the bents, the load to be carried, the materials available which are suitable for bents or stringers, respectively, etc. Crooked stringers are an unmitigated nuisance; and in a hasty bridge, when only a scrubby growth of timber is available, it may be impossible to secure reasonably straight pieces, except in very short lengths. In such a case the spans must be short. On the other hand, if bents must be built of round timber, and if good dimensioned material is available for stringers, the number of trestle bents may be reduced and the span-length increased. If the loads to be carried are moderate, longer spans will, of course, be permissible. The engineer must scan the situation and come to a decision based on his experience and common sense.

However, it is possible to state the usual limits of good practice. Under average conditions the proper economic span of hasty, military trestle-bridges is from 10 to 15 ft., averaging about 12 ft. For heavy standard bridges, designed to carry the greatest loads of the army, the maximum practicable span, when wooden stringers are employed, is 16 ft., which length requires 16-inch stringers. If I-beams be employed, this span may be increased to 22 ft. For any length over this, either strutted beams or some form of truss should be adopted. When very tall trestles are required, it will often be good economy to increase the span by using simple trusses, or by strutting.

In practice, the number of bents and the span of trestles are frequently determined by the material actually available for stringers. These must be able to carry the required loads; and, whenever possible, they are furnished in standard sizes and lengths for this purpose. Where the material for stringers has been cut to a given length, the span of the trestles is, of course, thereby fixed.

For moderate loads, tall trestles may be built in single stories, if material of suitable length is available. For the standard heavy trestles the following rules have been adopted:

Up to 16 ft., one-story bents with one-story bracing.

From 16 to 24 ft., one-story bents with two-story bracing.

Over 24 ft., two or more stories.

The need for longitudinal bracing between bents depends on the height, the span, and the nature of the traffic. If the height of the trestle exceeds 7 ft. and the span is greater than 10 ft., longitudinal bracing should be placed in each alternate bay. For considerable heights it is well to brace all bays, if practicable. In multiple-story bents each story should be separately braced.

Spar-Bridges

The term spar-bridge is a general designation for a military structure of rough (round) timber. Such bridges are built from necessity when better material is not available. They will continue, as in the past, to be a characteristic type in the operations of relatively small and poorly equipped