Manual of Military Training / Second, Revised Edition

by James A. Moss

The clear width of roadway of an ordinary highway bridge should not be less than 12 ft. for single track, or 20 ft. for double track.

The clear head room in ordinary military bridges should not be less than 9 ft. for wagons and cavalry; for highway bridges not less than 14 ft.

Ramps at the ends of a bridge, if intended for artillery, should not be steeper than 1 on 7. For animals, slopes steeper than 1 on 10 are inconvenient.

If the bridges are high, hand rails should be provided. A single rope may suffice, or it may have brush placed upon it to form a screen.

A guard rail should be provided along each side of the roadway, near the ends of the flooring planks. In hasty bridges it may be secured by a lashing or lashings through the planking to the stringer underneath. Otherwise it may be fastened with spikes or bolts.

[1112]. Spar bridges.—This name is applied to bridges built of round timbers lashed together. Intermediate points of support are provided by inclined frames acting as struts to transmit weight from the middle of the bridge to the banks. The single-lock and double-lock bridges with two and three spans of 15 ft., respectively, are the ones of most utility.

The first step in constructing a spar bridge is to measure the gap to be bridged and select the position of the footings on either bank. Determine the distance from each footing to the middle point of the roadway if a single-lock, or the two corresponding points of a double-lock bridge. Next determine and mark on each spar except the diagonals the places where other spars cross it. The marking may be done with chalk, or with an ax. If possible a convenient notation should be adopted. As, for example, in marking with chalk, a ring around the spar where the edge of the crossing spar will come, and a diagonal cross on the part which will be hidden by the crossing spar.

A simple way to determine the length of spars is the following: Take two small lines somewhat longer than the width of the gap, double each and lash the bights together. Stretch them tightly across the gap so that the lashing comes at the middle as at A, [Fig. 8]. Release one end of each and stretch it to the footing on the same side as indicated by the dotted lines. Mark each line at the footing C or C', and at the position chosen for the abutment sill, B or B'. Cut the lashing and take each piece of rope to its own side. The distances AB and AB' are the lengths between the transoms, and with 2 ft. added give the length of road bearers required. The distances AC and AC' are the lengths of struts from butt to top of transom, and with 3 ft added, give the total length of spars required.

For a double lock bridge, a piece of rope of a length equal to the length of the middle bay replaces the lashing. If the banks are not parallel, a measurement should be taken on each side of the bridge.

If desired, a section of the gap may be laid down on the ground in full size and the lengths of spars determined by laying them in place. This method, though given as standard by all authorities, requires more time and more handling of material than the other and gives no better results.