1、Designation: D 2555 06Standard Practice forEstablishing Clear Wood Strength Values1This standard is issued under the fixed designation D 2555; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in p
2、arentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.INTRODUCTIONThe development of safe and efficient working stresses for lumber, laminated timber, plywood,round timbers, and other solid wood products, each w
3、ith its own special requirements has, as acommon starting point, the need for an authoritative compilation of clear wood strength values for thecommercially important species.Also required are procedures for establishing, from these data, valuesapplicable to groups of species or to regional grouping
4、s within a species where necessitated bymarketing conditions. This standard has been developed to meet these needs and to provide, inaddition, information on factors for consideration in the adjustment of the clear wood strength valuesto the level of working stresses for design. Since factors such a
5、s species preference, species groupings,marketing practices, design techniques, and safety factors vary with each type of product and end use,it is contemplated that this standard will be supplemented where necessary by other appropriatestandards relating to specific work stresses for each such prod
6、uct. Practice D 245 is an example of sucha standard applicable to the interpretation of the clear wood strength values in terms of workingstresses for structural lumber.A primary feature of this practice is the establishment of tables presenting the most reliable basicinformation developed on the st
7、rength of clear wood and its variability through many years of testingand experience. The testing techniques employed are those presented in Test Methods D 143. Amongthe recognized limitations of such strength data are those resulting from the problems of samplingmaterial from forests extending over
8、 large regions, and the uneconomical feasibility of completelytesting an intensive sample. A practical approach to the improvement of strength data is through theapplication of the results of density surveys in which the specific gravity of the entire forest stand foreach species is determined on a
9、sound statistical basis. Through regression equations derived frompresently available strength data, revised strength values are established from the specific gravity-strength relationship for clear wood. This procedure greatly extends current capabilities to developnew estimates of strength and to
10、improve or verify estimates made in the past.1. Scope1.1 This practice covers the determination of strength valuesfor clear wood of different species in the unseasoned condition,unadjusted for end use, applicable to the establishment ofworking stresses for different solid wood products such aslumber
11、, laminated wood, plywood, and round timbers. Pre-sented are:1.1.1 Procedures by which test values obtained on smallclear specimens may be combined with density data fromextensive forest surveys to make them more representative,1.1.2 Guidelines for the interpretation of the data in terms ofassigned
12、values for combinations of species or regional divi-sions within a species to meet special marketing needs, and1.1.3 Information basic to the translation of the clear woodvalues into working stresses for different solid wood productsfor different end uses.1.1.4 For species where density survey data
13、are not as yetavailable for the re-evaluation of average strength properties,the presently available data from tests made under the samplingmethods and procedures of Test Methods D 143 or PracticeE 105 are provided with appropriate provision for their appli-cation and use. Because of the comprehensi
14、ve manner in whichthe density survey is undertaken, it follows that the re-evaluated strength data are intended to be representative of theforest stand, or rather large forest subdivisions.1This practice is under the jurisdiction of ASTM Committee D07 on Wood andare the direct responsibility of Subc
15、ommittee D07.01 on Fundamental Test Methodsand Properties.Current edition approved March 1, 2006. Published March 2006. Originallyapproved in 1966. Last previous edition approved in 2005 as D 2555 05a.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959
16、, United States.1.1.5 Some useful mechanical properties (tensile strengthsparallel and perpendicular to grain and modulus of rigidity fora longitudinal-transverse plane) have not been extensivelyevaluated. Methods are described for estimating these proper-ties by their relation to other properties.1
17、.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Do
18、cuments2.1 ASTM Standards:2D 143 Test Methods for Small Clear Specimens of TimberD 245 Practice for Establishing Structural Grades and Re-lated Allowable Properties for Visually Graded LumberD 2915 Practice for Evaluating Allowable Properties forGrades of Structural LumberE 105 Practice for Probabil
19、ity Sampling Of Materials3. Summary of Methods3.1 Two methods are presented for establishing tables ofclear wood strength properties for different species and re-gional subdivisions thereof in the unseasoned condition andunadjusted for end use. These are designated Method A andMethod B.3.1.1 MethodA
20、provides for the use of the results of surveysof wood density involving extensive sampling of forest trees,in combination with the data obtained from standard strengthtests made in accordance with Test Methods D 143. Theaverage strength properties are obtained from wood densitysurvey data through li
21、near regression equations establishing therelation of specific gravity to the several strength properties.NOTE 1Density surveys have been completed for only a limitednumber of species. Data are thus not currently available for the use ofMethod A on all commercial species. As such data become availab
22、le theywill be incorporated in revisions of this practice.3.1.2 Method B provides for the establishment of tables ofstrength values based on standard tests of small clear speci-mens in the unseasoned condition for use when data fromdensity surveys are not available. Separate tables are employedto pr
23、esent the data on woods grown in the United States and onwoods grown in Canada.4. Procedure for Establishing Clear Wood StrengthValues4.1 Method ASix steps are involved in establishingstrength values by the wood density survey procedure. Theseare: conducting the wood density survey, development of u
24、nitareas, determination of average specific gravity for a unit area,determination of strength-specific gravity relations, estimationof average strength properties for a unit area, and combiningvalues for unit areas into basic groups and establishing averagestrength properties and estimates of varian
25、ce for the groups. Inthese methods a basic group is a combination of unit areasrepresenting a species or a regional division thereof.4.1.1 Conducting Wood Density SurveyA well-designedand thorough wood density survey is required to provideneeded data on specific gravity for the reevaluation of stren
26、gthproperties. Such a survey requires consideration of the geo-graphic range to be covered, the representativeness of thesample, the techniques of density evaluation, and adequate dataanalysis.NOTE 2Detailed information on an acceptable method of conductingwood density surveys, together with survey
27、data, are presented in the U.S.Forest Service Research Paper FPL 27, “Western Wood Density SurveyReport No. 1.”4.1.2 Development of Unit AreasSubdivide the geo-graphical growth range of each species into unit areas thatcontain 1 % or more of the estimated cubic foot volume ofstanding timber of the s
28、pecies and are represented by reliableestimates of specific gravity of at least 20 trees. Make up unitareas of U.S. Forest Service Survey Units, or similar units orsubdivisions of units, for which reliable estimates of timbervolume are available. Develop unit areas objectively by meansof the followi
29、ng steps:4.1.2.1 Select a base survey unit or subdivision of a surveyunit to be grouped with others,4.1.2.2 Group with similar adjacent areas to make up a unitarea on the basis of a timber volume, and4.1.2.3 Determine the number of tree specific gravitysamples available in the proposed unit area.NOT
30、E 3The rules for developing unit areas should represent an effortto subdivide objectively and uniquely the range of a species into smallgeographic areas, which are assumed to be considerably more homoge-neous with respect to the mechanical properties of the species than is theentire range itself. Th
31、e number of unit areas associated with a species isa function of the volume of timber on the smallest usable areas and thenumber of tree specific gravity samples taken. In general, the larger therange and the greater the commercial importance of the species, thegreater are the number of unfit areas.
32、 One acceptable procedure forestablishing unit areas is presented in U.S. Forest Service Research PaperFPL 27, “Western Wood Density Survey Report No. 1,” Appendix C.4.1.3 Determination of Average Specific Gravity for a UnitAreaCalculate the average specific gravity of trees in eachunit area as the
33、simple average of individual estimates ofspecific gravity of trees within the unit area.4.1.4 Determination of Strength-Specific GravityRelationsFrom matched specific gravity and strength data onsmall clear specimens of wood, establish relationships of theform:y 5 a 1 bx (1)where:y = estimated stren
34、gth value,a, b = constants for the species, andx = specific gravity of the species.for each species, using standard statistical methods ofregression analysis. Equations for modulus of rupture, modulusof elasticity, maximum crushing strength, and maximum shear-ing strength are established in this man
35、ner. The distribution ofspecific gravity in the samples used to compute regressions2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary pa
36、ge onthe ASTM website.D2555062should be representative of the species and, in particular, shallrepresent the full specific gravity range. The nature of the truedistribution of specific gravity can be obtained from results ofwood density surveys. Obtain the data from specimens testedin accordance wit
37、h Test Methods D 143.4.1.4.1 Several methods are available for securing suitablesamples for obtaining data to compute strength-specific gravityrelationships, as follows: strength and specific gravity valuesfrom samples obtained in conformance with Test MethodsD 143 may be employed solely or in combi
38、nation with datasecured by sampling techniques described below or testsamples may be obtained from the forest resource in the formof trees, logs, or lumber. Select samples that are representativeof all growing stock from each of at least five differentlocations within the growth range of a species t
39、hat include thescope of environmental conditions of the range. This impliesthat the sample from a single location must be such that all ofthe growing stock from that location is represented.4.1.4.2 Where relationships between strength and specificgravity are shown to have a statistically significant
40、 difference atthe 5 % level within a species growth range, subdivide therange to permit the development of more accurate estimatingequations for each subdivision. Develop equations for subdi-visions of a species growth range only if specimens from atleast five distinctly different places in the prop
41、osed subdivisionare available and if the correlation coefficients from thestrength-specific gravity regressions are 0.50 or greater.4.1.5 Estimation of the Average Strength Properties for aUnit AreaGiven a set of strength-specific gravity estimatingequations for each species or subdivision thereof,
42、computeaverage strength properties for each unit area using theseequations and the average specific gravity for the unit area.4.1.6 Combining Unit Areas into Basic Groups and Devel-opment of Average Strength Properties and Estimates ofVariance for the GroupsCombine all unit areas containingtimber wh
43、ose properties are described by the same strength-specific gravity relationships to produce a basic group of unitareas. Develop the following information for these basicgroups:4.1.6.1 For each unit area, obtain, from reliable volumedata, the volume of the species being considered and estimatestrengt
44、h properties from appropriate equations. Determineaverage strength properties for a group of unit areas for aspecies or a subdivision thereof by the following equation:Y55 (iYiVi/V! (2)where:Y5= weighted average strength property for the group ofunit areas,Yi= average strength property for the ith u
45、nit area,Vi= percentage of standing timber volume of the speciesfor the ith unit area, andV = total percentage of standing timber volume of thespecies in the group of unit areas being combined.4.1.6.2 Compute the variability index, which is a measure ofthe homogeneity among average values for unit a
46、reas within agroup, by dividing the group average by the lowest unit areaaverage included in the group.4.1.6.3 Estimate a standard deviation, providing a measureof the dispersion of individual strength values about the groupaverage, for each basic group of unit areas using informationon variance obt
47、ained from density survey and standard strengthdata. Compute estimates of standard deviation for each prop-erty as:s 5 =b2sw21 sa2! 1 RMS (3)where:s = standard deviationb = slope of the strength-specific gravity relation,sw2= within-tree variance in specific gravity esti-mated from data used to obta
48、in strength-specific gravity relations,sa2= among-tree variance in specific gravity ob-tained from density survey data,(sw2+sa2) = estimate of total variance in specific gravity,andRMS = residual mean square from the strength-specific gravity relation.NOTE 4When a sampling technique is used that ens
49、ures only onespecimen will be taken per tree (such as a suitably designed mill sample),the quantity (sw2+ sa2) is automatically obtained as a total variance ofspecific gravity.NOTE 5An alternative procedure for developing average strengthvalues where all unit areas are contained within a single species orregional subdivision thereof consists of combining the volume weightedunit area specific gravities to establish a species or regional subdivisionspecific gravity and then computing the average strength properties bysubstituting the average specific gravity in the stren
