Deck or Flooring of Military Bridges

Dimensioned lumber, never less than 2 inches thick and preferably 4 to 5 inches (except for foot-bridges), is most desirable for decking. The construction of a deck of poles is a slow process; because it is a tedious task to collect and prepare sufficient material for a bridge of considerable length. Poles for decking should be at least 3 1/2 inches in diameter. They should be well spiked so that they will not rattle. As a pole deck is usually very rough, it will generally be advisable to chink the openings and cover first with brush and leaves and then with a layer of earth. The additional dead load thus brought upon the structure should not be overlooked.

Woven-brush mattresses, supported on deck poles at relatively wide intervals and covered with earth, make a satisfactory deck for foot-bridges.

The wear on decking resulting from military traffic is very great, hence light material is unsatisfactory. For heavy bridges on important routes a deck 5 inches thick, preferably of hardwood, is now regarded as standard. If only 2-inch material is available, a double thickness should be used. The distance between stringers in feet should not be more than the thickness of the decking in inches.

Width of Roadway

As the amount of material and the time required for construction increase with the width of the roadway, military bridges are made no wider than necessary, and are generally limited to a single line of traffic, with clearance for the passage of footmen, horsemen, or motor-cycles. As military traffic usually moves in trains and can be closely regulated, it requires a less width of roadway than unregulated civil traffic. For the standard bridges, a roadway of 10 or 11 feet is employed. If it be necessary to provide for the continuous movement of traffic in both directions, two bridges side by side may be adopted; or, if the traffic be very dense, three or four parallel bridges may be built. Two-way bridges are, of course, also employed. A 15 ft. roadway will pass two lines of traffic, but can hardly be called ample for the purpose, a width of 18 or 20 feet being better.

The maximum capacity of military bridges in service should be developed by efficient traffic control.

Side-Rails and Hand-Rails

Side-rails or wheel-guards of ample strength should never be omitted from any vehicle bridge. Hub-guards and hand-rails are generally provided for through-truss spans, including suspension bridges, largely for the protection of the bridge itself; but they are generally omitted in the case of deck spans. On the other hand, side-rails are unnecessary for foot bridges, except to secure the flooring; but hand-rails are generally provided.

Materials Employed in Military Bridging

Any available material must be utilized in the construction of military