Shear strength of spruce glued—laminated timber beams

Abstract

The effect of size on longitudinal shear strength has been well established for Douglas-fir glued–laminated (glulam) timber beams. The present study examined whether this phenomenon exists in glulam beams made of spruce. The experiment consisted of three projects in which beams of various sizes were tested under concentrated mid-span load. The project A beams had clear spruce webs and white elm flanges with cross-sectional dimensions varying from 25 × 25 mm to 75 × 75 mm. The project B beams had spruce glulam webs with Douglas-fir flanges; cross sections ranged from 20 × 100 mm to 90 × 200 mm. In project C, three groups of 10 replications of commercially representative sizes of glulam beams were made from stiffness-rated spruce–pine–fir lumber. The beam cross sections were 76 × 200 mm, 76 × 400 mm, and 127 × 400 mm.The results indicated that depth, width, and shear plane had significant effects on the longitudinal shear strength of the beams in project A. Depth, width, and shear span of the small glulam beams in project B also had highly significant effects on shear strength. However, no effects of depth and width on the shear strength of glulam beams in project C were found. Regression analysis showed no dependence of shear strength on sheared volume for the beams of all three projects. The three-parameter Weibull model also failed to predict the near-minimum shear strength of spruce glulam beams. The results suggested that the lower-bound shear strength of spruce glulam beams is a constant (regardless of beam volume) and could be used as a single characteristic value for glulam design in shear. Further review of published data indicates that this may also be the case for Douglas-fir glulam but with a lower characteristic value than for spruce.