the aid of only animals, simple tackle, and gin-poles or A-frames. There will be difficulty in transporting long pieces for such trusses. Where they must be constructed in the combat zone, the span, consequently, is limited to 40 or 50 ft. in the usual case. Larger trusses may occasionally be built in place; and when plant is available, as in the rear areas, much greater spans will be practicable.

Improvised trusses must be constructed of wood, preferably sawed material, and steel tie-rods or, in exceptional cases, cables. Accordingly, simple triangular (King post), Howe, and Pratt trusses (generally erect or through) will be the usual types. Where only light material is available, lattice or bow-string trusses may be employed. These types should be built as nearly as possible in accordance with civil practice in similar structures. Joints should be as simple as practicable, requiring a minimum of expert carpentering. Round timbers may be used in the simpler forms of trusses, if dimensioned stuff is not available. Deck spans may be employed for reasons similar to those which would dictate their adoption in civil practice.

Sectional Trusses and Girders

Sectional, portable, or "knock-down" trusses and girders, in wood and especially in steel, were the principal development in military bridging during the World War. They were demanded by the augmented density of traffic, and especially by the tremendous increase in military loads, which now include heavy motor-trucks, artillery of great calibers, tractors, and 30-ton tanks. The improvised bridges, long characteristic of military operations on a smaller scale, were, alone, inadequate to meet the unprecedented situation. It was recognized that there was needed some form of standardized superstructure, easily transported, rapidly erected, adaptable to varying spans, and capable of carrying the heaviest loads. This demand was best met by portable, sectional, steel girders and trusses; and various types of such structures were developed by the British, French, and American Armies.

As usual in such cases, a great variety of types was evolved; but, in the interests of efficiency, the number of such should be limited, and each should be made adaptable to varying conditions.

For the American Army, sectional steel girders (I-beams) for spans up to 30 ft., and sectional steel trusses, for spans from 33 to 90 ft., were designed. The British employed sectional trusses in spans up to 180 ft.

In order to permit transportation in motor trucks, the truss sections (panels or bays) were limited to 11 ft. 3 ins. Spans from 33 to 90 ft., in increments, could be erected with this equipage. The sections were connected for erection by bolts, no field rivets being used. These trusses can be employed with any type of support or pier. The truss is erected without falsework, being bolted together on one bank, and placed by launch-