22 
BULLETIN 1408, U. S. DEPARTMENT OF AGRICULTURE 
of the posts, to which they should.be securely drift bolted. The 
sills should be anchored to concrete piers or pedestals by bolts in the 
masonry and extending into the sill. The provisions to be made 
against undermining of the foundations by erosion are explained on 
page 20. Concrete foundations should be high enough to keep the 
timber sills above the ordinary water level, with such bearing area 
at the bottom as will prevent settlement. 
Spacing between stringers should be not more than 27 inches cen- 
ter to center, preferably between 18 and 24 inches. Inside stringers 
should have full bearing on the caps, and should be separated at the 
laps to avoid retention of moisture between them. The outside string- 
ers may be placed end to end over the center line of the cap. Double 
2 by 4 inch bridging should be used when the width of stringer is 
less than one-third of the depth. The number of lines used should 
depend upon the span, as follows : Spans under 15 feet, 1 line ; spans 
from 15 to 20 feet, 2 lines ; spans above 20 feet, 3 lines. Tables 1 and 
2 show sizes and number of stringers for various spans, allowable 
unit stresses, and widths of roadway. 
Floor planks, spiked to each stringer, should be not less than 3 
inches thick for public highway crossings and 2 inches for private 
crossings, but preferably 3 inches for a 5-ton loading. 
Table 1. — Size and number of stri?igers for timber trestles using Western larch, 
Pacific post oak, bur oak, bald cypress, mountain region Douglas fir, or red- 
wood for which a fiber stress of 1,200 pounds per ■square inch is assumed 
[Truck load 80 per cent on rear axle, axles spaced 12 feet, wheels spaced 6 feet impact 30 per cent] 
16-foot roadway 
14-foot roadway 
10-ton truck 
5-ton truck 
3-ton truck 
Span 
Stringers required 
Lumber 
in string- 
ers per 
lineal foot 
of bridge 
Stringers required 
Lumber 
in string- 
ers per 
lineal foot 
of bridge 
Stringers required 
Lumber 
in string- 
ers per 
lineal foot 
of bridge 
Feet 
10 
Number 
12 
9 
11 
9 
13 
10 
9 
11 
10 
13 
11 
9 
13 
10 
Inches 
4x12 
4x14 
4x14 
4x16 
4x14 
4x16 
6x14 
4x16 
6x14 
4x16 
6x14 
6x16 
6x14 
6 x 16 
Feet b. m. 
48.0 
42.0 
51.3 
48.0 
60.7 
53.3 
63.0 
58.7 
70.0 
77! 
72.0 
91.0 
80.0 
Number 
11 
8 
13 
9 
11 
8 
Inches 
3x10 
3x12 
3x10 
3x12 
3x12 
4x12 
Feet b. m. 
27.5 
24.0 
32.5 
27.0 
33.0 
32.0 
Number 
10 
7 
12 
8 
10 
Inches 
2x10 
3x10 
2x10 
3x10 
3x10 
3x12 
Feet b.m. 
16 7 
12 
17.5 
20.0 
14 
20.0 
25.0 
21.0 
16 
12 
10 
13 
11 
8 
12 
9 
7 
13 
10 
8 
11 
9 
8 
13 
10 
9 
13 
11 
9 
3x12 
4x12 
3x12 
4x12 
4x14 
4x12 
4x14 
4x16 
4x12 
4x14 
4x16 
4x14 
4x16 
6x14 
4x14 
4x16 
6x14 
4x14 
4x16 
6x14 
36.0 
40.0 
39.0 
44.0 
37.3 
48.0 
42.0 
37.3 
52.0 
46.7 
42.7 
51.3 
48.0 
56.0 
60.7 
53.3 
63.0 
60.7 
53.3 
63.0 
11 
8 
13 
9 
7 
10 
8 
3x10 
3x12 
3x10 
3x12 
4x12 
3x12 
4x12 
27 5 
18 
24.0 
32 5 
20 
27.0 
28.0 
30.0 
32.0 
22 
14 
11 
8 
12 
10 
6x 14 
6x16 
6x18 
6x16 
6x 18 
98.0 
88.0 
72.0 
96.0 
90.0 
12 
9 
3x12 
4x12 
36 
36.0 
24 
13 
10 
8 
12 
9 
3x12 
4x12 
4x14 
4x12 
4x14 
39 
40.0 
37 3 
26 
14 
11 
8 
12 
12 
9 
6x16 
6x18 
8x18 
6x18 
8x16 
8x18 
112. 
99.0 
96.0 
108.0 
128.0 
108.0 
48 
42.0 
28 
13 
10 
8 
4x12 
4x14 
4x16 
52 
46.7 
42.7 
