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Shear Strength of Glued-Laminated Timber Beams and Panels

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Shear Strength of Glued-Laminated Timber Beams and Panels Shear Strength of Glued-Laminated Timber Beams and Panels Douglas R. Rammer, Forest Products Laboratory, USDA Forest Service Abstract material. Researchers have questioned the validity Five sizes of glued-laminated timber beams and panels of predicting beam shear by ASTM shear-block testing are being tested in a five-point beam-shear test (Ethington and others 1979; Radcliffe and Suddarth configuration to determine the shear strength capacity 1955). In 1994, Soltis and Rammer (1994) established and size relationships. A total of 200 matched a relationship between ASTM D143 shear block and specimens will be tested: 100 loaded edgewise about beam shear by considering the size of the specimens. the strong axis and 100 loaded flatwise about the weak Although their research correlates beam shear to axis. Statistical methods will be used to estimate mean traditional testing methods, questions remain about and coefficient of variation considering censored data, shear design. One such question concerns the design test the significance of strength-size effect, and correlate of vertically laminated deck panels for timber bridges. between beam shear and ASTM D143 block shear. At the time this paper was written, only the beams loaded Deck panels loaded flatwise about the weak axis have a edgewise about the strong axis were completely tested shear design value less than panels loaded edgewise along with the smaller-sized beams loaded about the about the strong axis (AITC 1993; AFPA 1991). weak axis. This paper presents results for the strong When you consider the current ASTM standards, past axis tests and a general discussion about the weak axis and current shear research, and laminating effects, the tests. To date, edgewise specimens had similar size– difference in shear strength seems counter intuitive. strength relationships as were observed in previous glued-laminated shear studies. Objective The general objective of this study is to improve the Keywords: Glued-laminated, shear strength, beams, shear design criteria as it applies to deck panels. panels, design, size effect Specific objectives include the following: Introduction • Improve the beam-shear database to include axial Allowable shear strength values for wood construction glued-laminated deck combinations. are determined by adjusting ASTM D143 (1995a) • shear-block results, according to methods specified by Determine if there is a correlation between shear ASTM D245 (1995b) for visually graded lumber and strength and size for glued-laminated deck panels. ASTM D3737 (1995e) for glued-laminated timber Size parameters considered are beam width, shear area, or volume. 192 • Determine if the shear strength versus shear area The typical probability of no cracks in a piece of wood equation derived for horizontally laminated beams is given in Table 1, according to the Canadian is applicable to vertically laminated beams. In-Grade Testing Program. Based on these Canadian data and species, only the 39- by 254-mm (2- by • Clarify the design method for highway bridge 10-in.) material did not meet or exceed the one in decks by investigation of the failure mode in four condition for splits. In addition, the average panels bent about the weak axis. split length is 0.75 times the width for Douglas Fir, These objectives will be met through an experimental 0.93 the width for Hem-Fir, and 0.62 for Spruce-Pine- testing program and statistical analysis of the Fir (Foschi and others 1989). experimental results. Split limits are applied to the lamination material Background before fabrication. When fabricated in a beam, splits will be restrained from propagation by the surrounding The ASTM D3737 method for determining shear material and cannot develop a full-length split as strength of horizontally laminated glued-laminated assumed in the ASTM D3737 criteria. Therefore, material results in greater design values than does quality requirements, split probability, and length vertically laminated material. Lower design values characteristics along with split restraint after fabrication for vertically laminated members occur because ASTM seem to make the one in four split assumption D3737 assumes every fourth lamination has a through conservative. width check or split that limits the strength ratio to one-half. In a four-lamination beam, this results in a Since the early 1970s, shear research has moved from total reduction of one-eighth. Based on rational the ASTM shear-block to beam-shear experiments. In thinking, this approach seems inefficient in light of the the first comprehensive study on beam shear, Keenan ASTM D3737 §4.5, current shear strength research, (1974) indicated that shear strength of Douglas Fir and laminating concept. glued-laminated timber beams depended on the sheared Currently, ASTM D3737 §4.5 states the following area. Also, he observed that the two-beam theory about the quality of the laminating member prior to proposed by Newlin and others (1934) is not applicable fabrication: to glued-laminated timber beams. Lumber shall be free of shakes and splits that Foschi and Barrett (1976, 1977) applied Weibull’s make an angle of less than 45° with the wide theory of rupture to shear strength of wood. The face of the piece. Pitch pockets shall be Weibull theory is a statistical theory that predicts the limited in size to the area of the largest knot strength of material at a given probability of failure. permitted, and pitch streaks shall be limited Longworth (1977) used a simple-span, Douglas Fir to one sixth of the width of the lumber. glued-laminated timber beam with symmetrically By this passage, ASTM is requiring a higher quality of placed concentrated loads to show experimentally that material, with respect to splits, shakes, and checks. ASTM shear-block strength is unrepresentative of beam-shear strength, and shear strength varies with Lumber grading rules also specify a maximum beam size, sheared area, or volume. allowable split length as a function of the wide dimension of the member. For select structural (SS) Quaile and Keenan (1978) used a single-span beam test and No. 1 material, maximum split length is limited to investigate maximum shear in rectangular beams to the width of the wide face. For No. 2 material, the of specially designed glued-laminated material. This split length is limited to one and a half the member test specimen was successful in producing shear failures width (WWPA 1991). in 104 of the 108 specimens tested and gave further evidence that the ASTM shear-block test produces Table l—Probability of no crack in different material (Foschi and others 1989). 193 lower strength values than is evident in small, which is applicable to horizontally laminated beams, rectangular beams. Quaile and Keenan also investigated but only eight beams of one size were tested. the effect of growth ring orientation on shear strength. They concluded that radial shear strength values were Glued-laminated material has always boasted of the statistically different than tangential values at a 0.01 positive effects of lamination to resist bending forces: level of significance. This indicates that orientation, improved lumber utilization, more effective transfer either flatsawn or quarter sawn, might influence the of stresses around defects (knots), and reduced strength shear strength of the beam. This orientation effect was variability. Positive effects may also be experienced by also observed in Southern Pine shear-block results a deck panel resisting shear forces. A simplistic (Bendsten and Porter 1978). analogy is used to explain this reasoning. Keenan and others (1985) used a special test specimen The major advantage of a glued-laminated beam is its to determine the shear strength of glued-laminated ability to resist bending forces with the cross-sectional spruce beams. They concluded that shear strength is moment of inertia (full-composite action). Full-moment not a function of volume but of sheared area. resistance may only be accomplished if shear forces are transferred completely through the cross section, Rammer and Soltis (1994) experimentally defined the from one elemental point to the next point. Inability to beam-shear strength of Southern Pine and Douglas Fir transfer shear in one lamination will lead to glued-laminated timber beams. Strength was substantially reduced inertia values, and in the extreme determined for various-sized beams, 39 by 102 mm equal, the sum of the individual lamination inertia (2 by 4 in.) through 127 by 610 mm (5 by 24 in.), by values (no composite action). a five-point bending method. In a beam with full-width horizontal laminations and Based on the five-point bending and ASTM D143 loaded about its strong axis, shear strength will be shear-blocks results, Soltis and Rammer (1994) governed by the weakest lamination within the middle empirically derived an equation that related the two section of the beam. Delamination and a reduction in findings. Their equation considers the stress riser at the the moment of inertia will occur when the shear re-entrant comer of the ASTM shear block, different strength of the weak lamination is exceeded. If the load wood species, and the effect of beam size on shear is maintained or increased, this will lead to failure strength. The equation is of the remaining section by tensile rupture or shear. (1) In a deck panel (same beam as just noted, but loaded about weak axis), the weak lamination will not entirely control the shear strength of the beam. When the weak where lamination shear strength is exceeded, the lamination t=beam shear strength, MPa, will break and shed load to the surrounding stronger C = 2 = stress concentration factor to adjust the laminations. This load sharing or shedding process f will lead to increased performance The larger the deck ASTM shear-block strength to the true panel, the smaller the increase of load to the individual strength at failure, surrounding members and the greater the performance.
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