Assoc iateProfessor, University of Colorado,Denver ProfessorEmeritus, Universityof Colorado,Denver KLUWERACADEMICPUBLISHERS BOSTONIDORDRECHTILONDON Distributors for North, Central and South America: Kluwer Academic Publishers 10 1 Philip Dri ve Assinippi Park Norwell, Massachusetts 02061USA Telephone (781) 871-6600 Fax (781) 871-6528 E-Mail Distributors for all other countries: Kluwer Academic Publishers Group Distribution Centre Post Office Box 322 3300 AH Dordrecht, THE NETHERLANDS Telephone 3178 6392 392 Fax 31786546474 E-Mail services@wkap.nl> ~ "Electronic Services Library of Congress Cataloging-in-Publication Stalnaker, Judith J. Structural design in wood IJudith J.Stalnaker, Ernest C.Harris. -- 2nd ed. p.cm. Includes bibliographical references and index. ISBN 0-412-10631-0 1.Building, Wooden.2.Structural design.I. Harris, Ernest C. II.Title. IN PROCESS 624.1'84--dc 20 British Library Cataloguing in Publication Data available Cover design:Curtis Tow Graphics Copyright 1997 by Chapman &Hall Third printing by Kluwer Academic Publishers1999 96-15280 CIP Allrightsreserved.Nopart of thispublicationmaybe reproduced,storedin a retrieval system or transmitted in any form or by any means,mechanical, photo-copying, recording, orotherwise,withoutthepriorwrittenpermissionof thepublisher,KluwerAcademic Publishers,101Philip Drive, Assinippi Park, Norwell, Massachusetts 02061 Printed on acid-free paper. Contents Preface/xi 1.Introduction/1 1-1.Evolution of Timber Design/1 1-2.Material Properties/2 1-3.Typesof Construction/2 1-4.Hybrid Construction/8 1-5.Timber Bridges/8 1-6.Notes toStudents/9 References/10 2.Wood Structure and Properties/11 2-1.Wood asa Structural Material11 2-2.Problems in Use of Wood for Structures/11 2-3.Advantages of Wood asa Structural Material/11 2-4.Classification of Wood/12 2-5.Wood Structure/14 2-6.Juvenile Wood/17 2-7.Wood Axes/17 2-8.Factors Affecting Strength/17 2-9.Moisture Content/18 Example 2-1/20 Example 2-2/20 2-10.Specific Gravity/20 2-11.Time-Dependent Behavior of Wood/21 2-12.Strength-Reducing Characteristics/22 2-13.Thermal Properties of Wood/26 2-14.Testsand Properties of Interest tothe Structural Designer/27 References/30 Problems/30 3.Production andGrading of SawnLumber/32 3-1.Lumber Production/32 3-2.Standard Sizes of Lumber/32 3-3.Finish Designations/34 3-4.Cutting Patterns/34 3-5.Drying/36 3-6.Lumber Grading/37 3-7.Types of Grading/37 3-8.Definitions/38 3-9.Modem Grading Rules/39 3-10.Example of Visual Grading of Beams and Stringers/40 3-11.Grading Stamps/41 3-12.Caution toDesigner and Builder/41 3-13.Board Measure/42 Example 3-1/42 References/43 Problem/43 4.Loads andDesign Values/44 Part I. Loads/44 4-1.General/44 4-2.Dead Loads/45 Example 4-1/45 Example 4-2/46 Example 4-3/47 4-3.Vertical Live Loads/48 Example 4-4/49 Example 4-5/51 Example 4-6/51 Example 4-7/51 Example 4-8/51 4-4.Wind Loads51 4-5.Stability Under Wind Loads/53 v viCONTENTS 4-6.Load Combinations/54 Example 4-9/54 Example 4-10/55 4-7.Seismic Loads/56 Example 4-11/59 Part II.Design Values/59 4-8.Design Values/59 4-9.Base Design Values/59 4-10.Adjustment Factors/60 4-11.Tables for Base Design Values/62 Example 4-12/63 Example 4-13/63 Example 4-1464 Example 4-1564 Example 4-1664 Example 4-1765 Example 4-1865 4-12.Load and Resistance Factor Design/67 References/69 Problems/69 5.Connections-Nails, Screws, andBolts/72 5-1.Connection Design/72 5-2.General Principles/72 5-3.Nailsand Spikes/75 Example 5-1/77 Example 5-278 Example 5-378 Example 5-479 Example 5-579 Example 5-679 Example 5-780 5-4.Staples/80 5-5.Lag Screws/80 Example 5-8/83 Example 5-9/85 Example 5-10/85 5-6.Wood Screws/86 5-7.Bolted Connections/87 Example 5-11/89 Example 5-1290 Example 5-1390 Example 5-1490 Example 5 -1591 Example 5-1691 5-8.Connection Design by LRFD/92 Example 5-17/93 Example 5-18/94 Example 5-1994 Example 5-2094 Example 5-2195 Example 5-22/95 References/95 Problems/95 6.Selecting Sawn-Timber Beams100 6-1.Introduction/100 6-2.Design for Flexure-Laterally Supported Beams/100 Example 6-1/103 Example 6-2103 Example 6-3104 Example 6-4104 Example 6-5104 6-3.Design for Flexure-Laterally Unsupported Beams/105 Example 6-6/107 Example 6-7/108 6-4.Design of Beams for Shear/108 Example 6-8/111 Example 6-9/112 Example 6-10/112 Example 6-11/112 Example 6-12/114 6-5.Deflection/114 Example 6-13116 Example 6-14116 Example 6-15117 Example 6-16117 6-6.Design for Bearing Perpendicular toGrain/118 Example 6-17/119 Example 6-18/120 Example 6-19/120 6-7.Floor System Design/120 Example 6- 20/120 6-8.Beam Design by LRFD/124 Example 6-21/124 Example 6-22125 Example 6-23125 Example 6-24125 Example 6-25/126 References/126 Problems/127 7.Selecting Sawn-Timber Compression and TensionMembers/131 7-1. 7-2. Wood Columns Column Design Example 7-1/ Example 7-2 Example 7-3 131 /132 135 135 135 7-3.Round and Tapered Columns/136 Example 7-4/137 Example 7-5/137 Example 7-6/137 7-4.Spaced Columns/138 Example 7-7/140 7-5.Built-Up Columns/141 Example 7-8/142 7-6.Beam Columns/142 Example 7-9/144 Example 7-10/144 Example 7-11/145 Example 7-12/146 7 -7.Columns or Beam-Columns with Initial Curvature/147 7-8.Tension Members/147 Example 7-13/147 7-9.Combined Tension and Bending/150 7-10.Column Design by LRFD/150 Example 7-14/151 Example 7-15/151 Example 7-16/152 References/153 Problems/153 8.GluedLaminatedMembers/157 8-1.Glulams/157 8-2.Advantages of Glulams over Sawn Timbers/158 Example 8-1/159 8-3.Glu1am Production/160 8-4.Standard Sizes of Glulam/163 8-5.Limits of Curvature/164 8-6.Allowable Stresses and Adjustments/164 Example 8-2/166 Example 8-3/166 8-7.Suggested Design Procedure/167 Example 8-4/167 Example 8-5/168 Example 8-6/169 8-8.Biaxial Bending/170 Example 8-7/171 8-9.Cantilever Beam Systems/171 Example 8-8/173 Example 8-9/174 Example 8-10/176 8-10.Curved Glulams/176 Example 8-11/179 Example 8-12/180 Example 8-13/181 8-11.Tapered Glulams/184 Example 8-14/187 8-12.Members Both Tapered and Curved/188 CONTENTSvii 8-13.Three-HingedArches/188 Example 8-15/189 8-14.Glulam Columns/194 Example 8-16/194 Example 8-17/194 8-15.Glulam Design by LRFD/195 Example 8-18/195 Example 8-19/195 References/196 Problems/196 9.Bolts, Timber Connectors, and Special Weldments/199 9-1.Bolt Groups Subject to Moment199 Example 9-1/201 9-2.Connections with Moment-Bolts in Tension and Shear/202 Example 9-2/203 9-3.Stitch Bolts/204 9-4.Joist and Beam Hangers and Framing Anchors/204 9-5.Special Weldments/206 Example 9-3/206 Example 9-4/210 Example 9-5/212 9-6.Shear Plates and Split Rings/212 9-7.Shear Plates/214 9-8.Split Rings/214 Example 9-6/215 Example 9-7/216 Example 9-8216 Example 9-9216 9-9.Spiked Grids and Grid Plates/218 9-10.Nailer Plates and Toothed Plates/219 9-11.Drift Pins and Dowels219 References/219 Problems/219 10.Timber Trusses/221 10-1.Wood Truss Types/221 10-2.Light-Frame Trusses, Analysis/224 Example 10-1/227 10-3.Light-Frame Trusses, Member Design/229 10-4.Light-Frame Trusses, Connection Design/231 Example 10-2/233 10-5.Bracing and Erection of Light-Frame Trusses/236 viiiCONTENTS 10-6.Heavy-Timber Trusses237 10-7.Old-Style Heavy-Timber TrussesI238 Example 10-3I240 10-8.Modem Gusseted Heavy-Timber TrussesI242 10-9.Trusses Connected by Split RingsI244 10-10.Truss Deflections and CamberI245 10-11.Truss Design by LRFDI246 Example10-4I247 ReferencesI248 ProblemsI248 11.Plywood and Similar Wood Products/249 11-1.Plywood Production and ClassificationI249 Example11-1I254 11-2.Cross-Sectional PropertiesI254 Example11-2I257 Example11-3I258 11-3.Plywood Design for Bending and Axial LoadI258 Example 11-4I259 Example11-5I259 Example11-6I259 Example11-7I259 Example11-8I260 Example11-9I260 11-4.Plywood Design for Rolling ShearI260 Example 11-10I261 11-5.Plywood Design for Shear Through the ThicknessI261 Example 11-11I261 11-6.Roof Sheathing261 Example11-12263 Example11-13263 11-7.F100r SheathingI264 Example11-14I264 11-8.Wood-Based Fiber and Particle PanelsI264 11-9.LRFD for PlywoodI267 ReferencesI267 ProblemsI268 12.Diaphragm ActionandDesign/271 12-1.Lateral Loads on Structures271 12-2.Horizontal-Diaphragm Web DesignI273 12-3.Horizontal-Diaphragm Chord DesignI277 Example12-1I279 Example12-2I280 Example12-3I281 Example12-4I281 Example12-5I282 Example12-6I282 12-4.Shearwall DesignI282 Example 12-7I285 Example 12-8I286 12-5.Anchorage of Shear Panels to FoundationI287 Example 12-9I290 Example 12-10I290 Example 12-11I291 12-6.Strut DesignI291 Example 12-12I292 12-7.Wood Diaphragm with Masonry WallsI293 12-8.Plywood-Sheathed, Metal-Stud ShearwallsI294 12-9.Torsion in DiaphragmsI294 12-10.Other Loads on Wall SheathingI294 ReferencesI295 ProblemsI295 13.Built-Up and Composite Members/298 13-1.Plywood-Lumber BeamsI298 Example 13-1I299 13-2.Stressed-Skin PanelsI301 Example13-2I302 13-3.Sandwich PanelsI304 13-4.Wood Beams Reinforced with MetalI304 Example 13-3I305 Example 13-4I305 ReferencesI306 ProblemsI306 14.Formwork for Concrete/308 14-1.IntroductionI308 14-2.Plywood for FormworkI308 14-3.Design ForcesI309 14-4.TerminologyI310 14-5.Wall FormworkI311 Example 14-1I311 14-6.Formwork for Roof and F100r SlabsI313 Example14-2I313 ReferencesI314 ProblemsI314 15.Miscellaneous Structure Types/316 15-1.Light-Frame ConstructionI316 15-2.Pole BuildingsI319 Example15-1I327 15-3.Wood FoundationsI328 Example15-2I333 15-4.Timber BridgesI336 15-5.Bridge Deck TypesI340 15-6.Bridge Design CriteriaI343 Example 15-3I346 Example 15-4I350 Example 15-5I351 15-7.Composite DecksI353 15-8.ConclusionI355 ReferencesI355 ProblemsI356 16.WoodDurability, Protection, and Preservation/358 16-1.Causes of DeteriorationI358 16-2.Preservative TreatmentI364 16-3.Nonpressure Treatment MethodsI365 16-4.Pressure Treatment MethodsI365 16-5.Effectiveness of TreatmentI366 CONTENTSix 16-6.Using Proper Design Details toPrevent DecayI367 16-7.Termite ProtectionI370 16-8.Fire DamageI371 16-9.Design toMinimize Fire DangerI371 16-10.Evaluation and Repair of Wood StructuresI373 ReferencesI374 ProblemsI374 Appendix A377 Appendix B384 Appendix C Index/443 420 ERRATA P.xii:At the end of the last sentence "20036." should be "20036.)" P.61, left column, lines17and18:The words"and to modulus of elasticity, E"should be omitted P.78, left column, lines12 and13:The words"(Appendix Table C- 4)"should be omitted P.90:Near the end of Example 5-13, the lines should read Qg = 0.85 - (0.05 1 6)(12 - 4.125) = 0.78 N req.= 60801 (1.33x1.0 x 0.78 x 900) = 6.5 bolts P.112:In the caption to Figure 6-10,"6-9" should be "6-8" P.115:In Table 6-3, all fractionsshould have"L"in the numerator rather than "1" (occurs eleven times) P.143:Equation 7-10 and Equation 7-11need to have 1 at the end of each equation P.144:In Example 7-10, lines3&4, "applied at the face of the column." should be "applied at the center of the face causing an eccentricity of 3.75 in." P.144:In Example 7-10, 6th line, 3.2/09 should be 3.2/0.9 P.155:In Problem 7-40,"Chose" should be "Choose" P.159, right column,10th line from bottom:There is a missing parenthesis." ... x 21)1110" should be " ... x 211110" P.165, right column, line 16:"21"should be "20" P.268:In Problem 11-15, line 7"Structural IT"should be "Structural J" P.140:Example 7-7 lines 3 to 5, right column, should be: F'c= 1.25 (1.1) (1500) =2063 psi Cp = 0.349 F'c = 0.349 (2063) =720 psi lines10 to12 should be: F'c = 2063 psi asbefore Cp =0.345 F'c = 0.345 (2063) = 712 psi line 14 should be: Allow P = 2(712)(3.5 x 5.5) = 27,400Ib P.141:Example 7-7, line 1, left column, should be: Check net area under P = 27,400Ib line 6 should be: = 14.56 in2 for one 4x6 lines 8-9 should be Ic = 27,400/(14.56x2) = 941psi F*c= 2063 psiOK Preface Theprime purposeof thisbook istoserveasa classroomtextfortheengineeringorarchitec-turestudent.It will,however,alsobeusefulto designerswhoarealreadyfamiliarwithdesign in other materials(steel,concrete,masonry) but need tostrengthen, refresh, or update their capa-bilitytodostructuraldesigninwood.Design principlesforvariousstructuralmaterialsare similar,buttherearesignificantdifferences. Thisbook shows what they are. The book has features that the authors believe set it apart from other bookson woodstructural design.Oneof theseisanabundanceof solved examples. Another isitstreatment of loads. This bookwillshowhowactualmemberloadsare computed. The authors have found that students, more often thannot,have difficulty recognizing how load istransferred from one member toan-other-forexample,howtoproceedfroma specifiedintensityof floorliveloadandtype andthicknessof floormaterialtoknowingthe actual load per unit length reaching thebeam in question.Worked-outexamplesandstudent homework problems will illustrate the process. Anothersignificant featurethatwebelievesets this book apart is its inclusion of "structural plan-ning." Most textbooks show only the selection of member proportionsor number of connectorsin a joint tosatisfy a given,completely definedsit-uation.Thisbook,ontheother hand,showsthe thinking process needed todetermine whether or not the member is required in the first place. Fol-lowingthis,thespacingandcontinuityofthe memberaredecided,itsloadsaredetermined, and finallyitsshape and size areselected. Webelievethatillustratingstructuralplan-ning aswell asdetailed member and connection designisof considerablevalueinhelpingthe student makethetransitionfromtheoftensim-plisticclassroomexercisestoproblemsof the real world.Problems forsolution by thestudent follow the same idea. The first problems ineach subjectaretheusualtextbook-typeproblems, but in most chapters these are followed by prob-lemsrequiringthestudenttomakestructural planningdecisionsaswell.Thestudent maybe required, given a load source, to findthe magni-tudeoftheappliedloadsanddecideupona gradeof wood.Givenafloorplan,thestudent mayberequiredtodeterminea layout of struc-turalmembers.Theauthorshaveusedmostof theproblemsintheirclasses,sotheproblems have been tested. Thebook presentsmanyof thedesignexam-plesintheformof "computationsheets,"solu-tions in the form of actualdesign office compu-tations.Thisisintendedtoreinforcethe instructiongiveninChapter1regardingneat-ness and orderliness in designcomputations. Thebookrefersfrequentlytothreecodes-theNationalDesignSpecificationforWood Construction,theStandard for Load and Resistance Factor Design for Engineered WoodConstruction,andtheUniformBuildingCode. Whereverpossible,however,thebasicprinci-plesbehindcoderequirementsareexplained, andwheretheauthorsareawareof codeshort-comings,thattooispointedout.Wereferfre-quentlytocoderequirementsinthebelief that theorywithnoexposuretoreallifeisnotgood education.Whatisneededisbalancebetween thetheoreticalandthe practical (the latter is not a bad word). Thedesignmethodusedinthemajorityof thisbookisallowablestressdesign(ASD).In xi xiiPREFACE addition,boththeoryandexamplesusingthe newer design method, load and resistance factor design(LRFD),are included.Problems forstu-dentsolutionarespecified ina mannerthatal-lowsforsolutionbyeithermethod.If thein-structorsodesires,thebookcanbeusedfora coursedealingexclusivelywiththeLRFD method. In that case, it would be wise for the in-structortorequirestudentstousetheentire Standard for LRFD for Engineered WoodConstruction. In general, the book assumes that the user will be familiarwithstructuralanalysis.In fact,the authorsintendthatthebookwillreinforcethe principles of structural analysis using wood asa vehicle. The authors usethe book in a three semester-hourbeginningcourseandfindthatsufficient materialremainsthatasecondcoursecould be taught fromthe same book. The prerequisite for the first course should be mechanics of materials asaminimum,includingthesubjectsof shear and moment diagrams,flexuraldeflections,and axial forces in truss members. A more rigourous course in structural analysis, though desirable as a prerequisite, is not essential. Chapter 5(Connections-Nails,Screws,and Bolts)isintentionally inanunconventional po-sition. The authors realize that most books cover connectionsafter member design hasbeen pre-sented.Theyrealizealsothat(1)because of its positioninthecourse,connectiondesignis somethingmoststudentslearnaboutalmostas anafterthought(asthoughthesubjectisof lesser importancethanthedesign of members); and(2)structural failuresare farmore frequent intheconnectionsthaninthemembersthem-selves.Further,design of memberscan be done moreeffectivelyif thedesignerconsidershow the member will be connected, before makinga finalselectionofmembersize.Withthese thoughts in mind, the authors present the subject of connectionsbeforegoing on tothedesign of structural members. Those who prefer to present the subject in the conventionalsequencecanmerelygoonto Chapter 6,delaying the study of Chapter 5 until after Chapter 8 (glulam design).It should work equally well with either sequence. In the authors' beginning three-hour course in timber structure design, Chapters 1 through 7, 9, 10,11,most of 8,and parts of 12,13, and 14 are included. For students of limited means, it is possible to teachstructuraldesigninwoodusingthistext alone.However,thecoursecanbemuchmore meaningfulif thestudent can refer easilytothe NationalDesignSpecification,theUniform Building Code,or the Standardfor LRFD for Engineered Wood Construction.In our own classes, we require the student to obtain the National DesignSpecification aswellasthistextbook.(The NDSishandled byour university bookstore,or maybeobtainedfromtheAmericanForest& PaperAssociation,111119thSt.,N.W.,Suite 800, Washington, DC20036. JUDITH J.STALNAKER Ernest C.Harris Denver,Colorado 1 Introduction From ancient times,wood andstonehavebeen importantconstructionmaterials.Stonehasdi-minishedinimportance,butwoodisstillour most versatile building material. It isreasonable toclaimthatwoodandwoodproductsare amongourmoreimportantconstructionmate-rials,rankingalongwith structuralsteeland re-inforcedconcrete forbuildingconstruction.By weight, morewood isused in construction each year than cement or steel (1). 1-1.EVOLUTION OF TIMBER DESIGN Over the centuries,man learned touse wood ef-fectively,developingrulesof thumbtoaidin planningandbuildingwoodstructures.These ruleswerelearnedgradually,basedonlyon experience-bothsuccessfulandunsuccessful. Using these rules, skilled artisans were able to con-structinwood,producingdurable,long-lasting structures of utility, often of great beauty, and al-mostalwaysof adequatestrengthandservice-ability.Even today a large volume of wood con-struction depends, more or less,on such rules of thumb for itsdesign. More recently,however,engineers and archi-tectshavelearnedtodesignwoodstructuresin waysthatarebasedonengineeringprinciples. Thus today's designers, using more rigorous de-sign procedures, areable toensure that a partic-ulardesignwillachievethedesiredlevelof structuralsafetyandstiffness,aswellasecon-omy. As an example showing the economy pos-siblewith engineered design,thesameamount of woodthatwasneededinthepasttobuilda 320-ftz log cabin can be used today toconstruct a 3500-ft2 home, with those parts of the trees not suitedtolumberproductionbeingadequateto manufacturea30-year supplyof paperandtis-sue products for an average family(2). Muchprogresshasbeenmadeinmoving fromrule-of-thumbdesigntopresent-dayengi-neered designs. Yetour knowledge isnot static, andamongthedesignmethodsshowninthis bookaresomethatundoubtedlywillsomeday be replaced by newer and better methods. Asnew design methods evolve, greater relia-bility(withregardtosafety)andgreaterecon-omy (in terms of total volume of wood used) are theusualresult.Generally,andthisisunfortu-nate, the improved design methods are more dif-ficult to apply than the earlier methods they sup-plant,but thisisthe price tobe paid forgreater reliabilityandeconomy.Occasionallyanim-proved design concept proves so complex that it hastobe drasticallysimplifiedforthedesigner to use easily. When this isdone, the basic princi-plesinvolvedmaybecomeobscured.The "handbook engineer" isnot bothered by this, but themorecompetentthinkingdesignermaybe preventedfromdoingthebestpossibledesign job. Thatthestructuraldesignerwillplayanin-creasinglyimportantroleinalltypesof wood constructionisinevitable.Whether thedemand for wood construction and other wood products remainsconstantorincreases,thesupplyof readilyavailableandusabletreestoproduce woodof goodqualitywillalmostcertainlyde-crease.By one forecast,thedemand for various formsofwoodandwoodproductsusedfor buildingsisexpectedtoriseby1-4%peryear (3).Tomeet thisdemand will require better use ofourresources."Engineered"structureswill become more prevalent and those that use wood wastefully will become less common.Structural J. J. Stalnaker et al., Structural Design in Wood Chapman & Hall 19972STRUCTURAL DESIGNIN WOOD productsmanufacturedfromwoodwillreplace muchof thelumber weusetoday.Weseethis trend today in the introduction of wood products suchasflakeboard,waferboard,plastic/wood laminates, members made from thinly laminated wood, and shop-prefabricated structural compo-nents.Engineeringknow-howwillbeinvolved ineverystepwetaketoward making better use of wood in construction. 1-2.MATERIAL PROPERTIES Todesign effectively,a structuraldesigner must befamiliarwiththepropertiesandbehaviorof the material to be used.Unfortunately,the prop-ertiesand behavior of wood are unlike those for otherbuildingmaterialsandmuchmorecom-plex.Withthe possible exception of soils,wood hasmoreidiosyncrasiesthananyother material the structural designer uses. Thus, it is even more important in the case of wood than for other con-struction materials that the designer have a good knowledge of the peculiarities of the material. Laterchapterswillshowthatawoodde-signer's problemsstem partly fromthe fact that wood isa natural material-one over which,as yet,we havelittle control.Through forest man-agement,wearebeginningtocontrolcertain characteristicsof thelumberaforestwillpro-duce (3). However, we still cannot force a tree to produce a particular quality of material, nor can wecompel different trees in a forest to produce exactlythesamequalityofmaterial.Similar trees,grown in different localities, may produce wood of dissimilar properties. Consequently, we findthat the engineering properties of wood are extremely variable.Changes in the environment in which the wood is used further affect its prop-ertiesandbehavior.Forthesereasons,wood propertiesandbehavioraredealtwithatgreat length in Chapters 2,3,and 4. 1-3.TYPESOF CONSTRUCTION Consideringonlybuildings,thereareseveral typesof woodconstruction inusetoday.These include: Light-frame construction Post-and-beam construction GIulam construction Heavy-timber construction Pole building construction Wood shell and folded-plate construction Prefabricated-panel construction Log building construction Commonly,combinationsofthesetypeswith each other or with structural steel are used. light-Frame Construction Light-frame construction isthetypemost com-monlyusedforhousing.It isusedmostlyfor lowstructures,butfindsoccasionalusefor buildingsuptofourstorieshigh.Frequently, light-frame construction receives little or no en-gineeringdesign.Itsrequirementsarespelled outinsomedetailbyvariousbuildingcodes. Generally,knowingthecoderequirementsand working from just a floor plan and perhapsone ortwocross-sectiondrawings,skilledcarpen-tersoften constructsmalllight-framebuildings without engineering design assistance. The resulting structure will normally be satis-factoryastosafetyanddurability,butoftenit maybelackingineconomy.Withbothlabor costsandmaterialcostsincreasingsharply,re-search has been directed toward designing more efficientstructures.It islikelythatlight-frame construction of the futurewill require increased engineering designeffort tobringabout greater economywithoutsacrificingessentialstrength, stiffness, and durability. Today,buildersfrequentlyrequireengineer-ingdesignforcertain components,suchaslin-telsandheaders.Loadingconditionsforthese membersvaryfrombuildingtobuilding,so rule-of-thumbmethodsmaybeeitherinade-quate or wasteful. In fact, present building codes requirethatcertaintypesof light-framestruc-tures be engineered. Light-frameconstruction(shownbyFig.1-1) almost always uses lumber of 2-in. nominal thick-ness(actually 11/2in. thick),such as the familiar 2 X4s or 2 X8s. Walls use vertical members called studs,which are either 2X4sor 2X6sandare spacedat12in.,16in.,or24in.oncenters. Floor joists and roof joists or raftersare also2-in.nominal material placed with their wider di-RAFTER LOO R TWO 2X4 PLATES SHEATHING STUD TWO2X4 PLATES STUDS 2X4SOLE PLATE FLOORJOIST U''>'---FOUNDATION WALL Fig,1-1,Cross section throughlight-frame wall, mensionvertical.Whereinternalgeometryof thebuildingpermitsit,morerecent light-frame structuresusuallyhaveroofssupportedbyfac-tory-builttrussesmadeof2X4sor2X6s, Trussesandotherprefabricatedmembersmay alsoreplacesolidsawn lumber forfloor joists, headers,lintels,etc. INTRODUCTION3 The12-,16-,and24-in.spacingsmostcom-monly used intheUnited Statesare logical and economical,sincemostplywoodsheets,wall-boards,andinsulation boardshave a 4-ft width. Thus4ft,or48in.,istheusual"module"for building construction in wood. Post-and-Beam Construction Post-and-beam construction, also known astim-ber-frameconstruction,haditsbeginnings around200B.C.Themain,load-supporting membersinthistypeof woodconstructionare largerthanthoseoflight-frameconstruction, and both verticalmembers(posts)and horizon-talbeamsarefartherapartthanthestudsand joists of light-frame construction.Postsareusu-ally8 to12 ftapart,and horizontal membersat-tachedtothepostsarespacedverticallyinthe range from4 ftto8 ft(4).More closelyspaced floorjoistsandwallmaterialareconnectedto and supported bythe beams. If needed forstability, diagonal "knee" braces areused,asshownbyFig.1-2.Often,inolder buildings,thestructural memberswereexposed ontheoutsideof thewalls,andthespacesbe-tween posts,beams, and knee braceswere filled with brick or a kind of plaster. This led tothe fa-miliar Tudor style of architecture. Except for the pleasingappearance,however,thefilled-inma-terialwastroublesome.Asthewoodexpanded orshrunkwithmoisturechanges,wallcracks opened.Also,thewallfillinghadlittleinsulat-ingvalue,anditsgreatweightnecessitated heavier timber framingto carry the load. In later formsof post-and-beam construction, thebeamssupportwallmaterialconnectedto theoutsidefaceof thebeams.Inoldbarns,for example,verticalwoodplankswerenailedto the beams to form theoutside wall. Whenlight-frameconstructionwasdevel-oped,post-and-beamconstructiondeclinedin popularity,mostlybecauselight-framebuild-ingsweremuchcheaperandcouldbebuilt muchfaster.Manywillargue,however,thata light-frame building is not nearly as durable asa well-built post-and-beam structure. Post-and-beamconstructionisstaginga comebackinpopularity,andthiscanbeattrib-uted partly to the development of new insulating 4STRUCTURAL DESIGN IN WOOD POST Fig.1-2.Post-and-beam construction. wall panels and to prefabrication techniques that help combat the high cost of an otherwise labor-intensivetypeof construction.Usingprefabri-cated insulated panels for the exterior walls per-mitsplacing all main structural membersinside the building, protecting those members from the deterioratingeffectsof weatherandcondensa-tion. Purists insist that connections in thistypeof constructionbemadeusingonlywoodparts. Mortise-and-tenonjointsofseveralvarieties wereused in theolder buildings,and the joints wereheldtogetherbywoodpegs,insertedin holes through the members being joined. Refer-ence4showsmanytypesof suchconnections. Makingtheseconnections,however,istime-consumingandrequiresaveryhighlevelof carpentryskill.Theauthorsfeelthatsubstitut-ingmodemweldments(seeChapter9)with boltsorother modem timber connectorsmight producejustasgoodastructure,permitting constructiontoproceedmorequicklyandat lesstotalexpense.But,iftheappearanceof modem connections isoffensive,then mortise-and-tenon joints withwood pegswouldstill be theanswer. Glulam Construction Becauselarge,good-qualitytimbersarenot readily available today,glued laminated timbers (glulams) are commonly used instead. A glulam isa stack of planks (usuallyIltzin.thick)glued together under carefully controlled conditions to makeasinglememberof largercrosssection. (See Fig.1-3.) Thissubstitution isgood for sev-eral reasons:(1)fireresistanceof glulamsisat least asgood asthat of heavysawn timbers;(2) glulamsarelessaptthansawntimberstosplit aftertheyareinstalled;(3)glulamscanbe strongerandstifferthansawntimbersofthe samesize;and(4)theydonotwarpbadlyas sawn timbers frequently do. Heavy-limber Construction Heavy-timber construction isdefined bybuild-ingcodes.TheUniformBuildingCode(UBC) (5)definesitasconstructioninwhichtimber columnsarenotlessthan8in.inleast dimen-sionand beams(except forroof beams)arenot lessthan6in.wideby10in.deep.Of course, UBCdefinesheavy-timberconstructionin INTRODUCTION5 Fig.1-3.Light-frame construction used in conjunction with glulams. (Courtesy Overly Construction Co. Photograph by au-thors.) muchgreaterdetailthanisgivenhere.Heavy-timber construction requires engineering design. Structuresmeetingtherequirementsforheavy-timber construction are much more fire resistant thanothertypesofwoodconstruction.(See Chapter 16.) Inearlierbuildingcodes,heavy-timbercon-structionwasreferredtoas"millbuildingcon-struction." Typically,it wasused formanufactur-ing buildingsandstoragewarehouses.Architects often find these old buildingssuitable for conver-siontonewuses,developingattractiveoffices, shops,restaurants,theaters,andthelike inwhich the old timber construction is used to aesthetic ad-vantage. Many design engineers and architects are involvedinevaluatingtheoldconstructionand modifying structures to adapt them to modem use. Heavy-timberconstructionisnotlimitedto buildings,but may be foundinminestructures, docks, bridges,andother structures. Pole BuildingConstruction Pole-typeconstructiongoesbacktoprehistoric times in both Europe and North America. Exam-pIesof itsusein North America areseeninan-cientIndiandwellingsitesinthesouthwestern UnitedStates.Thedistinguishingfeatureofa pole-type building isthat itssupportingvertical members(roundpoles)areembeddedinthe ground,havingbeeneitherdrivenintothe ground or placed into excavated holes. Horizon-talbeamsandother framingareattachedtothe poles,asshown later in this book by Fig.15-5. Obviously,abigproblemwithbuildingsof thistypeisdecayof theembedded poles.How-ever,withtheuseofchemicallytreatedpoles, theproblemsof decayhavebeenalleviated but noteliminated.Asaresult,beginninginthe 1950s,polebuildingshaveenjoyeda rebirthin popularityintheUnitedStates.Forcertainap-plications,polebuildingsaresuitableandeco-nomically feasible. Pole buildings areused principally inagricul-ture,but insomelocalitiestheyareusedexten-sivelyforhousing.Theyareparticularlywell suited forhousingwherethe floormust bekept wellabovegroundleveltoavoidflooding. AlongtheU.S.Gulf Coast,forexample,pole-supported buildings are often better able to resist 6STRUCTURAL DESIGNINWOOD Fig.1-4.The Tacoma Dome erection sequence. Triangular sections of primary glulam beams and secondary purlinsareassembled onthe groundandthen lifted intoplace,one ringatatime.The dome waserected without dense interior scaffolding.(CoU11esyWestern WoodStrnctures, Inc.) theforceof hurricane-drivenwavesthanstruc-tureswhosesupportingwallsextendtoground level.The poles,of course,must have adequate bracing or stiffness toresist thewave forces. The pole acts as a column, transferring vertical load tothesoil.If the end area andperimeter of thepolearetoosmall,theholemaybedugto larger diameter and a concrete footing cast in the bottom to reduce the bearing pressure on the soil. Holedepthsusuallyrangefrom5 ftto8 ft,and footingdiametersarecommonlyfrom16in.to 24in.Polebuildingsarenormallyone-story structures,with polesspaced upto20ftineach direction.Heavytimberor lightframingisfas-tenedtothepolestoformwallsandroof,and floorif any.Withtrussesfortheroofstructure, spansofupto60ftarepractical,andafew longerspanshavebeenconstructed.Although pole buildings date back to ancient times, today's pole buildings are usually designed by engineers rather than by old rules of thumb (6). Pole construction can alsobea partialsolution toproblemsof freezingand thawingbeneaththe structure,asinpern1afrostareas.Beingapoor conductor, a wood pole carries little heat from the building aboveinto the frozen ground below,so it helps prevent thawing. If the structure ismounted on a platform at the top of the poles, the open cold-airspace below also helps prevent thawing of the soil.Polesinthistypeof environmentmustbe placedintoexcavatedholes,notdriven.Having their larger diameter at thebottom,polesprovide good resistance to being lifted from the ground by alternate cycles of freezing and thawing. Wood Shell and Folded-Plate Construction Woodshellsandfoldedplatesmayhaveboth economic and aesthetic advantages for roof con-struction.Roof shellscan beof variousshapes, themost common being the hyperbolicparabo-INTRODUCTION7 Fig.1-5.The 530-ft-diameter Tacoma Dome completely framed,with 2-inch timber decking being installed.(Courtesy Western Wood Structures, Inc.) loid.Other shapesarelimitedonly bytheinge-nuity and skill of thedesign engineer and by the capabilityof thematerialsusedtofollowcom-plex surfaces.Findingthemaesthetically pleas-ing,manyarchitects incorporate shell roofs into their designs.Theimportant thingtoremember isthat either shells or folded plates require engi- . neering design.Rulesof thumb willnot suffice. Todesign either,an engineer with proper know-howand experience should be engaged (7). Materialsforshellroofsincludebothboards and plywood sheets. In either case, the materials areusedintwoor more(usuallythree)layers. Thelayerscanbeconnectedtoeachotherby special nails, or glue,or both. Prefabricated-Panel Construction There aremanyvarietiesof prefabricated panel. Theobjectof prefabricationistoreducecost. Becauseofcarefullycontrolledfactorycondi-tions,prefabricated panelscanhavenearlyuni-form quality. They also can speed field erection, thussavingconstructioncost.Wherestressed-skinactionisprovided,theymayalsosavein total quantity of material used,thereby reducing thetotal dead load that thestructure must be de-signed tosupport. Stressed-skin panelsare those in which a ply-woodskinisgluedtosupportingmembers,so that the plywood sheet servesa doublepurpose: (1) It actsasa beam spanning from one support-ingmembertoanother;and(2)beingrigidly connectedtothosesupportingmembers,itacts together with them,thecombined member hav-ingstrength and stiffness exceeding thesums of thestrengthsor stiffnessesof theseparate parts. Stressed-skinpanelsmayhavetheplywoodon onesideonlyor mayhavebothtopandbottom plywood sheets attached tothe supporting mem-bers.Panelsareusuallymanufacturedflat,for usessuch aswall, roof,or floor panels or interior partitions.Buttheycanalsobemanufactured with one-way curvature. Stressed-skin panels always require engineer-ingdesignunlesstheyareusedstrictly fornon-load-carrying purposes. LogBuildingConstruction Logbuildings,inseveralforms,appeartobe makingacomeback,especiallyforresidential construction.Strangely, manyof themareengi-neeredstructures.Their popularity isenhanced, ofcourse,bytheirexteriorattractiveness.In many cases they are economically feasible,even 8STRUCTURAL DESIGN IN WOOD thoughtheyobviouslyusemorewoodthana light-framebuildingof equalfloorarea.There aretworeasonsforthis:(l) Lesstotallaboris involvedinpreparingandinstallingthewood components;and (2)wood issoeffective anin-sulatingmaterialthatextrawallinsulationis normally not required. 1-4.HYBRID CONSTRUCTION The authors use the word hybrid to describe any construction type that is mixed, and there are nu-merousexamplesof this.For example,incon-struction that isessentially light frame,wemay seesteelfloortrussessubstitutedfortheusual sawn-lumber floor joists, or floor trusses having woodtopandbottomchordsbutmetalweb members.Or wemayseelight-frame construc-tioninwhichheadersandlintelswillbewood trusses,or evensteelbeamsections.Other fre-quentcombinationsare:(l)acompletestruc-turalsteelframe,withwoodmembers(either joistsortrusses)spanningfromsteelbeamto steelbeamtosupporttheplywoodorwood compositefloor,or(2)woodjoistsorrafters supported by masonry bearing walls. So, even though several recognized construc-tion types were listed and defined above, there is absolutelynoneedthatastructureconform strictlytoanyoneofthosetypes.Whenever economy and serviceability are best served by a combination of types, then designers will proba-bly use it. 1-5.TIMBER BRIDGES Bridgesbuilteitherwhollyor partlyof timber includepedestrian,highway,andrailway bridgesandspecialtypessuchaspipebridges. Theyincludetrestles,simplegirder,arch,or trussbridges,and evensuspension bridges.Al-thoughwoodisanexcellent material for many types of bridge, its value for bridges isoften not clearly recognized, perhaps because of a fear of eitherimpermanenceorweakness.However, properlyconstructedandmaintainedwood bridges canbedurable.The useful lifeof mod-em glulam bridges isestimated to beashighas 50years,whereasforsteelorconcretebridges (becauseofcorrosionbydeicingmaterials)it may be much lower. Timber bridges are not nec-essarilyweak;theycancarryverylargeloads, such asheavily loaded (135-ton)logging trucks (8). Examples showing durability and capability tocarry heavy loadsand impact areseen in the large number of existing railroad trestlesand in themanycoveredbridgesthatstillcarryrural roadtraffic.Thelatterowetheirlonglife,of course,tothe factthat the main wood structural members and wood roadway deck are protected from the ravages of weather. According to refer-ence 7,bridges of wood have seen useful life of over 500 years! Timber (usually glued laminated) is a popular material for pedestrian bridges.In these,the en-tire superstructure-main girders and deck-are wood.Theselectionof woodoverothermate-rials is often made on the basis of aesthetics, but economy,durability,andeaseofmaintenance alsoplayimportantrolesindeterminingthe choice. Inrailway trestles,treated wood pilesare fre-quently used for the foundation,and treated tim-bers are used for all other structural parts, includ-ing thecross ties.Highway bridgesspanning up to about 30 ft can be built using sawn timbers for themainlongitudinalmembers(girders).When gluedlaminated(glulam)girdersareused,the practicalspanlimitishigher,spansaslongas 150fthavingbeenconstructed.Woodisused frequentlyforthe towers,stiffening trusses,and floor systems of suspension bridges. Arch bridges have become practicalwiththe developmentofglulams.Thistypeoftimber structurehasbeenusedforpedestrianbridges, highway bridges, and pipe bridges. The decision to use wood is often based on appearance of the bridgeandtheblendingof itsappearancewith thesurroundings.Bothtwo-hingedandthree-hingedarchesarepracticalusingglulams,but fixed-endarchesgenerallyarenot.Forlonger spans,woodtrussesarepractical.Theseare more commonly used in localities where timber isplentifulbutsteelisexpensiveandconcrete construction difficult. Bridgedecksarebuiltalsousingcomposite wood and concrete. In this type of deck the con-creteformsthewearing(top)surface.Wood membersbelowformthetensionpartofthe compositeflexuralsectionandtheconcrete above resists the compression. Thewood mem-bers alsoserveassupporting formwork tocarry theweightof thewetconcreteasthewearing surface is placed. 1-6.NOTES TO STUDENTS Design Codes Forsomeof you,thiswillbethefirstdesign course.Bydesignwemeantheprocessof choosing the type and quality of material touse, choosinglocationandspacingof themembers, andselectingshapeanddimensionsforeach member sothat itwill(1)carrytheload safely, (2)have the desired stiffnessand durability,and yet(3)beaseconomicalaspossible.Principles that you learned in previous courses(mechanics of materials,forexample)stillapply,butnow youwillhavetosupplementthoseprinciples with design rules agreed upon by committees of experienced engineers and researchers toensure that timber construction will be safe. These rules makeupastandardof practice,sometimesre-ferred to(loosely)as a "design code." Thestandardofpracticeprobablyhaving most meaning for this course is the National Design Specification for Wood Construction (NDS) (9).TheNDSmaybeoneof therequiredtexts for your course;if not,at least it isanavailable reference.It wouldbeanexcellentidea,espe-ciallyif thisisyourfirstdesigncourse,toread throughtheNDSnow,notstudyingitsrequire-mentsin great detail but just becoming familiar with thetypesof instruction itgivesandwhere to findthings in it. The NDSsets out the requirements for allow-ablestressdesignof woodstructures.Thereare twomethodsfor thedesignof woodstructures: (1)allowablestressdesign(ASD)and(2)load andresistancefactordesign(LRFD).The greater portionof StructuralDesigninWoodis devotedtoASD,withLRFDbeingcoveredin thelastsectionof manychapters.Thestandard of practice forLRFDdesign isthe Standard for LRFD for Engineered WoodConstruction(10). Astudent'sfirstexposuretousingadesign standard isoftena confusingexperience.Tryto realizenow,attheoutsetofthiscourse,that what these twostandardsdoismerelytodefine INTRODUCTION9 in some detail (for a particular material) how to applythedesignprinciplestowhichyouwere introducedinpreviouscourses.Realizingthis, youwillappreciatethetruepurposeof each re-quirementandwillfinditsapplicationmore meaningful. Accuracy Yourinstructorhasprobablycoveredthenext point before,but it willbear repeating.Just be-cause your calculator shows answers to seven or eight placesdoesnotindicatesucha high level of accuracyforourpurposeindesign.Thean-swersyourcalculatorshowsarenomoreaccu-rate than thevaluesyouenter into it.Loadsand allowablestresses,forexample,arerarely known tomore than threesignificant figures,so youshould not expect theanswers to besignifi-cant to more than three figures.In the process of solvingadesignproblem(if itismoreconve-nient than roundingoff at each step),youmight copyfiguresasthecalculatorshowsthem,that is,tomore than three figures.But to retain more thanthreefiguresintheanswertothedesign problemwouldbepurefoolishness.Byall means,roundofftheanswertothenumberof significant figures that is appropriate for the data with which you started. Occasionally,however,astudentlearnsthe above so well that rounding off goes too far,and in thiscasetheanswer maysuffer.Donot arbi-trarilyroundoff toone- or two-figureaccuracy earlyintheproblemandthenexpecttoretain three-figureaccuracy in the finalanswer. Order and Neatness Adesigner'scomputationsareusuallykeptas part of thecomplete,permanent recordof a de-sign project.This isdone for thedesigner's pro-tection,ofcourse.But,equallyimportant,the computationsmayfindadditionalfutureuse. Thestructuretowhichtheyapplymayneedto berevisedorextended;oritmayneedtobe adapted to a new use in which the loading differs from that of the original design. Consequently, it maybenecessaryforyou(orsomeoneelse)to readandunderstandyourcomputationsclearly andeasilyatsomedistantfuturedate.If your 10STRUCTURAL DESIGN IN WOOD original computationsareneat and orderly,and fullylabeledtoshowexactlywhatyouhadin mind, they will be easy to interpret. Engineers have long been noted forthe neat-nessand orderliness of their work, whether it be longhandcomputationsorworkingdrawings. However, being neat and orderly does not come naturally to most of us; we have to work at it.So whywaituntilprofessionalpracticebeginsto developthisnecessaryhabitof producingneat and orderly work? Wouldn't it make more sense todevelopitduringstudentyears?Todemon-stratewhattheauthorsconsiderproperform, severaloftheexamplesinlaterchaptersare shown as typical designer's computation sheets. A word of caution, however: Neatly recopying a previously made sloppy and disorderly compu-tation is wasting time. Furthermore, the copy may sufferfrominaccuraciesof thesloppyoriginal. Rather than making a preliminary version of your work, try tothinkfirst,then write.Orderly think-ing is even more important than orderly writing. REFERENCES 1.Smith, w.E, Principles of Materials Science and Engineering,McGraw-Hill, New York,1986. 2."TreeImprovement,"GeorgiaPacific,Portland,OR, 1981. 3.Stone.R.N.,andJ.F.Saeman,"World Demandand Supply of Timber ProductstotheYear2000,"Forest Products Journal,27(10), Oct.1977. 4.Benson,T.,BuildingtheTimberFrameHouse,Scrib-ner's, New York,1980. 5.UniformBuildingCode,InternationalConferenceof Building Officials, Wbittier, CA,1994. 6.Patterson,D.,PoleBuildingDesign,American Wood Preservers Institute, McLean, VA,1969. 7.WoodStructures,aDesignGuideandCommentary, American Society of Civil Engineers, New York,1975. 8.Gibas,D., "The Timber Bridge,anOld and New Con-cept," Forest Products Journal,33(11/12),Nov.lDee. 1983. 9.National Design Specification/or WoodConstruction, rev.1991Ed.,AmericanForest &Paper Association, Washington, DC,1993. 10.Standard forLoad and ResistanceFactorDesign for EngineeredWoodConstruction,AmericanSocietyof Civil Engineers, NY,1996. 2 WoodStructureandProperties 2-'.WOOD AS A STRUCTURAL MATERIAL Forstructuralapplications,woodismostcom-monly foundaseither sawn timbers,lumber,or glued laminatedmembers(glulams).Inthein-terestofeconomyandtopermitusingwood moreefficiently,increasingamountsofwood todayfindtheirwayintomanufacturedstruc-tural materials or members such as(1) plywood, hardboard,chipboard,flakeboard,waferboard, andplastic/woodlaminates(theseareknown collectively aswood composites); and (2)man-ufacturedmemberssuchasplywood-lumber beamsorwoodtrusses.Manyof thesewillbe discussed later in this book. 2-2.PROBLEMS IN USE OF WOOD FOR STRUCTURES Formoststructuralapplications,fortunately, wood'sadvantagesfaroutweighitsdisadvan-tages.However,structuraldesignersmust learn to cope with (1) wood's variability and (2) its re-sponse to environmental conditions. Variabilityisprobablythemoreseriousof these.Woodpropertiesvaryfromspeciesto species, from one position to another in the tree, fromone treetoanothergrowninthesamelo-cality,andbetweentreesgrowninonelocality andthosegrownin another.Generally,humans have little control over the quality of wood a tree produces,althoughstridesarebeingmadein thatdirectionbymeansof selectivetreefarm-ing. It is hoped that by such means straighter and faster-growingtreesmaybedeveloped,with morenearlyuniformpropertiesthanarefound in trees from natural forests(1). Themoisturecontentof woodinstalledina structuremaychangewithtime,eventually reachinganequilibriummoisturecontentthat depends on the average relative humidity of the surroundings.However, as the relative humidity within a building may not be constant, the equi-libriummoisturecontentmayvarywithtime. With any change of moisture content, wood will eithershrinkorswell-and warp.Collectively thesesizeandshapechangesareknownasdi mensionalinstability.Dimensionandshape changesduetomoisture change can be reduced or avoided by proper seasoning (drying)and by proper attention to details of design. Duration of loadingcausesstrength changes: Thelongera loadremainson awoodmember, the weaker the wood member becomes. Luckily, thestructuraldesignercaneasilyconsiderthis problem and compensate for it in the design pro-cedure. Duration of loading has negligible effect onmodulusof elasticitybut,becauseof creep, deflections are time-dependent. Finally,under someconditionswood'sdura-bility will be limited. Weathering, decay, insects, orfirecanobviouslylimittheusefullifeof a woodstructure.Yetthisproblem,too,canbe overcome byselectingtheproper kind of wood andbyproperdesign,treatment,construction, and maintenance. 2-3.ADVANTAGES OF WOOD ASA STRUCTURAL MATERIAL Wood'sprincipaladvantagesovertheother commonstructuralmaterialsare(1)itsecon-omy, (2) its appearance, and (3) its ease of work-ingandreworking.Other advantagesarebetter durability(forsomeapplications),ahigh 11 J. J. Stalnaker et al., Structural Design in Wood Chapman & Hall 199712STRUCTURAL DESIGNINWOOD strength/weightratio,andexcellentthermalin-sulating properties. Awoodstructureisfrequentlylesscostlyto construct than a similar structure in either struc-tural steel, reinforced concrete, precast concrete, ormasonry.Of course,acompletelyvalidcost comparison would have toinclude such costs as operation,maintenance,insurance,and thelike, over theentirelifeof thestructure.However,it should beobviousfromobservingstructuresall aroundusthat,evenwhenallthesefactorsare considered,woodisfrequentlythemosteco-nomical choice for buildingsuptothreeor four stories high. Awoodstructurecanbeeitheraesthetically pleasing or unattractive, depending entirely on the carethat gocsinto itsdesign.Early U.S.colonial structures provide excellent examples of attractive design and of construction details that ensure good durabilitydespitethefactthatwoodcandecay. Modemgluedlaminatedtimbermakespossible very attractive buildings. Wesee numerous exam-ples of thisincommercial and institutional build-ings, in high-quality residences,and even in inte-riortreatmentinlargerbuildingswhosemain structural members are of other matetials. Wood caneasilybecut,shaped,and finished in the field.Prefabrication at another locationis generallynotneeded,althoughprefabrication maybeemployedasatime-savingormoney-saving measure. Existing wood structures can be revisedoraddedtomoreeasilythansimilar structuresof any other material. If proper careistaken indetailsof design,in maintenance,and inselecting suitable preserva-tiveandfireretardanttreatments,woodwillbe durable.If untreated wood isallowed toremain moist inservice,however,it will decay.The se-crettoensuringdurabilityisfomfold:(1)Use proper species and grades,(2)avoid undesirable environmental conditions,(3)use proper design details,and (4)use whatever treatment is neces-sary and economically feasible. Whenwoodisusedforallcomponentsofa structure, the total weight isoften less than for a structure of other materials.Thisisparticularly trueincomparisontoreinforcedconcrete.The strength/weightratioof woodisadvantageous wherever dead load isan appreciable part of the total load. Fig.2-1.Proper detailing of joints in log building leadsto longlife.Upperphotographshowsold-timeconstruction. Lowerphotograph(Courtesy Goldeu LogHomesbyCMB, Arvada,Colorado) shows turned logsina modern log build-ing. 2-4.CLASSIFICATION OF WOOD Woodisclassifiedaseithersoftwoodorhard-wood,accordingtothespeciesoftreefrom whichitiscut.Thesearenotnecessarilygood names,but theyhave been used forsolong that they are accepted as part of the language by peo-plewhoworkwithwood.Softwoodsactually include some varieties that are quite hard,while hardwoodsincludespecieswhose woodisvery soft.Softwoodscomefromtheplantgroup knownasgymnosperms.Softwoodspeciesin-WOOD STRUCTURE AND PROPERTIES13 clude the needle-leaved trees.Most of these are cone bearing (conifers), and most are evergreen, whichmeansthat theydonot droptheir leaves forthewinterseason.Softwoodsalsoinclude thescale-leavedevergreens,mostof whichdo not bear cones. Treesproducinghardwoodsareof theplant groupknownasangiosperms.Allhardwood trees have broad leaves.Most nontropical hard-wood trees are deciduous; that is, they shed their leaves for the winter. Table 2-2. CommonName Northernredoak Southernredoak Black oak Pin oak Scarlet oak White oak Chestnui oak Sugar maple Blackmaple Redmaple Yellow birch River birch White ash Green ash Typical North American Hardwoods. ScientificNameLumber Name QuercusrubraRed Oak Q.JalcataRed Oak Q.velutinaRed Oak Q.palustrisRed Oak Q.coccineaRed Oak Q.albaWhiteOak Q.prinusWhite Oak Acer saccharumHardmaple A.nigrumHardmaple A.rubrumSoftmaple BetulaBirch alleghaniensis B.nigraBirch FraxinusamericanaWhiteash F.pennsylvanicaWhite ash Without getting into biologic structural differ-ences (except for one that isexplained later) the above definitions are about asprecise aswe can get.Perhapsabetterwayforourpurposeis merelytolist common North American species of each class. Table 2-1shows typicalsoftwood species and Table 2-2 typical hardwoods. Note:Thelist aboveisbynomeanscomplete. TheLatinnamesinthesecondcolumnof thesetablesarescientificnames.Thefirstpart of thescientificnameisthegenusandthesec-CommonName Ponderosa pine Lodgepolepine Longleaf pine Slash pine Virginia pine Pitch pine Shortleaf pine Slash pine Longleaf pine Loblollypine Red(or Norway)pine Black spruce Red spruce Whitespruce Sitka spruce Englemannspruce Blue spruce Douglasfir" Pacific silver fir Whitefir Eastern hemlock Westernhemlock Westernredcedar" Redwood Bald cypress Table 2-1.Important North AmericanSoftwoods. ScientificName Pinus ponderosa Pinuscontorta Pinus palustris Pinuselliottii Pinusvirginiana Pinusrigida Pinusechinata Pinuselliotti Pinus palustris Pinustaeda Pinusresinosa Piceamariana Picearubens Piceaglauca Piceasitchensis Piceaengelmannii Picea pungens Pseudotsugamenziesii Abies amabilis Abies concolor Tsugacanadensis Tsugaheterophylla ThujapUcata Sequoiasempervirens Taxodiumdistichum 'Not a tNe fir.(Somebotanists callittheDouglastree.) bNota tNe cedar.(TNe cedars-Cedrus species-are foundonlyinEuropeandAsia.) Note:Thelistaboveisbynomeanscomplete. Lumber Name Ponderosapine Lodgepole pine Longleaf yellow pine Longleaf yellowpine Southern yellowpine Southern yellowpine Southern yellowpine Southern yellowpine Southern yellowpine Southern yellowpine Norway(red)pine Easternspruce Easternspruce Easternspruce Sitkaspruce Engelmann spruce Engelmann spruce Douglasfir Whitefir White fir Eastern hemlock West coast hemlock Western redcedar Redwood Cypress 14STRUCTURAL DESIGNINWOOD ond part the species. Ordinarily, scientific names findlittle usebystructural designers.However, scientificnamesaretheonlyreliableonesfor defining precisely a particular species. Names in thefirstcolumnarecalledcommonnames. These are the names used by most people when identifying trees,but they are usually lessaccu-ratethanthescientificnames.Forexample,a speciesmayhavedifferentcommonnamesin differentregions;or,equallyconfusing,the samecommonnamemaybeappliedtomore than one species. The third column of each table givesstillan-other version-names applied by the lumber in-dustry.Thesearethenameswewillusein this book.Underlumberindustrynameswefind severaldifferentspecieslistedtogether under a singlename(speciesgroup).Thejustification forthisisthat thespeciesincluded under a sin-glelumber industrynamegrowin thesamelo-cality, have similar appearances,and havesimi-lar properties.Theimportantthingtonoticeis thatwhileonlythescientificnomenclatureis sufficiently accurate for the botanist, lumber in-dustry names are quite suitable for the structural designer's use. ThelistsinTables2-1and2-2arefarfrom complete.Forexample,thelumbernamered oak,forwhichTable2-2showsfivedifferent species,actuallyincludes14differentNorth Americanspecies.Similarly,thetableshows only two species under white oak, but 13species are actually included under that name. Other im-portant hardwoods(lumbernames),notshown inTable2-2,arehickory,beech,pecan,poplar, cottonwood,aspen,basswood,elm,walnut,and cherry. Toemphasizethat hardwoodsarenot neces-sarilyhard,consider thefamiliartropicalwood known asbalsa.Balsa isactually a hardwood! 2-5.WOOD STRUCTURE Featuresof wood structure includethoseof the wood-formingsubstanceandthoseofthe "whole wood."Featuresof thewoodsubstance areallmicroscopicandareof interest toindus-tries that use wood substance (rather than whole wood) as a raw material-the paper industry, for example.Onthe other hand,except for individ-ualwoodcells,mostwhole-woodfeaturesare visibletothenakedeyeor byaidof merelya handlens.Thesearethefeaturesthataffect wood'ssuitabilityforconstruction,andarethe ones that will be explained here. Wood Cell Wood'sbasic structural element isthecell.The hollow,tubularwoodcellsaremanytimes longerthantheyarewide.Forsoftwoods,the celllengthisabout100timesthewidth.Cells areapproximatelyrectangularincrosssection and have unsymmetrical tapered ends that over-lap,instaggeredposition,withthecellsabove andbelow.Thecell wallsconsist principally of celluloseandlignin.Celluloseaccountsfor 45-50% of theweight of completely drywood, andligninforabout20-30%. Another ingredi-ent,hemicellulose,amountsto20-25%(2). Minor components,whichvary fromspeciesto species,includenatural resins,oils,tannin,and alkaloids.Often,itistheseminorcomponents thatmakeaspeciessuitableorunsuitablefor some particular application. Cellulose gives the wood its strength:it is the load-carrying material. Lignin isthe "glue" that cementscellulosefiberstogether,fillingthe spacesbetween fibersandstiffeningthefibers. Thecellulose-lignincombinationisactually heavierthanwater;ithasaspecificgravityof 1.54.However,woodcellsarehollow,sothat thespecific gravity of wholewood rangesfrom 0.31to0.75formostwoodsusedinconstruc-tion.If thecellswerenothollow,woodwould sink rather thanfloat.A fewtropicalspeciesdo actuallysink.Ebony,forexample,weighs 70-80 Ib/ft3 (versus 62.4 for water). If celluloseformsthecellwallsinallwood speciesandistheconstituentthatgivesthe woodstructuralstrength,why,then,dowood properties(e.g.,strength)varysomuchfrom onespeciestoanother?Tounderstandwhy,we must consider furtherthestructure of thewood itself, in particular the arrangement and shape of its cells. Figure 2-2 is a drawing showing typical crosssectionsthrougha pieceof wood.Notice that most of the hollow, tubular cells are in a lon-gitudinalposition;that is,their longdimension is vertical, parallel to the trunk (stem). of the tree. T ...J ~ ~z-zO: SUMMERWOOD RESINCANAL -ww :;;>- >-00 -"' DURATIONOFLOAD Fig.2-9.Effectofloaddurationonstrength.(Courtesy Board of Regents,University of Colorado.) structuresmustconsider thateffect.Otherwise, toensureadesired levelof safety,thedesigner would have to be overly conservative, designing asthoughallloadsweretobeinplace forever. Toconsider theload-durationeffect,thestruc-turaldesignerusesa method based on therela-tionship shown by Fig.2-9.This graph is the re-sult of extensive research by the Forest Products Laboratory(9).Chapter4showshowthede-signertakesintoaccountthisload-durationef-fect. 2-12.STRENGTH-REDUCING CHARACTERISTICS Because woodstructuralmembersarecut from treesratherthanbeingformedfromahuman-madematerial,theywillhavesomestrength-reducingdefects,suchasthefollowing: (1) knots,(2)cross grain,(3) checks,(4)shakes, (5)compression wood,(6)wane,and (7)decay. Knots Knotsareformedbythechangeof woodstruc-ture that occurs where limbs grow from the main stemof thetree.Thelimb,extendingapproxi-matelyradiallyinthemaintrunk,hasitsown annualringsandrays,andthislocalarrange-mentofcellsinterruptsthenormalpatternfor themainportionofthetree.Theknothasa weakening effect that isillustrated by Fig.2-10. Thegraindirectionchangesseverelyasthe woodfiberspassaroundtheknot(crossgrain). Inthisregion,theappliedloadcausestensile WOOD STRUCTURE AND PROPERTIES23 AWAYFROMKNOT, AVERAGE TENSILE STRESS = PIA (A PRINCIPAL STRESS) p _ r ~AT KNOT, TENSILE STRESS PERPENDICULAR TOGRAINttl (INCOMBINATION WITH SHEARING STRESSESAND TENSI LESTRESS PARALLEL TO GRAIN) Fig.2-10.Effect of knot on stresses. stresscomponentsnonnaltothegrainofthe knot.Sincetensilestrengthnonnaltothegrain isverylow,theknotweakensthemembersig-nificantl y. Tensile strength isaffected most by theknot, but compressive strength is reduced also.Bend-ing strength is reduced too, the amount of reduc-tiondependingon wheretheknot islocated on the beam cross section. In drying,wood shrinks moreradiallythanlongitudinally,soknotsfre-quentlybecomelooseandmayevenfallout. Then,obviously,theyare asdamaging tobend-ing, compressive, or shear strength as they are to tensile strength. The manner in which the tree is cut to produce lumberinfluencesthedegreeofeffectaknot will have on strength.This will be illustrated in Chapter 3. Cross Grain Whenthelongitudinalaxisof thecellsisnot parallel to the edge of a piece of wood, the piece issaid tohave cross grain.Crossgrain involves both spiral grain and diagonal grain. Cross grain occursatknotsandotherlocations.Nearlyall lumber hasthisstrength-reducing characteristic tosomedegree.Figure2-11illustratesseveral casesof crossgrain,differingfromeachother accordingtotheorientation of theannualrings with the facesof the piece of lumber. Manytreesgrowinaspiralfashion,sothat thedirectionanddegreeof crossgrainvaries along the length of a piece of lumber. This spe-cial case of cross grain is called spiral grain. Spi-ralgrainmakeswoodverydifficulttowork. Further,asitsmoisturecontentchanges,the wood warps-all plane surfaces become twisted, warped surfaces. Spiral grain is common and is a hereditary trait insome trees. Diagonal grain iscaused by the way in which a piece of lumber is cut from the log, rather than being due to an inherent defect of the tree itself. However,therewillalwaysbevariationsin graindirectioninthetree(cellsarenotevery-where perfectly parallel),so it isnot possible in the mill to cut lumber so asto completely elimi-nate diagonal grain. Figure2-12showshowcrossgrainreduces thestrength of apieceof wood.Almost every-onehastriedtobendastickwithcrossgrain, onlytohaveit breakinthemannershownby Fig.2-12a.Thisbreak occursbecause theflex-ural stress (which is parallel to the surface) has a 24STRUCTURAL DESIGN IN WOOD

A8 .;;..- - .,,;j.i-: '"".= :/'0. cD F

G Fig.2-11.Typesof crossgrain.(Courtesyof USDA-For-est Service, Forest Products Laboratory.) CROSSGRAIN BENDINGFAI LURE " MOHR'S CIRCLE lalBENDING Ib)COMPR ESSION f. = Mell Fig. 2-12.Effect of cross grain on strength. component perpendicular tothegrain,andten-silestrengthinthatdirectionisverylow.The Mohr's circle in Fig. 2-12a shows how apprecia-bleperpendicular-to-graintensioncanoccur with small cross-grain angles. Similarly,Fig.2-12bshowshowacompres-sionmemberwithcrossgrainisweakened. Here. it is either the low strength in compression perpendicular tothegrainor the lowlongitudi-nal shear strength that precipitates failure. Strength at anangle tothe grain may be esti-mated by a simpleequation called Hankinson's formula.This equation,shown in later chapters, isused in designingmembersandconnections" wheretheforcestobetransferredareneither parallel to nor perpendicular tothe grain. Shakes and Checks Sometimesthegenerictermsplitsisusedto coverbothshakesandchecks.Shakesand checksarelongitudinalplanesofseparation (cracks)inthewood(seeFig.2-13).Shakes originate in the tree. They may be in the longitu-dinal/radial plane,in which casethey are called heart shakes or rift cracks.When they are in the longitudinal/tangentialplanetheyarecalled ring shakes, the plane ofthe break being parallel totheannual rings.Heart shakesarethought to be caused by tensile stresses in the tangential di-rection,thestresses resulting fromgrowth pres-sures within the tree. Ring shakes are thought to becausedbyshearduetowind.perhapscom-binedwithweaknessduetocelldamageby frost.Figure 2-14 shows types of shake. Checks occur as the wood seasons after the tree isfelled.Theymayoccurinthelogitself orin pieces of lumber cut from the log. Checks are lon-gitudinal cracks, usually in the longitudinal/radial plane but occasionally in the longitudinal/tangen-tial plane. They occur because of stresses resulting fromdifferentialshrinkageinthetangentialand radialdirectionsduringdrying.orfromuneven dryingindifferentportionsofthelumber.Too-rapiddryingcancauseseverechecking.Thus. checkingcanbereducedbycarefulattentionto drying procedures during lumber production. Figure 2-15shows the position and namesof typesof check.Surface checksmaysometimes be removed by planing:however,if drying con-Fig.2-13.Checks and shakes ina cottonwood log.(Photo-graph by authors.) tinues,theymayreappear.Themajorstructural effectof checksistoreducelongitudinalshear strength,oftenthedecidingfactorindesigning timber beams. CompressionWood Compression wood is a form of "reaction wood," inwhichanunsymmetricalgrowthpatternhas SURFACE SHAKE THROUGH SHAKE WOOD STRUCTUREANDPROPERTIES25 SURFACE CHECK TH ROUG H CH ECKS Fig.2-15.Typesof check. beencausedbylong-termbendingstressesin the live tree.For example, a tree that leans badly during its growth will have to resist a high bend-ingmomentcausedbythetree'sownweight. Compressivestressesduetothisbendingoccur onthelowerside,andtensilestressesdueto bendingoccurontheotherside.Thecompres-sivestresses,beingcombinedwiththedirect vertical compressive stress, PI A,arethehighest stressesfortheentirecrosssection.Thesehigh stressescausethetreetogrowthickerannual rings on that side,asshown by Fig.2-16. Cellwallsincompressionwoodarethicker, andspecificgravityishigherthanfornormal wood.Also,intheinternalmicrostructureof a compression wood fiber,the angle of inclination RING SHAKE HIGHERDENSITY BUT LOWERTENSILEAND IMPACT STRENGTH Fig.2-16.Compression wood. PITH SHAKE Fig.2-14.Typesof shake. 26STRUCTURAL DESIGNIN WOOD of itscomponent fibrilsisaltered in awaythat affectsphysicalproperties(10,11).Compres-sionwoodisundesirablemainlybecause,with moisturechange,itslongitudinal shrinkage and swellingareasmuchastentimesgreaterthan for normal wood. Wane Wane is the lumberman's term for the absence of wood.Often wane consists of bark remaining on thesurfaceof thefinishedlumber.Obviously, thestrengthisreducedbywane,sowaneis takenintoaccountintheallowabledesign stresses assigned to a given grade of lumber.In-clusionsof bark, called bark pockets,occasion-ally occur within the tree. These result from tree injurythatiscoveredbynewwood asthetree grows. The injury giving risetobark pockets is usually something minor, such asholes made by bird pecks or boring insects. Decay Decayedspotsrepresentportionsofthewood thatshouldbeconsideredtohavenostrength, sincethewoodfibersareinterruptedbythe decay.Further,decaycan easilyspreadtoadja-cent areas.Woodwith decay should not be used in new construction, but a designer may be called upon to evaluate the strength of an existing struc-tureinwhichdecayisalreadypresent.If the decay can be removed or its growth arrested,the structure can conceivably continue to be used. Allof theabovedefectsaffectthestructural valueof thewood,andtheyareconsideredin "grading," the process by which wood isevalu-ated and classified to indicate suitable allowable designstresses.Thiswill beshowninChapters 3 and 4.Other defects, not mentioned above, af-fect only appearance and durability. 213.THERMAL PROPERTIES OF WOOD Effect of Temperature onMechanical Properties Temperatureaffectsbothdimensionalstability and strength of wood. Wood expands as its tem-perature increases, asdo other construction ma-terials.Itscoefficientsof expansionvarywith direction, being largest radially and tangentially, andleastlongitudinally.Expressedaslength changeperunitof originallengthperdegree Fahrenheit,wood's radialand tangentialcoeffi-cients of expansion range from 0.000025X SG (specificgravity)fordensehardwoodsto 0.000045XSG for softwoodssuch asDouglas firorredwood(seereference7).Considering that thespecific gravity of thesewoodsisfrom 0.42forredwoodto0.68forcommondense hardwoods, the radial and tangential coefficients of expansionareseentobeabout0.000017to 0.000019 in'/in.fOF,or about three times as large asfor structural steel or concrete. Wood'slongitudinalexpansion coefficient is muchless.It isindependentof SGbutvaries withspeciesfromabout0.0000017to 0.0000025 (12), which isonly about 26-42% as muchasforothercommonconstructionmate-rials. Exposuretotemperaturesof 150For more forextended periodswillpermanentlyweaken wood(12).For temperaturesbetweenzeroand 150F,the 70strength of dry wood changes by '/3- 'h% per degree, being weaker at high tem-peratures and stronger at low.If the exposure to abnormaltemperaturesin thisrangeisnot pro-longed,wood'sstrengthpropertiescanbeex-pected toreturntoapproximatelytheir original values when conditions return to normal. Thermal Conductivity Wood's thermal conductivity is low compared to thatofothercommonstructuralmaterials.In other words, wood can do a better job of insulat-ing against heat loss.Often, therefore, less insu-lationisneededwithwoodconstructionthan with other types. The total thermal transmission of abuildingwall,ofcourse,dependsonthe propertiesandthicknessof eachof thevarious layers of material used. Engineering information on computingthe total thermal transmission (or resistance)of awallmaybefoundinsources such asthe ASHRAE Handbook (13). Thermal conductivity depends on several fac-tors, including direction of heat movement with respecttothegrain,specificgravity,moisture Table2-7.ThermalConductivity and RelativeInsulating Efficiency of Building Materials. Material Averagesoftwood Claybrick Glass Limestone Sandstone Concrete Steel Aluminum Insulating ConductivityEfficiencyCompared (BTU!hr!ft'!in.toWood thickness;oF)( %) 0.80100 4.816.5 5.514.7 6.512.3 12.06.6 12.66.3 312.00.25 1416.00.06 content,and typeand location of defects.Ther-malconductivityisaboutequalinradialand tangentialdirections,butisabout2.5timesas greatalongthegrain.Table2-7showsaverage valuesofthermalconductivityforwoodand othercommonlyusedconstructionmaterials andcomparestheirinsulatingefficiencies.No-tice that wood's thermal efficiency far surpasses thatof allotherlistedmaterialsthatmightbe used for structural members. 214.TESTS AND PROPERTIES OF INTEREST TO THE STRUCTURAL DESIGNER Strength and stiffnessare the wood properties of greatest interest to the structural designer. The de-signer is interested in other properties, of course, suchascreep,durability,hardness,dimensional stability, and thermal insulating qualities. Thestrength andstiffness propertiesof most interest in structural design are: 1.compressive strength parallel to the grain 2.modulus of elasticity parallel to the grain 3.tensile strength parallel to the grain 4.compressivestrengthperpendiculartothe grain 5.modulus of rupture (bending strength) 6.longitudinal shear strength (horizontal shear) 7.shear modulus For visually graded timbers (5in.X 5 in.nomi-nalandlarger),theabovepropertiesaredeter-WOOD STRUCTURE AND PROPERTIES27 mined by testingsmall clear specimensaccord-ingtomethodsspelledoutinASTMDI43(4) and ASTM D2555 (14) by the American Society forTestingandMaterials.Toobtainallowable properties,thestrengthpropertiesof thesmall clear specimens are then adjusted toaccount for defectsaccordingtotherulesin ASTMD245 (15). Priortothe1991NDS,specifiedallowables for dimension lumber (members with smaller di-mensionlessthan5in.)wereobtainedbythe smallclearspecimenapproach.Since1991al-lowablepropertiesofAmericansoftwooddi-mensionlumber havebeen determined bytests of full-size specimens. Standardtestsofsmallclearspecimensare described below. These tests are still used for de-terminingpropertiesof timbersandhardwood species of dimension lumber. Test for Compression Parallel to the Grain In this ASTM test a clear (defect-free) specimen measuring a full2X 2X 8 in.is used, with the longitudinal axis of the wood parallel to the 8-in. dimension.Longitudinalloadisapplied,in-creasing until the compressive stress iswell be-yond a defined elastic limit or until the specimen fails.Fromtheloadsappliedandlongitudinal strainsmeasuredduringthetest,astress-strain curvecanbeplotted,asshownbyFig.2-17. This curve shows several points of interest to the designer. Ultimate strength is the highest value of stress reached,shown by the ordinate at point A. "-" ... Modulus ofelasticity,also called Young's mod-A Fig. 2-17.Stress-strain curve, compression parallel to grain. 28STRUCTURAL DESIGNIN WOOD ulus, is the slope, E,of the initial straight-line part of the stress-strain curve. In the u.s. system, it is expressed in pounds per square inch (psi).(Mod-ulusof elasticity can also be foundfroma bend-ing test, but its value will differ slightly from that foundfromthecompressiontest.Thereasonis thatthemeasuredbeamdeflectionsareaffected byshear strains as well as by flexural strains.) Proportional limit is the value of stressat the upperendofthestraight-lineportionofthe curve, that is, the stress level above which stress isnolonger proportionaltostrain.Thepropor-tionallimitissometimeslooselyreferredtoas the elastic limit, but the two are not precisely the same.Point Bof thecurve in Fig.2-17isat the proportional limit. Noticethatatthebeginningof thetest,the curve of Fig.2-17 shows a slight reverse curva-ture.Thisisduetolocalcrushingof uninten-tional high spots as the ends of the specimen be-comeseatedontheplatenofthetesting machine.Tocorrectforthis,thestraight-line portion of the curve may be extrapolated down-wardtolocateanadjustedzeromark.Various typesof compression failureseen in this type of test are shown by Fig.2-18. Test for TensionParallel to the Grain The ASTM test for tensile strength parallel to the grainusesal-in.-squareclearspecimen18in. long,carefully tapered to measure only 3/8in.by 3/16in.atitsnarrowestportion.Astress-strain curve forthistestshowselasticactionalmost to thebreakingpoint.For defect-freewood,tensile strengthparalleltothegrain(longitudinally)is appreciablyhigher thancompressivestrengthin thesamedirection.However,designerscansel-dom utilize this high tensile strength,since wood " i I CRUSHING BROOMING SPLITTING WEDGESPLIT SHEARING COMBINED SHEARING AND SPLITTING Fig.2-18.Typesof failureundercompressionparallelto grain. insizespracticalforconstructionalwayshas strength-reducing defects, such as cross-grain and knots.Tensilestrengthsintangentialandradial directionsareverylow compared tothelongitu-dinal strength, about 3%and 5%, respectively. Test for CompressionPerpendicular to the Grain TheASTMtestforcompressionperpendicular to the grain illustrates clearly how much weaker woodisincompressionperpendiculartothe grainthanincompressionparallel tothegrain. Figure 2-19 shows schematically how this test is performed, and also a typical stress-strain curve. Fig.2-19.Compression perpendicular to grain. Noticethatthereisnotrueultimatestrength value.Asthewood nearest tothesurface of the specimen iscrushed, it merely becomes consol-idated and is harder to crush further.As the load increases, additional wood cells farther from the surfacealsocollapse and thestrengthincreases again.Thetest could proceed untiltheload ca-pacity of thetestingmachinewasreached.The woodspecimenwouldstillbethere,andcould accept moreload,but of courseit would be se-verely flattened.For thistype of loading(in de-sign,wecallit bearing),the failurecriterion is not reachinganultimate load,but rather reach-ing a limit beyond which the degree of distortion is considered unacceptable. Test for Flexure The ASTM static bending test defined byrefer-ence 4 uses a fu1l2-in.-square clear specimen 30 in.long,testedwithamidspanconcentrated loadandaspanof 28in.Theload-deflection curve for this test shows a definite ultimate load. Values that can be computed from this curve are given below. Modulusof rupture:Themodulusof rupture (MR)isthevalueof Me/Ionewouldcompute fb= Melt 18)ELASTICIb)AT ULTIMATE Fig.2-20.Bending stressvariation. WOOD STRUCTURE AND PROPERTIES29 usingthebendingmomentcausedbytheulti-mate load. Modulus of rupture is not the value of extreme-fiber bending stressat failure.Bending stressesunderelasticconditionsvaryasa straight line, asshown by Fig. 2-20a, whereas at ultimate load they vary asshown by Fig.2-20b. Nevertheless,theMRisausefulquantityfor comparing bending strengthsof different speci-mens, and it is used to establish allowable stress values for the designer's use. Modulusof elasticity:Byequatingthetheo-reticalexpressionformidspandeflectiontoan observed deflectionon thestraight-line portion oftheload-deflectioncurve,themodulusof elasticity, E,can be computed. this value of E is themorereliableonetouseforcomputingde-flectionofotherbeams,althoughitmaynot agree exactly with the values obtained from ten-sion or compression tests. Test for Shear Strength Parallel to the Grain Shear strength parallel to the grain is less than in anyother direction.In wood beams,thegrain is normallyparalleltothespan,andshear failure occursalongaplaneparalleltothespan,asin Fig. 2-21. (Wood beams will not fail by shearing at right angles to the span.) Since beams are usu-allyhorizontal,themeasuredultimateshear stressisoftencalled horizontalshear strength. Inthe ASTMtest forshear strength thereisno gradual yielding;failureissudden and onlythe ultimate load isobserved. ASTM D143definesother testsalso,includ-ingtensionperpendiculartothegrain,impact bending, and hardness. With each test, moisture contentofthewoodisalsodeterminedand recorded. Fig.2-21.Horizontal shear failure. 30STRUCTURAL DESIGN IN WOOD REFERENCES 1."TreeImprovement,"GeorgiaPacific,Portland,OR, 1981. 2.Thomas, R. J., "Wood Anatomy and Ultrastructure," in Wood:Its Structure and Properties,by F.F.Wangaard, PennsylvaniaStateUniversity,UniversityPark,PA, 1981. 3.Pearson,R.G.,andR.C.Gilmore,"Effectof Fast Growth Rate on the Mechanical Properties of Loblolly Pine,"ForestProductsJournal,30(5),May1980,pp. 47-54. 4.Standard Methods of Testing Small Clear Specimens of Timber,AmericanSocietyforTestingandMaterials, ASTM D143, West Conshohocken, PA,1994. 5.Tarkow,H.,"Wood and Moisture," inWood:ItsStruc-tureand Properties,byF.F.Wangaard,Pennsylvania State University, University Park, PA,1981. 6.Wood Handbook,U.S. Forest Products Laboratory, 1974. 7.WoodStructures,ADesignGuideandCommentary, American Society of Civil Engineers, New York,1975. 8.Bodig,J.,andB.A.Jayne,Mechanicsof Woodand WoodComposites,Van Nostrand Reinhold, New York, 1982. 9.Wood,L.W.,"Behaviorof WoodUnderContinued Loading," Engineering News Record,Dec.11,1947. 10.Stamm,A.J.,WoodandCelluloseScience,Ronald Press, New York,1964. II.Mark,R.F.,"MolecularandCellWallStructureof Wood," inWood:Its Structure and Properties,by F.F. Wangaard,PennsylvaniaStateUniversity,University Park, PA,1981. 12.TimberConstruction Manual,4th ed., American Insti-tute of Timber Construction, Wiley, NY,1994. 13.ASHRAE Handbook of Fundamentals, American Soci-etyofHeating,RefrigeratingandAir-Conditioning Engineers, Inc., New York,1992. 14.EstablishingClearWoodStrengthValues,ASTM D2555-88,AmericanSocietyforTestingandMate-rials, West Conshohocken, PA,1988. IS.Establishing Structural Grades and Related Allowable Properties forVisuallyGraded Lumber,ASTM D245-(01 (bl 93,AmericanSociety for TestingandMaterials,West Conshohocken, PA,1993. PROBLEMS 2-1.Awoodsampleweighs67.2grams(g)when removedfromacompressiontestspecimen. After drying in an oven until its weight is con-stant,itweighsonly56.4g.Whatwasthe moisturecontent(percent)atthetimeof the compressiontest?WastheMCaboveor below the fiber saturation point? 2-2.Theaveragespecificgravityof Douglasfir southmaybefoundfromAppendixTable Col.What istheaverageunit weight forthat species(pounds per cubic foot,or pet')? 2-3.Figure 2-22 shows two possible waysof nail-ingofboard-and-battensidingforexterior use.Whichmethodwouldbelesstrouble-some in service, and why? 24.Greenwesternlarch2XlOsareinstalledas floor joists in a heated building where the aver-agetemperaturewillbe6SOFandtherelative humidity25%.Thegreen2XlOsmeasured 9'h in.deep when installed. What will be their probable depth when the EMC is reached? 2-5.Planks of 2 X 12 coast D.fir will be used flat-wisetoformacoveroverawatertank.The plankswilldriedtoabout15%MCandwill measure111/4in.widewhen installed.Inuse theywillbe exposed forlongperiodsto90% relative humidity and80F temperature. Why should they not be placed in side-by-side con-tact with each other? How much space should be allowed between the planks? 2-6.Figure 2-23shows two alTangements for bolt-ing a wood 2X10 toa steel plate. The plank isinstalledgreen(unseasoned)andwilldry Fig.2-22.Two nailing methods for Problem 2-3. WOOD STRUCTUREAND PROPERTIES31 101Ibl Fig.2-23.Connections for Problem 2-6 .. outafteritisconnected.Whicharrangement ismorelikelytoresultinthemembersplit-ting,and why? 2-7.FigureI-Ishowed thecrosssection of a two-story light-frame wall. Assume that the sill is a 2X6,floor joists are2X12s,and the length ofthe studs in each story is 7 ftlOin. (See Ap-pendix Table B-1for actual dimensions.) (a)Recognizing that shrinkage inthickness of theplywoodfloorsheathingisnegli-gible,howmuchwillthedistancefrom thetopof theconcretefoundationwall to the underside ofthe second-floor ceil-ing joists decrease, assuming that (1)allwood isponderosa pine; (2)MCwas19%whenthewoodwas installed; (3)averagerelativehumidityinthe buildingandsurroundingswillbe 30%; (4)averagetemperatureofthewood will be70F;and (5)annualringsareintheworstposi-tion for vertical shrinkage. (b)What sort of trouble could thiscause? 2-8.A 2X 2 clear specimen is tested asa beam on a28-in.simplespan.Thebeamfailsundera midspan vertical concentrated load of 900 lb. Measurementsofactualdimensionsshowed thespecimen tobe2.02in.wideand1.96in. deep (vertically). What isthe modulus of rup-ture? 2-9.Assuming that the bending test of Problem 2-8tookfiveminutestoperform,whatwould betheprobablevalueof modulusof rupture fora similarspecimenunder a loadingof ten years' duration? 2-10.Whentheloadatthecenterof thebeamof Problem2-8was400lb,themidpointof the beamhaddeflected0.12in.Consideringthat theeffectof sheardeformationisnegligible, what is the modulus of elasticity, E? 2-11.For a seasoned wood whosespecific gravity is0.47,whatwouldbetheapproximate small-clearvaluesfor:(a)ultimatecom-pressivestrengthparalleltothegrain;(b) compressivestrength(proportionallimit) perpendicular tothegrain;and(c)modulus of elasticityincompressionparalleltothe grain? 3 ProductionandGrading ofSawnLumber 3-1.LUMBER PRODUCTION Lumber productionhasgrownfromtheoldaxe and two-person saw process to a sophisticated op-eration involvingspecialized machinery,suchas multiplebandsawsorlaser-beamcutterswith digital control, and even optimization techniques toincreasetheyieldof usefulmaterialfromthe log. To the engineer or wood scientist who wishes tospecializeinit,woodproductionpresents many interesting technical challenges. The struc-turaldesigner,however,needstoknowlittle about lumber production, except as it might affect either selection of material or the performance of that material in the completed structure. Where treesaregrown on tree farms,lumber qualityandyieldareinfluencedbythecare given to the trees-thinning and pruning, for ex-ample. Lumber production begins with logging, the fellingand trimmingof the trees. Following loggingthestepsin lumber production mayin-clude (but not necessarily in thisorder)debark-ing, air seasoning, rough sawing and finish saw-ing(bycircularsaws,multiplebandsaws,or laser),asshown in Fig.3-1,surfacing (by plan-ingorsanding),chemicaltreatment(preserva-tiveor fireretardant),andkilndrying.For the structuraldesigner,theimportantproduction featuresarethosedealingwithstandardsizes, type of surfacing,typesof "cuts,"and moisture condition. 3-2.STANDARD SIZES OF LUMBER Softwood lumber isnormallyavailablein even-foot lengths,from 6 ft to24 ft or 26 ft.In some 32 instances, the structural designer can save money foraclientbyavoidingdesignsthatcallfor lengthsthatarejustslightlylongerthanthese standard lengths.For example,if a floorsystem required a total of 48 joists, each 18 ft 3 in.long, thebuilderwouldhavetopurchase48pieces each 20 ftlong. A total of 84 lin ft of joist mate-rial would be wasted, unless a use could be found for the 1 ft 9 in. pieces cut off. Being made aware of the possible saving,the client or thearchitect mignt consider the wisdom of reducing the width of thestructurebythreeinchestopermitusing joists 18 ft long. Hardwoods are also available in 2-ft length increments, and occasionally it is pos-sible to obtain odd-foot lengths. Quotedlengthsof lumberareactualdimen-sions.However,widthsand thicknessesof lum-ber forstructuraluseareexpressed innominal dimensions.Thesearewhole-inchdimensions correspondingapproximatelytothesizeof the unseasoned,rough-cutpiecesfromwhichsur-facedlumber isproduced.For example, a piece of woodmeasuringalmost2in.thickby4in. wideintheroughconditionmaybesurfaced (smoothed) to produce a piece of dressed lumber measuring1.5in.by3.5in.,butthissmaller piece isstill called a 2X4. The thickness of wood removed to smooth the surfacesof rough lumber varieswiththerough dimensionandwiththemoistureconditionof thewood.Larger roughmaterialwillprobably notbeasstraightassmaller-sizedroughmate-rial, and surface defects may be larger. For either reason, a greater wood thickness may have to be removed from large sizesof rough lumber than fromsmallersizes.TheAmericanSoftwood J. J. Stalnaker et al., Structural Design in Wood Chapman & Hall 1997PRODUCTION AND GRADINGOF SAWN LUMBER33 Fig.3-1.Band saws reducing log toslabs. (Courtesy Western Wood Products Association.) LumberStandard(1)definesthefinaldimen-sions required for the finished lumber. Table 3-1showstheminimum standard sizes fordressedstructurallumberintheUnited States. Notice that there are two sets of standard dimensions-oneforlumberthatissurfaced after it is dry and one for lumber that is surfaced whilegreen.The reason forthisisthat alllum-ber eventuallyreachesanequilibriummoisture content (EMC)for the average temperature and relativehumiditywhereit isinstalled.Solum-ber surfaced greenismadethicker(andwider) thansimilarlumberthatissurfacedwhenitis dry(19%orlessMC).Theobjectofhaving thesetwostandardsistocausepiecesof each variety to be of the samesize when they reach a commonEMC,thesurfaced-greenlumber shrinkingasit driesandthesurfaced-drymate-Table3-1.Minimum StandardSizes forDressedLumberfor StructuralUse. Dimensionlumber Timbers Nominal 2' 3 4 5 or more Thickness(in.) Minimum SurfacedDry I! 2! 3! Surfaced Green 1ft, 2ft, 3ft, less than nom. Nominal 2" 3 4 6 8 10 12 14 16 5 or more aln 2-in.nominalthickness,widthsgreaterthan14-in.nominalareusuallynotavailable. Width(in.) Minimum SurfacedDrySurfacedGreen 1ft, 2! 2ft, 3ft, 94 II!134 jlessthan nom. 34STRUCTURAL DESIGNIN WOOD rialeitherswellingorshrinking,dependingon whether the EMC is more than or less than 19%. Withfewexceptions,nominal dimensionsof lumbernormallyavailableintheUnitedStates for structural purposes vary by 2-in. increments. Thus(except fornominal3-in.and5-in.mate-rialthat issometimesavailable)nominalthick-nessesand widths,in inches,are even numbers. TableB-1inthe Appendixof thisbookshows thestandard nominalsizesof sawn lumber and the actual dimensions expected at19% MC. Older Size Standards The dimensionsshown in Table 3-1and Appen-dix Table B-1were accepted asthestandard for theUnitedStatesinabout1970.Adesigner doingremodeling,repair,or preservationwork on older buildingsshould be aware that prior to 1970thestandarddimensionsweredifferent. Forlumbermanufacturedbefore1970,thick-ness and width (for lumber of up to12 in.nomi-nalwidth)weregenerallyI/Sin.to1/4in.more than shown by Tables 3-1and B-1. For example, a 2X 4 measured1% in.by 3%in.,asopposed to today's11/2in.by 31h in. However, in buildingsdating back tothe early part of this century or before, lumber dimensions maynot conform even tothat pre-1970standard. Forexample,inworkingonrestorationandre-modelingof anhistoric building,theauthorsen-countered existing first-floor joists that were actu-allyafull8in.deepby21f16- 21/4in.wide. Second-floorjoistswereonlyslightlydifferent. Eachwereprobablyreferredtoas2X8s.When adding modem 2 X 8s to reinforce the floor, it was necessarytoinstallshimsundertheendsof the new joists to make their tops flush with the tops of theoriginaljoists.Designersworkingwithold structures should always measure to determine the actual size of existing wood members. The problem raised bycombining lumberof older and present size standards iscompounded bythe effects of shrinkage.Structural members inplaceinoldstructuresprobablyhaveavery low moisture content, whereas new lumber being addedprobablyhashigherMC.Thus,some shrinkage of the newer lumber may be expected, and the amount of shrinkage may add to the dif-ference between old and new dimensions. 33.FINISH DESIGNATIONS Most lumber used in building design issurfaced on both facesand on both edges and isreferred toasS4S (surfaced on foursides).If a designer specifiesa nominal size of lumber but does not define which facesaretobe finished,the mate-rialsuppliedwillmostlikelybeS4S.If some other combination of surfaced and rough faces is desired,however,thedesignermayspecifyit. OthercombinationsaredefinedbysymbolsS forsidesandEforedges.Examplesof these combinations include: S2S1E (surfaced on two sides and one edge) S2S(surfaced on two sides only) S1S2E (surfaced on one side and two edges) If lumber other than S4S isdesired, the designer will have tospecify it. If the item is not stocked, aspecialmillordermayberequired,andthe builder mayhavetowait forthematerialtobe produced.A probableexceptionisroughmate-rial (not surfaced), which is frequently available in some of the larger standard sizes. 34.CUTTING PATTERNS Figure 3-2 shows three patterns in which lumber may be cut from the log. The difference between patterns isonly the position of theannualrings relative to the wide face of the piece. The impor-tanceof thisdifferenceisthatshrinkage,warp-ing,abrasionresistance,easeofpainting, appearance,andotherpropertiesalldifferac-(.1SLASHCUT (PLAIN SAWEDI C=::::3-1--- 0)))))))) ))) I IblRIFT CUT (QUARTER SAWEDI (elBASTARD CUT (BASTARD CUTI Fig.32.Types of cut (hardwood names in parentheses). PRODUCTIONANDGRADINGOFSAWNLUMBER35 cording to the position of the annual rings on the cross section. Figure 3-2a shows lumber cut so that the wide faceisnearlyparalleltotheannualrings.In softwoods, this is called slash cut lumber, and in hardwoods plain sawed.Figure 3-2b shows lum-ber in whichtheringsaremorenearlyperpen-dicular to the wide face. This is called rift cut for softwoods and quarter sawed for hardwoods. Of course,therewillbemanypiecesintermediate between thesetwo.Pieceswith the ringscross-ingat about 45degrees,asshown by Fig.3-2c, are known as bastard cut.Rift-cut lumber is also known as vertical grain or edge grain,and slash cut as fl