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Handbookam TimberEnttineering

BUREAU OF INDIAN STANDARDSMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARGNEW DELHI 110 002


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SP : 33-1986FI RST PUBLI SHED MARCH 1987First Reprint October 19880 BUREAU OF INDiAN STANDARDS

UDC 674(021)ISBN 81-7061-018-4

PRICE Rs 175.00

PRINTED IN INDIAAT KAPOOR ART PRESS, A38/3 MAYAPURI , NEW DELHI 110064AND PUBLI SHED BYBUREAU OF INDIAN STANDARDS. NEW DELHI 110002


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SPECIAL COMMITTEE FOR IMPLEMENTATION OFSCIENCE AND TECHNOLOGY PROJECTS (SCIP)

CHAIRMANDr H.C. VisvesvarayaNational Council for Cement and Building Material::. New DelhiMEMBERSSHRI A. K. BANERJEE

REPRESENTINGMetallurgical & Engineering Consultants (India) Ltd,Ranchi

SHRI J. D. CHATURVEDI Planning Commission, New DelhiDIRECTOR Central Building Research Institute (CSIR), RoorkeeSHRI GURNAMSINGH Ministry of Food & Civil Supplies (Finance Division)SHRI U. R. KURLEKAR Ministry of Food and Civil SuppliesDR M. RAMAIAH Structural Engineering Research Centre (CSIR),MadrasSHRI G. S. RAO Central Public Works Department (CDO), New DelhiSHRI A. CHAKRABORTY(Alternate)SHRI T. S. RATNAM Ministry of Finance (Bureau of Public Enterprises)SHRI V. RAO AIYGARI Department of Science & Technology, Nkw DelhiSHRI G. RAMAN Indian Standards Institution, New Delhi(Member Secretary)

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FOREWORDUsers of va rious civil engineering codes have been feeling the need for explanatoryhandbooks and other compilations based on Indian Standards. The need has beenfurther emphasized in view of the publication of the National Building Code ofIndia in 1970 and its implementation. The Expert Group set up in 1972 by theDepartment of Science and Technology, Government of India, carried out in-depthstudies in various areas of civil engineering and construction practices. During thepreparation of the Fifth Five-Year Plan in 1975, the Group was assigned the task ofproducing a Science and Technology plan for research, development and extensionwork in the sector of housing and construction technology. One of the items of thisplan was the production of design handbooks, explanatory handbooks and designaids based on the National Building Code and various Indian Standards and otheractivities in the promotion of the National Building Code. The Expert Group gavehigh priority to this item and on the recommendation of the Department of Scienceand Technology, the Planning Commission approved the following two projectswhich were assigned to the Indian Standards Institution:

a) Development programme on code implementation for building and civilengineering construction, andb) Typification for industrial buildings.A Special Committee for Implementation of Science and Technology Projects(SCIP) consisting of experts connected with different aspects was set up in 1974 to.advise the ISI Directorate General in identification and guiding the development ofthe work. Under the first programme, the Committee has so far identified subjectsfar several explanatory handbooks/compilations covering appropriate IndianStandards/ Codes/ Specifications which include the following:Design Aids for Reinforced Concrete to IS : 456-l 978 (SP : 16-1980)Explanatory Handbook on Masonry Code (SP : 20-1981)Explanatory Handbook on Codes of Earth 4.uake Engineering(IS : 1893-1975 and IS : 4326-1976) (SP : 2-1982)Explanatory Handbook on Indian Standard Code of Practice for Plain andReinforced Concrete (IS : 456-1978) (SP : 24-1983)Handbook on Concrete Mixes (SP : 23-1982)Handbook on Causes and Prevention of Cracks in Buildings (SP : 25-1984)Summaries of Indian Standards for Building Materials (SP : 21-1982)Handbook on Concrete Reinforcement and Detailing (SP : 34-1986)Functional Requirements of BuildingsFunctional Requirements of Industrial Buildings (SP : 32-1986)Foundation of BuildingsSteel Code (IS : 800)Building Construction PracticesWater Supply and Drainage with Special Emphasis on Plumbing (SP : 35-1987)Bulk Storage Structures in SteelFormworkFire SafetyConstruction Safety PracticesTall BuildingsInspection of Different Items of Building WorkLoading CodePrefabricationThis Handbook, formulated under this project, provides information on thefactors that influence timber engineering design and discusses them in detail. Itprovides explanatory review of Indian Standards on timber engineering and PartVI/Section 3 Wood of National Building Code of India 1983. The relevantliterature available on the subject has been considered while preparing theHandbook. The scope of the Handbook is, however, restricted to the use of solidtimber and does not cover wood products, such as plywood, laminated material and


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Wherever the symbol NBC is used, it refers to Part VI/Section 3 Wood of theNational Building Code of India 1983. Other important features of the Handbookhave been highlighted in the introduction.The Handbook, it is hoped, will be useful to designers of timber structures, fieldengineers and also laboratories engaged in design, research and testing of structuraltimber.The Handbook is based on the first draft prepared by Shri A.C. Shekar, Retired

Director, Forest Products Research, Forest Research Institute and Colleges, DehraDun. The draft was circulated for review to Inspector General of Forests, NewDelhi; Forest Research Laboratory, Bangalore; Dr A.N. Nayer, New Delhi;Directorate General of Technical Development, New Delhi; Western India PlywoodLtd, Cannanore; Indian Plywood Manufacturing Co Pvt Ltd, Dandeli; IndianPlywood Industries Research Institute, Bangalore; Forest Research Institute andColleges, Dehra Dun; Central Public Works Department, New Delhi; and PublicWorks Department (J&K), Srinagar, and views received have been taken intoconsideration while finalizing the Handbook.


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CONTENTSPage

1. INTRODUCTION2. TIMBER FOR STRUCTURAI, PIJKPOSES

2.1 General2.2 Growth and Development of Timber Engineering in India2.3 Timber Resources of India for Structural Purposes2.4 Selection and Identification of Structural Species2.5 Grading Practices, Standards and Preferred Sizes

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3. MECHANICAL AND PHYSICAL PROPERTIES OF1NDIAN WOODS 163.1 Methods of Test and Evaluation of Properties 163.2 Factor Affecting Strength Properties 193.3 Factors of Safety and Working Stresses 233.4 Reliability of Data and Confidence Limits 27

4. TIMBER PROCESSING TECHNIQUES4.1 Introduction4.2 Log Storage and Conversion4.3 Timber Seasoning4.4 Timber Preservation4.5 Fire-retardant Treatments4.6 Other Processes and Wood Products

303031::4040

5. TIMBER DESIGN-PART 1 425.1 General Considerations5.2 Beams (Flexural Members)5.3 Columns5.4 Combined Stresses

. TIMBER DESIGN-PART 2

4242505253

6.1 Timber Trusses6.2 Special Constructions6.3 Timber Fasteners ii:797. TEST METHODS FOR TIMBER STRUCTURES ANDCOMPONENTS

7.1 Introduction7.2 Destructive Tests7.3 Proof Tests7.4 Tests on Joints and Connectors

8586::

8. FABRICATION, CONSTRUCTION METHODS ANDPRECAUTIONS 878.1 General8.2 Fabrication and Assembly8.3 Light and Heavy Constructions8.4 Erection and Fixing

9. WOOD FINISHES, PROTECTION AND MAINTENANCE

87:;9090

9.1 Wood Finishes 90

85


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10. FUTURE POSSIBIL IT IES10.1 Introduction10.2 Glue-lam Engineering10.3 Plywood Engineering10.4 Other Forest Based and Biological Materials10.5 Prefab Techniques10.6 Computer Techniques

APPENDIX A INDIAN STANDARDS USEFUL FOR TIMBERENGINEERING

939:9494959595

APPENDZX B IMPORTANT INDIAN PUBLICATIONS IN 97TIMBER ENGINEERINGAPPENDIX C A DICHOTOMOUS KEY FOR IDENTIFICATION 99OF 25 COMMERCIAL TIMBERS OF INDIAAPPENDIX D SUMMARY OF SCHEME OF TESTS ON 102

SMALL CLEAR SPECIMENS OF TIMBERAPPENDIX E SUMMARY OF SCHEME OF TESTS ON 106

STRUCTURAL SIZES OF TIMBER BASEDON IS : 2408-1963APPENDIX F LIST OF TIMBER DESIGNS DEVELOPED AT 107

THE FOREST RESEARCH INSTITUTEAPPENDIX G STRUCTURAL SPECIES OF DIFFERENT 109

GROUPS, THEIR TRADE NAMES, OTHERINDIAN NAMES, AVAILABILITY,DURABILITY, TREATABILITY ANDREFRACTORINESS

APPENDIX H MECHANICAL PROPERTIES OF VARIOUS 130STRUCTURAL SPECIES

APPENDIX J TYPICAL NUMERCIAL EXAMPLE FOR 137CALCULATING THE COMPARATIVESUITABILI TY INDEX FROM THE BASICMECHANICAL PROPERTIES

APPENDIX K WORKED EXAMPLES 138


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1. INTRODUCTION1.1 Wood is one of the earliest materials used bymankind for building purposes and has continuedto attract attention in one form or other inspite ofmany competitive building materials and their useunder highly developed technologicalconsiderations. Wood is a biological materialfrom renewable resources, and extensively used instructural designs and other engineering fields.However, timber engineering developments areonly of recent origin in India. It is only in the pastthirty years, new design and test methods havebeen developed, a few types of properly designedwood construction have come up, and wood hasbeen recognized as an important building material(see National Building Code of India 1983).1.2 The present use of wood and other cellulosicmaterials, from forests, such as bamboo. thatch,reeds for residential and commercial types ofbuildings varies from about IO percent of the totalcost in cities and urban areas, to even 160 percentin hills and remote villages where modernarchitecture linked with economic improvements,has not yet made much impact. In some of thedeveloped countries of the west, use of timber hasbrought out several new types of architecturaldesigns also, and wooden structures are reportedto have stood the test of time for several years.Even in this countrv examnles exist in villages.

consolidate and update the information availablein this field and which is not full!, cover-cd in NBCand several Indian standards. It also provides ahandy document to engineers, architects andbuilders, and other technologists dealing in woodconstructions. The material contained is generallyrestricted to experiences in India and on Indianspecies only. but attention has also been drawnwherever necessary to some cases of design trendsin other countries which may prove useful forfuture development in this country also. Forobvious reasons of insufficient data andexperience in this c


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SP : 33(S&T)-1986perpendicular to the same known as longitudinal,radial, and tangential directions respectively (seeFig. I). Like some other materials, it is alsoelasto-plastic even at room temperatures. that is,it shows time-dependant strains under someminimum stresses known as limits-of-plasticflow.

t LONOITUOINAL

ABC0

ASFFijAONE

CROSS SECTIONlANOCf4T1ALFACESRADIALFACESHEART SlOlSAP SIDE

FIG. 1 PRINCIPAL DIRECTIONS AND SURFACES OFSYMMETRY IN TIMBER

2.1.2 Thus. its properties exhibit variations notonly due to different species of wood, differenttrees of same species, different localities andconditions of growth, but also due to methods oftest and conditions of materials at test. Hence inthe use of such a material with the requiredconfidence for safety and wide acceptability, therein an absolute need of a high degree ofstandardization to obtain test data based onstatistical and technological considerations, andthere is a need for a strict code to ensure economyand safety. In the following sections all theseaspects have been discussed mainly from the pointof view of a structural engineer using timber.2.1;3 During the last fifty years, the ForestResearch Institute, Dehra Dun, had systemati-tally est ablished identification characteristicsof different species of woods grown in India,evaluated several of their important propertiesand established suitability of various species fordifferent industrial and engineering purposes.From considertion of their availability and otherimportant properties, like mechanical andphysical properties, durability, refractoriness todrying, workability, glueability, etc, out of nearly300 commercial species tested so far, only about

85 have been identified as useful for engineeringstructures (see NBC 1983). However recentanalysis of strength data of more species haveindicated possibilities of utilizing many otherspecies for structural purposes under different

conditions. These aspects also have been discussedin the following sections.2.2 Growth and Development of TimberEngineering in India

2.2.1 Timber has been used in Indiastructurally as well as non-structurally from timesimmemorial for various types of house buildingpurposes. In the early days there was no scientificdata for proper designs, and consequently manyof the early attempts on timber structures havebeen attributed only to hit and try methods,craftsman techniques, using only durable species,particularly teak. Sometimes protection to somenon-durable woods against decay and insects wasgiven only by surface coating of oils, paints, etc,which were not fully effective under all types ofcircumstances. Occassionally, instances can alsobe observed when protection was given to woodby simple smoking and charring methods.However, by about 1920, mechanical tests ondifferent species under somewhat standardizedconditions began at the Forest Research Institute,Dehra Dun. In due course of time, by about 1935,these results yielded quite reliable data fordeveloping working stresses and designii:g ofsimple beams, columns and small trusses. Testswere also started on structural sizes of timber asearly as 1936 and about the same period a notewas also brought out by S. Kamesam onprotectional methods for structural timbersagainst fire, termites. borers and fungi.Subsequently conventional trusses of spansranging from 3 to 10 m have begun to be designedand were erected in different parts of the countryby different constructional agencies usingordinary bolts and metal bindings. In some of thearmy specifications, some of the strong anddurable woods were recommended based onstandardized designs of earlier British defencespecifications. These were largely used fortemporary constructions during World War II.However, with the creation of a new TimberEngineering Branch in 1950 at the ForestResearch Institute, Dehra Dun, smalldimensioned stock came into use for structures,using nails and bolts, etc. Dowel discs and otherconnectors were developed which weresubsequently utilized in the various types of lightand heavy constructions. The Indian species bythis time have been classified according to theirstrength, durability, refractiveness to drying, andother characteristics. On the basis of theseclassifications, and further studies on effects ofdefects, developments have taken place in gradingstructural timber not only according to theirstrength and durability but also according to theiravailability and possible end uses and whether fortemporary or for permanent constructions. Alsowith the increasing costs, and shortage ofnaturally durable timbers like teak, sal, deodar,etc; and with developments in the processingtechniques, the secondary species which wereoriginally considered unsuitable, were used under

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SP : 33(S&T)-1986proper treated conditions with the sameconfidence as conventional timbers. The designaspects of timber structures had been codified andincorporated in the National Building Code andseveral Indian standards have come up for use oftimber in building industry. Indian standards forspecial types of constructions also have come up.Considerable amount of published literature isnow available on research and developments thattook place in the last three decades in India. Moreimportant Indian specifications and technicalliterature are given in Appendices A and B. Withmore developments in wood technology,particularly plywood and laminated materials, itis hoped more timbers and new designs of timberproducts will come into use for structures. Thewide publicity and extensive demonstrationstructures are required in order to develop greaterconfidence, and introduce timber designs inseveral regional and institutional constructioncodes.2.3 Timber Resources of India forStructural Purposes

2.3.1 Indian has about 75.3 million hectaresor about 24 percent of land area under forests,which are unevenly distributed. All the forests arenot ftilly inventoried for the different speciesthough currently some efforts are going on forsystematic survey of forest resources in thedifferent regions. Much of the currently availableinformation in the National Building Code and inmany of the Indian Standards, as reported in theHandbook is based on the data supplied by thelocal forest departments on the basis of theirpreliminary surveys only.

2.3.2 These forests contain a variety of plantlife. Of these, woody plants alone constitute morethan five hundred species distributed in differentparts of the country. Commercial wood is knownas timber. All commercially utilizable wood, forexample, timber, comes from two main types oftrees known as gymnosperms and angios-perms. Trees of the former type are also knownas coniferous trees which have needle type leaves.Among the trees of the latter type, there are twofurther divisions: (a) dicotyledons and (b)monocotyledons, the names suggesting that theirseeds contain two lobes or single lobe.Dicotyledons are also refered to as broad-leavedtrees, and the monocotyledons produce meterialwhich have bundled type of fibres. like palmyrahand bamboos, etc, which are used generally in theround or circular sections as such for obtainingtheir full strength. Timber from coniferous trees issometimes referred to as softwood and that fromthe broad leaved trees is referred to as hardwoodthough the nature of the wood from the respectivetrees has nothing to do with softness or hardnessas the terms are commonly understood. There aremany hardwoods which are soft in nature likecotton wood (semul) or balsa. Similarly there arequite a number of softwoods which are fairly hard

like fir, deodar, etc. Hence the use of the termssoftwoods and hardwoods are beingdiscouraged in Indian standards for identifyingthe origin of the wood, and the words coniferousand non-coniferous or broad-leaved for treesand woods are being increasingly used in manypublications.2.3.3 All the species are known by their

botanical names (as Tectona grandis) consistingof two parts; the first is the generic name,indicating the genera from which the woodymaterial has come, and is always writtenbeginning with a capital letter. The second partindicates the name of the particular species, and iswritten beginning with a small letter. It is alwayspossible that under the same genera there can betwo or more species with distinct botanicalidentity (for example, Terminalia tomentosa,Terminalia ujuna, etc). However commercially,the woods are known by trade names and it ispossible the same trade name is used for morethan one species but belonging to the same genera(for example, Gurjan and several dipterocarpusspecies). A list of botanical names andcorresponding trade names of all species alongwith abbreviations used for the species are givenin IS : 1150-1976. In most of the Indianstandards, both the botanical and the trade namesare usually given. In the recent years, there havebeen some changes in botanical names in the lightof fresh knoweledge about the botanicalcharacteristics of the trees from which wood isobtained, and in accordance with internationalrules for naming the species. For this purpose thebotanical names are sometimes followed by namesof the authors who so named the species, such asM ang$era i ndica Linn and Albizzia proceraBenth but this is not a regular practice in alltechnical literature. IS : 399-1963 gives botanicalnames and trade names of commercial timbers.Some of the botanical names have subsequentlybeen changed and are reflected in NBC 1983.However, the trade names remain generallypermanent as they came to be used popularly andwidely both in commercial markets and technicalliterature. These timbers are also known by theirlocal names in regional markets.

2.3.4 The total recorded production of woodin the counfry is roughly estimated as 25 millionrn3 per annum of which approximately 10 millionrn is currently demanded by various industriesincluding saw mills and construction industry. Ofthis,. one can safely assume that approximately 2.5million m3 is now utilized in rural and urbanhouses and nearly 0.5 million rn3 for non-residential construction, and the balance of timberis used for other industries, such as plywood,railways, furniture, agriculture and pulp factories.It is, however, estimated that about 30 percent ofthe building materials is in the form of timberrequired for construction industry. On this basis,the current demand for timber for construction isexpected to be of the order of about 6 million m3

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SP : 33(S&T)-1986and this will increase more rapidly in the future.Although there are trends of increasing forestproductivity to meet the demands of variousindustries and building constructions, the gapbetween availability and demand in future is stillexpected to increase and hence there is a need ofgreater economy and rational utilization of timberby developing new techniques in structuralapplications of timber. A number of othersuggestions such as developing improved productsof wood for structural purposes, utilizing morespecies and the odd sizes of trees hithertoconsidered not usable, raising fast growing speciesin forests, obtaining quick rotations in felling thetrees, etc, are also under active consideration ofthe concerned authorities.

2.3.5 As known by the data suplied by variousforest departments and the Forest ResearchInstitute, nearly about 85 species ha.ve beenindentified in the National Building Code asuseful for structural purposese; and their primarycharacteristics, such as weight, availability,durability, refractoriness and treatability are giventherein. Recent analysis of strength data of morespecies indicate the possibility of using some morespecies. These have been indicated in Appendix Halong with other properties.

2.3.6 According to IS : 3629-1966 all thestructural species are classified into threegroups-super, standard and ordinary-basedon their modulus of elasticity for green timber asobtained under standard tests and also flexurestress in bending as obtained under workingstresses (see 2.3). But NBC classifies these intogroups A, B and C based on modulus ofelasticity and gives permissible stress in bending,tension and flexural. However, as structuralengineering design considerations of use of timberare based on permissible stresses obtained fromstandard tests, for all reactions like bending,compression and shear, these are also given inTable I for the three recognized groups alongwith standard specific gravity on which allproperties generally depend, and which generallyserves as a ready guidance for selection (see SlNo. 26 of Appendix B). According to a system ofgrouping several strength factors (see also 3.1.4)and also expressing composite strength figurescompared to teak as 100 the limiting compositestrength figure as used inIS : 399-1963 is alsogiven in the same table for the three groups.Earlier edition of IS : 833-1977 also suggests thatpermissible stress in bending of standard gradetimber, under use in inside locations, as additionallimiting factor for structural grouping. These andother corresponding permissible stresses are alsoindicated in the same table for comparison.Although, as mentioned in NBC, modulus ofelasticity alone may be generally sufficient forclassification of any species under differentgroups, the other criteria mentioned above alsoserve as a check and would be helpful to place anyspecies in the respective groups with greater

confidence. This has been necessary in somespecies, such as Carallia lucida, Glutatravoncorica, Pinus lon[f olia, some dipterocarpusand Terminalia species, Zanthoxl?lum rhetsa, etc.2.3.6.1 When the same species in differentlocalities fall in different groups, the group ofmajority localities is taken, such as Shorearobusta (Sal) of Assam, Bengal, Bihar and UP falls

under Group A but that of MP falls under GroupB. Similarly Tectona grandis (teak) of MP andOrissa falls under Group C but of Kerala,Maharashtra, UP and Bengal falls under GroupB. Hence the above two species are taken inGroup B. Similarly although Stereospermumchelonoides tested from Madras falls underGroup B yet the same tested from West Bengalfalls under Group C. Also other Stereos ermumspecies Ii ke Stereospermum scc* aveolens rom UPand Stereospermum xylocarpum from Madrasfall under Group C. Hence all the stereos-permum species have been shown under Group Conly. As grouping of species for structural speciesis associated with safety and economy, in the longrun, whenever there has been any doubt forappropriate grouping with regard to differentstrength characteristics and in relation to localityof growth, the safer side, that is, lower groupinghas been adopted. It may be noted that differentspecies of same genera may fall into differentgroups, such as Calophyllum tomemtosum inGroup B, and Callophyllum wightianum in GroupC. Similarly, Terminalia arjuna, T bellerica, T.bialata, T. chehula, T. manii, T. paniculata, andT. tomentosa all fall under Group B, butTerminalia m!riocarpa, and T. procera fall underGroup C.

2.3.6.2 In a seminar on timber and timberproducts for building purposes held in Dehra Dunin 1977, a suggestion was made that there shouldbe a fourth group, Group D also, for listing suchtimbers which are neither durable nor treatable,to be used for extremely temporary structures orother semi-structural purposes in service work.However, this has not been standardized as such,nor does it seem to stand logical permanentclassification, as any timber can be used for suchpurposes, and technological developments inwood preservation are always directed towardsimproving durability and treatability of all suchspecies.

2.3.6.3 When in any structure all the piecesof timber are from the same species, then it maybe useful to employ the working stresses of theparticular species as given in Appendix H.However, when a number of species are requiredto be used together, it is advantageous if all thespecies are of the same group and the workingstresses of the particular group for a givencondition as given in Table 2 would be justifiedfor maximum efficiency and economy. Themethods of deriving the working stresses arediscussed in 3.3 in greater detail.

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TABLE 2 PERMISSIBLE STRESSES FOR GRADE 2 TIMBER(Clause 2.3.6.3)

LOCATION PERMISSIBLESTRESSES, kgf/cmz/ A 3Group A(E = 126) Group B Group C(E=98 to 126) (E=56 to 98)

(I) (2) (3) (4) (5)Bending and tension along grain Inside locations 182 123 84

Outside locations 152 102 70Wet locations 120 81 60

Shear Horizontal, all locations* 12 9 6Along grain, all locations 17 13 9

Compression parallel to grain Inside locations 120 70 64Outside locations 106 63 56Wet locations 88 58 46

Compression prependicular to Inside locations 60 22 22grain Outside locations 46 18 17

Wet locations 38 15 14*The values of horizontal shears to be used only for beams. In all other cases shear along grain to be used.

2.3.7 For purposes of knowing the availabilityof the species India has been divided into fivezones and the approximate quantity available ineach zone is given in NBC 1983 and also inIS : 399-1963.a) North Zone - Jammu and Kashmir.

b)Punjab, Himachal Pradesh, Delhi, UttarPradesh and Rajasthan;East Zone - Assam, Manipur, Tripura,West Bengal, Bihar, Orissa, Sikkim, Bhutan,Andamans, North East Frontier Agency andNagaland;

cl

4

Centre Zone - Madhya Pradesh, Vidhar-bha areas of Maharashtra and the North-East part of Andhra Pradesh (Godavaridelta area);West Zone - Maharashtra (except Vidhar-bha areas), Gujarat and North-West part ofKarnataka;

e) South Zone -Tamil Nadu, AndhraPradesh (except the Godavari delta area),Kerala and Karnataka (except North-Westpart).However, it may be noted that the data onavailability in different zone is only approximateand collected quite sometime ago. In the recentyears the inter-zonal transport systems have beenimproved very much, and quite a lot of timbermovement has been feasible economically. Hence,the total approximate quantity available in thecountry as a whole has been estimated and the

principal areas of growth and availability alongwith trade names and other local names have beenindicated in Appendix G. ,Species which are in6

heavy demand for other important industries havealso been indicated accordingly.2.3.8 Species of wood are to be chosen alsoaccording to their durability. The durability ofspecies have been classified in IS : 399-1963according to the behaviour of the heartwoodsticks in grave-yard test, sometimes known as

stake tests. Test specimens of untreatedheartwood of different species of size 60 X 5 X 5cm are buried to half their length in the ground indifferent representative test centres in the country,containing different types of wood destroyingagents. The average results are analyzed and thespecies are classified for durability as below:High (that is, Class I of IS : 401-1982) (H) :Species, the test specimens of which indicateaverage life of 120 months or more, when testedas above.Moderate (that is Class 2 of IS : 401-1982)(M) : Species, the test specimens of which indicateaverage life of less than 120 months, but 60months or more when tested as above.Low (that is, Class 3 of IS : 401-1982) (L) :Species, the test specimens of which indicateaverage life of less than 60 months, when tested asabove.It may be indicated here that the above is notthe actual life for designed structures. The aboveis only a comparative classification of species forappropriate choice. Properly treated timbers givemuch longer life.2.3.9 Species are classified also fortreatability to indicate approximately the degreeof resistance offered by the heartwood of the

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species to penetration of the preservative underworking pressure of 10.5 kg/cm2 under conditionsof treatment normally employed for creosote-crude oil mixture and also water soluble mixture(see 4.4). There can be minor differences oftreatability with different types of preservatives.Sapwood of all species is very easily treatable.Treatability classification of different species is asfollows (see also IS : 401-1982):a>b)

cl4e)

Heartwood easily treatable;Heartwood treatable but completepenertration of preservative is not alwaysattained (IS : 401-1982 prescribes leastdimension as more than 6 cm);Heartwood only partially treatable;Heartwood refractory to treatment; andHeartwood very refractory to treatment,penetration of preservative being negligibleeven from side or end.

2.3.10 When species of wood are air-seasoned,as most of the constructional timders of largecross-sectional areas would be, they are likely tocrack and split. Under comparable normalconditions of seasoning, these are classified as ofhigh, moderate, and low refractoriness to air-seasoning, that is, to indicate likelihood of lossesin air seasoning, or when exposed to normalweather in outside locations. This classificationwill help comparative selection of appropriatespecies in the different areas.2.3.11 In the final selection of the species theNBC recognizes two choices as below dependingon their combined durability and treatability. Thiswill help engineers to adjust according to therequired economy in constructions, particularlywith reference to permanent or temporary, heavyor light constructions, and in selecting requiredtreatment processes, etc (see IS : 3629-1966).

2.3.11.1 First choice -The species can beany of the following:4b)c>4

Untreated heartwood of high durability(Class 1 of IS : 401-1982)Treated heartwood of moderate (Class 2)and low (Class 3) durability, and of a andb treatability classes.Heartwood of moderate durability (Class 2)and Class c treatability after appropriatepressure impregnation (see IS : 401-1982).Sapwood of all classes of durability afterthorough treatment with preservative.

2.3.11.2 Second choi ce - The speciesshould be of heartwood of moderate durabilityand Class d treatability.2.3.11.3 Although no specific mention ismade in NBC and IS : 3629-1966 recognizingselection of species of Class 3 durability and Class

HANDBOOK ON TIMBER ENGINEERING

Sp : 33(S&T)-1986c treatability for temporary structure when theirlife is not important.

2.3.12 Considering all the above, the timberresources for structural purposes in India demanda careful selection of species consistent with therequired economy. In recent years, the cost ofpopular and durable species ranged from Rs. 500to Rs. 1000 per cubic metre depending upon thesizes and grades. Some of the ornamental anddecorative varieties to be used for wall panelseven cost more, though plywood and other typesof panels are gradually coming into vogue. Thecost of sesoning has been around Rs. 75 per cubicmetre and the preservative treatment on theaverage costed Rs. 150 per cubic metre. However,it may be remembered that the economy of timberutilization depends not only on the fluctuatingcosts of material and treatment in the mark%,but also on the annual maintenance costs and iheminimum longivity expected of any structure.2.4 Selection and Identification nf StructuralSpecies

2.4.1 Wood identification is a highlyspecialized subject falling under the disciplineWood anatomy. Some of the popular species inthe various zones can be broadly guessed andidentified by their gro:,; features such as grain,texture, colour, hardness and weight aided by theknowledge of locality of origin of the species.However, a pen-knife and a hand lens of 10 timesmagnification would be helpful for furtherconfirmation of the timber by a closer study ofthe distribution of different types of cells. In orderto have more accurate confirmation it would beessential to take their sections (of the order ofmicrons in thickness) with the help of amicrotome, and study the same under amicroscope of higher magnification. Still thewood anatomists quite often feel that in somecases they would not be absolutely sure withregard to the species though they can confidentlyidentify the genus. In extreme cases, theherbarium specimen, that is, the leaves, fruits andflowers of the tree, from which the wood has beenobtained, will be required for establishingunquestionable identity. In general all the abovetests, alongwith the herbarium specimen, areconsidered always necessary for absoluteconfirmation.

2.4.2 Identification of commercial species ofIndia are usually done by what are called keysfor identification. There are two such system ofkeys in vogue. The first is called a dichotomouskey. A typical such key is given in Appendix Cwhich includes only some of the timbers but itshould be possible to construct such keysspecifically for the structural timbers belonging toa particular group. The Forest Research Institutehas published several keys for commoncommercial species available in various marketsor for species used for specific purposes such asfor motor lorry bodies. Some keys are constructed

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SP :33(S&T)-1986 bon the basis of features observed entirely bynaked eye or a hand lens, and some areconstructed on what can be observed undermicroscope. It is also possible to combine both. Itis, however, not possible to prepare P key of thistype for all available commercial timbers of theworld or India. Also when such a dichotomouskey is constructed for limited number of speciesagainst a background of common market or enduse, additional species cannot be included withoutchanging a large section of the same.

recorded by V-punching out the margin of theperforations corresponding to the features of thetimber printed on the card (see Fig 2). To identifya given timber, the whole pack of the printedcards are arranged together and a needle isinserted through the hole of the first featureobserved. On shaking out all such cards ofdifferent species which have the same feature fallout. A second feature observed on the timber tobe identified, is then selected and the aboveprocess of inserting a needle and shaking isrepeated on the fallen cards so that some morecards fall out which have the same features. Thusone after another all features are taken up andultimately only one card will fall out which has allthe features observed, and this card gives theidentitv of timber under consideration. Theadvantages of this type of keys are: (a) one can

2.4.3 A second system of key for identificationis through the help of marginally perforatedprinted pack of cards describing all features.IS : 4970-1973 describes this system of the keyalongwith all aids required for the same. In thissystem the diagnostic features of each timber are

SAJWOOD ANDHEARTWOOD DISTINCT LIGHT COLOURED 2lELLOW 3

I SHADESF BROWN 4 II SHADESF RED 6 1

=I SOFT TO VERY SOFT 6 II 36 EXCLUSIMLY SOLITARY Ii

I HEAW TO VERY HEAVY 13 II 41 FLAME LIKE I

FINE-TEXTURED 16MEMUII-COARSEJEXTVRED 19

7tlODERATELI BROAD 23

g FINE TO VERY FINE 26a 6ROAO 6 FINE 252 CONFLUENT-BROAD6 3 ilANDED -NARROW66 BANDED - 6ROAO

CLOSELY SPACED1 FEW WIDELY SPACED 27 1

8 HANDBOOK ON TIMBER E NGINEERING


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start with any obvious or unusual feature, (b) thefeatures can be used in any sequence and (c) anycard can be prepared for any new timber speciesand added to the pack and selection is also basedon negative characteristics, that is, cards oftimbers, in which the selected features are notpresent remain on the needle itself when shaken.2.4.4 Some people have tried to developidentification keys on the basis of chemicalconstituents of timber or mechanical and otherphysical properties of timber but these have notbecome popular as yet.2.4.5 In all cases of identifying biologicalspecimens, it is best in the end to establishidentification by comparison with authenticspecimen themselves and using all available keys.For this purpose purchasers and users of timberregularly are advised to keep in their custody a setof authentic specimen so certified by recognized

laboratories such as the Forest Research Institute,Dehra Dun, and Forest Research LaboratoryBangalore and Peechi (Kerala), etc.2.4.6 To be able to identify the different typesof common features that are to be looked for onthe different species in the *process of theiridentification and selection, description of thesame are given in 2.4.6.1 and 2.4.6.2.

2.4.6.1 Gross features4

b)

cl

Bark-The outer cover of a log, whichmany a time easily peels off on drying, isgenerally a rough surface. However, in somespecies like Ficus or Eucalyptus it is smooth.In Shorea species it shows fissures andcracks. In some species like Melia diamondshaped fissures are observed. The thicknessof the bark and its general appearance is ofsome diagnostic value.Pith - This is the soft core found near thecentre of the log as seen at the end. When indry condition, lot of radial cracks areobserved. The colour is commonly someshade of brown or grey. It is rarely of anydiagnostic value. The squarish ends withconcave sides is sometimes consideredtypical of teak.Sapwood and heartwood - When viewedon the cross-section of a log, two clear zonesappear, the one comparatively lighter inshade and nearer the bark side is known assapwood and the inner zone near the pith,is known as heartwood. These two zonescontain cells with different functions in thegrowth of the tree. However, in a felled treeor a log stored in a depot, colour distinctionis not always a true criteria for distinguish-ing them. In Abies pindrow (fir) and Piceasmithiana (spruce) there is no colour distinc-tion. Hardwoods like mango, semul, do notshow the colour difference. In some species,for example, Shorea robust, the sapwoodchanges its colour from reddish brown near

HANDBOOK ON TIMBER ENGINEERING

d)

e)

r)

SP : 33(S&T)-1986the heartwood region to nearly white nearerthe bark. However, from anatomical pointof view and also strength point of view thereis no difference between sapwood and heart-wood. The sapwood is a very easily treat-able than heartwood, and so is alwaysadvantageous from the point of preser-vation and tretment of timber.Growth ri ngs - Most timbers show on thecross-section a number of concentric marksapproximately annualar in shape, that is,circular around the pith and ultimatelytaking contour shape as of bark. Sometimesthey are faint and sometimes they are pro-minently visible to the eye. The width of thering consists of two parts, the wood formedin the early part of growing season known asearlywood and (sometimes referred in lite-rature as springwood) and the wood formedduring later part of growth season knownas latewood(sometimes referred in literatureas summerwood). The two parts differ intexture, colour and general structure. Thetime interval between two such consecutivegrowth rings is usually a single alterationof growth season which is usually one year.Hence they are also often known as annualrings and thus the number of rings providea reliable idea of the age of the tree. Some-times there can be false rings, and in someregions the growing season is not regularlyannual. Sal, gurjan, jaman, and mango donot show any distinct annual or even sea-sonal growth rings because the growth iscontinuous and one seasons growth mergesinto next seasons growth. In conifers likechir, deodar, and in some broad leaved treeslike mulberry, teak, toon, the rings aredistinct and give a correct idea of the rate ofgrowth. Fast growth indicates rings widerthan 2.5 mm, and 8 rings or more percentimetre indicate slower growth. Width ofrings has a significant effect on strength andsometimes they are of diagnostic value.Grain and texture-These are somewhatloosely understood terms to indicate thedirection, alignment and size of the cellsas seen on the surface of piece of timber.These are also of some diagnostic value.The grain which is concerned with orienta-tion of cells with reference to the axis of thetree is usually described straight, cross,spiral, interlocked, wavy, irregular, etc.The texture of wood is commonly describedas coarse, fine, even or uneven to indi-cate the size, proportion and distribution ofvarious wood elements. Sandalwood andhaldu are fine textured woods; mango,hollock and kokko are coarse textured.Other characteristics, such as colour, odour,hardness and weight as their names suggest,help in identification of very commonlyknown timbers but one should be very

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SP : 33(S&T)-1986cautious in applying these to any unfamiliarwood.

2.4.6.2 M inute structure qf ~vood4

b)

c)

Depending upon the function expected ofvarious types of cells during the growth of atree, they are classified as vessels fibres,trachieds, parenchyme, etc. The size, pro-portion and arrangement of the variouswood elements constitute the characteristicpattern, and celluler structure seen undera lens or microscope on the basis of whichidentity of wood is established. These arealso responsible for making the specificspecies usable for specific end uses, andparticularly building purposes wherestrength, nailability, glueability, insulatingproperties, paintability and treatabilityform important considerations.Pores or vessel s -They appear on thecross-section of broad-leaved trees as appro-ximately a circular or oval openings orholes. Hence they are called pores. In coni-ferous woods they are not visible and sothey are called non-porous. The pores mayby distributed in a ring form or diffused. Inring porous woods, those of early wood aredistinctly larger than those of late woodsuch as in teak, mulberry and toon. Hence aring is clearly seen. In diffused porous wood,such as sal, haldu the pores are uniformlydistributed. Sometimes there will be othertypes of groupings such as radial chains asin ebony and lambapatti, or in obliquegrouping such as in poon. The pores may beminute in size as in Gardenia sp. or largeas in semul and kokko. In the sapwood,pores may be empty but in heartwood theysometimes get filled with deposits andinclusions called tyloses which are formedby ingrowth of adjoining cells as a part ofheartwood formation. They appear asfoam-like structures. They are consideredindicative of natural durability but reducethe easiness of seasoning and treatment.Fibres - This is a somewhat misleadingterm. As commonly understood any thread-like or filament-like material of plant originis a fibre. However, wood anatomists recog-nize this as vertically aligned, narrow elon-gated, thick walled, wood cell with taper-ing ends occuring only in hard woods andwhich impart mechanical strength to the treein its growth, and make up the bulk of thewood by weight. They appear not indivi-dually but collectively in dark colouredpatterns. Like vessels they are absent inconifers and the corresponding longitudinalelements in conifers are called tracheidswhich are described below. However in pulptechnology the words long fibres and shortfibers are used for the filament likematerials in conifers and hardwoods res-pectively.

4

e)

f)

g)

h)

Tracheids - In conifers they have dual roleof conducting the sap and also givingstrength to the tree. They have pits on theirwalls like vessels, but have thick walls andtapered ends like fibres. The early woodhave thinner walls, larger pits, larger lumenand the later wood contains thicker walls,less pits and smaller lumen, thus giving riseto prominent growth rings.Paranchyma cel l s - Somewhat short, re-ctangular cells with thin walls, and pitsdesigned for storage and conduction of foodmaterial during growth. Generally they arealong the direction of growth of the tree,and hence known as longitudinal paranc-hyma. But sometimes they are formedamong the cells running in radial directionand are correspondingly called radial para-nchyma. The collective patterns of thesecells are of important diagnostic value.Rays - Horizontally oriented cells knownas medullay rays running in radial directionappear as light coloured ribbons of variouspatterns giving distinct features. Broad,high rays are characteristic of oak, beech,etc. Fine inconspicuous rays are features ofall conifers and some hardwoods like laurel,benteak and ebony.Ripple marks-When viewed on the tan-gential surface, some wavy lines caused byrays running at right angles to the longi-tudinal direction are sometimes observed onsome woods such as bijasal kanju and satin-wood, and are very helpful in identificationof timbers.Resi n canals and gum ducts - Intercellularcanals, serving as repositories of wasteproducts like resin and gums, either in verti-cal or in horizontal directions observeddistinctly under a lens in some conifers likechir, katl, and in hardwoods like sal, vella-vine and hopea. They are of varying shadesof brown and varying sizes, which seems tobe very good diagnostic feature as only afew timbers possess these.

2.4.7 After identification of any species ofwood, another important factor which influencesselection is identifying the common defects andassessing their influence on the stress-bearingcapacity of the material. This is discussed in detailin 2.5 which deals with grading. Several Indianstandard specifications have indicated the extentof permissibility of the defects for respective uses.Brief descriptions of some common defects,required to be identified for structural purposesare given below [see IS : 3364 (Parts 1 and 2)-19761:

4

b)

Checks - Identified by the separation offibres along the grain observed on the longi-tudinal or end surfaces.Decay - Also referred as rot is commonlyidentified as brownish or greyish patches on


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c)

4

e)

0g)

h)

the surfaces, and their condition is judgedquantitatively by their surfaces. As these arecaused by fungii which eat away the cellwall, the strength is affected unlike sapstainin which the contents of cell and not cellwall are eaten. [see 3.2.4.5 and item (j)below].Flutes - These appear on the surface of logsas longtitudinal depressions, deeper andwider at the butt-ends and gradually dimi-nishing in depth and width as it reachesupwards.Grain orientation --Spiral grain is identi-fied on the logs by the direction of surfacegrain, or fine hair splits between them asthey spiral along the length either clockwiseor anticlockwise as seen from the smallerend, that is, top of the log (also known asright handed or left handed). In cut tim-ber, this appears as inclined in both radialand tangential surfaces and there can besome confusion as to the exact cause of anyinclination of the grain to its longitudinalaxis. Any grain appearing as generally notparallel to the longitudinal edges, are knownby the, general term cross grain and inclu-des different types such as diagonal grain(caused due to material not sawn parallelto the axis of a straight log) inclined grainand slanting grains (caused due to naturalorientation in the growing tree).Holes-These are caused by insects, birdsand mechanical injuries, and are classified invarious specifications according to theirdiameters.In-bark - Identified as patches of barkembedded in the wood partly or wholly.Knots -These are left over portions ofbranches, separated from the main stem.They are classified according to their condi-tion as live knots (syn. intergrown knots,tight knots, sound knots) and dead knots(syn. unsound knots or decayed knots).Loose knots, encased knots, pith knotsand hollow knots are all self-explanatory.They are classified according to their sizeand shape as circular, oral, spike, pin knots,small knots, and large knots which can alsobe recognized as their names imply.Reaction Wood- In conifers, this abnor-mality in wood is found on the underside ofbranches and leaning trunks and knownas compression wood. It is characterized byheavily lignified trachieds rounded anddistributed along the growth rings. Theyare denser and darker than surroundingtissues. In hardwoods this abnormal wood isfound on the upper side of leaning trees andbranches, and known as tension wood. Itis characterized by little or no lignification.The machine surfaces with such abnormality

SP : 33(S&T)-1986tend to be wooly. Converted timber contain-ing reaction wood develops distortions dueto high longitudinal shrinkage and reducesstrength considerably.Rot - See decay above. The word dry rotis commonly used to indicate wood that iscrumbly, and not as is erroneously consi-dered that it is grown on dry wood. Patho-logists use this word to indicate action ofsome types of fungii such as M ercul i uslacrymans and similarly the word wet rot asdue to action of some fungii like concopheracerebella. As these attack only the cellulosewithout affecting lignin, the left over portionappears as brown and-hence they are gene-rally known as brown rot. Certain fungiilike Polyporus hirsutus can eat away allcomponents of cell wall, so that the left outportions appear whitish and so popularlytermed as white rot. Study of action ofdifferent types of fungii, their identificationand action on different species is a highlyspecialized subject of the pathologist, but foran engineer, particularly with reference toidentify and evaluate the condition of thematerial by the nature of the attackedportion.Shakes and spli t s - Shakes are seen only onthe cross-section of the material as a partialor complete separation of wood tissues andare known as heart shakes (extending inradial direction on the heart region of thecross-section) as cupshakes (occuringbetween growth rings) and as starshakes(emerging from the pith as a number ofradial shakes and appearing as a star).Splits are seen as separation of fibres bothon the cross-section as well as on the longi-tudinal surface.Wa??e-- This appears as lack of woodmaterial on edges or surfaces and indicatespresence of original sapwood with or with-out bark.Warp --- Sometimes also referred as distor-tion, this is a general term for defects deve-loped in seasoning or drying such as bow(face concave or convex along the grain),cup (face concave or convex across thegrain), spring (curvature in the longitudinalplane) and twist (spiral distoration alongthe grain).

2.4.8 Some of the above defects can bea propriately measured and assessed [seeI8 : 3364 (Part 1 and 2)-l 9781 and theirpermissible limits form the subject of individualspecifications. However in NBC, for the purposeof structural timbers, completely prohibiteddefects and permissible defects with limitationsfor different grades have been identified includingpermissible location of these defects. Loose grain,splits, compression wood in conifers, heartwoodrot, sap rot, and warps are completely prohibitive

HANDBOOK ON TIMBER E NGINEERING 11


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SP : 33(S&T)-1986defects. Wanes, knots, worni holes, pin holes,other than those due to live power-post beetles,checks, shakes, slope of grain are permitted tospecified limits. It will be useful for any personusing timber to be familiar with various othertypes of defects, whether natural or caused bysubsequent processing techniques, or externalagencies like fungii and insects by reference tostandard literature and relevant specifications.2.5 Grading Practices, Standards andPreferred Sizes

2.5.1 Grading a piece of timber for therequired end use has been ad,opted in some formor other ever since timber was being utilized.Users of tool handles, builders of early structures,all knew mostly by trial and error methods, whenand where to avoid defects like knots, splits andeven sapwood, etc. Thus grading has come tomean as intelligent choice of timber for specificpurposes. With the possibilities of improving theproperties of timber and timber products, andwith improvements in conversion and processingtechniques, grading rules can and should bealtered from time to time for rational utilizationof timber keeping in view the primary factors ofsafety and also economy. At this stage it may bealso kept in view that grading of timber, even byvirtue of the terminology employed, has asignificant impact on timber trade and itseconomy in general; for example, what can begrade I, II and III in terms of strength, if termedas select grade, Grade 1 and Grade II has adifferent impact on the economy. Similarly ifclassification of the whole lot is made into two orthree groups as against four or five, then again ithas an impact on price structure commercially.Sometimes a defect which is not accepted for onepurpose, may be preferable for some otherpurpose (for example, a piece with too muchdistorted grain unsuitable for structures, can besuitably cut and matched for giving a beautifuldecorative appearance). Thus, grading of timber isa complex problem, and so several purchasers andsellers prepare their own grading specifications.Thus in spite of bestefforts for unifying severalgrading specifications and paractices, the railway,the MES and the PWD still continue to have theirown specifications, various forest departments inthe country have their own grading specificationsand there have also been some differences inmethods of measurement of lengths of logs andconverted timber, girths and diameters of logs,and also in the calculation of volumes andmeasurements of defects. Even in foreigncountries the Lumber ManufacturingAssociations, dealing with some specified speciesand recommending the same for specific end uses,have their own grading rules. In other countrieswhere grading is practised on large scale,recognized inspecting organizations have alsocome up which issue certificates of grading andtake upon themselves the responsibility for qualityof the material. In the context of such varied

considerations in grading, some attempts havebeen made in FAO grading rules, as also adoptedin some IS .specifications, to standardize methodsof measurements and quantify the defects andevaluate their cumulative effects, which cannot beexactly predicted otherwise [IS : 3364 (Parts Iand 2)-19761. Such recent attempts on the part oftechnologists to bring a mathematical pattern forexactness, for a subject which otherwise is tooarbitrary and complicated to avoid disputes, canbe viewed only to serve as a general guide withwide limitations and tolerances.

2.5.2 Systems of Grading- In India, atpresent there are several systems of grading aspractised by various sellers and purchasers, and asalso codified in various Indian Standardsaccording to end uses of timber such as thoseintended for further conversion (IS : 190-l 974and IS : 1326-1976), for cut sizes (IS : 1331-1971and IS : 2377-1976), for teak squares (IS : 3731-1966), for measurements and calculation ofvolumes [IS : 2134-1981, IS : 3364 (Parts 1 and2)-1976 and IS : 2184-19731, for specific uses suchas lorry bodies, aircraft, etc (IS : 2176-1979 andIS : 1329-1975), and for constructional timbers(IS : 3629-1966). All these specifications can bebroadly classifed as follows:

4

b)cl

4

In

Grade classification based purely and some-times arbitrarily on (1) dimensions and (2)general appearance.Grade classification based on the bestultimate use of logs or converted material.Grade classification based purely on defectsand rough estimates of our turn of utiliz-able material.Grade classification based purely on evalua-tion of units of defects and fixing thenumber of units permissible for a specifiedvolume in each grade.spite of a high degree of standardization as. . .indicated above, and also attempts to unify andcodify some of the common practices, the presentstatus of grading technology cannot be said tohave attained any perfect stage acceptable to all,and yet it is widely felt that some sort of gradingis very necessary fqr the selection and payments tobe made for the timbers. On a recent survey ofdemand and stocking position, the preferred sizesof converted timber, standardized (IS : 1331-1971) and codified in NBC are given in Table 3.However, IS : 489 1-1968 gives preferred cut sizesand tolerances of structural timbers furtherdivided for: (a) roof timbers, (b) roof purlins,rafter and floor beams, (c) partition framing,covering, and (d) centering. These are given inTables 4 to 6.

Preferred sizes for doors, windows, ventilatorframes, etc. can be found in IS : 1003(Part l)-1976, IS : 1003(Part 2)-1966, IS : 2191(Parts 1and 2)-1980, IS : 2202(Parts I and 2)-1980 andIS : 4021-1976.12 HANDBOOK ON TIMBER ENGINEERING


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TABLE 3 STANDARD THICKNESS ANDWIDTHS OF CUT TIMBER NORMALLYAVAILABLE IN MARKETSTHICKNESS WIDTH

(cm) (cm)1.0 4, 5 and 6 to 12 in steps of 2 cm1.5 4, 5 and 6 to 18 in steps of 2 cm2.0 4, 5 and 6 to 24 in steps of 2 cm2.5 4, 5 and 6 to 30 in steps of 2 cm3.0 4, 5 and 6 to 14 in steps of 2 cm4.0 4, 5 and 6 to 30 in steps of 2 cm5.0 8, 5 and 6 to 30 in steps of 2 cm6.0 8 to 30 in steps of 2 cm

1::: 8 0 to o 30 0 in n stepsteps of f 2 2 cmm12.0 12 to 30 in steps of 2 cm14.0 16 to 30 in steps of 2 cm16.0 . 16 to 30 in steps of 2 cm18.0 18 to 30 in steps of 2 cm20.0 20 onlyUnder each cross-section, the lengths may vary from 1 metreand up, in steps of 5 cm. All dimenssions are given at amoisture content of 20 percent in accordance with IS0

recommendations.NOTE - Many saw mills still cut timber according to therequirements of purchasers. The above is only a rough guideof availability from depots where cut sizes are stocked.

SP : 33(S&T)-19862.5.3 Measurements and Evaluation - Whileit is not intended to describe here in detail all themethods of measurement, a brief mention ofsome, with particular reference to structuraltimbers, may be useful. Hence these are discussedbelow.

2.5.3.1 Lengths of logs- To be measuredby tapes in metres, rounded off to the nearestlower 5 cm. When the ends are not cut at rightangles to the axis of log, the measurement shouldbe made up to the point which will give a full flatcut.

2.5.3.2 Girths and diameters - In the caseof standing trees calipers are employed formeasurement of diameters, and tape formeasurement of girth, usually at breast-height,that is, 1.5 metres from ground level. In the caseof felled logs, girths are incurred by tape at midlength of the log. When diameters are measured,which is not common, it is done at the smallerend, usually by averaging the smallest and largestdiameters at that end.

2.5.3.3 Bark allowances - All measure-ments are usually done under bark, but where it

TABLE 4 PREFEREDCUT SIZES OF STRUCTURAL TIMBERS FOR ROOF TRUSSES (FROM 3 TO 20 METRES)(Clause 2.5.2)

THICKNESS WIDTH, cmcm A/ \2.0 4 5 6 8 -2.5 4 5 6 8 10 12 14 163.0 4 5 6 8 10 12 14 164.0 - - 6 8 10 12 14 165.0 - 6 8 IO 12 14 166.0 - 8 10 12 14 168.0 10 12 14 16

NOTE - Preferred lengths of timber are : 1, 1.5, 2, 2.5 and 3m.

TABLE 5 PREFERRED CUT SIZES OF STRUCTURAL TIMBER FOR ROOF PURLINS, RAFTERS,FLOOR BEAMS, ETC.(CIouse 2.5.2)

THICKNESS WIDTH, cmcm 76 8 10 12 14 16 -8 10 12 14 16 - -

10 - - 14 16 18 20,NOTE- Preferred lengths of timber are : 2, 2.5, 3 and 3.5m.

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SP : 33(S&T)-1986TABLE 6 PREFERRED CUT SIZES OF STRUCTURAL TIMBERS FOR PARTITIONFRAMING AND COVERING

(Clause 2.5.2)THICKNESS WIDTH, cmcm / h 11.0 5 8 IO -

1.5 5 8 10 12 162.0 82.5 83.0 - - 84.0 4 5 6 85.0 - 5 6 86.08.0 - -

NOTE - Preferred lengths of timber are : 0.5, I, I.5 and 2m

10 12 16IO . 12 16 20 24IO I2 16 20 24IO I2 I6 20 24IO I2 I6 20 24- I2 I6 20 24IO I2 I6 20 24

has to be measured over the bark, appropriateallowance is made for the same depending on thespecies. On the averge this is I/ 16.2.5.3.4 Converted timber - Lengths aremeasured by tape or scale and rounded off tolowest 5 cm. The width and thickness, measuredat the narrowest places, are rounded off to thenearest centimetre or millimetre as per individualspecification. The face of converted timber arerecognized as wide face and narrow face unlessthey are of square cross-section. The edges,particularly in planks, are sometimes referred tonarrow faces but the words edge of a face, as instructural grading terminology, refers tointersection of two perpendicular faces, that is,corner of a face.2.5.3.5 Volume calculations - Volume isgenerally obtained by reference to prepared tables[IS : 2134-1981 and IS : 336 (Parts 1 and 2)-19761based on quarter-girth formula for logs, that is,volume = (l/4 girth)2 X length. But in view of thepresent day developments in selviculturalpractices and wood technology, the general trendis to calculate volume on the basis of true

volume of log considered as a cylinder (&h).2.5.3.6 Conversion of sizes to the requi redmoisture content - The nominal sizes prescribedin IS : 133 -197 1 for stocking in depot tire givenat 20 percent moisture content as required insome international standards, but taking intoconsideration the shrinkage effects, the sizeobtainable at the required moisture content (thatis, equilibrium with the atmospheric humidity) isgiven by the formula:

whered,,, = given dimension at the given standardmoisture content, m;

&m = required dimension at the observedmoisture content m; andK = a factor, representing shrinkage perunit moisture content on the basis ofdimensions at the standard moisturecontent m. This depends upon fibresaturation point, normal shrinkagevalues as determined by standardprocedures which, in turn, is rough-ly estimated as dependent on specificgravity p, by the formula K = 0.41 p.But for ready use, K is recommendedto be 0.2 for all species of specificgravity below 0.6 and K = 0.3 forspecies of specific gravity above 0.6.When the specific gravity is notknown K can be equated to 0.25 asthe nearest approximation.

NOTE- Where greater accuracy is required in individualspecies the exact figures of change in percent, per unitmoisture content (even separately for radial and tangentialdirections, if so required) may be calculated from the followingformula:

f=f-P -f-20- -SP 100Where

K = the required change in percent per unit mois-ture content corresponding to the approximatevalue of 0.2 or 0.3;f= fibre-saturationquestion; point of ihe species in

p = I2 percent or 0 percent as the case may becorresponding to the shrinkage value takentnyge of both may be taken, if necessary);S,, = standard shrinkage value from green to dry(p = 12) or over-dry g=ed by method given in I 0) as calcula-: 1708-1969.

The values off and S, may be obtained from the various&ublications of the Forest Research Institute and Colleges,hra Dun.

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2.5.3.7 Dqficts - Methods of measurementof defects have been standardized inIS : 3364(Part 1 and 2)-1976 and according totheir sizes,. shapes or numbers, etc, the unitsaregiven, which represent approximate degrade ofthe utilizable material. These defects are discussedvery briefly in the following clauses,. Foridentifying the defects see 2.4.7. All measure-ments of defects are made correct to 1 mm andthe units are evaluated correct to seconddecimal place on the basis of IO-metre length ofthe material; For defects not covered in thefollowing clauses, and which are suspected tocause degrade, they may be measured andevaluated as equivalent defects, guided by theirappearance, size, location and distribution withreference to the whole piece and the purpose towhich it is put.a)

b)

cl

4

e)f-l

Check - Measured by length and width ofmaximum separation of fibre up to 2 mm.When such checks are numerous on anysurface they are evaluated on the basis ofaverage length and the effective areaexpressed as a percentage of whole area onwhich they appear. For affected areas from20 to 50 percent and for checks ranging inlength from 5 to 50 cm, the defect unitsrange from 0.01 to 1.60. Checks more than2 mm are generally evaluated as shakes.Decay or rot -This is estimated aspercentage of affected area to entire area ofthe surface. The units of defects for decayranging from 1 to 50 percent varies from0.01 to 0.5.Flutes-These are measured by theirlengths on the longitudinal surface of logsand depths at their maximum points whichis sometimes expressed as percentage of thegirth of the log. Defect units for lengthsvarying from 0.5 to 5 metres for differentdepths ranging from 5 to 25 cm, the unitsof defects vary from 0.04 to 1.66.Ho/es-Except pin holes, others are ex-pressed by their diameters. When in largenumbers, they are expressed as so many per100 cm? to limit their occurance in variousgrades. In terms of defect values for varyingconcentrations of holes from 1 to 5 per 100cm, and for the diameter of the largest holeranging from 5 to IO mm, the defect valuesrange from 0.01 to 0.90.lnbark - Measured and evaluated in thesame way as knots.Knots - The maximum diameter of a knotis taken as its size though some practicesexist in some places to take the average ofthe maximum and the minimum, or theaverage of maximum and of the one per-pendicular to it at the pith centre of theknot. When a group of knots exist together,the mean of the maximum diameters of all

HANDBOOK ON TIMBER ENGINEERING 15

g)

h)

j)

SP : 33(S&T)-1986knots at the area are taken. For number ofknots ranging from 1 to 15, and the averagemaximuq diameter of knot varying from 5to 20 cm, the defect values vary from 0.1 to4.50, for other sizes and number propor-tionate values can be taken. Double valuesare taken for unsound and decayed knots.Shu~Yr .-~ Except star shakes. all the othershakes are measured by the length andmaximum width of the opening. It is presu-med that the width is generally proportio-nal to the depth also. However, in NBC, forstructural grades, only the depth at itsmaximum point is recommended to bemeasured. For length of shake from 2 to 20cm and for the width ranging from 0.2 to 2.5cm, (provided the area of cross-section doesnot exceed 0.5 ml). the defect values rangefrom 0.06 to 1.74. For areas of cross-sectionexceeding 0.5 m2, the values are halved.In the case of star shake, the large shake istaken for consideration and the defect valuesare multiplied by half the number of shakesof the group.Splits - In the case of splits which appearon two surfaces, the length on the longitu-dinal surface and depth on the perpendicularsurfice (that is, the end surface in cross-section) are measured. For lengths rangingfrom 0.25 to 4 metres, and for depths from1 to 4 cm, the defect values range from 0.06to 1.86.Cross grai l 1 - This is measured by the slopeof the grain with reference to the edge of themajor on which it appears. When present onboth the adjacent surfaces of a piece oftimber the combined slope is given by

1 I2= +J-x A2 Lt

k) Di stort i on or w arp-This is identified byany deviation in converted timber from atrue plane surface causing departure fromits original planes. It includes bow, cup,spring, twist and any combination thereof.

1- = combined slope, andXwhere

1 1A and 2 = slopes on the adjacentsurfaces

The slope ranging from 5 to 50 aregiven defect values ranging from 0.2 to 1 OO.In the case of spiral grain for logs, the samevalues as above are given.

2.5.4 Guiding Principlei - Reference may bemade to IS : 6534-1971 for full details, specialterminology, and code of ethics for inspection andreinspection. It should be kept in mind by all


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SP : 33(S&T)-1986concerned that all systems and procedures ofgrading discussed above are somewhat arbitrary,based only on available experience, brought on,as close as possible, to a technoiogical pattern,and cannot be claimed to be of exact assessmentof the quality of material. Thus, grading of awhole consignment becomes preferable to gradingof individual pieces. In a lot. where most of thepieces fall in a particular grade, any singleindividual piece having one of the defects slightlyin excess of permissible limits for a particulargrade may not necessarily qualify for the expectedproperties of the grade, as the location,distribution and combination of the defects play amajor role. In this connection the discretion of thegrader comes into play quite often. A smalllatitude of 5 percent is generally allowed forpersonal judgement of the inspectors and graderswho have to exercise a balance of considerationbetween the availability of the material, end use,urgency of requirement, and costs involved, etc.Experience and integrity of graders and inspectorswill add to a great extent the acceptance of thematerial by all concerned.

working stresses of common grade (Grade II) and5/6 times those of standard grade. Thepermissible sizes of some defects under thedifferent grades as accepted in NBC are given inTable 7. In addition to the above considerationfor grading of structural timbers, the other criteriawhich limit the use of timbers are given below:4

b)

Light timber having a weight less than 75percent of averge weight indicated in co1 12of Appendix H are rejected.Defects not permissible in any grade areloose grain, splits, reaction wood, decay,knot and holes with live infestation.

cl

3.3.1

Wanes are permitted provided they are notcombined with knots and they do notadversely influence bearing areas andnailing edges, and general appearance.MECHANICAL AND PHYSICALPROPERTIES OF INDIAN WOODSMethods of Test and Evaluation ofProperties

2.5.5 Stress Grading and StructuralGrading - These two terms are often usedsynonimusly. However, sometimes a smalldifference is conceived; the former referring to asystem by which the materials or species as awhole, are graded purely by consideration of theprincipal stresses that come into play, (that is,evaluating how the species, the sizes together withdefects can bear the required stresses or whatpercentage of basic stress it can bear). The latter,that is, structural grading is a system by which thematerial graded is on the basis of evaluation ofsupposed effects of visible defects on generalstrength of the material, irrespective of what typeof stresses are likely to arise when employed in thestructure. Obviously the distinction is on1.y narrowand theoretical in nature particular19 for visualselection. Stress grading machines arein vogue inother countries, but they have not yet beenintroduced in India even for research anddevelopment activities. Machine grading dependson the principle of relationship between stiffnessand strength, and brings out not only cumulativeeffects of all visible defects, but also effects of anyhidden features in structural members.

2.5.5.1 The structural grading of Indianspecies is covered by IS : 3629-1965 andIS : 1629-1971. In India three grades arerecognized. The select grade contains defectswhich do not reduce strength by more than 12.5percent of fundamental, that is, ultimate stresses;the Grade I also known as standard gradecontains defects which reduce strength by morethan 12.5 percent but not more than 25 percentand Grade II also known as common gradecontains defects which reduce strength by morethan 25 percent but not more than 37.5 percent.The safe working stresses of selection grade are716 times those of standdrd grade (Grade I) and

3..1.1 Accurate evaluation of basic propertiesof timber, which exhibits wide variations, is animportant base for establishing design functions.For obtaining a confident average of a givenspecies, the samples should be representative ofdifferent portions of a tree, different trees,different localities in India, and the tests shouldbe conducted under highly standardizedprocedures to obtain maximum reliability andrepeatability of the data. For this purposes, testson timber are conducted: (a) in small clearspecimen in accordance with IS : 1708-1969 and(b) on structural sizes in accordance withIS : 2408-1965. The former are conducted as aroutine on all species in the green condition andin the dry condition. Greater emphasis is,however, given on the results of green conditionas the results are more reliable, being free frominfluence of any drying stresses introduced in thetimber. Also from the point of design, they aresafer values because the strength properties ingreen timber are lower compared to strength ofdry timber, though eventually the larger sectionsof timbers used in structures become dry andattain a moisture content in equilibrium withatmospheric humidity of the locality where theyare placed. The standardized details of sizes oftest specimens, rate of loading, surface on whichload is applied, and the properties evaluated asroutine are summarized in Appendix D but theexact procedures required to be essentialyfollowed are described in greater detail inIS : 17081969. A set of formulae employed toevaluate the properties obtained in different testsare also given in Appendix D for ready referenceand convenience. The data obtained on the basisof the above procedures and formulae, whenobtained in accordance with the samplingprocedures prescribed in IS : 2455-1974, are

16 HANDBOOK ON TIMBER ENGINE ERING


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TABLE 7 PERMI SSIBLE DEFECTS FOR CUT SiZES OF TIMBER FOR STRUCTURAL USE

SL DEFECTS GRADE 1NO.(1) (2) (3)

(Ch4.w 23.5. I)All dimensions are in millimetres

GRADE 2

i) Wane Shall be permissible at itsdeepest- portion up toa limit of I / 8 of the widthof the surface on whichit occursii) Worm holes Other than those due to Other than those due to Other than those due topowder post beetles are post beetles are permis- powder post beetles arepermissible sible permissible

iii) Slope of grain Shall not be more than I Shall not be more than in Shall not be more than I inin 20 I5 I2iv) Live knots:

WIDTH OF PERMISSIBLE MAXIMUM SIZEWIDE FACES OF LIVE KNOT ONOF CUT SIZES

TO TIMBER Narrow Faces Remainingand l/4 of Central Halfthe Width of the WidthFace Close of the Wide

1;I50200250::400450500550600

v) Checks andshakes

WIDTH OF THEFACE O F THE

TIMBERMax

75I:200250:gg500550600

to Edges ofCut Size ofTimber(2)IOI3I9222527:;::3638

Faces

(3)

PERMISSlBILE DEPTH PERMISSIBLEDEPTH PERMISSIBLE DEPTHMUX Max Max

(4) (5)Shall be permissible at its Shall be permissible at itsdeepest portion up to deepest portion up toa limit of I /6 of the width a limit of I /4 of the widthof the surface on which of the surface on whichit occurs it occurs

PERMISSIBLE MAXIMUM SIZE PERMISSIBLE MAXIMUM SIZEOF LIVE KNOT O N OF LIVE KNOT ON

Narrow Faces Remainingand I /4 of Central Halfthe Width of the WidthFace Close of the Wide

Narrow Faces Remainingand l/4 of Central Halfthe Width of the WidthFace Close of the Wideto Edges of FacesCut Size ofTimberto Edges ofCut Size ofTimber

Faces

(3)25::::100II5I31I50165181200

GRADE 3

(6) (7)29:r: :I:

Z::87

:: 114123ZG 132141105 150IO8 I56I I4 I59

(4)

::2

1::171198225270300-

presented according to a system prescribed in the Institutes where systematic studies are carried outsame standard. While most of the engineering as a routine on all species, follow the abovelaboratories are not always able to follow the procedures,above presented procedures for sam ling and and the data presented by theselayout of the tests, and presentation oP data, the organizations can be taken as most authentic.regular laboratories like Forest Research However, in organizations like most of thecolleges, engineermg laboratories, where they doHANDBOOK ON TIMBER ENGINEERING 17


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SP : 33(S&T)-1986not or cannot follow the detailed testingprocedures prescribed in IS : 24551974, forvarious reasons they do adopt the standard sizesand rates of loading, etc, which will give data on alimited base and can be considered suffi-cient for comparison with any acceptable limitsof confidence and for further evaluation for pur-pose of local design considerations, if any.

3.1.2 The tests of structural sizes given in 3.1.1are described in greater detail in IS : 2408-1963.The main object of testing in the larger sizes,according to the methods prescribed in thisstandard is to serve as a cross check on the designdata wherever necessary and wherever such largesized material can be spared for such tests. Theyserve to check the established relations betweenthe strength functions based on small clearspecimens, and the permissible stresses instructural timber. When such tests are done on alarge number of timbers and data becomeavailable for study, they can also be used todevelop and improve upon the grading rules fordifferent structural timbers and different strengthgroups. In a way they serve to determine the effectof defects and other variables on the strength oftimber in structural sizes and study all relevantfactors as are necessary for specific structuraldesigns. As described in IS : 2408-1963, while themajor tests are conducted on large-sized materialfor bending, compression parallel to grain, andcompression perpendicular to grain, the minortests are conducted according to IS : 1708-1969011 defect-free small clear specimens cut frommatched portions of material intended for majortests or from undamaged portions of materialalready subjected to such major tests. The

scheme of tests for major tests and formulae aregiven in Appendix E.3.1.3 From the data thus obtained, theworking stresses, also known as permissiblestresses are derived. These are discussed in 3.3alongwith factors of safety, etc, and they are thestresses actually used in designs.3.1.4 The above strength data. calledsuitability indices, are used for a quick andcomparative assessment of timber species in aquantitative manner for various industrial andengineering purposes (XV U/.VO .3.6 and Table 2).These are based on basic strength data onphysical and mechanical properties, evaluated onsmall clear specimen by selecting mechanical andphysical properties. which play principal orauxiliary roles for particular end use. such asstrength as a beam, suitability as a post, toolhandles, agriculatural implements, etc. Theselected basic properties are given appropriateweightage according to relative important rolethey play. The variations in green and dryconditions are also taken into consideration. Theaverage value obtained by adding the computedand adjusted values of the concerned properties isusually compared with a similar value of a wellknown species commonly used for the purpose inview, usually with teak, which is widely used andwell known in its behaviour. It should beremembered that these suitability indices are notto be used for design purposes but they serve onlyfor comparative selection of different speciesbased on limited and arbitrary considerations. Atypical example of deriving such indices is givenin Appendix J. The values for a few selectedspecies are given in Table 8. These figures have

TABLE 8 COMPARATIVE SUITABIL ITY INDI CES IN TERMS OF TEAK AS 100 FOR SOME IMPORTANTPROPERTIES (FIGURES ARE ADJ USTED TO NEAREST 5)(Clause 3. I 4)

SLNo.

(I)i)

ii)iii)iv)v)

vi)vii)

viii)ix)x)

SPECIES AND TRADE NAME STRENGTH SUITABILITY SHOCKAS A BEAM AS A POST RESISTINGCOMPARA- ABILITY

TIVE INDEX ST%T

SURFACEHARDNESS

REFRAC-TORINESS

(SPLITINGCO-EFFI-CIENTS)

NAIL ORSCREW

HOLDINGPROPERTYCOMPARA-

TIVE INDEX(3)loo80

115100

(2)Adina cordifolia (haldu)Cedrus deodara (deodar)Dalbergia sissoo, (sissoo)Dysoxyium malabaricum

(white cedar)Hopea glabra (hopea)Mang$era indica (mango)h4imusop.y littoralis.(bullet wood)

(3) (4) (5) (6) (7)80 80 70 100 6080 85 55 65 8595 90 130 120 8090 95 110 90 80130 130 145 205 55 14575 75 90 85 60 100145 145 145 230 105 150

Ougeini n dalber& oides(sandan)Shoree robusta

(salIXykn xylocarpa (irul)

80 80 I05 I30 80 120II5 110 130 I50 90 I25105 110 80 180 60 120

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been in wide use in various forest departmentsand depots for comparative evaluation only.3.2 Factors Affecting Strength Properties

3.2.1 Being a biological material, naturally thecell size, the thickness, and orientation of woodcells have necessarily some influence on thestrength properties and these depend on inherenthereditary characteristics and growth condition(such as soil. climate, locality and selviculturalmethods employed). It has not been possible tostudy these effects in any systematic manner. butempirically some conclusions have been arrivedthat material of the same species do not have verysignificant difference from commercial points ofview, and their densities would reflect the seriousdifferences, if any. It may be remembered that inactual use, considerable factors of safety areemployed on a statistical average. Factors like ageof tree, position of wood in the tree, season offelling, natural or plantation grown wood,condition of the tree when felled (that is, whetherdead or green or twisted or containingabnormalities like streaks, etc), also come upoften, but for engineering purposes, whenmaterial has to be selected from depots and yardsit would not be possible to lay much importanceto the above as these pertain to the quality of thetree in standing condition only. However, most ofthe laboratories engaged on systematic tests fromselected trees in the forests, keep appropriaterecords of the same. The only criteria of growingcondition which can be judged at the time ofselection and use of timber for engineeringpurposes is the rate of growth as reflected by thewidth of the growth rings on the cross-section oftimber pieces. In the same species, too wide ringsor too narrow rings may indicate slightly lowerstrength than those with average growth. Presenceof distortions or abnormalities in the woodstructure as identified in growth rings may causeundue concentrations of stress at those places.However, keeping in mind that the width of thering may also vary from bottom to top or frompith to periphery, this should not be taken as tooserious a criterian for strength particularly ascompared to specific gravity. By and large studieson variation of strength due to causes as discussedabove have so far been oriented more to studyvariations in specific gravity which in turn reflectsthe strength.

3.2.2 Specific gravity of wood is the basicproperty which has considerable influence on thestrength properties, so much so in the event ofnon-availability of actual strength data, the samecan be computed by the formulae developed onthe basis of studies on more than 150 species ofIndia, and confirmed on quite a large number ofspecies subsequently tested. The general formulais given by:

S = Kp

HANDBOOK ON TIMBER ENGINE ERING

whereS = strength, andp = standard specific gravity based oneven-dry weight and volume at test(see IS : 1708-1969)

K and n are constants of the particular property.The formulae with K and n values are given inTable 9. Similar formulae were obtained also forworking stresses for standard grade of timberunder different locations. These are given in Table10. It may be seen from these formulae that inmost of the properties n is unity, thus indicating alinear relationship. However, it may be noted thatall properties do not vary with specific gravity inthe same manner to expect the sameproportionate increase or decrease as n, thepower index for p in some of the properties, is notunity. No data is available in India for variationof strength with specific gravity within the rangeof any single species, but in USA a formula asgiven below is used.

whereS,=&=4=

strength corresponding to p,,strength corresponding to p2, andon an average 1.7 (varying between1.25 and 2.25) for various properties

3.2.3 Moisrw e Content - This affects thestrength of wood in dry condition. In greencondition, that is. well above the so called fibresaturation point (fsp) which is between 20 and 30percent for most of the species, the strength is notaffected and is the lowest obtained on the species.In the dry condition, the strength increases as themoisture content is reduced and is given by theformula :Log snl =a-bm

where S, is the strength at moisture content m,and a and b are constants for the species andproperties in question. The formula is sometimesknown as Madison formula, and althoughevolved for timbers in USA, the same has beenfairly successfully used for Indian timbers also. Byknowing strength at any two given moisturecontent, the values of a and b can be calculatedand used for knowing strength at any othermoisture content. Usually, by knowing strength Sof green timber, that is, S above the fibresaturation point, ,f moisture content (S beingconstant at any moisture content in the greencondition) and also by knowing strength in drycondition at any determined moisture content, thestrength value at the required air-dry condition(12 percent) is computed for presentation ofstandard data; and for purposes of comparison of

19


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SIP : 33(S&T)-1986.

TABLE 9 EFFECT OF SPECIFIC GRAVITY ON ULTIMATE STRESSES

PROPERTYStatic BendingModulus of ruptureFibre stress at elastic limitModulus of elasticityI mpact bendingFibre stress at elastic limitHeight of dropModulus of elasticityCompression parable1 to grainCompression stress as elastic limitMaximum Crubling stressModulus of elasticityCompression perpendicul ar to grainCompression stress at elastic limitCompression stres at elastic limitShearRadialTangentialTension to grainRadialTangentialHardnessEndRadialTangential

(Clause 3.2.2)UNIT

kg/cm2kg/cm1000 kg/cm*

kg/cm*cm1000 kg/cm

kg/cm2kg/ cm21000 kg/cm*

kg/cm*kg/ cm2

kg/cm*kg/ cm2

kg/ cm2kg/cm2

Load in kg(standardsteel ball

GREENCONDI

Is Code Sp33(Timber)

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