The methodology for conducting a stress analysis of newly designed timber structures is well known. However, few data are available on the actual or allowable stress for repairing damaged structures. Consequently, design engineers often propose a repair scheme (e.g., gluing) without any knowledge of its structural effectiveness. To partially fill this void, a stress analysis was conducted on epoxy-repaired truss joints (Journal of Structural Engineering, Feb. 1986). Tests were conducted on epoxy-bonded truss joints made of various species of wood to determine actual glue-line shear stress recorded in pounds per square inch (psi). Summary information for independent random samples of southern pine and ponderosa pine truss joints is given in the accompanying table.

a. Estimate the difference between the mean shear strengths of epoxy-repaired truss joints for the two species of wood with a 90% confidence interval.
b. Construct a 90% confidence interval for the ratio of the shear stress variances of epoxy-repaired truss joints for the two species of wood. Based on this interval, is there evidence to indicate that the two shear stress variances differ? Explain.

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Southern Pine Ponderosa Pine Sample Size 100 47 Mean Shear Stress, psi 1,312 1,352 Standard Deviation 422 271