Date of Award
Master of Science (MS)
Schiff, Scott D.
The reference design values published in the National Design Specification (NDS) for Wood Construction are derived from full-scale testing of lumber samples performed in the 1980s. This testing program is commonly known as the In-GradeTest Program. Selective annual sample tests of visually graded Southern Pine lumber from 1994 to 2010 revealed an overall decreasing trend in the mechanical properties. Because of this alarming observation, a new round of full-scale In-Grade test of visually graded Southern Pine was initiated in 2010. The new test data indicated significant reductions in certain design values published in the current design code (2005 NDS). The new reference design values have been adopted by the 2012 NDS. Compared to the 2005 NDS, the 2012 NDS reference design values for modulus of elasticity (MOE) and modulus of rupture (MOR) were reduced by approximately 0.0 to 14.3% and 11.4 to 41.7%, respectively. This suggests that the underlying reliability of structures constructed recently using Southern Pine might not meet the minimum target flexural reliability speculated in the design code. The main goal of this study was to assess the reliability of flexural members constructed using visually graded Southern Pine lumber and designed using the 2005 NDS design values to determine if they meet the minimum target reliability of wood construction.
The new MOE and MOR data were obtained from the Southern Pine Inspection Bureau (SPIB). Probability distribution fitting was performed to determine the best-fit statistical distributions for the new MOE and MOR data. Five distributions were considered: Normal, Lognormal, Gumbel, Frechet and Weibull distributions. The fitted distribution parameters were used to assess the reliability of visually graded Southern Pine floor joists subjected to uniformly distributed dead and live loads.
Two scenarios were considered in the reliability analyses conducted in this study. The first scenario assessed the reliability of flexural members designed using the 2005 NDS reference design values which are derived from the 1978 In-Grade test data. The second scenario assessed the reliability of flexural members designed using the new reference design values for visually graded Southern Pine lumber which are derived from the new (2010) In-Grade test data. The analysis results showed that the reliability of Scenario 1 designs (i.e. designs based on the 2005 NDS values) are lower than that of Scenario 2. However, the overall influence of reductions in new reference design values of visually graded Southern Pine on the reliability or safety of bending members is not as significant as expected. This is because the design of flexural members, in particular for No. 2 and better grades, often is controlled by the serviceability limit state (deflection) and not the strength level limit state.
Using both the 2005 NDS and 2012 NDS design values, maximum span lengths for floor joists for common ranges of live load-to-dead load ratios, joist spacings and joist dimensions were computed and tabulated in a series of tables. These tables can be used by practitioners as design guides to quickly determine if the floor joists designed based on the 2005 NDS are at-risk or required rectification. Since shear failure usually does not control in the design of floor joists, only the bending strength and serviceability (deflection) limit states were considered in the maximum span tables. Comparison between the maximum span lengths determined from the 2005 NDS and 2012 NDS revealed that the reduction in allowable span lengths is a function of lumber grade, in which the reductions in maximum span lengths for lower grade lumbers are more significant than that of higher grade lumbers. There are no reductions for the maximum span lengths of Select Structural (SS) grade lumber while the maximum span lengths of No. 3 & Stud grade floor joists are reduced by 7.7 to 13.4%.
Yang, Mengyu, "Flexural Strength Reliability of Visually Graded Southern Pine Dimensional Lumber" (2013). All Theses. 1746.