1、Designation: D6815 09Standard 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 the case of revisio
2、n, 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 and creep effects of
3、 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 intended to demonstrat
4、e 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. Quantification of spe
5、cificduration 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 duration of load relation
6、shipis 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 must be satisfied:
7、 (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 phenomena not descr
8、ibed 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 Establishing Structural Grade
9、s and Re-lated Allowable Properties for Visually Graded LumberD1037 Test Methods for Evaluating Properties of Wood-Base Fiber and Particle Panel MaterialsD2915 Practice for Evaluating Allowable Properties forGrades of Structural LumberD3043 Test Methods for Structural Panels in FlexureD4442 Test Met
10、hods for Direct Moisture Content Measure-ment of Wood and Wood-Base MaterialsD4761 Test Methods for Mechanical Properties of Lumberand Wood-Base Structural MaterialD5457 Specification for Computing Reference Resistanceof Wood-Based Materials and Structural Connections forLoad and Resistance Factor D
11、esignE4 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/AF and is an careful selection with the desire to assure that the productcan meet all
12、 six acceptance criteria.6.4 Allowable Property AdjustmentIf the retest provesthat the product meets all the acceptance criteria defined in 6.3,all time-dependent member and connection properties definedby the NDS shall be reduced by the percent change in stresslevel used in the retest(s).7. Report7
13、.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 other specific process par
14、ametersinvolved 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 Records of test environmen
15、tal 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 Statistical calculations, including
16、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 available, aprecision and bias s
17、tatement will be included.9. Keywords9.1 creep rate; creep-rupture; duration of load; fractionaldeflection; lumber; structural composite lumber; structural-usepanelsD6815 094APPENDIXES(Nonmandatory Information)X1. COMMENTARY ON DURATION OF LOAD EFFECTS IN WOOD PRODUCTSX1.1 ScopeX1.1.1 Appendix X1 pr
18、ovides 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 duration of load(DOL) and creep adjus
19、tment 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-ating DOL effects in these and othe
20、r 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 board(X1.5.1-X1.5.3) indicate that wo
21、od 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 This specification does not address th
22、e 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 conditionsrequires separate evaluation and is cons
23、idered 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 well as wood productmanufacturers
24、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 structuralsize lumber. A historica
25、l 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 products, anappropriate procedure for co
26、nfirming 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 structural lumber asderived from the r
27、esults 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 level degradation phenomena(chemic
28、al 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 days) appear to fit within aprojec
29、tion 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 different than those of solidwood may exp
30、erience 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 traditionally dried solidwood (pos
31、sible 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 degradation phenomena underload not significa
32、ntly 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 Solid Sawn StructuralSize LumberX1
33、.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, tem-perature, relative humidity and repeated loading on the dura-tion of load response of lumber. The majority of this work hasbeen p
34、reviously summarized by Karacabeyli and Soltis (5), andKaracabeyli and Barrett (6) based on the studies conducted atthe Forest Products Laboratory, Madison, WI and at ForintekCanada, Vancouver, BC. The summary in Appendix X1 in-cludes only those studies conducted under constant load inbending.X1.3.2
35、 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 grades included SelectStructural
36、, 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 095various studies. All studies were conducted in constant 20C(68F
37、), 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 standard short-term flexure testsmethods,
38、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 determined using the Equal RankAssumption,whi
39、ch 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 (Note X1.1).NOTE X1.1The following examp
40、le 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 specimens. Assume that the strength of t
41、he 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 the 2nd percentile (1 out of 50) of it
42、s 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 strength of Piece B is estimated as 1
43、300 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 time to failure (x-axis) willbe paired w
44、ith 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 respectively. The broad band of data observ
45、edis 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. The lumber average trend line is simi
46、lar 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 define the minimumperformance requirement
47、s 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 Table X1.1.This range characterizes the d
48、uration 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 ratio for threemonths of constant load is
49、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 approach is interpreted to mean thatin
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