Evaluation of glulam shear strength using a full-size four-point test method. CIB-W18 paper 34-12-2

The shear strengths of structural glued laminated timber (glulam) have been traditionally evaluated in the United States based on the procedures set forth in ASTM Standards D3737 and D2555 using small block shear values of clear wood specimens. For most glulam products, the design shear stresses so derived are conservative. In recent years, the demand to optimize the design shear stress has been increased due to a higher design shear stress offered by competing structural wood composites, such as laminated veneer lumber (LVL) and parallel strand lumber (PSL). Since 1997, APA has conducted a series of full-size shear tests on glulam manufactured with Douglas fir, Southern pine, and Spruce-Pine-Fir. A four-point load method with a clear distance between the edge of the reaction bearing plate to the edge of the nearest curved load bearing block of at least 2 times the specimen depth was used to test all specimens. Based on this experience, the full-size shear test method has been adopted in ASTM D3737 as a standard test method for determining the horizontal shear strength of glulam. This paper provides detailed descriptions of the test methods, experimental results, and data analyses. The test results obtained from this study indicate that the characteristic shear strength values based on full-size shear tests are approximately 70 % of the values determined from small block shear tests. However, the allowable horizontal shear stress could be increased by a factor of at least 1.25, including a 10 % reduction to allow for occasional seasoning checks. This increase can be attributed in part to the difference in the procedures used to derive the design value between the full-size and small block shear tests