Timber Structures. Sawn Lumber Glulam Plywood. Sawn Lumber. Highly variable Classified by grade Visually or Mechanically graded Grade depends on number and location of defects Defects – shakes, checks, splits and knots.
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Member can be any length that can be shipped in the following cross-sections:
The allowable stress depends on the species and grade. The following table gives allowable stress values in psi:
Adjustment Factors are used to modify the allowable stress for various use conditions.
The following table lists the adjustment factors and their applicability:
Consider a wood frame house with dimensions shown. The first floor will be supported by joists 16” c.c., which, in turn, will be supported on wood sills on basement walls at the exterior, and on spine beams on the interior. The spine beams, in turn, will be supported on columns placed 20’ c.c. in order to leave maximum clear space in the basement. Service live load will be assumed as 40 psf, and service dead load, 15 psf. We will assume plywood structural flooring, and No. 2 Southern Pine for the joists and spine beam. The self weight of the joists and spine beams will be assumed to be included in the dead load.
CD = Load Duration Factor
= 1.0 for maximum intensity live load for 10 years, p. 3 notes
CL – Beam Stability Factor – for sawn lumber only, CL = 1.0 for the following conditions (d and b are nominal dimensions):
If these conditions are not met CL must be calculated (eq. p. 6 notes).
CF - Size Factor
CV – Volume Factor – Only applies to Glulam, p. 3 notes
Cfu – Flat use Factor – allowable stress factors are based on
loading the narrow edge. Flat use factor modifies the allowable stress when the load is applied to the wide face.
Cr = 1.15
2 x 12 joist
2 x 6 wood sill
2 x 12 joist
Spine beam showing loading from joists
equation p. 6 notes