Synopsis
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1897 Excerpt: ...--For a common road bridge, the load is assumed to be a maximum when covered with men, estimated at 120 lbs. to the sq. ft., plus the weight of the bridge, usually taken at about 80 lbs. per lineal ft. For reasons which are evident, the bridge should be as short as possible, with good approaches. Swampy, high, or steep banks should be avoided. 248.--Bridges usually take their names from some part of their construction, as Trestle, Truss, Pile, Suspension, or Floating Bridges. The distance between supports (determined by the strength of the balks to bear the desired load) is called the bay or span, and the corresponding part of the bridge the span. The superstructure, consisting of the stringers or balks, the floor, the side-rails and the fastenings, is of the same nature for each kind, as shown in PI. 35, Fig. 1. The ends of the balks rest on cross pieces of the supports called transoms, on the balks (of which there are usually five) are laid chess or poles, forming the floor; on top of the floor, over the outside balks, are laid side-rails or poles, which are securely fastened every i or 5 ft. to the ballts beneath by rack lashings. Hand-rails (Fig. 2) should always be provided on each side of the roadway. The usual width of military bridges is 9 ft. in the clear, between side-rails; 6 ft. will answer for Infantry in column of two's, and Cavalry by file; 2.5 ft. for Infantry in single file. 249. For determining the strength of the materials to be used, all errors should be on the side of safety. The practical method is to place the ends of the timber on low supports, as far apart as they will he in bridge; as many men as can, then step on it and jump up and down; or it is otherwise arranged so as to bring as great a weight upon it as it will have to bear a...
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