1. Timber Structures
Sawn Lumber
Glulam
Plywood

2. 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

3. Sawn Lumber is available in lengths up to 20’ to 25’ with the following cross sectional dimensions:
Nominal Size
Actual Size
Boards
1 x (2→12)
¾” x 1 ½ →11 ½
Dimension
Lumber
2 x 2
1 ½ x 1 ½
2 x 4
1 ½ x 3 ½
2 x 6
1 ½ x 5 ½
2 x 8
1 ½ x 7 ¼
2 x 10
1 ½ x 9 ¼
2 x 12
1 ½ x 11 ¼
4 x 4
3 ½ x 3 ½

4. Glulam Composed of strips of sawn lumber glued together
Can form larger sections than can be obtained from trees
Can optimize placement of wood

5. Glulam Width, in. Laminations 2 ½ 3 1/8 5 1/8 6 ¾ 8 ¾ 10 ¾ 6 to 18
Member can be any length that can be shipped in the following cross-sections:
Western Species
1 ½” thick laminations
Southern Pine
1 3/8” laminations
Width, in.
Laminations
2 ½
3 1/8
5 1/8
6 ¾
8 ¾
10 ¾
6 to 18
4 to 16
4 to 24
5 to 32
6 to 42
7 to 54 3 5
8 ½
10 ½
4 to 18
4 to 26
5 to 35
6 to 46
7 to 56

6. Plywood Made from sheets of wood peeled from logs
Glued together to from large panels
Typically 4’ by 8’
Available in ¼” to 1 1/8” thicknesses
Use American Plywood Association literature for strengths

7. Allowable Stress Design of Timber
ASD design still in use in timber
Service load stress ≤ allowable stress
The allowable stress depends on the species and grade. The following table gives allowable stress values in psi:

8. Allowable Stress Adjustment Factors
Adjustment Factors are used to modify the allowable stress for various use conditions.
The following table lists the adjustment factors and their applicability:

9. Timber Beam Design Example p. 7 notes
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.

10. Floor Plan 60’-0 c.c. 20’-0 c.c. 20’-0 c.c. 20’-0 c.c. 20’-0 c.c.
20’-0 c.c.
Spine beam
20’-0 c.c.
20’-0 c.c.
20’-0 c.c.

12. Allowable Stress Values, psi

14. Adjustment Factors CD = Load Duration Factor
= 1.0 for maximum intensity live load for 10 years, p. 3 notes CM Ct

15. Adjustment Factors
CL – Beam Stability Factor – for sawn lumber only, CL = 1.0 for the following conditions (d and b are nominal dimensions):
d ≤ 2b → no lateral support required (2x4)
d ≤ 4b → both ends held in position to prevent overturning (2x8)
d ≤ 5b → one edge of beam laterally supported for entire length. (2x10)
d ≤ 6b → compression edge laterally supported for entire length… (2x12)
d ≤ 7b → both edges continously laterally supported (2x14)
If these conditions are not met CL must be calculated (eq. p. 6 notes).

16. Adjustment Factors CF - Size Factor
CV – Volume Factor – Only applies to Glulam, p. 3 notes
Typ.
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.
Flat use

17. Adjustment Factors
Cr – Repetitive use factor – For sawn lumber whenever three or more beams in contact or not more than 24” c.c. share a load
Cr = 1.15
Cc – Curvature factor – applies only to glulam
Cf – Form factor – For circular or diamond shape cross-sections
CH –Shear Stress Factor – Can increase allowable shear stress of beam if location and extent of defects is known

18. Detail of Beam Resting on Sill
Critical shear for design is a distance d from face of support
20’-0
2 x 12 joist d
2 x 6 wood sill d
basement wall

19. Can consider composite section when calculating deflection if sub- floor is properly connected to joists
plywood flange
2 x 12 joist

20. Floor Plan 60’-0 c.c. 20’-0 c.c. 20’-0 c.c. 20’-0 c.c. 20’-0 c.c.
20’-0 c.c.
Spine beam
20’-0 c.c.
20’-0 c.c.
20’-0 c.c.
Spine beam showing loading from joists

21. Available Glulam Width, in. Laminations 2 ½ 3 1/8 5 1/8 6 ¾ 8 ¾ 10 ¾
Western Species
1 ½” thick laminations
Southern Pine
1 3/8” laminations
Width, in.
Laminations
2 ½
3 1/8
5 1/8
6 ¾
8 ¾
10 ¾
6 to 18
4 to 16
4 to 24
5 to 32
6 to 42
7 to 54 3 5
8 ½
10 ½
4 to 18
4 to 26
5 to 35
6 to 46
7 to 56

22. Allowable Stress Values, psi

23. Adjustment Factors

24. Cp = Column stability factor
CD = 1.0 for continuous lateral support
CD ≤ 1.0 no intermediate lateral support
equation p. 6 notes

25. Southern Pine Span Tables
What is the maximum span for Grade 2, 2 x 10 floor joists, spaced 16” c.c., carrying 40 psf live load?
Choose a ceiling joist to carry a live load of 20 psf for a span of 15 ft. The joist will be spaced at 24” c.c.
An existing floor system is composed of Grade 1, 2 x 12’s, spaced 12” c.c. The joists span 14 ½’. What is the maximum allowable live load based on the Southern Pine Span Tables?

26. Biaxial Bending of Beams Example
Choose a W-shape beam for the factored loading shown using grade 36 steel. The simple beam span is 25 feet and it is continuously braced against instability. It is not necessary to check shear and deflection for this problem. 4 5
2 k/ft 3
3 k/ft