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Publication Number: FHWA-HRT-04-098
Date: APRIL 2005

Covered Bridge Manual

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FOREWORD

The Transportation Equity Act for the 21st Century (TEA-21) as amended by the TEA-21 Restoration Act established the National Historic Covered Bridge Preservation Program (NHCBPP). This program includes preservation of covered bridges that are listed, or are eligible for listing, on the National Register of Historic Places. It includes research for better means of restoring and protecting covered bridges. It also includes technology transfer to disseminate information on covered bridges as a means of preserving our cultural heritage. The development of the Covered Bridge Manual is one of the research projects funded through NHCBPP.

The broad objectives of the NHCBPP research program are to find means and methods to restore and rehabilitate historic covered bridges to preserve our heritage using advanced technologies, and to assist in rehabilitating and restoring these bridges. The specific objectives of this research project are to provide comprehensive support to those readers involved with maintaining, assessing, strengthening, or rehabilitating any covered bridge.

The manual is intended primarily for engineers and historic bridge preservationists to provide technical and historical information on preservation of covered bridges. It will also be of interest to others involved with these bridges—including lay people, owners, and contractors.

The manual is separated into several sections with a number of chapters devoted to the specifics of each. The sections include background, description of bridge components, technical engineering issues, existing bridges, and references. The appendices include multiple case studies of existing bridge rehabilitation and construction of new authentic covered bridges.

This manual does not supersede any other. This publication is the final version of the manual.

NOTICE

This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document. This report does not constitute a standard, specification, or regulation.

The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers' names appear in this report only because they are considered essential to the objective of the document.

QUALITY ASSURANCE STATEMENT

The Federal Highway Administration (FHWA) provides high-quality information to serve Government, industry, and the public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement.

 

1. Report No

FHWA-HRT-04-098

2. Government Accession No.

N/A

3. Recipient's Catalog No.

N/A

4. Title and Subtitle

Covered Bridge Manual

5. Report Date

April 2005

6. Performing Organization Code

N/A

7. Authors(s)

Phillip C. Pierce, P.E., Robert L. Brungraber, P.E., Ph.D.,

Abba Lichtenstein, P.E., and Scott Sabol, P.E.

Chapter 19 was authored by J.J. Morrell, Department of Wood Science and Engineering, Oregon State University and S.T. Lebow, U.S. Forest Products Laboratory, Madison, WI.

8. Performing Organization Report No.

FAU-OE-CMM-04

9. Performing Organization Name and Address

Phillip C. Pierce, P.E.

6738 County Highway 14

Treadwell, NY 13846

10. Work Unit No. (TRAIS)

N/A

11. Contract or Grant No.

DTFH61-00-C-00116

12. Sponsoring Agency Name and Address

 Office of Infrastructure R&D
Federal Highway Administration
6300 Georgetown Pike
McLean VA 22101-2296

13. Type of Report and Period Covered

Final Report

October 2000– November 2004

14. Sponsoring Agency Code
15. Supplementary Notes

Contracting Officer’s Technical Representative (COTR), Sheila Rimal Duwadi, P.E., Office of Infrastructure Research and Development (R&D); John O’Fallon, P.E., Office of Infrastructure R&D

16. Abstract

This manual provides guidance to those involved with all aspects of the work, from initial inspection and evaluation, through the engineering of rehabilitation, to construction issues. Broadly speaking, this manual covers general terminology and historic development of covered bridges. The manual also addresses loads, structural analysis, connections, and design issues. The last sixchapters contain discussions of evaluation, maintenance, strengthening, and preservation of existing covered bridges; historic considerations of existing structures; and provide a state-of-the-art guide on wood preservatives for covered bridges. Historic preservation requirements as they relate to the U.S. Department of Interior standards for these important and unusual structures also are provided. The appendices include an extensive series of case studies.

The manual focuses on the nuances of the engineering aspects of covered bridges, including some issues not addressed currently by national bridge specifications. The chapter on timber connections provides a comprehensive discussion of covered bridge joinery and represents an important contribution to covered bridge engineering.

17. Key Words

Covered, Bridge, Manual, Design, Construction, Rehabilitation, Historic, Preservation

18. Distribution Statement

No restrictions. This document is available to the Public through the National Technical Information Service; Springfield, VA 22161

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

341

22. Price

N/A

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized (art. 5/94)

 

PREFACE

This manual attempts to fill in gaps in the literature about the nuances of covered bridges. It deals with quirks about them known to the team of four experienced engineers who have prepared this manual. Yet, the relatively small number of covered bridges in the United States and their geographic dispersion makes it impractical for any team to have first-hand knowledge of all aspects of all covered bridges. There are, no doubt, some issues that have not been included herein.

For readers who have attempted to document the strength of covered bridges, this manual may not contain the answers to all questions. There are several things about covered bridges that continue to defy explanation: How have they survived as long as they have, subject to the abuse of vehicles weighing substantially more than the vehicles familiar to the builders of these bridges? How does an engineer explain the discrepancy between theoretical weakness and observed performance?

Some research projects have focused specifically on various aspects of covered bridges, and research continues. Yet the relatively small number of covered bridges makes the potential return on investment in that research relatively limited, and other research awaits funding.

Having offered the above caveat, we hope this manual is interesting and useful.

 

SI* (Modern Metric) Conversion Factors


TABLE OF CONTENTS

Chapter 1. Introduction

Chapter 2. Covered Bridges: Form, Use, and Terminology

The “Typical” Covered Bridge

The Typical Setting

Component Terminology

Facts about Covered Bridges

Chapter 3. Historical Development of Covered Bridges

The Development of Truss Concepts in Europe

Early Truss Construction in the United States

The First Covered Bridge in the United States

Patents and Covered Bridges

Prevalence, Prominence, Demise, and Resurgence

Chapter 4. Types of Longitudinal Trusses

Kingpost

Queenpost

Multiple Kingpost

Burr Arch

Town Lattice

Long Truss

Howe Truss

Other

Chapter 5. Floor Systems

Floor Beams

Stringers

Decking

Running Planks

Replacement Floor Systems

Chapter 6. Ancillary Features

Roof

Rafters

Engineering Challenges Related to Rafters

Portals

Siding

Bracing

Traffic Railing

Chapter 7. Foundations

Types of Foundations

Common Conditions of Foundations

Foundation Challenges

Support Features

Chapter 8. The Engineering Challenge

Overview

Why Do They Stand?

Chapter 9. Design and Analysis Specifications

American Association of State Highway and Transportation Officials

The National Design Specifications for Wood Construction

Specifications for Minimum Loads

Other Building Specifications

Glued-Laminated Timber Specifications

Chapter 10. Issues Related to Wood

Terminology

Wood Typical to Historic Covered Bridges

Wood Availability for Repairs

Seasoned Versus Green Wood

Sawing / Sizing / Finishing

Nuances of Glulam Components

Wood Preservatives

Chemically Treated Wood

Fire Retardants

Protective Finish Treatments

Chapter 11. Loads

Dead Load (Self-Weight)

Vehicular Loads (Live Load)

Snow Load

Wind Load

Other Loads

Load Combinations and Load Duration

Chapter 12. Force and Stress Analysis Issues

Truss Analysis—Basics

Floor Analysis

Bracing Analysis

Chapter 13. Design Issues

Material Properties/Allowable Stresses

NDS Penalty for Tension Stresses in Large Members

Horizontal (or Along-the-Grain) Shear

Nuances of Loading Location As It Relates to Horizontal Shear Stresses

Chapter 14. Connections

Introduction

Truss Connections—Trusses Other Than Town Lattice

Truss Connections—Town Lattice Trusses

Arch Bearing Pads and Arch Connections to Sandwiched Truss Elements

Overhead Bracing

Floor Connections

Roofing and Siding

Chapter 15. Evaluating Existing Bridges

Field Examination and Inspection

Wood Species Identification

Timber Grading

Individual Member Assessment Through Strength Ratio Concepts

Analytical Evaluations for Vehicle Capacities (Load Rating)

Analytical Evaluation for Pedestrian Density Capacities

Load Testing

Destructive Testing of Components Removed From Bridges

Chapter 16. Repairing and Strengthening Existing Structures

Repair of Trusses

Less Authentic Repair and Strengthening of Trusses

New Methods of Strengthening Trusses

Floor System Repair and Strengthening

Summary of Recommended Actions

Example Projects

Chapter 17. Preserving Existing Covered Bridges

Roofing and Siding

Roadway Drainage

Foundation Support Details

Regular Cleaning

Fire Protection

In Situ Chemical Treatment

Painting or Treating Siding

Retightening Connectors

Chapter 18. Historic Considerations With Existing Structures

National Register of Historic Places

Historic American Engineering Record

State Historic Preservation Offices

The Advisory Council on Historic Preservation

U.S. Secretary of Interior’s Standards for Historic Preservation

Chapter 19. Initial Preservative Treatment of Wood in Covered Bridges

Introduction

Evaluating the Need for Preservative Treatment

Types of Deterioration

Preventing Fungal and Insect Attack

Methods of Preservative Treatment

Environmental Considerations for Treatment

Treatment Specifications

Methods for Improving Performance of Treated Wood

Preservative Chemicals

Preservatives That are Effective Above Ground with Partial Exposure and Occasional Wetting

Conclusions

Appendix A. Those Intriguing Town Lattice Timber Trusses

Introduction

Description of the Truss

The Problem

Alternative Approaches

The Latest Chapter

Appendix B. The Trials and Successes of Covered Bridge Engineering and Construction—Hamden Covered Bridge, Delaware County, NY

Abstract

Introduction

Engineering and Construction Challenges

How Tight Should the Counters Be?

Completed Bridge

Appendix C. Rehabilitation of Fitch’s Covered Bridge

History of the Bridge and Condition Prior to Current Project

Modifications/Rehabilitation as Part of Current Project

Original and New Construction Techniques

Problems and Solutions

Research Conducted

Costs

Final Project Photos

Appendix D. Rehabilitation and Resiting of the Brown’s River Covered Bridge, Westford, VT

Bridge Description and Project Background

Engineering and Construction Challenges

Appendix E. A Tale of Two Bridges

Speed River Covered Bridge, Guelph, Ontario, Canada

Twin Bridges, North Hartland, VT

Appendix F. Smith Covered Bridge Over the Baker River

Introduction

Bridge Dimensions and Details

Main Truss and Arch Configuration

Floor System Configuration

Superstructure Materials

Substructure Configuration

Superstructure Fabrication and Erection

Construction Costs

Project Photographs

Appendix G. Caine Road Covered Bridge, Ashtabula County, OH

Appendix H. Replacement of the Mill Covered Bridge, Tunbridge, VT

Mill Bridge at Tunbridge, VT

Appendix I. Rehabilitation of the Paper Mill Covered Bridge, Bennington, VT

Appendix J. Replacement of the Power House Covered Bridge, Johnson, VT

References

Other Resources

Nontechnical Covered Bridge Information

Technical Information—Books / Articles Relevant to Covered Bridges

Articles Related to Covered Bridges

Covered Bridge Societies


LIST OF FIGURES

Figure 1. Chiselville Bridge, Sunderland, VT

Figure 2. Eagleville Bridge, Washingtion County, NY

Figure 3. A classic historic covered bridge, the Taftville Bridge, Woodstock, VT

Figure 4. Typical covered bridge, Upper Falls Bridge, Weathersfield, VT

Figure 5. Pony truss covered bridge, Comstock Bridge, East Hampton, CT

Figure 6. Bridge diagram—general terminology

Figure 7. Diagram of queenpost truss—general terminology

Figure 8. Floor system with stringers and floor beams—Warren Bridge, VT

Figure 9. Floor system with floor beams, but without stringers—Hutchins Bridge, VT

Figure 10. Running planks—Hutchins Bridge, VT

Figure 11. An unusual upper lateral system—Seguin Bridge, VT

Figure 12. Lower lateral system—Williamsville, VT

Figure 13. Unusual ship knee braces—Village or Great Eddy Bridge, in Waitsfield, VT

Figure 14. Shelter panel covering of truss ends—Fitch’s Bridge, Delaware County, NY

Figure 15. A shelter panel separate from the trusses—Hamden Bridge, Delaware County, NY

Figure 16. Framing of the independent shelter panel—Hamden Bridge, Delaware County, NY

Figure 17. Foundations

Figure 18. Bolster beams—Worrall’s Bridge, Rockingham, VT

Figure 19. Blenheim Bridge—longest clear span in the United States

Figure 20. Rare, old double-barrel covered bridge—Pulp Mill Bridge, Middlebury, VT

Figure 21. Classic use of a natural formation as an abutment—Red Bridge, Morristown, VT

Figure 22. Date carved in an end post of the Westford Bridge, Westford, VT—may be original to the bridge

Figure 23. Salisbury Center Bridge—Herkimer County, NY

Figure 24. Brown Bridge—Shrewsbury, VT

Figure 25. Diagram of kingpost truss

Figure 26. Diagram of kingpost truss with subdiagonals

Figure 27. Diagram of queenpost truss

Figure 28. Diagram of multiple kingpost trusses

Figure 29. Shear failure of a vertical at a notch—Mill Bridge, Tunbridge, VT

Figure 30. Example of a broken tail from ice impact—South Randolph Bridge, VT

Figure 31. Bowing of bottom chord due to impact from ice floes—South Randolph Bridge, VT

Figure 32. Diagram of conventional and modified Burr arch

Figure 33. Connection of arch to post—Wehr Bridge, Lehigh County, PA

Figure 34. Diagram of Town lattice truss

Figure 35. Diagram of Long truss

Figure 36. Long truss bottom chord wedge—Downsville Bridge, Delaware County, NY

Figure 37. Wedges between counter and floor beam in a Long truss—Hamden Bridge, Delaware County, NY

Figure 38. Diagram of Howe truss

Figure 39. Diagram of Paddleford truss

Figure 40. Transverse floor beams and longitudinal decking—Fitch's Bridge, Delawary County, NY

Figure 41. A floor with stringers, floor beams, and transverse decking—Comstock Bridge, East Hampton, CT

Figure 42. Mortise-and-tenon connection in floor beam—Downsville Bridge, Delaware County, NY

Figure 43. End notches of floor beams used in a Town lattice truss—West Dummerston Bridge, VT

Figure 44. Timber dowel reinforcement of post—Mill Bridge, Tunbridge, VT

Figure 45. Installation of distribution beams—Union Village Bridge, VT

Figure 46. Typical transverse plank decking with running planks—Salisbury Center Bridge, Herkimer County, NY

Figure 47. Nail-laminated decking being removed—Fitch's Bridge, Delaware County, NY

Figure 48. Glue-laminated floor beams and decking system under construction—Hamden Bridge, Delaware County, NY

Figure 49. Running plank installation—Taftsville Bridge, VT

Figure 50. Independent floor system—Chiselville Bridge, VT. Note that the pier cap does not support the timber truss, which must still support the weight of the covering and snow

Figure 51. Classic gable roof—Forksville Bridge in Sullivan County, PA

Figure 52. A flat roof bridge—Hogback Bridge, Madison County, IA

Figure 53. Example of rafter ties—Northfield Falls Bridge, VT

Figure 54. Unusual detailing of a portal—Upper Falls Bridge, Weathersfield, VT

Figure 55. Example of portal extension—Wehr Bridge, Lehigh County, PA

Figure 56. Lateral brace connection at tie beam—Fitch's Bridge, Delaware County, NY

Figure 57. Traditional knee brace—Salisbury Center Bridge, Herkimer County, NY

Figure 58. Alternative and stronger style of knee brace—Hamden Bridge, Delaware County, NY

Figure 59. Another alternative knee brace—Hopkins Bridge, Enosburgh, VT

Figure 60. Check brace at bottom chord—Brown's River Bridge, Westford , VT

Figure 61. Check brace at top chord—Quinlan Bridge, Charlotte, VT.

Figure 62. Chin brace—Elder's Mill Bridge, Watkinsville, GA

Figure 63. Interior curbing—West Dummerston Bridge, VT

Figure 64. Squeeze timber approach railing—Hamden Bridge, Delaware County, NY

Figure 65. Approach railing and bridge curb—Paper Mill Bridge, Bennington, VT

Figure 66. Transition from approach railing to inside curb—Mill Bridge, Tunbridge, VT

Figure 67. Original stone high abutment in good condition—Upper Falls Bridge, Weathersfield, VT

Figure 68. Supplemental pile bents—Hamden Bridge, Delaware County, NY

Figure 69. Abutment stem and wing wall identification

Figure 70. Badly cracked and shifted stones in the bottom of an abutment stem—Halpin Bridge, Middlebury, VT, before its replacement

Figure 71. Parged stone abutment stem

Figure 72. Damage to stone wall caused by tree roots

Figure 73. Modified original stone abutment—Fitch’s Bridge, East Delhi, Delaware County, NY

Figure 74. Bearing block installation—Brown’s River Bridge, Westford, Vermont

Figure 75. Bolster beam—Fuller Bridge, Montgomery, VT

Figure 76. Hold-down anchor—Paper Mill Bridge, Bennington, VT

Figure 77. Thetford Center Bridge, exterior view, Thetford, VT

Figure 78. Thetford Center Bridge, interior view, Thetford, VT

Figure 79. Surface evidence of powder post beetles

Figure 80. A covered bridge with painted siding—Wehr Bridge, Lehigh County, PA

Figure 81. Example of bridge without protective treatment (the Westford Bridge in Vermont before its recent rehabilitation)

Figure 82. H20 design truck vehicle (after AASHTO standard specifications)

Figure 83. HS20 design truck vehicle (after AASHTO standard specifications)

Figure 84. Lane load configuration (after AASHTO standard specifications)

Figure 85. Snow load on covered bridges can cause failure—Power House Bridge, Johnson, VT

Figure 86. A covered bridge destroyed by wind—Bedell Bridge between Haverhill, NH, and Newbury, VT, 1979

Figure 87. Another example of collapse by wind—Smith Bridge at Brownsville, VT

Figure 88. The Smith Bridge before collapse.

Figure 89. Typical eccentric joint in a timber truss (Brown’s River Bridge, Westford, VT, before its recent rehabilitation)

Figure 90. Three-dimensional image of computer simulation—unloaded. Note the short transverse elements at the truss lattice intersections that depict the trunnel connectors

Figure 91. Three-dimensional image of computer simulation—distorted from load

Figure 92. A heavy timber Burr arch—Wehr Bridge, Lehigh County, PA. 124

Figure 93. A lightweight arch attached only to the inside of the truss—Salisbury Center Bridge, Herkimer County, NY

Figure 94. Large bolster beam supported on bearing blocks—Hall Bridge, Rockingham, VT

Figure 95. A bolster formed of concrete—Village or Great Eddy Bridge, Waitsfield, VT

Figure 96. Distribution beam system—Worral’s Bridge, Rockingham, VT

Figure 97. No X lateral system and no knee braces—Comstock Bridge, East Hampton, CT

Figure 98. An extremely strong upper lateral and knee brace system—Hamden Bridge, Delaware County, NY

Figure 99. American Chestnut (allowable stresses are not in the NDS)—Comstock Bridge, East Hampton, CT

Figure 100. Example of tensile failure of a bottom chord element in a World War II timber building

Figure 101. Example of a tensile failure of a bottom chord in a covered bridge

Figure 102. A horizontal shear failure—Mill Bridge, Tunbridge, VT, before its recent collapse due to ice floe impact

Figure 103. Simple lap joint, with through-plane connectors

Figure 104. Simple lap joint, with in-plane connectors

Figure 105. Grain orientation of rectangular in-plane connector

Figure 106. Lap joint with bypassing leaves and end grain bearing surfaces

Figure 107. Lap joint with tapered halves and connectors

Figure 108. Bolt-of-lightning joint

Figure 109. Eccentricity in member layout and prying at connectors

Figure 110. Eye-and-wedge clamping bolt, hand-forged

Figure 111. A double-leaf lap joint, with through connectors

Figure 112. Butt joint with steel fish plates

Figure 113. Butt joint with fish plates—wooden plates

Figure 114. Butt joint with bars and rods splice

Figure 115. Simple lap joint for compression members

Figure 116. Simple bearing joint at angled notch

Figure 117. Top and bottom chord connections to vertical

Figure 118. Truss vertical at bearing seat with critical shear face

Figure 119. A sistered diagonal

Figure 120. Example of pegs added to increase the shear capacity

Figure 121. Long truss with counter timbers

Figure 122. Town lattice trusses with identical versus mirrored web members

Figure 123. Town lattice truss connections need to be inspected carefully

Figure 124. Sistered lattice web at chord connection

Figure 125. Built-up end post for a Town lattice terminated vertically—Paper Mill Bridge, Bennington, VT

Figure 126. A solid-sawn end post—Fuller Bridge, Montgomery, VT

Figure 127. An inclined end treatment—Bartonsville Bridge, Rockingham, VT

Figure 128. The corresponding interior end post—Bartonsville Bridge, Rockingham, VT

Figure 129. Intermediate posts—Worrall’s Bridge, Rockingham, VT

Figure 130. Bearing blocks beneath the bottom chord where tails have been removed—Paper Mill Bridge, Bennington, VT

Figure 131. Load sharing between arch and superimposed truss

Figure 132. Tie beam to top chord connection

Figure 133. A set of upper braces

Figure 134. Added verticals at tie beams in Town lattice truss

Figure 135. Tie beam to top chord connection details, first diagram

Figure 136. Tie beam to top chord connection details, second diagram

Figure 137. Tie beam to top chord connection details of failed joint

Figure 138. Siding nailers spaced away from truss elements

Figure 139. Fitch’s Bridge, Delaware County, NY

Figure 140. Brown’s River Bridge, Westford, VT

Figure 141. Racked two-span continuous bridge—West Dummerston, VT, before its recent rehabilitation

Figure 142. Example of sag—Station Bridge, Cambridge, VT, before rehabilitation

Figure 143. A chord butt joint in trouble—Station Bridge in Northfield, VT

Figure 144. Field tests for deflection measurement comparison against computer prediction—Brown Bridge, Shrewsbury, VT

Figure 145. Dial gauges used to measure the load response

Figure 146. Three-element test specimen

Figure 147. Tensile tests on lattice elements

Figure 148. Replacement versus sister elements in Long truss—Downsville Bridge, Delaware County, NY, during its major rehabilitation in 1999

Figure 149. Sister elements in Town lattice—Silk Road Bridge, Bennington, VT

Figure 150. Sistered post repair in Long truss—Downsville Bridge, Delaware County, NY, during its major rehabilitation in 1999

Figure 151. Partial post replacement in multiple kingpost truss—Union Village Bridge, VT

Figure 152. Extra chord elements added to an existing Town lattice truss—Cedar Swamp Bridge, between Salisbury and Cornwall, VT

Figure 153. Metal elements added to an existing bottom chord—Lincoln Bridge, Woodstock, VT

Figure 154. An arch element added to a Town lattice structure—Scott Bridge, Townshend, VT

Figure 155. A single bolt connects these single-piece arch segments to the truss post—Wehr Bridge, Lehigh County, PA

Figure 156. Steel plates at heel connection of queenpost truss—Power House Bridge, Johnson, VT, before its collapse in 2001

Figure 157. Spliced lattice tail replacements—River Road Bridge, Troy, VT

Figure 158. Use of steel bolts in chord replacement—Kingsley Bridge, Shrewsbury, VT

Figure 159. Installation of metalwork beneath a timber floor beam—Wehr Bridge, Lehigh County, PA

Figure 160. Strengthening by using glue-laminated deck panels and floor beams—Hamden Bridge, Delaware County, NY

Figure 161. Good siding details—Fitch’s Bridge, Delaware County, NY

Figure 162. Good siding details around a window—Fitch’s Bridge

Figure 163. Unfortunate approach grading directing drainage into a covered bridge—West Hill Bridge, Montgomery, VT

Figure 164. Consequences of the poor entrance drainage in figure 163

Figure 165. A trench drain at a bridge entrance—Fitch’s Bridge, Delaware County, NY

Figure 166. Sacrificial timbers beneath truss elements

Figure 167. Accumulation of debris and asphalt encasing timber elements

Figure 168. Bridge at the Green, Arlington, VT

Figure 169. Comstock Bridge, East Hampton, CT

Figure 170. Museum at Shushan Bridge—Washington County, NY

Figure 171. Brown Bridge,Shrewsbury, VT

Figure 172. Lower Cox Bridge, Northfield, VT

Figure 173. Side view of a typical covered bridge supported by Town lattice timber trusses. This is the Fuller Bridge, Montgomery, VT, during its recent reconstruction

Figure 174. Depicts a portion of the bottom chords (upper and lower portions) of a Town lattice truss. The sections demonstrate the termination of one of the four chord elements at a given location along the truss

Figure 175. Depicts the “rat’s nest” of wiring accompanying the installation of the 46 transducers. The equipment is shown mounted on top of the lower bottom chord element. The upper bottom chord element is in the foreground. The edge of the nail-laminated deck is shown along the right edge (a curb timber had been removed)

Figure 176. The temporary platform suspended beneath the bottom chords to enable access for strain gage installation. The siding was removed to facilitate the testing, but the bridge was scheduled to undergo a complete rehabilitation after the testing, which required siding removal for other purposes

Figure 177. View of the Hamden Covered Bridge prior to work in 2000. Note the mid-span timber bent added in the 1970s to help support the sagging trusses. Also note theexternal top chord braces

Figure 178. How NOT to relocate a covered bridge for repair work.

Figure 179. Successful relocation of the bridge using rollers on a false work system. The siding and roofing at the ends awaits final bearing installation

Figure 180. A classic “Bolt-of-Lightning” splice. It relies on end bearing and shear to transfer the forces. The cuts include vertical mitering to allow the bearing blocks to settle into a tight fit, without falling out, should the joint tend to stretch

Figure 181. One-hundred-thirty-foot- (39.6 meter) long one-piece bottom chords—monsters to fabricate and deliver and a handful to work with

Figure 182. Internal bracing. It is certainly hard to pick out, but the major light-colored diagonal member is the Knee Brace. It is bolted to a lighter color Tie Beam and behind it with a Principal Rafter. The darker members are the original upper lateral system

Figure 183. Diagonal to post connections at the top chord indicating significant overstress

Figure 184. Wedges used in the connection between vertical post and chord—you have to look closely on the left side of the right post. In the writer’s opinion, their value is principally to distribute the high side-grain bearing stresses in the post to a larger area

Figure 185. Typical “folding wedges” of a coun1 transverse member in the middle of the photo is the Floor Beam. The counter on the left bears against the wedges, then the Floor Beam against the side of the post

Figure 186. Near final exterior view. Guide rail not yet installed and final seeding and mulch also not yet in place

Figure 187. Final inside view. Note the lighter-colored replacement members

Figure 188. Start of work

Figure 189. Deterioration at end

Figure 190. Older “hung” floor

Figure 191. Partially removed 1976 floor

Figure 192. South Abutment prior to work

Figure 193. North Abutment prior to work

Figure 194. Longer trusses and new recess in existing abutment

Figure 195. Reworked internal bracing system

Figure 196. Partially installed deck

Figure 197. Installing cedar shakes

Figure 198. Siding details during installation

Figure 199. Approach railing configuration at north entrance

Figure 200. Elevation view of completed bridge

Figure 201. View of south entrance and railing configuration

Figure 202. Internal view of completed bridge

Figure 203. As the bridge sat for 13 years

Figure 204. Replacement top chords

Figure 205. Hidden rot inside top of old post

Figure 206. Bridge moved, but not down off temporary cribs

Figure 207. View of shear dowel reinforcement

Figure 208. Reconstructed east abutment

Figure 209. Underway on dollies

Figure 210. On top of the bypass bridge—ready to be slid sideways

Figure 211. Sliding sideways

Figure 212. Bridge perspective view

Figure 213. Typical bridge section

Figure 214. Raising the trusses

Figure 215. Hartland Bridge on the move

Figure 216. Observers watching a bridge move past

Figure 217. Bridge in place awaiting final lowering

Figure 218. Overall view of bridge showing overhanging portal, metal roof, outboard sidewalk, main truss, and arch visible through open side, and granite-faced abutment

Figure 219. Main truss and arch during construction showing shouldered and wedged timber connections

Figure 220. Underside of bridge showing floor beams, metal deck clips, bottom lateral bracing, arch springing area, and steel fabrication at the location where the arch passes through the bottom chord

Figure 221. Final bridge

Figure 222. Existing site with poor alignment

Figure 223. New abutment construction on new alignment

Figure 224. New stub abutment stem

Figure 225. Truss assembly

Figure 226. Three chord elements

Figure 227. Adding siding before truss lifting and installation

Figure 228. North truss in position and secured

Figure 229. Both trusses erected

Figure 230. Installation of roof trusses and purlins

Figure 231. Installation of wood shingles

Figure 232. Completed bridge

Figure 233. Original view

Figure 234. Destroyed March 4, 1999

Figure 235. View in December, 2002

Figure 236. A view of the original bridge

Figure 237. Wooden peg reinforcement of post corbel

Figure 238. Reinforcement rod

Figure 239. Closeup of the failed corbel after the repair

Figure 240. Bar-and-rod connector tensile splice

Figure 241. Transition from approach rail to inside curb

Figure 242. Bridge being moved into position

Figure 243. Oxen power and capstan

Figure 244. Completed bridge

Figure 245. New pad at East Abutment—truss erection underway

Figure 246. Entrance of completed bridge

Figure 247. The bridge before work

Figure 248. Modified knee braces

Figure 249. Cranes and truck positioning the trusses

Figure 250. Bridge following reopening

Figure 251. Opening day parade

Figure 252. Completed replacement truss

Figure 253. Bridge before collapse

Figure 254. Bridge after collapse from snow

Figure 255. Double bottom chord arrangement with scarf joint

Figure 256. Connection between deck and bottom chord

Figure 257. Heel connection details

Figure 258. Connection of tie beam, truss vertical, and rafter plate

Figure 259. Final product

Figure 260. Power house replica, Johnson, VT

Figure 261. Old Stone Fort Bridge, Schoharie County, NY


LIST OF TABLES

Table 1. Locations and dates of covered bridges by State

Table 2. Truss configurations—numbers and lengths

 

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