. For example, repairs have been made for

which a 60-ft-long bottom chord was needed. That length 

can be challenging to purchase today, as well as to transport 

long distances to the site. If it is a single timber that is 

needed, a local wood lot or forest may have a tree that can 

yield timber of the required length. Purchasing the tree from 

the landowner, milling the timber, then allowing it to dry 

(obtaining a moisture content similar to the rest of the 

timber in the bridge) is a solution to a problem not often 

considered with other construction projects.

Reclaimed material is becoming more readily available. It 

can be used for smaller members, such as joists and girders, 

in the same species, grade, and quality as the material that 

was in the original bridge. Reclaimed timber may be 

available in larger dimensions, although these sizes may be 

more difficult to find. Bear in mind that if only a dozen 

large timbers are needed for a project, they are available 

somewhere. It may take time to source them, but it is 

definitely worthwhile because it increases the longevity of 

the repair and preserves the original integrity and historic 

character of the bridge without other augmentations.

Strength Enhancement

Strength enhancement is primarily considered during 

rehabilitation and when load upgrades are needed. For 

rehabilitation, the bridge is basically performing but has 

suffered some deterioration or maybe some failure of local 

members and needs to be repaired. The repairs can be made 

using traditional timber repair methods as previously 

discussed. The members that have deteriorated or been 

damaged will be repaired or replaced. Load upgrades are 

required when the bridge cannot support anticipated loads 

based on engineering analysis. In cases like these, bridges 

need to be strengthened. There are several means to increase 

load capacity. One approach that is often overlooked but can 

account for slight increases in allowable design values of the 

existing timbers is through visual grading of the timbers.

Visual Grading to Determine Strength of 

Bridge Members

Material properties are important to the structural engineer 

when wood members carry loads, which is especially 

important with covered bridges. To determine if the existing 

timbers can carry the required structural loads, it is 

important to know the appropriate strength and stiffness 

properties of the wood. Identification of the wood species is 

an important factor, not only to calculate the wood’s 

strength and stiffness but also to determine if there will be 

significant differential shrinkage if other woods are used for 

repairs. For wood in good condition, with no decay or insect 

damage, an estimate of strength can be made by determining 

the species of the wood and visually grading the wood 

members.

The first step in visual grading is identifying the wood 

species. Although some individuals can identify wood 

species in the field using a hand lens, the most reliable 

means to accurately identify species is done by examining 

anatomical features of the wood under a microscope.

Typically, to determine the species of a wood member, a 

sample as small as 0.5 to 1 in. long or a small core (0.5 in. 

or less in diameter) should be taken from an inconspicuous 

location. A representative number of samples should be 

taken from every framing member type (not from every 

member) to be graded. These samples can be analyzed for 

species identification by a university or a wood consultant 

(for a fee).

After the species is identified, then width and thickness, 

knot size, knot location, and slope of grain can be measured 

in the field and used to determine the allowable grade.

Knowing the grade enables strength properties to be 

assigned for design and structural analysis. For new wood 

construction, structural engineers rely on design values 

referenced in building codes to determine an acceptable 

Figure 5.13—High-quality lumber procured for bridge 

repairs (Blair Bridge, 1829, 1869, Grafton County, 

New Hampshire).

Figure 5.14—High-quality timber procured for 

bridge repairs (Blair Bridge, 1829, 1869, Grafton

County, New Hampshire). Note the end checks 

(which are the result of the timbers drying) and 

proper storage with spacers to allow for air 

circulation.
Guidelines to Restoring Structural Integrity of Covered Bridge Members

39

species, size, and grade for a particular load condition. For 

existing covered bridges, engineers often rely on current 

codes and standards to determine adequacy of the wood 

members. However, current standards are generally based 

on lower quality material than is found in many historic 

covered bridges, that is, current codes and standards allow 

for larger knots than are found in older material. It is not 

that the older material is much better than new material but 

more that the new material is allowed to have larger defects, 

which decreases the design properties for the member.

Because many older bridges were constructed before 

building codes or design values for wood products were 

established (and, thus, before grade stamps were used), 

engineers inexperienced with historic structures or materials 

are often in a quandary when determining what design 

values are appropriate. Frequently, a species and grade are 

assumed, leading to wood members being declared 

structurally deficient. The result is often an overly 

conservative estimate of design values and unnecessary 

replacement, repair, and retrofit decisions, accompanied by 

higher project costs.

Although estimates of strength and stiffness can be made by 

knowing the species, establishing the grade, and therefore 

the design properties, is more involved. In situ grading of 

structural members in a covered bridge should not be 

conducted unless the individual is familiar with appropriate 

standards and has the knowledge of how grades are 

established within the design codes.

Knots and slope of grain are generally considered the most 

significant strength-limiting characteristics in lumber and 

timber that affect both the structural grade and material 

properties. Three major strength-reducing effects arise from 

the presence of a knot: (1) part of the timber cross section is 

decreased because harder, denser, but structurally weaker 

knotwood takes the place of the regular wood fibers; (2) a 

stress concentration, and subsequent decrease in capacity, is 

induced by the material inhomogeneity of the knot 

surrounded by the rest of the timber; and (3) the growth 

pattern of the trunk is disrupted by the branch that caused

the knot, which results in considerable distortion of the grain 

angle around the knot. This grain angle distortion can allow 

for the development of tensile stresses perpendicular to the 

grain and the formation of checks and microfractures as the 

wood dries.

The location of the knot has an impact on member strength.

Therefore, within industry-developed grading rules, there 

are knot size limitations based on the location of the knot.

Centerline knots on the wide face of a member have the 

least impact on grade and therefore have the largest 

allowable knot size. Edge knots generally increase localized 

tensile stresses and therefore have smaller knot size 

limitations. This information can be useful to the structural 

engineer. Not only is the position of a knot known in a 

particular timber but also the position along the length of the 

timber is known. For example, the grade-limiting knot may 

be on the bottom face of the bottom chord near midspan.

Being in tension, that knot size and position will influence 

the performance of the bottom chord, and the grade is 

particularly important for that case. In contrast, if the gradelimiting knot is on the upper face near the end of another 

bottom chord, the influence on structural performance is 

much smaller. Based on grading rules, each knot (if the 

same size) would result in the same structural grade being 

assigned, even though the knot near the end on the upper 

face has much less influence on the performance of 

the chord.

One reason knots have such impact on the strength capacity 

of a timber is because of the distorted grain angle that 

occurs as the tree grows around the branch. When logs are 

milled into lumber and timber, the areas of distorted grain 

can be cut in such a way that segments of grain “run out” at 

one or several locations along the lumber’s length rather 

than extend parallel along the entire length of the board 

(Fig. 5.15). The same effect can occur if the timber is milled 

at an angle that is not parallel to the grain or if the entire log 

is twisted because of spiral growth patterns in the tree.

Areas of cross grain, or where the grain runs out, create 

deviations in the way stresses are transmitted throughout the 

piece and concentrate stresses at the point at which the wood 

fibers have been discontinued, which significantly weakens

the member.

Slope of grain is generally measured as a ratio of rise to run, 

that is, the number of inches the grain slopes upward or 

downward within a given distance (generally 8, 10, 12, or 

15 in.) that is parallel to the long axis of the member 

(Fig. 5.16). Because seasoning (drying) checks in timber 

generally follow the slope of grain, determining the slope of 

grain on painted timbers can be achieved relatively easily by 

examining drying checks. All visible faces of the timber 

should be examined for slope of grain because not all faces 

will exhibit the same extent of slope of grain. Twist in a 

Figure 5.15—Large edge knot in a diagonal brace (Eldean 

Covered Bridge, 1860, Miami County, Ohio). Note how the 

grain deviates around the knot and “runs out” at the edge 

of the member. This creates a weak location in the 

member.
General Technical Report FPL–GTR–252

40

member is also indicative of slope of grain when seasoning 

checks are absent (Fig. 5.17). Although not always 

conclusive, this approach closely correlates with results 

achieved using laboratory methods for measuring slope of 

grain, which are not practical for field use.

Seasoning checks, which are separations between wood 

fibers that do not fully penetrate the width or thickness of a 

member, are common in structural timbers and rarely affect 

the performance of a wood member. Splits are separations 

of wood fibers that extend completely through the width or 

thickness of a wood member. Short splits typically do not 

affect the performance of a wood member, but long splits 

should be evaluated by the engineer if there are concerns 

about shear strength in beams or buckling in columns.

Increasing Load Capacity by Augmenting or 

Reinforcing Existing Members

Increasing the load capacity of a bridge can be achieved by 

adding supplemental supports or augmenting existing 

members with stronger materials. The trusses are the key 

elements that support the bridge, but frequently, the limiting 

factor in carrying load is the deck itself. Simply increasing 

the number of floor joists may allow for a significant

increase in capacity, providing the deck is adequate. Adding 

an arch to a trussed bridge can also increase capacity. The 

Cornish-Windsor Bridge over the Connecticut River had 

four radial arches installed in the 1980s to augment the 

Town lattice trusses. Adding arches is rarely done because it 

significantly alters the historic character of the bridge. It is 

more common to use stronger materials to augment the 

existing timbers or connections where they are deficient.

Steel U-bolts placed on opposite sides of the timbers can be 

used to augment the connection capacity between the 

timbers. The goal is to tie the chord together when minor 

damage or deterioration is present in or near the connection,

but this does not warrant a full repair. The timber shown in 

Figures 5.18 and 5.19 is a bottom chord with a splice 

between two timbers. The steel U-bolts allow for increasing 

the strength of the poor connection by maintaining tightness 

between the timbers. Because the bottom chord is in tension, 

the connection wants to open under load; the U-bolt helps to 

keep the bottom chord timbers together and decreases sag in 

the bridge. Rather than steel U-bolts being used, connections 

in the original timbers are often joined with timber joints 

Figure 5.16—Slope of grain as evidenced by the 

presence of seasoning checks (Seguin Covered Bridge,

1850, Chittenden County, Vermont).

Figure 5.17—Slope of grain in a vertical post as indicated 

by slight twisting at the top of the post (Pine Bluff 

Covered Bridge, 1915, Putnam County, Indiana).

Figure 5.18—U-bolt tying together a scarf joint in the 

bottom chord (Taftsville Covered Bridge, 1836, Windsor 

County, Vermont).

Figure 5.19—U-bolt tying together a scarf joint in the 

bottom chord (Taftsville Covered Bridge, 1836, Windsor 

County, Vermont).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

41

using shear blocks, such as the lightning bolt splice shown 

in Figure 5.20.

The use of bolts, lag screws, and nails to reinforce 

connections and splices is evident on prior repairs to 

covered bridges. Fully threaded structural fasteners are 

gaining acceptance in North America after being introduced 

in Europe prior to 2010. These fasteners have the advantage 

of being manufactured from high-strength steel and are selftapping, that is, they do not require predrilling. With design 

values approaching those for bolts, these fasteners provide a 

less labor-intensive reinforcement option that does not alter 

the appearance of the bridge as the installation of steel plates 

or multiple bolts often do. Because they are being used in 

new construction and rehabilitation of timber structures, it is 

reasonable to expect that these fasteners will be increasingly 

considered as a viable repair strategy for reinforcing and 

strengthening traditional joinery.

Increasing Load Capacity Using Laminated 

Timber

There are a number of ways that mechanically laminated 

decking or glulam beams and deck panels have been used in 

covered bridges. Truss chords or floor beams made of 

glulam with higher allowable design values than solid 

timber is one way to increase load capacity. Although the 

use of glulam may not satisfy the intent of The Secretary of 

the Interior’s Standards for the Treatment of Historic 

Properties in terms of in-kind replacement, it is a viable 

solution for increasing the capacity of a bridge that must 

carry vehicular traffic.

Glulam is made of various wood species, typically Douglasfir or southern pine. It can be engineered and manufactured 

to provide specific design properties, often greater than 

those possible with high-quality solid timbers. For 

architectural effect, it is possible to have glulam fabricated 

with rough edges to better replicate the weathered, textured 

surface of the timbers they are replacing. In addition to the

higher design properties of glulam, fabricating a single 

continuous truss chord without splices eliminates the 

strength reduction associated with the loss of net section 

from the timber shear blocks and butt joints between 

adjacent chord segments.

If multiple glulam segments are joined with steel plates, care 

should be taken to ensure that moisture does not get trapped 

between the steel and the glulam, leading to decay. Because

only a few joints will be used, careful installation of the 

joint is critical to ensure proper load transfer and to limit the 

opportunity for moisture to be trapped in the joint. Glulam 

can be produced in less time than it may take for a large 

solid timber to dry to a moisture content at which it can be 

installed in the bridge. If exposure level is high and 

durability a concern, glulam (as with solid timber) can be 

pressure-treated to increase durability.

Often somewhat hidden from public view, glulam truss 

chords and floor beams are unobtrusive and can be installed 

using many of the same methods used for solid timbers 

(Fig. 5.21). Glulam connections often consist of steel angles 

and plated connectors, rather than traditional timber joinery, 

to take advantage of the higher design properties (Fig. 5.22).

Glulam can also be used for bedding timbers (Figs. 5.23 

and 5.24).

Solid lumber planks were traditionally used for decking, 

occasionally with two lines of running planks parallel to the 

long axis of the bridge for wheels (Fig. 5.25). The running 

Figure 5.20—Lightning bolt splice used to join two 

timbers in a bottom chord (Blair Bridge, 1829, 1869, 

Grafton County, New Hampshire).

Figure 5.21—Glued-laminated timber joists for increasing 

the load-carrying capacity of the deck (Cornwall-Salisbury 

Covered Bridge, 1865, Addison County, Vermont).

Figure 5.22—Installing a glulam bottom chord in a Town 

lattice truss ca. 1988 (Cornish-Windsor Covered Bridge,

1866, Sullivan County, New Hampshire, and Windsor 

County, Vermont. Used with permission from DCF 

Engineering).
General Technical Report FPL–GTR–252

42

planks could more easily be replaced than deck boards after

physical abrasion from traffic decreased their thickness.

Running planks were considered sacrificial; that is, 

maintenance of the deck included periodic replacement of 

these planks. Decking designed to carry greater loads relied 

on the composite action of the decking rather than on each 

deck plank carrying the load.

Mechanically laminated decking places the deck boards on 

edge, then secures them to one another with nails, and in 

later versions screws, to provide a stiffer and stronger deck 

than simple plank decks. The same construction technique is 

used to construct the arches in covered bridges to give them 

composite action so they act as a unit rather than several 

loose boards. Mechanically laminated decks have been used 

for several decades. The composite action can increase the 

capacity of the decks considerably. Since the early 1990s, 

stress-laminated decks have been in use (Figs. 5.26 and 

5.27). The difference between the traditional mechanically 

laminated deck connected with nails or screws and a stresslaminated deck is that, with stress-laminated decks, the deck 

boards are joined with steel rods that extend across the 

entire width of the deck. The threaded rods are stressed in 

tension and clamped along the edges of the deck to squeeze 

the deck boards together. The resulting increase in friction 

between the boards improves the composite action 

compared with the use of many nails or screws. Glulam 

panels that are fabricated off site can also be used as 

replacement decking to increase the capacity of the bridge.

Increasing Load Capacity by Reinforcing with 

Fiber-Reinforced Polymers

Fiber-reinforced plastics or polymers have been used for a 

number of years to strengthen timber, concrete, steel, 

masonry, and stone structural members. Typical applications 

include column-to-beam connections, seismic retrofitting, 

repair of corrosion-damaged beams and columns, bridge 

decks, piles, precast prestressed concrete shells, and roof 

structures. FRPs are considered to have a number of 

advantages, including a wide range of products with 

specified tensile strengths; low mass; ease of fabrication;

custom colors, coatings, and geometry; resistance to 

corrosion; and low transportation costs. Additionally, FRPs 

can be made from recycled plastics. However, FRPs have 

some disadvantages. These disadvantages include high 

initial costs and the need for highly trained and specialized 

Figure 5.23—Glulam bottom chord and floor beams 

being installed ca. 1988 (Cornish-Windsor Covered 

Bridge, 1866, Sullivan County, New Hampshire, and 

Windsor County, Vermont. Used with permission from

DCF Engineering.)

Figure 5.24—Glulam used as a bedding timber (CornishWindsor Covered Bridge, 1866, Sullivan County, New 

Hampshire, and Windsor County, Vermont).

Figure 5.25—Running planks (Parvin Bridge, 1921, Lane 

County, Oregon).

Figure 5.26—Stress-laminated deck viewed from below 

(Seguin Covered Bridge, 1850, Chittenden County, 

Vermont).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

43

engineers to design the structural systems and address 

potential creep, rupture, and shrinkage issues.

Research was conducted by the Constructed Facilities 

Center and the Institute for the History of Technology and 

Industrial Archaeology of West Virginia University to 

develop methods to strengthen structural wood members of 

historic covered bridges using Glass-Fiber-Reinforced 

Polymer (GFRP) composite materials. Laboratory 

experiments were conducted to test the effects of GFRP 

composite materials (both plates and rebar) on the bending 

and shear capacities of structural members. The specific 

objectives were to develop methods to strengthen truss and 

arch members and floor beams and to identify the adhesives 

necessary to fix the GFRP composite materials in place. The 

experiments were intended to result in repairs that comply 

with The Secretary of the Interior’s Standards for the 

Treatment of Historic Properties.

The results of the testing were somewhat mixed. In tension 

tests, GFRP rebar embedded in wood members performed 

relatively well. In small-scale bending tests, strength and 

stiffness were improved by bonding a GFRP plate to the 

tension face of the member. However, considerable surface 

preparation was necessary to ensure an adequate bond.

Bonding of the GFRP plate also required routing a cut-out 

for the placement of the plate along the tension side of the 

test specimens. Initial bending tests with GFRP rebar at the 

top and bottom of the test specimens did not improve 

member performance. This method may be more suitable 

for compression members in trusses. In shear tests, the shear 

capacity of GFRP members decreased slightly compared 

with a solid timber control specimen.

For all laboratory tests, specimens were tested indoors at 

near constant moisture content and temperature. The effect 

of temperature and moisture fluctuation on the adhesive was 

not considered and could be a factor in performance in 

practical application. As indicated from the research, 

obtaining a high-strength bond between the composite and 

the wood is difficult, and this would be even more true in 

the field with the weathered, uneven surface of existing 

timbers. The surface preparation to obtain a good bond 

requires a very high level of skill and quality control, some 

of which is difficult to maintain under field conditions.

Additionally, finding a steady, reliable source of composite 

material has been challenging because of market limitations.

Until these issues are resolved, it is unlikely that FRPs will 

provide a reliable means of increasing the load capacity of 

covered bridges.

Increasing Load Capacity through PostTensioning

The capacity of a covered bridge can be increased, or at 

least restored to its original capacity, by post-tensioning.

Post-tensioning involves adding steel cables or rods to carry 

some of the load that the bottom (tension) chord may not be 

able to carry because of its species, size, or grade. The 

Goodpasture Covered Bridge in Oregon (Fig. 5.28) carries 

very significant highway loads, including logging trucks.

One option that allowed for maintaining the historic 

integrity and aesthetics of the bridge while increasing load 

capacity was to insert post-tensioning cables (strands). Steel 

rods could also have been used; using rods would have 

simplified the anchorage details and made it much easier to 

retension should it be necessary to make adjustments. In 

either case, as with any significant repair, proper shoring 

and staging was required to prevent damage to the bridge 

Figure 5.27—Stress-laminated deck as the wearing 

surface (Doyle Road Covered Bridge over Mill Creek,

1868, Ashtabula County, Ohio).

Figure 5.28—Overview (Goodpasture Covered Bridge,

1938, Lane County, Oregon).

Figure 5.29—First steel truss being inserted into a bridge 

(Goodpasture Covered Bridge, 1938, Lane County, Oregon. 

Used with permission from OBEC Consulting Engineers).
General Technical Report FPL–GTR–252

44

during repairs. Steel trusses were erected inside the bridge to 

provide temporary support (Figs. 5.29 and 5.30).

The ends of the bottom chords had steel plates attached to 

them to serve as the anchors for the cables (Figs. 5.31 and 

5.32). The cables, each made up of several strands, were 

threaded through steel guides along the length of the bridge 

to prevent abrasion that could damage the cables or bottom 

chord timbers (Fig. 5.33). The contractor did not have 

access under the bridge and could not replace the floor 

beams. Therefore, the strand guides were attached to the 

bottom chords about midway between each pair of floor 

beams. After the strands were installed, hydraulic jacks were 

used to apply 20,000 lb of tension to the strands. The 

tensioning process transferred some of the tension that was 

carried by the timber bottom chord, which was highly 

stressed in tension, into the cables (Fig. 5.34). As a result, 

the bottom chord was no longer the controlling member in 

the bridge and the rated capacity increased from 15 to 

44 tons to support loads associated with heavy trucks.

One final note on post-tensioning; the tension in the cables 

(or rods) can be monitored remotely (and relatively 

inexpensively) to determine if the bridge is being 

overloaded or the tension in the cables (or rods) is changing 

with time because of temperature fluctuations or creep.

Increasing Clearance

Lack of adequate clearance, either within the bridge or 

above the waterway, has resulted in damage to bridge 

members from oversized vehicles and loss of several bridges 

from flooding. Glulam beams have been used to essentially 

lower the deck from its original position, thereby increasing 

the height of vehicles that can pass through the bridge.

Clearance is increased because the floor beams are attached 

to the bottom of the glulam, thus lowering the deck. This 

repair also offers the advantage of increasing the capacity of 

the bridge. Although effective, inserting the glulam 

dramatically alters the interior bridge aesthetics (Fig. 5.35).

Reducing Risk Caused by Overload

The use of glulam or steel beams installed underneath, 

attached to the abutments but not in contact with the bridge, 

acts as a safeguard if the bridge becomes overloaded. The 

bridge can be repaired without altering the original 

construction. But if load-carrying capacity is marginal, the 

glulam or steel beams serve as a restraint to carry the 

intended loads if excessive deflection occurs because of a 

Figure 5.30—Steel support trusses erected in a bridge 

prior to installing post-tensioning hardware 

(Goodpasture Covered Bridge, 1938, Lane County, 

Oregon. Used with permission from OBEC Consulting 

Engineers).

Figure 5.31—Steel end plate at the end of the bottom 

chord serving as the anchor for the steel strands 

(Goodpasture Covered Bridge, 1938, Lane County, 

Oregon. Used with permission from OBEC Consulting 

Engineers).

Figure 5.32—Steel end plate that serves as the anchor for 

the steel strands, viewed from below (Goodpasture 

Covered Bridge, 1938, Lane County, Oregon).

Figure 5.33. Steel strands threaded through a guide to 

prevent abrasion of either the cables or the bottom chord 

timbers (Goodpasture Covered Bridge, 1938, Lane 

County, Oregon).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

45

temporary overload. This approach is completely reversible 

and does not affect the integrity of the original bridge.

Adding girders below a bridge is not always effective 

because the stiffness of the bridge trusses is typically much 

higher than the stiffness of girders (glulam or steel) of 

reasonable depth. It takes significant deflection to transfer 

the load from the trusses to the supplemental girders.

Engineering analysis can determine the merits of this option.

Although the girders may be designed for the total load, the 

trusses will still carry a significant proportion of the load.

The supplemental girders are effective only if the trusses 

deflect enough for the girders to pick up some of the load.

The installation of glulam or steel beams underneath a 

bridge eliminates some of the space between the bottom of 

the bridge and high water and creates an opportunity for 

debris to accumulate against the bridge during a flood. 

Many bridges have been lost as a result, and this possibility 

should be accounted for when considering this type of 

augmentation.

Increasing Durability

When conditions do not allow wood to dry and the 

likelihood of wood decay or insect attack is high, the use of 

pressure-treated timbers should be considered to improve 

durability (Fig. 5.36). Pressure treating timber is discussed 

in numerous publications that give the benefits and 

drawbacks of using chemically treated wood. Some wood 

species can be treated quite easily, such as southern yellow 

pine, whereas others are difficult to treat and rely on 

mechanical means, such as incising Douglas-fir to improve 

penetration of treatment into the wood (Fig. 5.37).

Although durability is most commonly referenced in terms 

of the ability of wood to resist biological attack, durability 

can also be increased by mechanical means. Vehicular 

damage to portal openings and knee braces, which add 

stiffness to the connections between the roof framing and 

trusses, is especially common. In spite of height limits being 

clearly posted, vehicles either knowingly or inadvertently 

enter a bridge that does not have adequate clearance, 

causing damage to the bridge.

An option for increasing internal clearance is the use of 

natural knee braces (Figs. 5.38 and 5.39). A natural knee 

brace comes from the base of a tree where the roots extend 

or from the juncture of a branch with the trunk. It forms a 

very strong brace that increases clearance for over-sized 

vehicles.

In summary, there are several options for repairing a 

covered bridge. The first question should be does the bridge 

need to be repaired based on its current condition and 

Figure 5.34—Jacking the steel strands to introduce 

tension into the cables (Goodpasture Covered Bridge,

1938, Lane County, Oregon. Used with permission from

OBEC Consulting Engineers).

Figure 5.35—Root Road Covered Bridge, 1868, 

Ashtabula County, Ohio.

Figure 5.36—Pressure-treated bedding timbers used for 

increased durability (Shoreham Bridge, 1897, Addison 

County, Vermont).

Figure 5.37—Pressure-treated glulam repair (Parvin 

Bridge, 1921, Lane County, Oregon).
General Technical Report FPL–GTR–252

46

intended use? If so, maintaining the historic character of the 

bridge should be the driving force behind any repair 

strategy. Rehabilitating the bridge because of deterioration 

with time, enhancing the load capacity, increasing clearance, 

and increasing durability to provide a long service life and 

to extend the maintenance cycle depends on understanding 

the bridge condition and behavior and how to properly

support the bridge during repair. A bridge that is properly 

repaired can provide decades of reliable performance before 

significant repairs are needed again.

References

Anthony, R.W.; Dugan, K.D.; Anthony, D.J. 2009. A 

grading protocol for structural lumber and timber in historic 

structures. Grant Number MT-2210-05-NC-05.

Natchitoches, LA: National Center for Preservation 

Technology and Training.

Fischetti, D.C. 2009. Structural investigation of historic 

buildings, a case study guide to preservation technology for 

buildings, bridges, towers and mills. Hoboken, New Jersey:

John Wiley & Sons, Inc.

Graton, M.S. 1978. The last of the covered bridge builders. 

Plymouth, New Hampshire: Clifford-Nicol, Inc.

Lebow, S.; Kirker, G.; White, R.; Amburgey, T.; Barnes, 

M.H.; Sanders, M.; Morrell, J. 2012. Guide for in-place 

treatment of wood in historic covered and modern bridges.

Gen. Tech. Rep. FPL–GTR–205. Madison, WI: U.S. 

Department of Agriculture Forest Service, Forest Products 

Laboratory.

Pierce, P.C.; Brungraber, R.L.; Lichtenstein, A.; Sabol, S.; 

Morell, J.J.; Lebow, S.T. 2005. Covered bridge manual.

Publication No. FHWA-HRT-04-098. McLean, Virginia:

U.S. Department of Transportation, Federal Highway 

Administration.

Wiedenhoeft, A.C.; Kretschmann. D.E. 2014. Species 

identification and design value estimation of wooden 

members in covered bridges. Gen. Tech. Rep. FPL–GTR–

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

Service, Forest Products Laboratory.

Figure 5.38—Natural knee brace; the curvature of the 

brace follows the grain (North Hartland Twin Bridge,

2001, Windsor County, Vermont). Figure 5.39—Natural knee braces (North Hartland 

Twin Bridge, 2001, Windsor County, Vermont).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

47

Chapter 6: Maintenance

Covered bridges often suffer from a lack of regular 

maintenance. This can lead to deterioration and failure of 

wood roofing and siding, which protect the structural 

timbers and trusses, and in extreme circumstances, failure of 

critical structural members. Routine inspection and 

maintenance can significantly extend the service life of a 

covered bridge and may require only a day each year to 

conduct a thorough visual inspection and limited probing to 

identify potential problem areas. Cleaning and maintaining 

the painted surfaces are essential maintenance tasks.

Decreasing the effects of wood decay or insect attack 

through the use of remedial wood preservative treatments 

can be very cost effective, and installing fire protection 

monitoring, alarm, or sprinkler systems can prevent the loss 

of a bridge from careless acts or vandalism.

Cleaning

Dirt, loose vegetation, grit, and litter can accumulate 

anywhere on a bridge where wind or water drainage takes it.

The buildup of debris traps additional moisture and creates a 

favorable microenvironment for active wood decay.

Removing the debris through periodic cleaning with 

brooms, shovels, leaf blowers, or other readily available 

cleaning implements is a simple but essential task in

maintaining a bridge.

Many covered bridges are not on paved roads or at least 

may not have paved approaches. Gravel, dirt, and grit 

accumulate on the approach, and then foot and vehicular 

traffic aid in moving some of the debris onto the bridge.

Precipitation further moves the debris onto the wearing 

surface, running boards, supporting timbers, and trusses 

(Figs. 6.1 and 6.2). Wind blows loose vegetation (leaves, 

pine needles, grass, etc.) into areas of the trusses that are not 

well drained for this larger debris (Figs. 6.3 and 6.4). With

time, it is trapped and allows for moisture to accumulate and 

be absorbed into the wood at that spot. Left in place long 

enough, the trapped, moist material can allow the growth of

grass, shrubs, and even trees. Identifying and removing 

vegetation and trapped debris is easy and highly effective in 

protecting the bridge (Fig. 6.5).

Some bridges have features that limit the accumulation of 

debris by providing natural drainage below the guardrail 

(Fig. 6.6), between the wearing surface decking (Fig. 6.7), 

or other areas. It is important to maintain the historic 

character of the bridge, but if opportunities are available to 

improve drainage and limit accumulation of debris that do 

not change the historic character of the bridge, they should 

be considered to decrease maintenance demands. Installing a 

grate or a cattle guard at an approach will significantly 

decrease the quantity of gravel and loose vegetation that 

accumulates on the bridge by providing a drainage 

mechanism that intercepts debris before it reaches the bridge 

(Fig. 6.8).

Cleaning under the bridge is equally important. Debris 

flowing downstream can collect on piers, abutments, rocks,

and low-flow areas (Fig. 6.9). With sufficient build-up, the 

debris may contact the bedding timbers, lower chords of the 

trusses, or siding. As with accumulated debris on the bridge, 

debris and vegetation that is in contact with the wood can 

result in active wood decay. Potentially more serious, 

sufficient build-up of debris can restrict flow of the 

waterway during periods of high rainfall, run-off, or 

flooding that can erode the material around the piers and 

abutments. The loss of material, or scour, can result in 

destabilization of the bridge supports and has resulted in 

collapse of bridges. Bridge siding and lower chord members 

are frequently loosened, damaged, or lost as the result of 

impacts by brush and other debris carried down waterways 

by high water.

Wildlife can also present maintenance challenges with 

covered bridges. Rodents and mammals may nest and 

inhabit the less accessible areas of a bridge because of the 

protection the closed areas provide against predators. This 

can be under the bridge superstructure, on the abutments, 

between truss chord members, behind lattice work, and on 

the roof framing. Various insects can also inhabit a bridge 

but seldom cause damage except for (most commonly) 

termites, carpenter ants, and carpenter bees. Various 

Figure 6.1—Debris build-up on bridge wearing surface 

and at the approach (Century Bridge, 1868, Ashtabula 

County, Ohio).

Figure 6.2—Debris build-up on wearing surface (Century 

Bridge, 1868, Ashtabula County, Ohio).
General Technical Report FPL–GTR–252

48

Figure 6.3—Vegetation debris build-up (Century Bridge,

1868, Ashtabula County, Ohio).

Figure 6.4—Debris build-up between boards and 

adjacent to siding (Ballard Road Covered Bridge, 1883, 

Greene County, Ohio).

Figure 6.5—Debris accumulation and incipient vegetation 

colonization (Houck Covered Bridge, 1880, Putnam 

County, Indiana).

Figure 6.6—Gap under guardrail allows debris to be 

washed away. Compare with guardrail in Figure 6.2 that 

rests directly on the wearing surface (Willard Covered 

Bridge, 1871, Windsor County, Vermont).

Figure 6.7—Gaps between the decking that allow for 

drainage of water and fine debris (Goodpasture 

Covered Bridge, 1938, Lane County, Oregon).

Figure 6.8—Steel grate on approach to bridge to 

intercept debris before it reaches the wearing surface 

of the bridge (Baker’s Camp Covered Bridge, 1901, 

Putnam County, Indiana).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

49

inhibitors can be installed in some locations to discourage 

bird and rodent access to the bridge. Nail strips and other 

devices have been used successfully to prevent birds and 

rodents from building nests on the upper surfaces of 

exposed roof framing members and top chords of trusses 

(Fig. 6.10).

Vegetation and Moisture Management

After cleaning, the two most cost-effective measures for 

extending the life of covered bridges are controlling water 

and vegetation management. These measures can greatly 

improve the environment around the bridge. If organic 

debris and vegetation are cleared from around the perimeter 

of covered bridges, debris build-up within the interior is 

removed and maintained, and the interior of the bridge has

minimal exposure to moisture, the occurrence of and rate of 

deterioration caused by decay fungi will typically be 

slowed. Moisture management is often incorporated into the 

construction of the bridge. That is the purpose of the roof 

and siding. Additionally, the roof eaves and the portals may 

extend several feet beyond the trusses to add more 

protection from wind-blown precipitation (Fig. 6.11).

Without vegetation management, covered bridges can be 

engulfed in the invasive trees, shrubs, and grasses that 

spring up without periodic maintenance (Fig. 6.12). This 

overgrowth creates a moist microclimate conducive to decay 

fungi through a combination of denser shade, decreased

airflow around the bridge members, and increased nitrogen 

production. Denser shade decreases surface temperatures of 

bridge members and allows water vapor released from 

transpiration to condense more rapidly on the members.

Concentrations of leafy vegetation also can decrease

airflow, which decreases rates of evaporation. Dense 

clusters of vegetation drop leaves that release nitrogen as 

they decompose and allow growth of decay fungi.

As previously discussed, vegetation and inorganic debris 

can accumulate along the edges of the roadbed, between the 

running boards on the road surface, between truss members, 

and at bridge supports and abutments. Although some 

people view vegetation cover as part of the aesthetic 

character of the bridge, it was not the intent of the bridge 

builder nor is it in the interest of the current bridge steward 

(Fig. 6.13). Increased vegetative cover often attracts insects, 

rodents, and larger animals that can damage structural 

timbers, roofing, and siding. Dense vegetation with 

accumulated debris also increases the risk of fire.

Siding and Roofing

Siding and roofing protect the structural members of the 

bridge. Failure to maintain the roof and siding provides an 

Figure 6.9—Debris build-up under bridge (Stevenson 

Road Bridge, 1877, Greene County, Ohio).

Figure 6.10—Nails used to discourage birds and rodents

(Lowell Covered Bridge, 1945, Lane County, Oregon).

Figure 6.11—Overhangs on the roof and portals protect 

the truss members and decking from moisture damage 

and decay (Lincoln Gap Covered Bridge, 1879, 

Washington County, Vermont).

Figure 6.12—Vegetation (West Engle Mill Road Covered 

Bridge, 1877, Greene County, Ohio).
General Technical Report FPL–GTR–252

50

opportunity for moisture to find its way to the structural 

members, possibly resulting in decay or insect attack.

Missing or loose shingles or boards allow precipitation into 

the interior space of the bridge. Repairing or replacing loose 

or missing shingles and boards should be a routine part of 

periodic maintenance.

The siding, and occasionally the roof, may have been 

painted. However, some covered bridges were never painted 

or finished with a coating, and the application of a coating in 

such cases should only be considered if the structure will 

otherwise be compromised because of nondurable wood 

species being exposed to the elements (Figs. 6.14 and 6.15).

In most cases, adherence to The Secretary of the Interior’s 

Standards for the Treatment of Historic Properties is the best 

practice for preserving historic fabric, context, and 

authenticity. Weathered wood may not be as durable as 

painted wood, but if it is considered a characterizing feature 

of a historic bridge, it should not be painted without 

considering the change of context.

Extending the service life of the protective roofing and 

siding may require periodic painting (of wood that was 

originally painted) to prevent the wood from cracking or 

splitting caused by excessive weathering. Peeling paint and 

failed coatings offer insufficient protection to the wood 

substrate, resulting in shorter service periods and more 

frequent maintenance (Figs. 6.16 and 6.17).

There are many causes of paint failure and often multiple 

conditions that contribute to the degradation of coatings and 

wood substrates. Frequently, these conditions can be 

addressed through appropriate selection and application of 

coatings, but this requires correctly identifying the cause(s).

As such, some understanding of wood and coating 

deterioration is necessary when considering the need for 

repair or replacement of wood substrate materials and/or 

reapplication of an exterior coating. Mechanisms of wood 

deterioration were discussed in Chapter 2, and there are 

excellent sources of information on paint failure listed in the 

References section.

Siding and roofing that was never painted or has been 

improperly maintained will weather. Weathering of wood is 

the result of the action of cyclic wetting and drying, 

exposure to ultraviolet (UV) light, and erosion of the wood 

through wind-blown debris (a process similar to sand 

blasting). Weathering is a long-term process and is a factor 

in the deterioration of wood siding and roofing when 

maintenance has been allowed to lapse and protective 

coatings have failed or have begun to fail. However, 

weathering affects intact paint as well; weathering processes 

can lead to soiling of intact paint surfaces and to chalking 

and erosion of the paint. Where paint is lost, weathering 

gradually erodes the exposed, uncoated wood fibers. The 

process is slow enough, however, that exterior woodwork 

Figure 6.13—Vegetation removal as part of bridge 

maintenance (West Engle Mill Road Covered Bridge,

1877, Greene County, Ohio).

Figure 6.14—This bridge may never have been painted, 

or the paint has weathered away (Benetka Road Covered 

Bridge, ca 1900, Ashtabula County, Ohio).

Figure 6.15—It is possible that this bridge was never 

painted (Durgin Covered Bridge, 1869, Carroll County, 

New Hampshire).

Figure 6.16—Paint failure on the siding (Rolling Stone 

Covered Bridge, 1915, Putnam County, Indiana).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

51

can often survive several decades without a protective 

coating before significantly decreasing serviceability.

If extensive weathering has occurred or if the wood is 

continually wet because of environmental conditions that 

have not been addressed (for example, poor drainage), new 

coatings will not adhere well to the wood substrate. If 

coatings are to be applied to weathered surfaces, an 

acceptable compromise between surface preparation and 

preservation of the historic, weathered wood substrate and 

coating performance and durability must be achieved. In the 

same manner, decisions regarding the repair or replacement 

of weathered wood surfaces should strike a balance between 

preserving the authentic appearance of a historic bridge and

ensuring that siding and roofing continue to protect the 

structural integrity of the bridge.

If a bridge has not been painted in several decades, testing 

for lead paint is recommended. Prior to conducting repairs 

on the bridge, it may be necessary to do abatement of the 

lead, paying careful attention to prevent contamination of 

the water below the bridge. Regardless of the presence of 

lead, a paint analysis should be conducted if there are 

questions regarding the bridge’s historic paint scheme or if 

there is evidence of coating incompatibility.

For most bridges, the reapplication of a coating to exposed 

wood surfaces is a consideration when deterioration in the 

form of peeling paint, weathered wood substrates, or even 

failures of the building envelope have begun to appear and 

there are concerns about the long-term serviceability of the 

wooden members. If moisture problems and subsequent 

deterioration were caused by a lack of paint maintenance, 

there is generally no need for extensive replacement of 

weathered siding or roofing, unless it has deteriorated or is 

no longer able to protect the structural timbers.

The current best practice for exterior coatings applied to 

wood substrates is to either replace or sand all weathered 

wood surfaces to expose fresh wood prior to coating 

application. This approach, although appropriate for 

ensuring the longevity of the coating, fails to account for the 

visual impact that sanding and/or replacing weathered wood 

fabric will have on the appearance of a historic bridge. The 

need to sand exposed wood surfaces and remove the rough 

texture of the wood surface prior to reapplication of a 

coating can therefore be a complex issue to consider.

Proper application procedures for paints and stains are 

critical. Coatings should be applied to sound wood that is

free of dirt, flaking paint, and loose wood fibers. After

determination of lead content of any existing paint has been 

made, appropriate paint removal efforts can be undertaken, 

if necessary. These steps may include hand-scraping areas 

of peeling paint, low-grit media blasting of the old surfaces, 

or the use of infrared heating devices that allow for the 

softened paint to be scraped from the wood surface.

There is considerable technical literature available regarding 

coatings and coating applications on wood substrates. Some 

of these resources are listed in the References. Because 

technologies are always evolving, it may be difficult to find 

information on new products; however, the preponderance 

of research conducted during the past 80 years indicates that

it is not necessarily the nature of the product but the 

thoroughness of the surface preparation, sound application 

procedures, and routine maintenance that result in a longlasting protective coating. Therefore, decisions regarding a 

specific coating may be less significant than those regarding 

surface preparation, application, and regular maintenance.

Regarding the siding and roofing, although the intent should 

be to maintain the historic character of the bridge in 

accordance with The Secretary of the Interior’s Standards 

for the Treatment of Historic Properties, there may be valid 

reasons for using alternate materials. The most common 

area of historic bridges to warrant material substitutions is 

the roof. In some climates, wood shingles may not be 

readily available that equal the quality of the historic 

shingles. The maintenance and replacement costs may 

warrant consideration of a material with a longer service 

life. Additionally, concerns about fire may warrant 

consideration of fire-retardant shingles or a composite or 

metal roof covering. Such substitutions can be made 

successfully without jeopardizing the visitor experience of 

the bridge (Fig. 6.18).

Fire Protection

The USDA Forest Service’s report Evaluating FireDamaged Components of Historic Covered Bridges

provides decision makers with a wealth of information on 

the effects of fire on covered bridges. Discussions about fire 

and bridges are generally separated into the following 

topics:

• Assessing fire damage

• Repairs to fire-damaged timbers

• Fire prevention and control

Figure 6.17—Paint failure on the siding (Stevenson Road 

Bridge, 1877, Greene County, Ohio).
General Technical Report FPL–GTR–252

52

After a fire has occurred, either naturally or by vandalism 

(including arson and unintentional starts from camp fires or 

smoking), assessing the extent of damage is similar to the 

assessment process described in Chapter 2. Thinner wood 

elements, such as siding and roofing, that have developed 

char are probably in need of replacement. Larger structural 

timbers may have surface scarring, char, or a loss of 

material. The structural engineer typically conducts an 

analysis of the remaining sound wood to determine if repair, 

reinforcement, or replacement is required using the same 

repair procedures discussed in these guidelines.

From a maintenance perspective, fire prevention and control 

systems are key to protecting bridges, particularly in remote, 

isolated areas, where access to a water supply or fire station 

is limited and where bridges are subject to frequent 

vandalism. Fire alarms, sprinkler systems, and lightning 

rods are effective strategies to decrease loss caused by fire 

(Figs. 6.19 and 6.20). Fire-retardant treatments can be 

applied to the timbers and wood members to decrease the 

burning potential. Removing vegetation near the bridge can 

help prevent fires from spreading to a bridge and eliminate 

hiding places for vandals. Also, automated perimeter lights 

can deter vandals. Many of these measures can be included 

in routine inspection and maintenance programs to afford 

better protection of the bridge.

Remedial Treatments to Retard Decay and 

Insect Attack

Biological growth is an indication of a favorable 

environment for wood decay (Fig. 6.21). It does not mean 

that decay fungi are active, or even present, but it is a visual 

sign that warrants closer inspection. If the wood is soft and 

easily penetrated by an awl, the decayed wood should be 

assessed for continued service as discussed in Chapter 2. If 

the decay area is small and does not compromise the 

structural integrity of the timber or the bridge, remedial 

chemical treatments can be used to extend the service life 

for perhaps decades before the timber needs to be replaced.

There are two general paths to extending the life of wood: 

nonchemical means (through moisture management and 

maintenance previously discussed) and chemical means 

(through the use of wood preservatives). Wood 

preservatives are generally grouped into two categories: 

preservatives used for in situ, remedial treatment of inservice wood and preservatives used for the pressure 

treatment of new wood used to replace deteriorated 

elements.

In situ treatments are typically applied to the wood surface 

and cannot be forced deeply into the wood. However, they 

can be inserted into the center of large wooden members via 

treatment holes, which are then plugged with a dowel to 

allow for periodic reapplications. These preservatives are 

available as liquids, rods, or pastes. Pressure-treated wood, 

on the other hand, generally has much deeper and more 

uniform preservative penetration, depending on the wood 

species and treatment process but cannot be applied in situ.

Pressure-treated timber can be used as replacement material 

when the original timber, such as a bedding timber, has 

deteriorated to the point where repair is no longer feasible.

Figure 6.18—Replacement of original roof with metal 

roof, perhaps to limit maintenance costs (Seguin 

Covered Bridge, 1850, Chittenden County, Vermont).

Figure 6.19—Fire Box (Cornish-Windsor Covered Bridge,

1866, Sullivan County, New Hampshire, and Windsor 

County, Vermont).

Figure 6.20—Fire alarm system (Durgin Covered Bridge,

1869, Carroll County, New Hampshire).

Figure 6.21—Biological growth found on this floor beam 

indicates favorable conditions that could lead to decay 

(Stewart Bridge, 1930, Lane County, Oregon).
Guidelines to Restoring Structural Integrity of Covered Bridge Members

53

Used as a common remedial chemical treatment, borate rods 

can be inserted into holes drilled in the wood where 

deterioration is likely (Fig. 6.22). Borates are low-level 

toxicity preservatives that are used to improve the durability 

of both new and in-service wood products. Borates are not 

an alternative to pressure-treated wood that will be in 

ground contact. They require moisture to migrate through 

the wood; therefore, they are placed where the wood is 

likely to be frequently damp, such as near the exposed end 

grain, near the bottom face of members close to ground 

contact and at exposed, wood–wood connections where 

members meet, including repair splices.

Borates effectively control termites, carpenter ants, a variety 

of beetles, and other wood-boring insects. Topical borate 

treatments (liquids) applied to the surface offer no 

protection to the interior of large-dimension members and 

are not recommended. Borate rod installation is not 

complicated and can be completed by anyone with basic 

carpentry skills and appropriate tools. To install borate rods, 

holes are drilled on a nonvisible face of a timber or near 

vulnerable joinery, the rods are inserted and the holes are 

filled with either a pressure-treated wood plug or a plastic 

threaded plug (to allow the insertion of additional rods 

during future inspection cycles). These rods are typically 

effective for 3 to 10 years depending on environmental 

conditions, but they should be regularly inspected and used 

as part of a cyclical, long-term maintenance program.

Pressure-treated wood may be considered as a replacement 

option when conditions would result in deterioration of 

untreated replacement wood; examples include belowground timbers, where termite infestation is likely, or places 

in which maintenance is likely to be difficult because of

remote location or lack of access. Specifying the correct 

pressure-treated wood depends on its specific use. To help 

guide selection of pressure-treated wood, the American 

Wood Protection Association (AWPA) developed Use 

Category System (UCS) standards.

Connections and Fasteners

The bridge should be visually inspected for missing or loose 

connections. In particular, treenails (pegs) used in the 

lattice-work of trusses and traditional timber joinery are 

often collected as souvenirs. The people who collect the 

pegs do not recognize that the pegs are essential to the 

structural performance of the bridge (Fig. 6.23).

As discussed in Chapter 2 on condition assessments, 

corroded metal fasteners can indicate moisture and salt 

issues. Light surface corrosion (rust) is generally not 

problematic and need only be monitored (Fig. 6.24).

Severely corroded metal that has either a loss of material or 

a build-up of corroded metal may require cleaning or 

warrant replacement of the steel connectors to avoid 

potential failure of the connection (Fig. 6.25).

Repairing and restoring a covered bridge properly is the 

primary means of keeping the bridge in service and 

preserving the craft that went into building the bridge for 

Figure 6.22—Material interface between the base of the 

arch and the concrete abutment where remedial 

treatments, such as borates, can extend the service life of 

the timber (Durgin Covered Bridge, 1869, Carroll County, 

New Hampshire).

Figure 6.23—Missing treenail connection (Benetka Road 

Covered Bridge, ca 1900, Ashtabula County, Ohio).

Figure 6.24—Minor surface corrosion (rust) (Shoreham 

Bridge, 1897, Addison County, Vermont).

Figure 6.25—Severely corroded bolts (Potter’s 

Covered Bridge, 1871, Hamilton County, Indiana).
General Technical Report FPL–GTR–252

54

future generations to appreciate. It is an investment that 

requires decision makers to understand the roles of 

condition assessment and engineering analysis in selecting 

cost-effective shoring and repair strategies. Maintenance is 

the means of protecting that investment.

References

Kukay, B.; Todd, C.; Jahn, T.; Sannon, J.; Dunlap, L.; 

White, R.; Dietenberger, M. 2016. Evaluating fire-damaged 

components of historic covered bridges. Gen. Tech. Rep.

FPL–GTR–243. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory.

Lebow, S.; Kirker, G.; White, R.; Amburgey, T.; Barnes, 

M.H.; Sanders, M.; Morrell, J. 2012. Guide for in-place 

treatment of wood in historic covered and modern bridges.

Gen. Tech. Rep. FPL–GTR–205. U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory.

Williams, R.S.; Knaebe, M.T.; Feist, W.C. 1996. Finishes 

for exterior wood: selection, application, and maintenance.

Madison, WI: Forest Products Society.
Guidelines to Restoring Structural Integrity of Covered Bridge Members

55

Chapter 7: Bibliography

Adeli, H., ed. 2008. Historic bridges: evaluation, 

preservation and management. Boca Raton, FL: CRC Press.

Allen, R.S. 1959. Covered bridges of the middle Atlantic 

states. Brattleboro, VT: Steven Greene Press.

Allen, R.S. 1970. Covered bridges of the middle West. 

Brattleboro, VT: Steven Greene Press.

Allen, R.S. 1974. Covered bridges of the Northeast. 

Brattleboro, VT: Steven Greene Press.

Allen, R.S. 1970. Covered bridges of the South. Brattleboro, 

VT: Steven Greene Press.

Anthony, R.W.; Dugan, K.D.; Anthony, D.J. 2009. A 

grading protocol for structural lumber and timber in historic 

structures. Grant Number MT-2210-05-NC-05.

Natchitoches, LA: National Center for Preservation 

Technology and Training.

ASCE. 1976. American wooden bridges. ASCE Historical 

Publication No. 4. New York: American Society of Civil 

Engineers Committee on History and Heritage of American 

Civil Engineers.

ASCE. 1982. Evaluation, maintenance and upgrading of 

wood structures, a guide and commentary. Alan Freas, 

Chairman. New York: American Society of Civil Engineers.

AWC. 2015. National design specification for wood 

construction. Washington, DC: American Wood Council.

Barna, E. 1996. Covered bridges of Vermont. Woodstock, 

VT: The Countryman Press.

Cassens, D.L.; Feist, W.C. 1986. Finishing wood exteriors: 

selection, application, and maintenance. Agricultural 

Handbook No. 647. Washington, DC: U.S. Department of 

Agriculture, Forest Service.

Christianson, J.; Marston, C.H., executive editors. 2015. 

Covered bridges and the birth of American engineering. 

Historic American Engineering Record. Washington, DC: 

National Park Service.

Cockrell, B. 2008. Oregon’s covered bridges. Charleston, 

SC: Arcadia Publishing.

Cockrell, B.; Cockrell, N. 1978. Roofs over rivers: a guide 

to Oregon’s covered bridges. Beaverton, OR: Touchstone 

Press.

Conwill, J.D. 2004. Covered bridges across North America. 

St. Paul, MN: MBI Publishing Company.

FHA. 1993. Design, construction, and quality control 

guidelines for stress laminated timber bridge decks.

Publication No. FHWA-RD-91-120. McLean, VA: U.S. 

Department of Transportation, Federal Highway 

Administration.

Fischetti, D.C. 2009. Structural investigation of historic 

buildings, a case study guide to preservation technology for 

buildings, bridges, towers and mills. Hoboken, NJ: John 

Wiley & Sons, Inc.

Foo, H.C.; Akhras, G. 1996. Strengthening requirements of 

old, timber Warren trusses. Journal of Performance of 

Constructed Facilities. 10(3): 127-134.

FPL. 2010. Wood handbook: wood as an engineering 

material. Gen. Tech. Rep. FPL–GTR–190. Madison, WI: 

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

Products Laboratory.

Gasparini, D.; Bruckner, J.; da Porto, F. 2006. Timedependent behavior of post-tensioned wood Howe bridges. 

ASCE Journal of Structural Engineering. 132(3): 418-429.

Gasparini, D.; Nizamiev, K.; Tardini, C. 2015. G. W. 

Whistler and the Howe bridges on the Nikolaev Railway, 

1842-1851. Journal of Performance of Constructed 

Facilities. 30(3).

Gasparini, D.; Simmons, D. 1997. American truss bridge 

connections in the 19th century. Part I: 1829-1850. Journal 

of Performance of Constructed Facilities. 11(3): 119-129.

Gasparini, D.; Simmons, D. 1997. American truss bridge 

connections in the 19th century. Part II: 1850-1900. Journal 

of Performance of Constructed Facilities. 11(3): 130-140.

Goldstein, E.W. 1999. Timber construction for architects 

and builders. New York: McGraw-Hill.

Graton, M.S. 1978. The last of the covered bridge builders. 

Plymouth, NH: Clifford-Nicol, Inc.

Haupt, H. 1869. General theory of bridge construction: 

containing demonstrations of the principles of the art and 

their application to practice; furnishing the means of 

calculating the strains upon the chords, ties, braces, countergraces, and other parts of a bridge. New York: D. Appleton 

and Company.

Kasal, B.; Tannert, T., ed. 2010. In situ assessment of 

structural timber. State of the Art Report of the RILEM 

Technical Committee 215-AST. Dordrecht, Heidelberg, 

London, New York: Springer.

Kirker, G.T.; Clausen, C.A.; Blodgett, A.B.; Lebow, S.T. 

2013. Evaluating naturally durable wood species for repair 

and rehabilitation of above-ground components of covered 

bridges. Gen. Tech. Rep. FPL–GTR–224. Madison, WI: 

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

Products Laboratory.

Kukay, B.; Todd, C.; Jahn, T.; Sannon, J.; Dunlap, L.; 

White, R.; Dietenberger, M. 2016. Evaluating fire-damaged 

components of historic covered bridges. Gen. Tech. Rep.

FPL–GTR–243. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory.
General Technical Report FPL–GTR–252

56

Lebow, S.; Anthony, R.W. 2012. Guide for use of wood 

preservatives in historic structures. Gen. Tech. Rep. FPL–

GTR–217. Madison, WI: U.S. Department of Agriculture,

Forest Service, Forest Products Laboratory.

Lebow, S.; Kirker, G.; White, R.; Amburgey, T.; Barnes, 

M.H.; Sanders, M.; Morrell, J. 2012. Guide for in-place 

treatment of wood in historic covered and modern bridges.

Gen. Tech. Rep. FPL–GTR–205. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory.

Lewandoski, J.; et al. 2006. Historic American roof trusses. 

Becket, MA: Timber Framers Guild.

Long, S.H. 1836. Description of Col. Long’s bridges, 

together with a series of directions to bridge builders.

Concord, NH: John F. Brown Printers.

National Society for the Preservation of Covered Bridges. 

2003. Burlington charter for the preservation of historic 

covered bridges. Burlington, VT: First National Best 

Practices Conference for Covered Bridges. 

http://www.coveredbridgesociety.org/downloads/burlingtoncharter_06jun2003.pdf.

O’Grady, P. 1998. Replicate: the rebuilding of the Corbin 

covered bridge in Newport, New Hampshire. Concord, NH: 

Town and Country Reprographics, Inc.

Pellerin, R.F.; Ross, R.J., ed. 2002. Nondestructive 

evaluation of wood. Madison, WI: Forest Products Society.

Phares, B.; Wipf, T.; Sievers, R.; Hosteng, T. 2013. Covered 

bridge security manual. Gen. Tech. Rep. FPL–GTR–223. 

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

Service, Forest Products Laboratory.

Pierce, P. 2013. Reflections on load capacity of historic

covered bridges. Timber Framing. 109: 20-24.

Pierce, P.C.; Brungraber, R.L.; Lichtenstein, A.; Sabol, S.; 

Morell, J.J.; Lebow, S.T. 2005. Covered bridge manual.

Federal Highway Administration Publication No. FHWAHRT-04-098. McLean, VA: U.S. Department of 

Transportation, Federal Highway Administration.

Reece, M.B. 2010. Strengthening historic covered bridges to 

carry modern traffic. New York: Nova Science Publishers, 

Inc.

Ritter, M. 1990. Timber bridges: design, construction, 

inspection, and maintenance. Publication No. EM 7700-8. 

Washington, DC: U.S. Department of Agriculture, Forest 

Service. 944 p.

Ross, R.J.; Brashaw, B.K.; Anderson, S.J. 2012. Use of laser 

scanning technology to obtain as-built records of historic 

covered bridges. Res. Pap. FPL–RP–669. Madison, WI: 

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

Products Laboratory. 21 p.

Ross, R.J.; Brashaw, B.K.; Wang, X.; White, R.H.; Pellerin, 

R.F. 2004. Wood and timber condition assessment manual.

Madison, WI: Forest Products Society.

Town, I. 1821. A description of Ithiel Town’s improvement 

in the construction of wood and iron bridges: intended as a 

general system of bridge building. New Haven, CT: S. 

Converse.

Walker, C.E. 1959. Covered bridge ramblings in New 

England. Contoocook, NH: Published by C. Ernest Walker.

Weeks, K.D.; Grimmer, A.E. 1995. The Secretary of the 

Interior's standards for the treatment of historic properties, 

with guidelines for preserving, rehabilitating, restoring & 

reconstructing historic buildings. Washington, DC: U.S. 

Department of the Interior, National Park Service, Cultural 

Resource Stewardship and Partnerships, Heritage 

Preservation Services.

Wendichansky, D.; Pumarada-O’Neill, L.F., ed. 2002. 

Rehabilitating and repairing the buildings and bridges of the 

Americas: hemispheric workshop on future directions. 

Reston, VA: American Society of Civil Engineers.

Whipple, S. 1847. A work on bridge building consisting of 

two essays, the one elementary and general, the other giving 

original plans and details for iron and wooden bridges.

Utica, NY: H. H. Curtiss, Printer.

Whipple, S. 1872. An elementary and practical treatise on 

bridge building, enlarged and improved edition of the 

author’s original work. 2nd ed. New York: D. Van 

Nostrand.

Wiedenhoeft, A.C.; Kretschmann, D.E. 2014. Species 

identification and design value estimation of wooden 

members in covered bridges. Gen. Tech. Rep. FPL–GTR–

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

Products Laboratory.

Williams, R.S.; Knaebe, M.T.; Feist, W.C. 1996. Finishes 

for exterior wood: selection, application, and maintenance.

Madison, WI: Forest Products Society.

Wright, D.W.; Caswell, Jr., W.S.; Brock, D. 2009. World 

guide to covered bridges. Concord, NH: The National 

Society for the Preservation of Covered Bridges, Inc.

Zelinka, S.L. 2013. Corrosion of fasteners in wood treated 

with newer wood preservatives. Gen. Tech. Rep. FPL–

GTR–220. U.S. Department of Agriculture, Forest Service, 

Forest Products Laboratory.

Zelinka, S.L. 2013. Guide for materials selection and design 

for metals used in contact with copper-treated wood. Gen.

Tech. Rep. FPL–GTR–227. U.S. Department of Agriculture,

Forest Service, Forest Products Laboratory.