Wood Handbook, Wood as an Engineering Material

a minimum of one-half the thickness of the main member.

The single-bolt value must then be modified for any varia-

tion from these reference conditions. The theoretical ap-

proach, after 1991, is more general and is not limited to

these reference conditions.

The theoretical approach is based on work done in Europe

(Johansen 1949) and is referred to as the European Yield

Model (EYM). The EYM describes a number of possible

yield modes that can occur in a dowel-type connection

(Fig. 8–5). The yield strength of these different modes is

determined from a static analysis that assumes the wood

and the bolt are both perfectly plastic. The yield mode that

results in the lowest yield load for a given geometry is the

theoretical connection yield load.

Equations corresponding to the yield modes for a three-

member joint are given in Table 8–16. (Equations for

two-member allowable values are given in the NDS.) The

nominal single-bolt value is dependent on the joint geom-

etry (thickness of main and side members), bolt diameter

and bending yield strength, dowel bearing strength, and di-

rection of load to the grain. The equations are equally valid

General Technical Report FPL–GTR– 190

Table 8–13. Percentage of basic bolt-bearing stress used for calculating

allowable bolt loads

Ratio of bolt

length to

diameter

(L/D)

L/D adjustment factor by classa

Parallel

to grain

Perpendicular

to grain

1 2 3 1 2 3 4

1 100.0 100.0 100.0 100.0 100.0 100.0 100.0

2 100.0 100.0 100.0 100.0 100.0 100.0 100.0

3 100.0 100.0 99.0 100.0 100.0 100.0 100.0

4 99.5 97.4 92.5 100.0 100.0 100.0 100.0

5 95.4 88.3 80.0 100.0 100.0 100.0 100.0

6 85.6 75.8 67.2 100.0 100.0 100.0 96.3

7 73.4 65.0 57.6 100.0 100.0 97.3 86.9

8 64.2 56.9 50.4 100.0 96.1 88.1 75.0

9 57.1 50.6 44.8 94.6 86.3 76.7 64.6

10 51.4 45.5 40.3 85.0 76.2 67.2 55.4

11 46.7 41.4 36.6 76.1 67.6 59.3 48.4

12 42.8 37.9 33.6 68.6 61.0 52.0 42.5

13 39.5 35.0 31.0 62.2 55.3 45.9 37.5

aClass determined from basic bolt-bearing stress according to Table 8–14.

Table 8–14. L/D adjustment class associated with basic bolt-bearing stress

Basic bolt-bearing stress for species group (MPa (lb in–2))

Loading direction Softwoods Hardwoods

L/D adjustment

(Table 8–13)

Parallel <7.93 (<1,150) <7.33 (<1,063) 1

7.93–10.37 (1,150–1,504) 7.33–9.58 (1,063–1,389) 2

>10.37 (>1,504) >9.58 (>1,389) 3

Perpendicular <1.31 (<190) <1.44 (<209) 1

1.31–2.00 (190–290) 1.44–2.20 (209–319) 2

2.00–2.59 (291–375) 2.21–2.84 (320–412) 3

>2.59 (>375) >2.84 (>412) 4

Table 8–15. Factors for adjusting
basic bolt-bearing stress
perpendicular to grain for bolt
diameter when calculating
allowable bolt loads

Bolt diameter
(mm (in.))

Adjustment

factor

6.35 (1/4) 2.50

9.53 (3/8) 1.95

12.70 (1/2) 1.68

15.88 (5/8) 1.52

19.05 (3/4) 1.41

22.23 (7/8) 1.33

25.40 (1) 1.27

31.75 (1-1/4) 1.19

38.10 (1-1/2) 1.14

44.45 (1-3/4) 1.10

50.80 (2) 1.07

63.50 (2-1/2) 1.03

>76.20 (>3 or over) 1.00