STEUCTUEES USED IN DRAINING AGRICULTURAL LAND 
23 
Table 2. — Size and number of stringers for timber trestles using tanbark oak, 
whit-e oak, Cuban- pine, longleaf pine, coast region Douglas fir, and shortleaf 
pine (treated) for ichich a stress of 1,600 pounds per square inch is assumed 
[Truck load SO per cent on rear axle, axles spaced 12 feet, wheels spaced 6 feet, impact 
SO 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 
Xumber 
12 
9 
11 
9 
13 
10 
8 
11 
9 
Inches 
3x12 
4x12 
4x12 
4x14 
4x12 
4x14 
4x16 
4x14 
4x16 
Feet b. m. 
36.0 
36.0 
44.0 
42.0 
52.0 
46.7 
42.7 
51.3 
48.0 
Number 
12 
8 
10 
11 
8 
Inches 
2x10 
3x10 
3x10 
3x12 
3x10 
3x12 
Feet b.vi. 
20.0 
20.0 
25.0 
21.0 
27.5 
24.0 
Number 
8 
Inches 
2x10 
Feet b. m. 
13.3 
12 
9 
2x10 
15.0 
14 
1] 
2x10 
3x10 
18.3 
17.5 
16 
13 
10 
8 
11 
8 
3x10 
3x12 
4x12 
3x12 
4x12 
32.5 
30.0 
32.0 
33.0 
32.0 
12 
9 
2x10 
3x10 
20.0 
22.5 
18 
13 
9 
10 
10 
14 
11 
12 
11 
8 
13 
1 
\l 
8 
14 
11 
9 
9 
4x14 
6x14 
4x16 
6x14 
4x14 
4x16 
4x16 
6x14 
6x14 
4x16 
6x14 
6x16 
6x14 
6x16 
6x18 
6x14 
6x16 
6x18 
8x16 
60.7 
63.0 
53.3 
70.0 
65.3 
58.7 
64.0 
77.0 
64.0 
69.3 
77.0 
72.0 
91.0 
80.0 
72.0 
98.0 
88.0 
•81.0 
96.0 
10 
3x10 
3x12 
25.0 
21.0 
20-- --- --- 
7 
13 
10 
8 
11 
9 
3x12 
4x12 
4x14 
3x12 
4x12 
4x14 
4x12 
4x14 
36.0 
36.0 
32.7 
39.0 
40.0 
37.3 
41.0 
42.0 
11 
8 
3x10 
3x12 
27.5 
24.0 
22-- - - 
12 
9 
3x10 
3x12 
30.0 
27.0 
24 
14 
10 
8 
11 
9 
3x10 
3x12 
4x12 
3x12 
4x12 
35.0 
30.0 
32.0 
26 
13 
10 
8 
11 
8 
7 
4x12 
4x14 
4x16 
4x14 
4x16 
6x14 
52.0 
46.7 
42.7 
51.3 
42.7 
49.0 
33.0 
36.0 
28 
12 
10 
3x]2 
4x12 
36.0 
40.0 
Handrails should be secured to the outside stringers by at least 
two bolts, spaced as far apart as the depth of the stringer permits. 
The usual type of handrail has one rail spiked to the top of posts 
and two side rails, one at the top of the post, the other midway be- 
tween the floor and the top of posts. A wheel guard should be on 
each side of the roadway and be wide enough to prevent contact be- 
tween vehicles and handrails. 
For small drains, or large ones where the discharge is not exces- 
sive or subject to considerable increase, a culvert is more economical 
than a bridge ; but in deciding between them it must be remembered 
that a culvert must be capable of passing more than the normal flow, 
otherwise weeds and debris may clog the intake. Pipe culverts 
should never be less than 15 inches in diameter. Except for very 
shallow fills, two parallel lines are not recommended, because of 
their cost and the greater possibility that the waterway may become 
obstructed. On many drainage ditches, culverts can not be used 
because of floods which occasionally greatly exceed the normal flow, 
and in some districts because small motor boats are used in main- 
tenance work. 
