Research Brief #25

Relationship Between Tension-Perpendicular-to-grain Stress and Shear Strength
of Southern Pine Glued Laminated Timber
Louisiana Forest Products Development Center
Louisiana State University
Baton Rouge, LA

Research Objectives

Evaluate experimentally the influence of tension-perpendicular-tograin stresses on shear strength in a commercial grade Southern Pine glulam beam (22F grade).

Investigate the distribution of shear and tension-perpendicular-tograin stresses near the critical section of test specimens using a finite element method.

Evaluate validity of Norris' interaction equation for combined shear and tension-perpendicular-to-grain, if required, propose new failure criteria based on experimental data.

Synopsis of Results

• The experimental results indicated that the average pure shear strength and pure tension perpendicular to-grain strength were around 819 psi and 306 psi, respectively.
• The average values of shear strength decreased with increase in the tension-perpendicular-to-grain stresses.
• Norris' interaction equation can be used to describe the experimental data appropriately, but the parabolic fitting gave a conservative prediction.
• Finite element analysis showed that shear and tension-perpendicular-to-grain stress distributions were nonlinear, and the maximum shear and tension-perpendicular-to-grain stresses were not occur at the same locations.
• While frequently occurring near the glue line of the samples, the failure was observed mainly by the weakness of the wood fibers, rather than that of the adhesive.

Conclusions

The average shear strength decreased gradually with the increase of tension-perpendicular-to-grain stress. Statistical analysis showed that Norris' interaction equation could be used to predict the average values of shear strength at any tension-perpendicular-to-grain stress levels, whereas the parabolic fitted lines gave a conservative prediction on the interaction behavior. The finite element analysis showed that the distributions of the tension-perpendicular-to-grain and shear stresses were nonlinear in the critical section. The failure of the glued laminated timber specimens was suggested to be caused by the combined effects of tension-perpendicular-to-grain and shear stresses, rather than other failure modes. The observation that the failure frequently occurred near the glue line of the samples was explained by the local stress concentration near the glue line and possible temperature effect. Further research is required to clarify the validity of using Norris' interaction equation to relate the shear strength to the pension-perpendicular-to-grain stress at a point of critical section.

Researchers

Yong Chen, Research Assistant
Department of Civil and Environmental Engineering.

Vijaya K.A. Gopu, Professor and Coordinator
Structural Engineering & Mechanics Program,
Department of Civil and Environmental Engineering, LSU

Target Group

Residential and commercial housing builders Louisiana economic development and public policy officials.

Methodology

• The tension-perpendicular-to-grain stress on shear strength of Southern Pine glulam timber was evaluated experimentally at 11 different tension-perpendicular-to-grain stress levels. The difference between two consecutive tension-perpendicular-to-grain stress levels was 10% of the average value of the pure tension perpendicular-to-grain strength and 3 replications were applied.
• Finite element analysis was conducted using a public FEM program, MSC/NASTRAN in the UNIX system, and four-node plane bilinear isoparametric elements were used for the calculation of the shear and tension-perpendicular-to-grain stress distributions in the critical section.
• Validity of Norris' interaction equation was tested using two nonlinear statistical models and the fitness of Norris' interaction equation to the experimental data was compared with two parabolic fitting schemes