1、Designation: D6815 09 (Reapproved 2015)Standard Specification forEvaluation of Duration of Load and Creep Effects of Woodand Wood-Based Products1This standard is issued under the fixed designation D6815; the number immediately following the designation indicates the year oforiginal adoption or, in t
2、he case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This specification provides a procedure for testing andevaluating duration of load an
3、d creep effects of wood andwood-based materials relative to an accepted duration of loadadjustment model. This specification was created for productsthat are currently covered by a consensus standard (forexample, lumber, structural composite lumber, and structural-use panels). This procedure is inte
4、nded to demonstrate theengineering equivalence to the duration of load and creepeffects of visually graded lumber as specified in Practice D245for a product under evaluation used in dry service conditions.This procedure is not intended to evaluate the performance ofproducts under impact loading. Qua
5、ntification of specificduration of load or creep factors is beyond the scope of thisspecification. For further guidance regarding the applicabilityof this specification refer to X1.1 in the Commentary.1.2 Use of the procedure in this specification to determineequivalence to the Practice D245 duratio
6、n of load relationshipis limited to solid wood and wood-based products whose longterm load behavior is similar to that of solid wood. Equivalencedemonstrated in this specification is dependent upon evaluationof a products 90-day (minimum) creep-rupture performance.In this evaluation, three criteria
7、must be satisfied: (1) adequatestrength over a 90-day period, (2) decreasing creep rate, and (3)limited fractional deflection.Asummary of the development ofthese criteria and the underlying assumptions behind them isprovided in the Commentary in Appendix X1 and AppendixX2.1.3 Long term degradation p
8、henomena not described by acreep-rupture model are not addressed in this specification (seeCommentary X1.2.4).2. Referenced Documents2.1 ASTM Standards:2D9 Terminology Relating to Wood and Wood-Based Prod-uctsD198 Test Methods of Static Tests of Lumber in StructuralSizesD245 Practice for Establishin
9、g Structural Grades and Re-lated Allowable Properties for Visually Graded LumberD1037 Test Methods for Evaluating Properties of Wood-Base Fiber and Particle Panel MaterialsD2915 Practice for Sampling and Data-Analysis for Struc-tural Wood and Wood-Based ProductsD3043 Test Methods for Structural Pane
10、ls in FlexureD4442 Test Methods for Direct Moisture Content Measure-ment of Wood and Wood-Based MaterialsD4761 Test Methods for Mechanical Properties of Lumberand Wood-Base Structural MaterialD5457 Specification for Computing Reference Resistance ofWood-Based Materials and Structural Connections for
11、Load and Resistance Factor DesignE4 Practices for Force Verification of Testing MachinesE6 Terminology Relating to Methods of Mechanical TestingE177 Practice for Use of the Terms Precision and Bias inASTM Test Methods2.2 Other References:ANSI/AFand is an careful selection with the desire to assure t
12、hat the product canmeet all six acceptance criteria.6.4 Allowable Property AdjustmentIf the retest proves thatthe product meets all the acceptance criteria defined in 6.3, alltime-dependent member and connection properties defined bythe NDS shall be reduced by the percent change in stress levelused
13、in the retest(s).7. Report7.1 The report content depends on the type of tests con-ducted. As a minimum, the report shall include the followinginformation:7.1.1 Description of the material under evaluation, includ-ing species, grade (or grade combination), specimen geometry,and grain orientation, and
14、 other specific process parametersinvolved in its manufacture.7.1.2 Description of the sampling and matching protocolused.7.1.3 Descriptions of the test setup, including detaileddrawings, the span, and the deflection measuring apparatus.7.1.4 Description and frequency of calibration procedures.7.1.5
15、 Records of test environmental conditions.7.1.6 Test data, including (1) specimen moisture content, (2)applied loads, (3) deflection measurements at various testdurations, (4) test specimen time-to-failure, (5) creep rate, and(6) fractional deflection for each surviving test specimen.7.1.7 Statistic
16、al calculations, including parametric statisticson short-term bending tests (if applicable) and description ofprocedure used to calculate the five percent point estimate.8. Precision and Bias8.1 The precision of the provisions in this specification havenot yet been determined. When data become avail
17、able, aprecision and bias statement will be included.9. Keywords9.1 creep rate; creep-rupture; duration of load; fractionaldeflection; lumber; structural composite lumber; structural-usepanelsD6815 09 (2015)4APPENDIXES(Nonmandatory Information)X1. COMMENTARY ON DURATION OF LOAD EFFECTS IN WOOD PRODU
18、CTSX1.1 ScopeX1.1.1 Appendix X1 provides general background informa-tion on the underlying assumptions used in establishing thecreep-rupture (duration of load) evaluation procedures in thisspecification. The procedure in this specification was originallydeveloped to provide for the evaluation of dur
19、ation of load(DOL) and creep adjustment factors for structural compositelumber (SCL) products. Much research has since been con-ducted on SCL products to demonstrate their long-term loadperformance. It was considered important to provide theengineering community with a standard procedure for evalu-a
20、ting DOL effects in these and other wood products. It is theintent of the Committee to limit the application of the conceptsin this specification to products that exhibit DOLeffects similarto solid wood. Creep-rupture tests of sawn lumber, structuralcomposite lumber, plywood, and oriented strand boa
21、rd (X1.5.1 X1.5.3) indicate that wood products whose strength iscontrolled by the properties of the wood fibers, wood strand orother wood elements in the product exhibit degradation mecha-nisms generally similar to those of solid wood used to establishthe DOL relationship in Practice D245.X1.1.2 Thi
22、s specification does not address the conditions ofextremely rapid loading or impact loading. Consequently thesections in Practice D245 related to this type of loading cannotbe applied to new products evaluated with this specification.Verification of the DOL adjustment for impact load conditionsrequi
23、res separate evaluation and is considered beyond thescope of this specification.X1.2 BackgroundX1.2.1 The phenomenon of creep-rupture, usually called theduration of load (DOL) effect in wood and wood-basedproducts has been of particular interest to the wood science andtimber engineering community as
24、 well as wood productmanufacturers concerned with the introduction of new buildingproducts and implementation of new codes for engineeringdesign in wood. Since the early 1970s, a significant amount ofwork has been conducted on measuring and empiricallymodeling the time-dependent strength behavior of
25、 structuralsize lumber. A historical perspective of this issue and a reviewof the major test studies conducted are provided by Barrett (4).X1.2.2 If new engineered wood products are to use theduration of load adjustments recommended in the design codesfor solid sawn lumber and other wood-based produ
26、cts, anappropriate procedure for confirming the applicability of suchuse is needed.X1.2.3 Through the use of a 90-day creep-rupture experi-ment the procedures of this specification allow a comparison ofthe 90-day term load performance of a wood or wood-basedproduct to that observed in solid sawn str
27、uctural lumber asderived from the results of extensive tests on lumber ofstructural sizes.X1.2.4 Typically, creep-rupture models are empirical, rely-ing on events observable only at a macro level. This type ofmodel, in the context of the proposed short term test, is onlysensitive to the actual micro
28、 level degradation phenomena(chemical bonds) leading to failure when that degradation leadsto creep or rupture during the test. The traditional DOLbehavior as presented in Practice D245 is based on observationand judgement of solid wood only. In that model, relativelyshort-term test results (like 90
29、 days) appear to fit within aprojection that can cover a longer period of degradation.However all degradation phenomena embodied in that state-ment of DOL are those of solid wood with limited processing.Materials or combinations of materials that may degrade underload and time with mechanisms differ
30、ent than those of solidwood may experience a different failure history than thatpredicted by the Practice D245 model. This specification is notdesigned to project duration of load performance beyond theperiod of the test for processing methods or materials havingdegradation mechanisms different from
31、 traditionally dried solidwood (possible examples of this may be chemically modifiedwood products or wood-plastic composites). Some compositematerials, such as plywood and glued laminated beams fabri-cated by traditional methods, may have test data and/or fieldexperience that demonstrates degradatio
32、n phenomena underload not significantly different from (or superior to) solidwood. Longer time intervals at the appropriate load levels aresuggested where the failure mechanisms leading to measurablefailure are not well understood or where field experience islimited.X1.3 Duration of Load Results for
33、 Solid Sawn StructuralSize LumberX1.3.1 Beginning in 1983, coordinated duration of loadprograms were initiated in the United States and Canada toinvestigate the effects of grade, species, loading mode,temperature, relative humidity and repeated loading on theduration of load response of lumber. The
34、majority of this workhas been previously summarized by Karacabeyli and Soltis (5),and Karacabeyli and Barrett (6) based on the studies conductedat the Forest Products Laboratory, Madison, WI and at ForintekCanada, Vancouver, BC. The summary in Appendix X1 in-cludes only those studies conducted under
35、 constant load inbending.X1.3.2 In total over 4600 individual lumber specimens fromover 40 separate test groups representing four wood species(Douglas-fir, western hemlock, white spruce, and southernpine) in various grades and sizes were placed under constantlong-term load in bending. The range of g
36、rades included SelectStructural, No. 2 and better and a test series with three qualitylevels labeled as High, Medium and Low. Beam sizes includednominal 2-by-4, 2-by-6, and 2-by-8in. lumber. Time underconstant load ranged from one week to four years among theD6815 09 (2015)5various studies. All stud
37、ies were conducted in constant 20C(68F), 50 % relative humidity or ambient in-door conditions.X1.3.3 Time-to-failure data collected from each of thesestudies was analyzed using the Stress Ratio approach. Thisapproach involved testing matched sets of members. The firstset was tested according to stan
38、dard short-term flexure testsmethods, usually producing failure in one to five minutes. Thesecond set was then loaded to produce a constant stress in allmembers, usually to some fractile in the distribution of theshort-term strength, and times-to-failure were recorded. Stressratios were then determi
39、ned using the Equal RankAssumption,which assumes that the order of failure for the constant loadmembers is the same as that for the standard short-term tests.The stress ratio was then calculated as the ratio of the appliedconstant load stress to the ranked stress from the standardshort-term tests (N
40、ote X1.1).NOTE X1.1The following example is intended to illustrate the use ofthe equal rank assumption as a basis for determining stress ratios and doesnot relate to the actual lumber test data or duration of load estimatesprovided in this Commentary. Start with a short-term test consisting of100 sp
41、ecimens. Assume that the strength of the weakest five specimenswas 1000 psi, 1100 psi, 1200 psi, 1300 psi, and 1400 psi, respectively.Assume that the stress level chosen for the long-term testing of 50specimens is 1000 psi. In the long-term test group, the first piece to fail(call it Piece A) is at
42、the 2nd percentile (1 out of 50) of its group; thesecond piece (call it Piece B) is at the 4th percentile, and so on. The equalrank assumption estimates that the (unknown) short-term strength of PieceA is the same as the 2nd percentile piece in the control group or 1100 psi.Similarly, the short-term
43、 strength of Piece B is estimated as 1300 psi(same as the 4th percentile of control group). This procedure assumes thatweve actually loaded Piece A to 91 % of its short-term strength(1000/1100) and that weve loaded Piece B to 77 % (1000/1300) of itsshort-term strength. So, when Piece A fails, its ti
44、me to failure (x-axis) willbe paired with a stress ratio of 0.91. Similarly, when Piece B fails, its timeto failure will be plotted with a stress ratio of 0.77.X1.3.4 Stress ratio versus time-to-failure plots for thesestudies are shown in Figs. X1.1 and X1.2 for logarithmic andreal time scales respe
45、ctively. The broad band of data observedis considered to be representative of the duration of loadbehavior of structural solid sawn wood based on the StressRatio approach.X1.3.5 Comparison of the average, minimum and maximumstress ratios for the lumber data to the Madison Curve is shownin Fig. X1.3.
46、 The lumber average trend line is similar to theMadison Curve for the time period of1htoapproximately 1year after which the two lines begin to diverge.X1.4 Duration of Load Evaluation ProcedureX1.4.1 The evaluation procedure in this specification usesthe structural lumber stress ratio results to def
47、ine the minimumperformance requirements expected for wood and wood-basedproducts. Fig. X1.4 shows the observed stress ratio results forstructural lumber when Fig. X1.2 is redrawn to reflect asix-month constant load time period. The minimum, average,and maximum stress ratios observed are shown in Tab
48、le X1.1.This range characterizes the duration of load behavior ofstructural solid sawn lumber. All wood and wood-based prod-ucts should meet these minimum stress ratio values if theirduration of load behavior is to be considered “like structurallumber”.X1.4.2 From Table X1.1, the minimum stress rati
49、o for threemonths of constant load is 0.55. This result is interpreted tomean that a wood member stressed to 55 % of its ultimateshort-term strength for a three month time period should notfail if its duration of load performance is characteristic ofstructural lumber. Since the strength of any one particular pieceof lumber is not known with absolute certainty it is necessaryto measure the short-term strength of a large number of piecesto characterize its bending strength distribution and determineits 5 % tolerance limit (TL). Based on a non-parametricestimate of the 5 % TLthis
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