The intensity of pressure at 26 feet 9 inches is 481.5 pounds per square foot.

The distribution of pressures is shown by the diagram on Fig. 111. The maximum bending moment is slightly below the point midway between the rangers and for a 12–inch strip is 10,500 inch-pounds.

Assuming 3 inch sheeting the maximum fiber stress is:

f = Mc
I = 10,400 × 1.5 × 12
12 × 27 = 568 pounds per square inch.

The working strength of yellow pine as given in Table 59, is 1200 pounds per square inch. Thinner sheeting should therefore be used.

TABLE 59
Working Unit Stresses for Timber
The most used value in the Building Codes of Baltimore, Boston, Cincinnati, Chicago, District of Columbia, and New York City
WoodTension, lb. sq. in.Compression With Grain, lb. sq. in.Compression Across Grain, lb. sq. in.Transverse Bending, lb. sq. in.Shear With Grain, lb. sq. in.Shear Across Grain, lb. sq. in.
Yellow pine12001000600120070500
White pine80080040080040250
Spruce and Va. pine.80080040080050320
Oak10009008001000100600
Hemlock60050050060040275
Chestnut6005001000800 150
Locust 120010001200100720
As published in American Civil Engineers Pocket Book.

Assuming 2–inch sheeting, the fiber stress is 1,300 pounds per square inch. This stress is too large. By reducing the ranger spacing slightly the stress can be brought within the required limits.

Assuming a ranger spacing of 3 feet 9 inches the depth to the upper ranger is changed to 23 feet and the maximum stress in the 2–inch sheeting becomes 1,140 pounds per square inch, a satisfactory result. The results for the computations for the other ranger spacings are shown in Table 60. The spacing of the rangers at the sheeting junctions is controlled by convenience and is not computed so long as it is obviously safe.

3. Size of Rangers.—The rangers will be assumed as 16 feet long with two end cross braces and one intermediate cross brace for each ranger. Starting as before at the bottom of the trench.

The area of the panel below the ranger and between cross braces is 24 square feet.