Bending, Shear, and Connection Tests for Nail Laminated Timber Evaluation


Executive Summary

Mechanically laminated timber (MLT) is a broad category of wood-based engineered panelized products that includes nail laminated timber (NLT) and dowel laminated timber (DLT), which are both used in the Canadian construction market. There are currently no published product standards or engineering design provisions for MLT products. To address this gap, the development of a product standard (CSA O125, Mechanically laminated timber – Production and qualification specifications) was initiated along with corresponding design specifications that will ultimately be incorporated in CSA O86, Engineering design in wood [1].

To support the development of CSA O125, a testing program was proposed that included laminate material tests, full size NLT bending tests of three different panel arrangements in single and double span arrangements, and short-span bending tests of seven different configurations. The tests were executed at the University of Northern British Columbia in collaboration with the University of Alberta. The objective was to evaluate the feasibility and applicability of a mechanical procedure to determine the strength and stiffness related layup factors for custom NLT with butt joints for CSA O125. The program was also designed to verify the layup factors for prescriptive NLT that were adopted, based on research from Germany [2], during the CSA O125 drafting process, and to propose new layup factors based on the test results.

Material tests were undertaken on 2-by-6-inch (38 x 140 mm) and 2-by-8-inch (38 x 184 mm) lumber using a three-point bending test setup in accordance with ASTM D198-21, Standard test methods of static tests of lumber in structural sizes [3]. For both sizes, 30 randomly selected lumber pieces were tested in bending on edge at a test span of 18 times the laminate depth. Load-deformation data and failure loads were recorded, and characteristic values for bending stiffness and bending strength were determined based on ASTM D2915-17, Standard practice for sampling and data-analysis for structural wood and wood-based products [4].

Full-size NLT panels made from 2-by-6-inch (38 x 140 mm) and 2-by-8-inch (38 x 184 mm) lumber were tested in single- and double-span boundary conditions under two-point loads. The single-span tests were based on the third-point bending test in accordance with ASTM D198-21 [3], while the double-span tests were based on the proposed test setup in CSA O125. The panels were tested at spans of 18 times the panel thickness. Three different arrangements were tested with each configuration having five replicates. Load-displacement data and failure loads were recorded, and characteristic values for bending stiffness and bending strength were determined based on the proposed method in CSA O125. Bending strength and stiffness layup factors were derived from the characteristic values and material properties were obtained from the results of the material tests.

A total of 41 short-span bending tests were performed on seven different specimen arrangements. Specimens consisted of three to five laminations with a different number of butt joints within the specimens. Each configuration had between three and eight replicates. Specimens were tested at spans of six and ten times the specimen depth. Shear strength layup factors were derived from the characteristic values and material properties were obtained from the results of the material tests.

Results from the testing program showed that the double-span bending test setup proposed for CSA O125 is sufficient but could be replaced by a simpler single-span third-point bending test setup. Results also showed that the system effect reduces the negative impact of butt joints on the bending properties of NLT panels. The variation of the measured bending moment capacity and stiffness of NLT panels was much lower than that of lumber laminate, leading to higher characteristic values compared with the measured characteristic bending strength or the specified bending strength of lumber of the same grade. Finally, results showed that under simple support conditions and single-point load, the effect of butt joints on the shear capacity could not be determined in this testing program.