TENSILE STRENGTH of FINGER JOINTS in PITH-ASSOCIATED and NON-PITH-ASSOCIATED SOUTHERN PINE 2 by 6'S

ABSTRACT Finger joints in lumber containing p it h - associated wood apparently cause significant reductions in the strength of large glued-laminated timber beams. This probably results from the interaction of the inherent lower strength of the pith-associated wood and strength reductions intrinsic to the joint design. To more fully establish the effect of these factors and thus lead to design criteria for higher strength laminated timber beams, the tensile strength of finger- jointed southern pine 2- by 6-inch dimension lumber with and without pith-associated wood was studied. One hundred and fifty tension tests were made on southern pine lumber selected to meet or exceed the AITC 301-67 tensionlamination grade. These included 60 control (non-jointed) specimens and 90 finger-jointed specimens. Thirty of the controls and 60 of the jointed specimens contained a significant amount of pith-associated wood but still met the dense classification for structural lumber. The remaining specimens were free of pith-associated wood. The average tensile strength of finger- jointed lumber free of pith-associated material was significantly greater than that for jointed lumber which contained pith-associated material. Finger- jointed lumber consisting of one pith-associated and one non-pith-associated board had strength properties similar to finger-jointed lumber manu factured from two boards containing pith- associated wood. Control lumber which contained pith-associated material had an average tensile strength 34 percent less than that which was free of pith-associated material. The estimated 5 percent exclusion limit, in terms of tensile strength for specimens free of pith-associated material, was 4,700 p.s.i. for the finger-jointed specimens and 5,600 p.s.i. for control specimens. Although the results represent data on a single lumber sample, they are helpful in establishing tension grades of lumber for use inhigh-strength laminated timber members. TENSILE STRENGTH OF FINGER JOINTS IN PITH-ASSOCIATED AND NON-PITH-ASSOCIATED SOUTHERN PINE 2 BY 6's by R. C.MOODY, Engineer Forest Products Laboratory 1 Forest Service U.S. Department of Agriculture lNTRODUCTlON In the past few years, structural finger joints be restricted. have largely replaced scarf joints in the manu- Limited data on the strength of scarf-jointed facture of laminations for glued-laminated mem- structural lumber were presented by Zehrt (11) bers. From a production standpoint, the use of and data obtained during proof-loading experi- finger joints results in a savings of both material ments on finger-jointed lumber were presented and labor. In producing end joints, a section of by Strickler, Pellerin, and Talbott (8). However, lamination equal to the length of the joint is no data on the strength of commercially made wasted, A scarf joint is about 1-1/2 feet long in finger- jointed structural lumber are known to nominal 2-inch-thick material, but a finger joint be published. The bending strength of various is only about 1 to 2 inches long, thus saving end joints in small clear specimens has been lumber. A savings of labor costs is also realized investigated (3,9). Tension testa on small clear because the manufacture of finger joints is more finger- jointed specimens have also been con- adaptable to high-speed production. ducted (3,6); however, little is known about Results of tests on large glued-laminated either the tensile or bending strength of produc- timber beams (2)2 indicated that failure of a tion lumber with various end joints. significant number of the southern pine beams The purpose of the study was to evaluate the involved finger joints in the outer tension lamina- tension - parallel - to - grain strength of finger- tion. These joints were made lumber containing jointed structural lumber which either free the pith (heart) or a significant amount of non- of pith - associated wood or contained pith - dense wood near the pith, which will be hereafter associated wood. Material without joints was referred to as pith-associated wood. The low- included to evaluate the efficiency of the end density wood near the pith had low strength joints and to obtain an indication of the tensile properties and, because the strength of finger strength of material meeting tension lamination joints is less than that of clear straight-grained grades. non-jointed wood, the effect of pith- A parallel-to-grain tension test instead of a associated wood and finger joints could seriously bending test was used as the tension test more reduce the strength of jointed lumber. If this is closely simulates loading conditions in the outer the case, the amount of pith-associated wood in few tension laminations of large glued laminated boards used in critical tension laminations should timber beams. 1 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 2 Undertined numbers In parentheses refer to Literature Cited at the end of this report. RESEARCH MATERIAL containing thirty 12-foot-long specimens, were made with finger joints near midlength. One group consisted of NPA boards finger-jointed One hundred fifty 12-foot-long, nominal 2 by together; a second of PA boards finger- 6 specimens of southern pine structural lumber jointed and a third group made by were selected from a commercial lumber supply finger-jointing PA and NPA boards together. for this study. All lumber was graded to be To minimize production variables between Dense (7) and to meet the requirements of AITC groups of material, the following procedure was 301-67 (1). Selection and grading were done by used to manufacture the specimens with con- personnel of a laminating 3 tinuous finger - jointing equipment : “End-joint Seventy-five of the boards were graded to con- three noli-pith-associated 2 by 6’s, followed by tain the pith or a significant amount of pith- three pith-associated 2 by 6’s, followed by three associated material. The - remaining 75 were non-pith-associated, followed by three pith- selected to be free of the pith or a significant associated, etc.” By each 2 by 6 at mid- amount of pith-associated material. For future length after finger-jointing, each set of six con- reference, the will be referred to as secutive 2 by 6’s produced two specimens that either the pith-associated (PA) or the non-pith- were either (a) completely non-pith-associated, associated (NPA) material. (b) completely pith-associated, or (c) one-half Precise instructious regarding grading of non-pith-associated and one-half pith-associated. pith-associated wood were not given at the time The number of specimens selected were: of selection; therefore, all hoards were sub- sequently reclassified at the Laboratory prior to MateriaI Control Finger-jointed testing. Criteria used for this reclassification NPA 30 30 will be discussed in another section. FA 30 30 Sixty boards, 30 each of the PA and NPA material, were randomly selected for testing NPA-PA -- 30 without finger joints and will. be referred to as controls. The remaining 90 boards, 45 of each type of material, were used for the finger- RESEARCH METHODS jointed specimens. Each of the 150 boards was identified and the end of each board was arbitrarily When received at the Laboratory, all specimens assigned an A or B designation. The PA boards were stickered and stored in an atmosphere of were assigned Nos. 1-75, while the NPA boards 75° F. and 65 percent relative humidity to con- were assigned Nos. 76-150. Control specimens dition for at least 3 weeks before testing. were assigned Nos. 46-75 and 121-150. After the material was selected, a cross section about 2 inches long was sawn from each Visual end of the boards and properly identified. The cross sections were used to determine specific During the period, the percentage gravity and for reclassification purposes, of board cross section occupied by knots and associated grain deviation was determined. The width of knots and associated grain deviations MANUFACTURE OF steeper than 1 in 16 were measured on each wide FINGER-JOINTED SPECIMENS face of the boards. These measurements included grain deviations across the wide face of the Forty-five boards each of the PA and NPA boards and estimations of grain deviations through material were finger-jointed, using production the thickness of the boards. Estimations were equipment and procedures. The finger-joint di- made of the effective width of spike knots. Knots mensions were 1.11-inch length, 0.03-inch tip, whose associated grain deviation opened on an and 0.25-inch pitch. Three different groups, each edge were considered as edge knots. 3 Acknowledgment is made to the Koppers Co., Inc., Unit Structures Div., Peshtigo, Wis., who furnished alI research material for this study without cost to the Laboratory. FPL 138 2 the knot data, each 12-foot, 2 by 6 board 3. End sections not containing the pith and was regraded according to the AITC 301-67 (1) at least six annual rings and one-third or the 301+ requirements (2). The AITC 301-67 summerwood in the nearest 1 inch from the pith requirements for tension laminations for glued- were classified as non - pith - associated. Es- laminated beams were: sentially, this requirement extends the density 1. Growth rate requirements shall apply to rule used for grading lumber (7) to every inch the full length, and pieces of exceptionally of end section. weight shall be excluded. No distinction was made between the first two 2. Knots shall not occupy more than one- types, i.e., those containing the pith and pith- quarter of cross section. associated, except in describing the failures. In 3. The general slope of grain shall not exceed subsequent discussion of data, both types of 1 in 16 except where more: restrictive slope of material are referred to as pith-associated.

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