spans, mainly for railroad bridges on the lines of communication. Their purpose, of course, is to reduce the time required for pier construction. They differ in no essential from civil structures for similar situations.

Concrete

Concrete is utilized for culverts, for foundations of framed trestles, occasionally for abutments of bridges on important routes of communication, and in the repair of old masonry bridges. For piers, piling or crib-work is almost invariably preferable, even in the case of large and important bridges. Arches and girder spans of concrete will seldom be used. In the battle zone concrete finds very little application in bridgework.

Stone

Stone is employed as rip-rap for the protection of piers and abutments, and as crib-filling. Stone masonry will only exceptionally be used for abutments or wing and retaining walls; for there will be but few situations in which either concrete or rubble-concrete would not be preferable. If no cement is available, line-mortar or dry-stone walls are sometimes employed.

Paint

Paint and other protective materials are sparingly used in military bridging. Ordinarily the time for which the structures will be required does not justify the employment of these preservatives. Steelwork should always receive a shop coat, but field coats will usually be omitted.

Joints and Fastenings

As military bridges must be constructed in the least possible time and usually by unskilled labor, intricate joints, involving expert carpentering, are to be avoided. Generally a plain butt joint, requiring no carpentering, other than squaring the end of a timber, should be used. The fastenings ordinarily employed for hasty bridges include lashings of rope, marline, or wire; dogs; fish-plates or "scabbing"; drift bolts; through (screw) bolts; and spikes.

Lashings are employed in the absence of other materials, or, as in the floating equipage (which is generally lashed throughout), when the bridge is to be later dismantled and the material salvaged. They are not a very secure form of attachment, especially when they actually carry the load, as in the case of the cap of a spar trestle. In such structures it is advisable to dap or notch the timbers together. Marline gives a tighter lashing than rope, but not such a strong one; and small rope is preferable to large. Lashings do not stand racking well, and often are not sufficiently durable even for military uses.

Dogs are easily made of any iron available. They should not be employed to carry a load, but only to hold timber in position.

Drift bolts are commonly employed as fastenings for all parts of a bridge except the deck.  They  are  readily  made  up  in any desired length, and are