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Guidelines to Restoring Structural Integrity of Covered Bridges

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Guidelines to Restoring Structural Integrity of Covered Bridges United States Department of Agriculture Guidelines to Restoring Structural Integrity of Covered Bridge Members Ronald W. Anthony Forest Service In cooperation with the Forest Products Laboratory United States Department of Transportation General Technical Report Federal Highway Administration FPL–GTR–252 January 2018 A FHWA-NHCBP (National Historic Covered Bridge Preservation) publication Abstract These guidelines are designed for decision makers This study is part of the Research, Technology, and (selectmen, county commissioners, city planners, Education portion of the National Historic Covered preservation officers, etc.) that have responsibility for Bridge Preservation (NHCBP) Program administered by repairing and maintaining existing covered bridges to help the Federal Highway Administration. The NHCBP them understand what goes into making effective decisions program includes preservation, rehabilitation, and about how, and when, to repair a covered bridge. The restoration of covered bridges that are listed or are purpose of these guidelines is to present the steps necessary eligible for listing on the National Register of Historic for decision makers to identify effective rehabilitation Places; research for better means of restoring and techniques for restoring the structural integrity of covered protecting these bridges; development of educational bridge members. The intent is to retain the maximum aids; and technology transfer to disseminate information amount of historic fabric while ensuring public safety and on covered bridges in order to preserve the Nation's minimizing future maintenance requirements. To make cultural heritage. 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 This study is conducted under a joint agreement informed of what to consider when conducting an between the Federal Highway Administration–Turner engineering analysis, and (4) be aware of various repair Fairbank Highway Research Center and the Forest options that can meet the long-term goals for the bridge. Service–Forest Products Laboratory. Only after the decision maker knows the type of bridge, its Federal Highway Administration Program Manager— condition, and to what loads it is subjected can a repair Sheila Rimal Duwadi, P.E. strategy be developed. How to support the bridge during the repair phase, what repairs are appropriate, and how to Forest Products Laboratory Program Manager– maintain the bridge after repairs are implemented are all Michael A. Ritter, P.E. critical to ensuring a long service life for the covered bridge. Keywords: covered bridges, historic, guidelines, repair, restore, structural integrity English unit Conversion SI unit factor January 2018 inch (in.) 25.4 millimeter (mm) Anthony, Ronald W. 2018. Guidelines to restoring structural integrity of foot (ft) 0.3048 meter (m) covered bridge members. FPL–GTR–252. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 56 p. pound (lb), mass 0.45359 kilogram (kg) A limited number of free copies of this publication are available to the ton 0.90718 tonne (t) (×103 kg) 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. 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: [email protected]. USDA is an equal opportunity provider, employer, and lender. Guidelines to Restoring Structural Integrity of Covered Bridge Members Ronald W. Anthony, President and Wood Scientist Anthony & Associates, Inc., Fort Collins, Colorado Contents Executive Summary ............................................................ 2 Chapter 5: Repairs ............................................................ 34 Rehabilitating the Bridge Using Traditional Timber Acknowledgments .............................................................. 4 Repairs ................................................................... 34 Strength Enhancement .................................................. 38 Chaper 1: Introduction ........................................................ 5 Visual Grading to Determine Strength of Purpose of the Guidelines ............................................... 5 Bridge Members .................................................... 38 Need for Repairs to Covered Bridges .............................. 5 Increasing Load Capacity by Augmenting or Truss Types ..................................................................... 5 Reinforcing Existing Members .............................. 40 The Secretary of the Interior’s Standards for the Increasing Load Capacity Using Laminated Timber .... 41 Treatment of Historic Properties .............................. 6 Increasing Load Capacity by Reinforcing with Fiber- Where do You Start? ....................................................... 8 Reinforced Polymers ............................................. 42 References ....................................................................... 8 Increasing Load Capacity through Post-Tensioning ..... 43 Increasing Clearance ..................................................... 44 Chapter 2: Condition Assessment ....................................... 9 Reducing Risk caused by Overload .............................. 44 Mechanisms of Deterioration .......................................... 9 Increasing Durability .................................................... 45 What to Look For .......................................................... 11 References .................................................................... 46 Where to Look............................................................... 12 What Tools to Use ......................................................... 12 Chapter 6: Maintenance.................................................... 47 Challenges when Assessing Condition .......................... 16 Cleaning ........................................................................ 47 References ..................................................................... 18 Vegetation and Moisture Management ......................... 49 Siding and Roofing ....................................................... 49 Chapter 3: Engineering Analysis ...................................... 21 Fire Protection .............................................................. 51 Construction Details ...................................................... 22 Remedial Treatments to Retard Decay and Timber Joinery and Connections ................................... 23 Insect Attack .......................................................... 52 Considerations for Engineering Analysis of Timber Connections and Fasteners ............................................ 53 Joinery and Connections ........................................ 24 References .................................................................... 54 Considerations for Engineering Analysis of a Burr Arch ............................................................... 25 Chapter 7: Bibliography ................................................... 55 Chapter 4: Supporting the Bridge for Repairs .................. 28 Cribbing, Shoring, Staging, and Rigging ...................... 28 Other Considerations ..................................................... 31 References ....................................................................
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