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Timber Bridges Design, Construction, Inspection, and Maintenance

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Timber Bridges Design, Construction, Inspection, and Maintenance Timber Bridges Design, Construction, Inspection, and Maintenance Michael A. Ritter, Structural Engineer United States Department of Agriculture Forest Service Ritter, Michael A. 1990. Timber Bridges: Design, Construction, Inspection, and Maintenance. Washington, DC: 944 p. ii ACKNOWLEDGMENTS The author acknowledges the following individuals, Agencies, and Associations for the substantial contributions they made to this publication: For contributions to Chapter 1, Fong Ou, Ph.D., Civil Engineer, USDA Forest Service, Engineering Staff, Washington Office. For contributions to Chapter 3, Jerry Winandy, Research Forest Products Technologist, USDA Forest Service, Forest Products Laboratory. For contributions to Chapter 8, Terry Wipf, P.E., Ph.D., Associate Professor of Structural Engineering, Iowa State University, Ames, Iowa. For administrative overview and support, Clyde Weller, Civil Engineer, USDA Forest Service, Engineering Staff, Washington Office. For consultation and assistance during preparation and review, USDA Forest Service Bridge Engineers, Steve Bunnell, Frank Muchmore, Sakee Poulakidas, Ron Schmidt, Merv Eriksson, and David Summy; Russ Moody and Alan Freas (retired) of the USDA Forest Service, Forest Products Laboratory; Dave Pollock of the National Forest Products Association; and Lorraine Krahn and James Wacker, former students at the University of Wisconsin at Madison. In addition, special thanks to Mary Jane Baggett and Jim Anderson for editorial consultation, JoAnn Benisch for graphics preparation and layout, and Stephen Schmieding and James Vargo for photographic support. iii iv CONTENTS CHAPTER 1 TIMBER AS A BRIDGE MATERIAL 1.1 Introduction .............................................................................. l- 1 1.2 Historical Development of Timber Bridges ............................. l-2 Prehistory Through the Middle Ages ....................................... l-3 Middle Ages Through the 18th Century ................................... l-5 19th Century ............................................................................. l-7 20th Century ............................................................................. l- 17 1.3 The Future of Timber as a Bridge Material .............................. l- 17 1.4 Selected References .................................................................. l-20 CHAPTER 2 TYPES OF TIMBER BRIDGES 2.1 Introduction ............................................................................ 2- 1 2.2 Beam Superstructures ............................................................... 2-2 Log Beams ................................................................................ 2-2 Sawn Lumber Beams ................................................................ 2-3 Glued-Laminated Timber Beams ............................................. 2- 5 Laminated Veneer Lumber Beams ........................................... 2- 6 2.3 Longitudinal Deck Superstructures .......................................... 2- 8 2.4 Trusses...................................................................................... 2- 10 2.5 Trestles ...................................................................................... 2- 12 2.6 Glulam Deck Arches ................................................................ 2- 13 2.7 Suspension Bridges ................................................................... 2- 15 2.8 Decks ........................................................................................ 2- 17 Sawn Lumber Planks ................................................................ 2- 17 Nail-Laminated Lumber ........................................................... 2- 18 Glued-Laminated Timber ......................................................... 2- 20 Composite Timber-Concrete .................................................... 2- 20 2.9 Stress-Laminated Timber ......................................................... 2- 22 2.10 Timber Substructures ............................................................... 2- 25 Abutments ............................................................................... 2- 25 Bents ........................................................................................ 2-25 2.11 Selected References ................................................................. 2- 28 v CHAPTER 3 PROPERTIES OF WOOD AND STRUCTURAL WOOD PRODUCTS 3.1 Introduction .............................................................................. 3- 1 3.2 Structure of Wood .................................................................... 3-1 Microstructure . .. 3-2 Macrostructure .......................................................................... 3-2 3.3 Physical Properties of Wood .................................................... 3-5 Directional Properties ............................................................... 3-6 Moisture Content ...................................................................... 3-6 Dimensional Stability ............................................................... 3-8 Density ...................................................................................... 3-11 Specific Gravity ........................................................................ 3-12 Thermal Expansion ................................................................... 3-12 Coefficients of Friction ............................................................. 3-13 Electrical Conductivity ............................................................. 3-13 Pyrolytic Properties .................................................................. 3-14 Natural Durability ..................................................................... 3-15 Chemical Resistance ................................................................. 3-16 3.4 Mechanical Properties .............................................................. 3-17 Elastic Properties ...................................................................... 3-17 Modulus of Elasticity.......................................................... 3-18 Shear Modulus .................................................................... 3-18 Poisson’s Ratio ................................................................... 3-18 Strength Properties . 3-18 Compression . 3-19 Tension . 3-20 Shear . 3-21 Bending . 3-22 Torsion . 3-22 Shock Resistance . 3-23 3.5 Factors Affecting the Strength and Other Properties of Wood . .. 3-23 Anatomical Factors . 3-23 Specific Gravity .................................................................. 3-23 Slope of Grain ..................................................................... 3-24 Knots ................................................................................... 3-24 Abnormal Wood ................................................................. 3-25 vi Compression Failures ........................................................ 3-27 Shake and Pitch Pockets ..................................................... 3-28 Environmental Factors .............................................................. 3-29 Moisture Content ................................................................ 3-29 Temperature ........................................................................ 3-30 Decay and Insect Damage .................................................. 3-30 Ultraviolet Degradation ...................................................... 3-31 Service-Factors ......................................................................... 3-31 Duration of Load ................................................................ 3-31 Creep ................................................................................... 3-32 Fatigue ................................................................................ 3-32 Treatment Factors ............................................................... 3-33 3.6 Properties of Sawn Lumber ...................................................... 3-33 Product Standards ..................................................................... 3-34 Lumber Manufacture ................................................................ 3-35 Lumber Species .................................................................. 3-35 Lumber Sizes ...................................................................... 3-36 Lumber Grading ....................................................................... 3-41 Visual Stress Grading ......................................................... 3-42 Mechanical Stress Grading ................................................. 3-46 Quality Control and Marking ................................................... 3-47 Material Specifications ............................................................. 3-48 3.7 Properties of Glued-Laminated Timber (Glulam) ................... .3-49 Product Standards ..................................................................... 3-50 Manufacturing Criteria for Glulam ........................................... 3-51 Standard Glulam Sizes ........................................................ 3-52 Adhesives ............................................................................ 3-53 Joints . .. 3-54 Appearance . 3-55 Grades of Glulam . 3-55 Bending Combinations . 3-56 Axial Combinations . 3-57 Quality Control and Marking . 3-57 Material Specifications . ....... 3-58 3.8 Selected References . 3-60 vii CHAPTER 4 PRESERVATION AND PROTECTION OF TIMBER BRIDGES 4.1 Introduction .............................................................................
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