bridges. Even bamboo has been successfully employed to carry heavy loads.

Timber

Timber can be adapted to the requirements of bridge construction more easily and rapidly than any other material. It is also the most generally available of all bridge materials. For civil constructions it has the disadvantage of lack of permanence—a consideration having little or no weight in military operations.

Timber, accordingly, is the favorite material of the military bridge builder. Dimensioned (sawed) timber is greatly preferable to round timber (unsawed logs and poles) ; first, because it can be much more quickly incorporated in the bridge; second, because it makes a stronger and better structure; and third, because it lends itself better to standardized designs and methods of construction. A few standard sizes of lumber (sawed timber), sufficient to meet the requirements of the standardized designs, are preferable to a great number of miscellaneous sizes that are difficult to adapt to any design.

Military bridges must be and are frequently constructed of round timber, but this is from necessity and not from choice. Round timber is seldom used (except for piles or posts of framed trestles), if dimensioned material is available.

Piling

Piles are frequently employed for abutments, trestles, and piers in situations where the nature of the bottom does not permit the use of framed trestles. The use of piles necessitates a pile-driver, and consequently at the front should be avoided if possible, although frequently necessary there. Framed trestles, which require no plant for their erection, are preferable, even if it be obligatory to rip-rap the bottom in order to increase its bearing power, or to prevent scour. Nevertheless there will be situations, even at the front, where piles must be used; and portable pile-drivers will be a part of the engineer equipment of every army.

Steel

Any steelwork which involves field riveting will find little application in military bridging, even on the lines of communication, for the reason that its erection demands special plant and skilled labor. Rolled sections (I-Beams) are frequently employed as stringers in trestle bridges on the lines of communication, and even at the front, when it is necessary to carry very heavy loads, such as tanks.

Portable, demountable, steel truss-bridges, capable of very rapid erection, and designed for both light and heavy traffic, were employed in the World War. The designs were very ingenious, and such structures will undoubtedly be used to a great extent in future.

Special fabricated-steel girders arc frequently employed for long stringer-