Covered Bridge Manual PUBLICATION NO. FHWA-HRT-04-098 APRIL 2005 Research, Development, and Technology Turner-Fairbank Highway Research Center 6300 Georgetown Pike McLean, VA 22101-2296 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. Technical Report Documentation Page 1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. FHWA-HRT-04-098 4. Title and Subtitle 5. Report Date April 2005 Covered Bridge Manual 6. Performing Organization Code 7. Author(s) 8. Performing Organization Report No. 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. 9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) Phillip C. Pierce, P.E. 6738 County Highway 14 11. Contract or Grant No. Treadwell, NY 13846 DTFH61-00-C-00116 12. Sponsoring Agency Name and Address 13. Type of Report and Period Covered Turner-Fairbank Highway Research Center (HR D1-07) Final Report Federal Highway Administration October 2000– November 2004 6300 Georgetown Pike 14. Sponsoring Agency Code McLean, VA 22101-2296 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 six chapters 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 18. Distribution Statement Covered, Bridge, Manual, Design, Construction, No restrictions. This document is available to the Rehabilitation, Historic, Preservation public through the National Technical Information Service (NTIS), Springfield, VA 22161 19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of 22. Price Unclassified Unclassified Pages 341 Form DOT F 1700.7 (8-72) Reproduction of completed page authorized 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. ii SI* (MODERN METRIC) CONVERSION FACTORS APPROXIMATE CONVERSIONS TO SI UNITS Symbol When You Know Multiply By To Find Symbol LENGTH in inches 25.4 millimeters mm ft feet 0.305 meters m yd yards 0.914 meters m mi miles 1.61 kilometers km AREA in2 square inches 645.2 square millimeters mm2 ft2 square feet 0.093 square meters m2 yd2 square yard 0.836 square meters m2 ac acres 0.405 hectares ha mi2 square miles 2.59 square kilometers km2 VOLUME fl oz fluid ounces 29.57 milliliters mL gal gallons 3.785 liters L ft3 cubic feet 0.028 cubic meters m3 yd3 cubic yards 0.765 cubic meters m3 NOTE: volumes greater than 1000 L shall be shown in m3 MASS oz ounces 28.35 grams g lb pounds 0.454 kilograms kg T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t") TEMPERATURE (exact degrees) oF Fahrenheit 5 (F-32)/9 Celsius oC or (F-32)/1.8 ILLUMINATION fc foot-candles 10.76 lux lx fl foot-Lamberts 3.426 candela/m2 cd/m2 FORCE and PRESSURE or STRESS lbf poundforce 4.45 newtons N lbf/in2 poundforce per square inch 6.89 kilopascals kPa APPROXIMATE CONVERSIONS FROM SI UNITS Symbol When You Know Multiply By To Find Symbol LENGTH mm millimeters 0.039 inches in m meters 3.28 feet ft m meters 1.09 yards yd km kilometers 0.621 miles mi AREA mm2 square millimeters 0.0016 square inches in2 m2 square meters 10.764 square feet ft2 m2 square meters 1.195 square yards yd2 ha hectares 2.47 acres ac km2 square kilometers 0.386 square miles mi2 VOLUME mL milliliters 0.034 fluid ounces fl oz L liters 0.264 gallons gal m3 cubic meters 35.314 cubic feet ft3 m3 cubic meters 1.307 cubic yards yd3 MASS g grams 0.035 ounces oz kg kilograms 2.202 pounds lb Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T TEMPERATURE (exact degrees) oC Celsius 1.8C+32 Fahrenheit oF ILLUMINATION lx lux 0.0929 foot-candles fc cd/m2 candela/m2 0.2919 foot-Lamberts fl FORCE and PRESSURE or STRESS N newtons 0.225 poundforce lbf kPa kilopascals 0.145 poundforce per square inch lbf/in2 *SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380. (Revised March 2003) iii TABLE OF CONTENTS Chapter 1. Introduction ................................................................................................................................3 Chapter 2. Covered Bridges: Form, Use, and Terminology.........................................................................7